chore(core): merge v2 into queued controls

Merge v2 at 3e9b009642 while preserving queued-control dispatch behavior and regression tests. Update the execution test fixture to the opaque LayerNode replacement API.

packages/cli/src/index.ts

@@ -98,12 +98,13 @@ Effect.gen(function* () {
   Effect.provide(Config.layer),
   Effect.provide(Updater.layer),
   Effect.provide(
-    LayerNode.compile(LayerNode.group([Global.node, AppProcess.node, Npm.node]), [
-      [
-        Global.node,
-        Global.layerWith(process.env.OPENCODE_CONFIG_DIR ? { config: process.env.OPENCODE_CONFIG_DIR } : {}),
+    LayerNode.compile(LayerNode.group([Global.node, AppProcess.node, Npm.node]), {
+      replacements: [
+        Global.node.replace(
+          Global.layerWith(process.env.OPENCODE_CONFIG_DIR ? { config: process.env.OPENCODE_CONFIG_DIR } : {}),
+        ),
       ],
-    ]),
+    }),
   ),
   Effect.provide(
     Observability.layer({

packages/cli/src/server-process.ts

@@ -30,12 +30,13 @@ export const run = Effect.fnUntraced(function* (options: Options) {
   return yield* processEffect(options).pipe(
     Effect.provide(Updater.layer),
     Effect.provide(
-      LayerNode.compile(LayerNode.group([Global.node, AppProcess.node]), [
-        [
-          Global.node,
-          Global.layerWith(process.env.OPENCODE_CONFIG_DIR ? { config: process.env.OPENCODE_CONFIG_DIR } : {}),
+      LayerNode.compile(LayerNode.group([Global.node, AppProcess.node]), {
+        replacements: [
+          Global.node.replace(
+            Global.layerWith(process.env.OPENCODE_CONFIG_DIR ? { config: process.env.OPENCODE_CONFIG_DIR } : {}),
+          ),
         ],
-      ]),
+      }),
     ),
     Effect.provide(NodeServices.layer),
   )

packages/core/src/effect/app-node-builder.ts

@@ -1,20 +1,11 @@
 import { buildLocationServiceMap } from "../location-services.js"
 import { LocationServiceMap } from "../location-service-map.js"
 import { LayerNode } from "@opencode-ai/util/effect/layer-node"
-import { makeGlobalNode } from "@opencode-ai/util/effect/app-node"
 
-export function build<A, E>(root: LayerNode.Node<A, E, any>, replacements: LayerNode.Replacements = []) {
-  // Only build the location service map if it's actually needed
-  if (!LayerNode.hasUnbound(root, LocationServiceMap.node) || hasReplacement(replacements, LocationServiceMap.node))
-    return LayerNode.compile(root, replacements)
-
-  const locationMap = buildLocationServiceMap(replacements)
-  const locationMapNode = makeGlobalNode({ service: LocationServiceMap.Service, layer: locationMap, deps: [] })
-  return LayerNode.compile(root, replacements.concat([[LocationServiceMap.node, locationMapNode]]))
-}
-
-function hasReplacement(replacements: LayerNode.Replacements, node: LayerNode.Node<unknown, unknown, any>) {
-  return replacements.some(([source]) => source.name === node.name)
+export function build<A, E>(root: LayerNode.Graph<A, E>, replacements: LayerNode.Replacements = []) {
+  return LayerNode.compile(root, {
+    replacements: [LocationServiceMap.node.replace(buildLocationServiceMap(replacements)), ...replacements],
+  })
 }
 
 export * as AppNodeBuilder from "./app-node-builder.js"

packages/core/src/instance.ts

@@ -55,6 +55,7 @@ import { ToolOutput } from "./tool-output.js"
 import { Vcs } from "./vcs.js"
 
 export * as Instance from "./instance.js"
+export { Service, byLocationNode, type Interface } from "./instance/service.js"
 
 const nodes = [
   Location.node,
@@ -110,9 +111,9 @@ const nodes = [
   Vcs.node,
   // Start repository watches only after boot-critical filesystem and Git work.
   LocationWatcher.node,
-] as const satisfies readonly Node.LocationNode<unknown, unknown>[]
+] as const satisfies readonly Node.LocationGraph<never, unknown>[]
 
-export const graph = LayerNode.group<typeof nodes>(nodes)
+export const graph = LayerNode.group(nodes)
 
 export type Services = LayerNode.Output<typeof graph>
 export type Error = LayerNode.Error<typeof graph>
@@ -141,29 +142,23 @@ export interface Options {
 // source still honors explicit plugin operations from wellknown and
 // host-injected config.
 const vanillaReplacements: LayerNode.Replacements = [
-  [Config.node, Config.configured({ project: false, global: false })],
-  [InstructionDiscovery.node, InstructionDiscovery.configured({ project: false, global: false })],
+  Config.node.replace(Config.configured({ project: false, global: false })),
+  InstructionDiscovery.node.replace(InstructionDiscovery.configured({ project: false, global: false })),
 ]
 
 // One instance is one compiled, fresh copy of the graph standing on a directory.
 export function layer(ref: Location.Ref, options: Options = {}) {
   const startedAt = performance.now()
   // Ordered: vanilla defaults, then caller replacements (which win over the
-  // defaults), then bound pairs (which win over everything).
-  const allReplacements: LayerNode.Replacements = [
+  // defaults), then instance bindings (which win over everything).
+  const replacements: LayerNode.Replacements = [
     ...(options.discovery === false ? vanillaReplacements : []),
     ...(options.replacements ?? []),
-    [Location.node, Location.boundNode(ref, { discovery: options.discovery })],
-    [InstancePlugins.node, InstancePlugins.bound(options.plugins ?? [])],
+    Location.node.replace(Location.boundNode(ref, { discovery: options.discovery })),
+    InstancePlugins.node.replace(InstancePlugins.bound(options.plugins ?? [])),
   ]
-  // Apply replacements during hoist, not afterward: replacements can
-  // introduce new tagged dependencies (Location.boundNode depends on
-  // Project), and the hoist walk is the only pass that can still slice
-  // those back out.
-  const location = LayerNode.hoist(graph, Node.tags.values.global, allReplacements)
 
-  return LayerNode.compile(location.node).pipe(
-    Layer.fresh,
+  return LayerNode.compile(graph, { replacements, shared: Node.tags.values.global }).pipe(
     Layer.tap(() =>
       Effect.logInfo("location services booted", {
         directory: ref.directory,
@@ -171,6 +166,5 @@ export function layer(ref: Location.Ref, options: Options = {}) {
         durationMs: Math.round(performance.now() - startedAt),
       }),
     ),
-    Layer.provide(LayerNode.compile(location.hoisted)),
   )
 }

packages/core/src/instance/service.ts

@@ -0,0 +1,42 @@
+export * as Instance from "./service.js"
+export type { Services } from "../instance.js"
+
+import { Context, Effect, Layer, Option, Scope } from "effect"
+import type { Session } from "@opencode-ai/schema/session"
+import { makeGlobalNode } from "@opencode-ai/util/effect/app-node"
+import type { Services } from "../instance.js"
+import { LocationServiceMap } from "../location-service-map.js"
+
+/** Selects Session capabilities; implementations own caching and lifetime. */
+export interface Interface {
+  readonly provide: (
+    session: Session.Info,
+  ) => <A, E, R>(effect: Effect.Effect<A, E, R>) => Effect.Effect<A, E, Exclude<R, Services>>
+  /** Borrow a cached instance without initializing one when it is absent. */
+  readonly provideIfLoaded: (
+    session: Session.Info,
+  ) => <A, E, R>(effect: Effect.Effect<A, E, R>) => Effect.Effect<Option.Option<A>, E, Exclude<R, Services>>
+}
+
+export class Service extends Context.Service<Service, Interface>()("@opencode/Instance") {}
+
+const layer = Layer.effect(
+  Service,
+  Effect.gen(function* () {
+    const locations = yield* LocationServiceMap.Service
+    return Service.of({
+      provide: (session) => Effect.provide(locations.get(session.location)),
+      provideIfLoaded: (session) => (effect) =>
+        // Scope the borrowed reference without replacing the caller's Scope.
+        Effect.scopedWith((scope) =>
+          Effect.gen(function* () {
+            const context = yield* locations.contextEffectOption(session.location).pipe(Scope.provide(scope))
+            if (Option.isNone(context)) return Option.none()
+            return Option.some(yield* effect.pipe(Effect.provide(context.value)))
+          }),
+        ),
+    })
+  }),
+)
+
+export const byLocationNode = makeGlobalNode({ service: Service, layer, deps: [LocationServiceMap.node] })

packages/core/src/persistent-pty/index.ts

@@ -112,7 +112,7 @@ export interface Interface {
 
 export class Service extends Context.Service<Service, Interface>()("@opencode/PersistentPty") {}
 
-export const configured = (options: Options = {}) =>
+const makeLayer = (options: Options = {}) =>
   Layer.effect(
     Service,
     Effect.gen(function* () {
@@ -361,8 +361,14 @@ export const configured = (options: Options = {}) =>
     }),
   )
 
-export const layer = configured()
-export const node = makeGlobalNode({ service: Service, layer, deps: [Bus.node, Global.node] })
+export const layer = makeLayer()
+export const configured = (options?: Options) =>
+  makeGlobalNode({
+    service: Service,
+    layer: options === undefined ? layer : makeLayer(options),
+    deps: [Bus.node, Global.node],
+  })
+export const node = configured()
 
 const request = (daemon: DaemonTransport, value: object, start = false) =>
   daemon.request(value, start).pipe(Effect.mapError(unavailable))

packages/core/src/plugin/host.ts

@@ -80,7 +80,8 @@ export const make = Effect.fn("PluginHost.make")(function* (plugin: Interface, p
   const response = <A, E, R>(effect: Effect.Effect<A, E, R>) =>
     effect.pipe(Effect.map((data) => ({ location: locationInfo(), data })))
 
-  return {
+  // Keep the instance graph's inferred types independent of Session handles.
+  const context: Plugin.Context = {
     app,
     location: locationInfo(),
     options: {},
@@ -206,7 +207,8 @@ export const make = Effect.fn("PluginHost.make")(function* (plugin: Interface, p
           .subscribe()
           .pipe(
             Stream.filter(
-              (event): event is EventManifest.ServerEvent | RpcEvent => EventManifest.isServer(event) || isRpcEvent(event),
+              (event): event is EventManifest.ServerEvent | RpcEvent =>
+                EventManifest.isServer(event) || isRpcEvent(event),
             ),
           ),
     },
@@ -449,7 +451,8 @@ export const make = Effect.fn("PluginHost.make")(function* (plugin: Interface, p
       wait: (input) => runtime.session.wait(input.sessionID),
       context: (input) => runtime.session.context(input.sessionID),
     },
-  } satisfies Plugin.Context
+  }
+  return context
 })
 
 export function storage(kv: KV.Interface, pluginID: string): Plugin.Context["storage"] {

packages/core/src/plugin/runtime.ts

@@ -62,7 +62,7 @@ const require = <A, E, R>(cell: Cell, f: (runtime: Interface) => Effect.Effect<A
 
 const defaultCell = makeCell()
 
-export const layerWithCell = (cell: Cell) =>
+export const layerWithCell = (cell: Cell): Layer.Layer<Service> =>
   Layer.succeed(
     Service,
     Service.of({

packages/core/src/plugin/skill/opencode.md

@@ -11,6 +11,9 @@ truth. Follow links from that page when the question needs more detail. Fetch
 <https://opencode.ai/v2/docs/> first when you need to discover the relevant
 documentation page.
 
+A machine-readable documentation index is available at
+<https://opencode.ai/v2/llms.txt>.
+
 ## Version policy
 
 Always answer for OpenCode V2 unless the user explicitly asks about V1,
@@ -152,6 +155,8 @@ before answering. Refer to this guide when the user wants to build a plugin. It
 covers hooks, transforms, tools, plugin context capabilities, and package
 entrypoints. Plugins can also extend the TUI; for those, fetch the
 [CLI plugin guide](https://opencode.ai/v2/docs/build/plugins/cli).
+For custom methods and events shared with other plugins or clients, fetch the
+[RPC guide](https://opencode.ai/v2/docs/build/plugins/rpc).
 
 ## [Service](https://opencode.ai/v2/docs/troubleshooting#check-the-background-service)
 
@@ -220,6 +225,16 @@ exposes typed Effects, Streams, and decoded OpenCode schema values. Its
 `Service` API can discover, start, stop, and authenticate with the local
 background service from a Node application.
 
+## [SDK](https://opencode.ai/v2/docs/build/sdk)
+
+For questions about embedding OpenCode directly in an application, fetch the
+full [SDK guide](https://opencode.ai/v2/docs/build/sdk) before answering. The SDK
+hosts OpenCode in the application without opening an HTTP listener.
+
+Use the [Effect SDK guide](https://opencode.ai/v2/docs/build/sdk/effect) for
+Effect applications. For Cloudflare Durable Objects, use the
+[Cloudflare SDK guide](https://opencode.ai/v2/docs/build/sdk/cloudflare).
+
 ## [Troubleshooting](https://opencode.ai/v2/docs/troubleshooting)
 
 OpenCode runs a client and a background server. Start by determining whether a

packages/core/src/session.ts

@@ -1,7 +1,7 @@
 export * as Session from "./session.js"
 export * from "./session/schema.js"
 
-import { Cause, Effect, Layer, Schema, Context, RcMap, Stream } from "effect"
+import { Cause, Effect, Layer, Schema, Context, Stream } from "effect"
 import { ListAnchor } from "@opencode-ai/schema/session"
 import { and, desc, eq } from "drizzle-orm"
 import { Project } from "./project.js"
@@ -10,6 +10,7 @@ import { Location } from "./location.js"
 import { SessionMessage } from "./session/message.js"
 import { PromptInput } from "@opencode-ai/schema/prompt-input"
 import { Bus } from "./bus.js"
+import { Instance } from "./instance/service.js"
 import { Database } from "./database/database.js"
 import { SessionProjector } from "./session/projector.js"
 import { SessionMessageTable } from "./session/sql.js"
@@ -238,20 +239,16 @@ const layer = Layer.effect(
     const global = yield* Global.Service
     const execution = yield* SessionExecution.Service
     const store = yield* SessionStore.Service
+    const instances = yield* Instance.Service
     const locations = yield* LocationServiceMap.Service
     const fs = yield* FSUtil.Service
     const jobs = yield* Job.Service
     const environments = yield* SessionEnvironment.Service
-    const sessions = yield* Session.make((ref) => locations.get(ref))
+    const sessions = yield* Session.make()
     const admission = yield* SessionInbox.Service
     const closeTransport = Effect.fn("Session.closeTransport")(function* (session: SessionSchema.Info) {
-      const location = Location.Ref.make({
-        directory: session.location.directory,
-        workspaceID: session.location.workspaceID,
-      })
-      if (!(yield* RcMap.has(locations.rcMap, location))) return
       yield* SessionModelTransport.Service.use((transport) => transport.close(session.id)).pipe(
-        Effect.provide(locations.get(location)),
+        instances.provideIfLoaded(session),
       )
     })
     const isDurableSessionEvent = Schema.is(SessionEvent.Durable)
@@ -403,7 +400,7 @@ const layer = Layer.effect(
       prompt: (input) => sessions.forSession(input.sessionID).prompt(input),
       generate: Effect.fn("Session.generate")(function* (input) {
         const session = yield* result.get(input.sessionID)
-        const generate = yield* SessionGenerate.Service.pipe(Effect.provide(locations.get(session.location)))
+        const generate = yield* SessionGenerate.Service.pipe(instances.provide(session))
         return yield* generate.generate(input)
       }),
       command: Effect.fn("Session.command")(function* (input) {
@@ -412,7 +409,7 @@ const layer = Layer.effect(
           const plugins = yield* PluginSupervisor.Service
           yield* plugins.flush
           return yield* Command.Service
-        }).pipe(Effect.provide(locations.get(session.location)))
+        }).pipe(instances.provide(session))
         const delivery = input.delivery ?? "steer"
         yield* commands.execute({
           name: input.command,
@@ -535,6 +532,7 @@ export const node = makeGlobalNode({
     Project.node,
     SessionExecution.node,
     SessionStore.node,
+    Instance.byLocationNode,
     SessionInbox.node,
     LocationServiceMap.node,
     SessionProjector.node,

packages/core/src/session/execution.ts

@@ -4,7 +4,7 @@ import { Cause, Context, Effect, Exit, Layer } from "effect"
 import { Bus } from "../bus.js"
 import { Database } from "../database/database.js"
 import { Job } from "../job.js"
-import { LocationServiceMap } from "../location-service-map.js"
+import { Instance } from "../instance/service.js"
 import { makeGlobalNode } from "@opencode-ai/util/effect/app-node"
 import { SessionEvent } from "./event.js"
 import { SessionRunCoordinator } from "./run-coordinator.js"
@@ -35,7 +35,7 @@ export interface Interface {
   readonly awaitIdle: (sessionID: SessionSchema.ID) => Effect.Effect<void>
 }
 
-/** Routes execution from a Session ID to the runner owned by that Session's Location. */
+/** Routes execution from a Session ID to its selected instance's runner. */
 export class Service extends Context.Service<Service, Interface>()("@opencode/SessionExecution") {}
 
 type InterruptReason = "user" | "shutdown"
@@ -48,12 +48,12 @@ export function terminal(exit: Exit.Exit<void, SessionRunner.RunError>, reason?:
   return { type: "failed" as const, error: toSessionError(failure) }
 }
 
-/** Process-local execution: drains run in this process, routed through the Session's Location graph. */
+/** Process-local execution: drains run in this process using the selected instance. */
 export const layer = Layer.effect(
   Service,
   Effect.gen(function* () {
     const store = yield* SessionStore.Service
-    const locations = yield* LocationServiceMap.Service
+    const instances = yield* Instance.Service
     const bus = yield* Bus.Service
     const jobs = yield* Job.Service
     const db = (yield* Database.Service).db
@@ -90,7 +90,7 @@ export const layer = Layer.effect(
       const result = yield* SessionRunner.Service.use((runner) =>
         runner.drain({ sessionID, force, continuation, promotable }),
       ).pipe(
-        Effect.provide(locations.get(session.location)),
+        instances.provide(session),
         Effect.tapCause((cause) =>
           Cause.hasInterruptsOnly(cause)
             ? Effect.void
@@ -172,7 +172,7 @@ export const layer = Layer.effect(
 export const node = makeGlobalNode({
   service: Service,
   layer,
-  deps: [SessionStore.node, LocationServiceMap.node, Bus.node, Database.node, Job.node],
+  deps: [SessionStore.node, Instance.byLocationNode, Bus.node, Database.node, Job.node],
 })
 
 /** Low-level compatibility layer for callers that only need durable Session recording. */

packages/core/src/session/session.ts

@@ -1,11 +1,11 @@
 export * as Session from "./session.js"
 
-import { DateTime, Effect, Fiber, Layer, Schema, Scope } from "effect"
+import { DateTime, Effect, Fiber, Schema, Scope } from "effect"
 import type { Agent } from "@opencode-ai/schema/agent"
 import type { Model } from "@opencode-ai/schema/model"
 import { Event } from "@opencode-ai/schema/event"
 import { Bus } from "../bus.js"
-import { Location } from "../location.js"
+import { Instance } from "../instance/service.js"
 import { PluginSupervisor } from "../plugin/supervisor-service.js"
 import { Shell } from "../shell.js"
 import { ShellResult } from "../shell/result.js"
@@ -33,26 +33,19 @@ import { SessionRevert } from "./revert.js"
 import { SessionSchema } from "./schema.js"
 import { SessionStore } from "./store.js"
 
-export type Services =
-  | PluginSupervisor.Service
-  | Reference.Service
-  | SessionPrompt.Service
-  | SessionRevert.Service
-  | Shell.Service
-  | Skill.Service
-
 type PromptRequest = SessionPrompt.Input & {
   id?: SessionMessage.ID
   resume?: boolean
 }
 
 /**
- * Build once in the host Scope: `const sessions = yield* Session.make(servicesFor)`.
+ * Build once in the host Scope: `const sessions = yield* Session.make()`.
  * Use `sessions.forSession(id)` for handles that share host services and reload current state.
  */
-export const make = Effect.fn("Session.make")(function* (servicesFor: (ref: Location.Ref) => Layer.Layer<Services>) {
+export const make = Effect.fn("Session.make")(function* () {
   const bus = yield* Bus.Service
   const store = yield* SessionStore.Service
+  const instances = yield* Instance.Service
   const execution = yield* SessionExecution.Service
   const admission = yield* SessionInbox.Service
   const scope = yield* Scope.Scope
@@ -174,7 +167,7 @@ export const make = Effect.fn("Session.make")(function* (servicesFor: (ref: Loca
               const preparation = yield* SessionPrompt.Service
               const references = yield* Reference.Service
               return { item: yield* preparation.prepare({ sessionID, messageID, input }), references }
-            }).pipe(Effect.provide(servicesFor(session.location))),
+            }).pipe(instances.provide(session)),
           )
           // Commit a staged revert only after preparation succeeds, before admitting new work.
           if (session.revert) yield* SessionRevert.commit(bus, session)
@@ -205,7 +198,7 @@ export const make = Effect.fn("Session.make")(function* (servicesFor: (ref: Loca
         const plugins = yield* PluginSupervisor.Service
         yield* plugins.flush
         return yield* Shell.Service
-      }).pipe(Effect.provide(servicesFor(session.location)))
+      }).pipe(instances.provide(session))
       const started = yield* shell
         .create({
           command: input.command,
@@ -256,7 +249,7 @@ export const make = Effect.fn("Session.make")(function* (servicesFor: (ref: Loca
     input: { id?: SessionMessage.ID; skill: Skill.ID; resume?: boolean },
   ) {
     const session = yield* get(sessionID)
-    const skills = yield* Skill.Service.pipe(Effect.provide(servicesFor(session.location)))
+    const skills = yield* Skill.Service.pipe(instances.provide(session))
     const skill = yield* skills.get(input.skill)
     if (!skill) return yield* new SkillNotFoundError({ skill: input.skill })
     yield* bus.publish(
@@ -355,14 +348,12 @@ export const make = Effect.fn("Session.make")(function* (servicesFor: (ref: Loca
     if (yield* execution.isActive(sessionID)) return yield* new BusyError({ sessionID })
     return yield* SessionRevert.Service.use((revert) =>
       revert.stage({ session, messageID: input.messageID, files: input.files }),
-    ).pipe(Effect.provide(servicesFor(session.location)))
+    ).pipe(instances.provide(session))
   })
   const clear = Effect.fn("Session.revert.clear")(function* (sessionID: SessionSchema.ID) {
     const session = yield* get(sessionID)
     if (yield* execution.isActive(sessionID)) return yield* new BusyError({ sessionID })
-    yield* SessionRevert.Service.use((revert) => revert.clear(session)).pipe(
-      Effect.provide(servicesFor(session.location)),
-    )
+    yield* SessionRevert.Service.use((revert) => revert.clear(session)).pipe(instances.provide(session))
     return yield* execution.wake(sessionID)
   })
   const commit = Effect.fn("Session.revert.commit")(function* (sessionID: SessionSchema.ID) {

packages/core/test/agent.test.ts

@@ -22,8 +22,8 @@ const globalLayer = Layer.succeed(Global.Service, Global.Service.of(global))
 
 const it = testEffect(
   AppNodeBuilder.build(LayerNode.group([Agent.node, Bus.node, Location.node]), [
-    [Global.node, globalLayer],
-    [Location.node, locationLayer],
+    Global.node.replace(globalLayer),
+    Location.node.replace(locationLayer),
   ]) as unknown as Layer.Layer<unknown, never>,
 )
 

packages/core/test/bus-session-routing.test.ts

@@ -21,7 +21,7 @@ import { testEffect } from "./lib/effect"
 
 const it = testEffect(
   AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node, SessionProjector.node]), [
-    [Bus.node, Bus.configured({ persist: true })],
+    Bus.node.replace(Bus.configured({ persist: true })),
   ]),
 )
 const a = Location.Ref.make({ directory: AbsolutePath.make("/a") })

packages/core/test/bus.test.ts

@@ -100,12 +100,14 @@ const tail = (bus: Bus.Interface, input: { aggregateID: string; after?: number }
 
 const it = testEffect(
   AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node, Location.node]), [
-    [Location.node, locationLayer],
-    [Bus.node, Bus.configured({ persist: true })],
+    Location.node.replace(locationLayer),
+    Bus.node.replace(Bus.configured({ persist: true })),
   ]),
 )
 const itWithoutLocation = testEffect(
-  AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node]), [[Bus.node, Bus.configured({ persist: true })]]),
+  AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node]), [
+    Bus.node.replace(Bus.configured({ persist: true })),
+  ]),
 )
 const itWithoutPersistence = testEffect(AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node])))
 
@@ -631,8 +633,7 @@ describe("Bus", () => {
       const continueRead = yield* Deferred.make<void>()
       let pause = true
       const eventLayer = AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node]), [
-        [
-          Bus.node,
+        Bus.node.replace(
           Bus.configured({
             persist: true,
             beforeAggregateRead: () =>
@@ -640,7 +641,7 @@ describe("Bus", () => {
                 ? Deferred.succeed(readStarted, undefined).pipe(Effect.andThen(Deferred.await(continueRead)))
                 : Effect.void,
           }),
-        ],
+        ),
       ])
 
       yield* Effect.gen(function* () {
@@ -1318,7 +1319,7 @@ describe("Bus", () => {
   it.effect("log replays across configured read pages", () =>
     Effect.gen(function* () {
       const eventLayer = AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node]), [
-        [Bus.node, Bus.configured({ persist: true, logReadPageSize: 2 })],
+        Bus.node.replace(Bus.configured({ persist: true, logReadPageSize: 2 })),
       ])
 
       yield* Effect.gen(function* () {
@@ -1351,8 +1352,7 @@ describe("Bus", () => {
       const releaseRead = yield* Deferred.make<void>()
       const firstRead = yield* Ref.make(true)
       const eventLayer = AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node]), [
-        [
-          Bus.node,
+        Bus.node.replace(
           Bus.configured({
             persist: true,
             beforeAggregateRead: () =>
@@ -1363,7 +1363,7 @@ describe("Bus", () => {
                 }),
               ),
           }),
-        ],
+        ),
       ])
 
       yield* Effect.gen(function* () {

packages/core/test/catalog.test.ts

@@ -25,7 +25,7 @@ const locationLayer = Layer.succeed(
 )
 const catalogLayer = AppNodeBuilder.build(
   LayerNode.group([Catalog.node, Bus.node, Credential.node, Integration.node]),
-  [[Location.node, locationLayer]],
+  [Location.node.replace(locationLayer)],
 )
 const it = testEffect(catalogLayer)
 
@@ -48,7 +48,7 @@ describe("Catalog", () => {
   it.effect("derives availability from active credentials without changing provider state", () => {
     const integrationID = Integration.ID.make("test")
     const localCatalogLayer = Layer.fresh(
-      AppNodeBuilder.build(LayerNode.group([Catalog.node, Credential.node]), [[Location.node, locationLayer]]),
+      AppNodeBuilder.build(LayerNode.group([Catalog.node, Credential.node]), [Location.node.replace(locationLayer)]),
     )
 
     return Effect.gen(function* () {
@@ -78,7 +78,7 @@ describe("Catalog", () => {
     const providerID = Provider.ID.make("remote")
     const localCatalogLayer = Layer.fresh(
       AppNodeBuilder.build(LayerNode.group([Catalog.node, Credential.node, Integration.node]), [
-        [Location.node, locationLayer],
+        Location.node.replace(locationLayer),
       ]),
     )
 
@@ -108,7 +108,7 @@ describe("Catalog", () => {
     const providerID = Provider.ID.make("remote")
     const localCatalogLayer = Layer.fresh(
       AppNodeBuilder.build(LayerNode.group([Catalog.node, Credential.node, Integration.node]), [
-        [Location.node, locationLayer],
+        Location.node.replace(locationLayer),
       ]),
     )
 

packages/core/test/codemode.test.ts

@@ -35,7 +35,7 @@ describe("CodeMode", () => {
       Effect.scoped,
       Effect.provide(
         AppNodeBuilder.build(Tool.node, [
-          [Location.node, Location.boundNode({ directory: AbsolutePath.make("/project") })],
+          Location.node.replace(Location.boundNode({ directory: AbsolutePath.make("/project") })),
         ]),
       ),
     ),

packages/core/test/codemode/instructions.test.ts

@@ -85,7 +85,7 @@ describe("CodeModeInstructions", () => {
       execute: () => Effect.succeed({ output: "zeta" }),
     }
     const layer = AppNodeBuilder.build(Tool.node, [
-      [Location.node, Location.boundNode({ directory: AbsolutePath.make("/project") })],
+      Location.node.replace(Location.boundNode({ directory: AbsolutePath.make("/project") })),
     ])
 
     return Effect.gen(function* () {

packages/core/test/config/command.test.ts

@@ -43,10 +43,10 @@ const it = testEffect(
   AppNodeBuilder.build(
     LayerNode.group([Command.node, Bus.node, FSUtil.node, AppProcess.node, Location.node, ShellSelect.node]),
     [
-      [Mcp.node, emptyMcpLayer],
-      [Config.node, emptyConfigLayer],
-      [Location.node, testLocationLayer],
-      [ShellSelect.node, shellLayer],
+      Mcp.node.replace(emptyMcpLayer),
+      Config.node.replace(emptyConfigLayer),
+      Location.node.replace(testLocationLayer),
+      ShellSelect.node.replace(shellLayer),
     ],
   ),
 )
@@ -340,17 +340,16 @@ describeNative("ConfigCommandPlugin native watcher", () => {
               ShellSelect.node,
             ]),
             [
-              [
-                Location.node,
+              Location.node.replace(
                 Layer.succeed(
                   Location.Service,
                   Location.Service.of(location({ directory: AbsolutePath.make(path.join(tmp, "project")) })),
                 ),
-              ],
-              [Global.node, Global.layerWith({ config: global, home: path.join(global, "home") })],
-              [ShellSelect.node, shellLayer],
-              [Credential.node, emptyCredentialNode],
-              [WellKnown.node, emptyWellknownNode],
+              ),
+              Global.node.replace(Global.layerWith({ config: global, home: path.join(global, "home") })),
+              ShellSelect.node.replace(shellLayer),
+              Credential.node.replace(emptyCredentialNode),
+              WellKnown.node.replace(emptyWellknownNode),
             ],
           ),
         ),

packages/core/test/config/compaction.test.ts

@@ -40,13 +40,12 @@ const it = testEffect(
   Layer.merge(
     config,
     AppNodeBuilder.build(LayerNode.group([SessionCompaction.node, SessionModelRequest.node, Config.node, Bus.node]), [
-      [
-        llmClient,
+      llmClient.replace(
         Layer.mock(LLMClient.Service)({
           stream: () => Stream.make(LLMEvent.textDelta({ id: "summary", text: "summary" })),
         }),
-      ],
-      [Config.node, config],
+      ),
+      Config.node.replace(config),
     ]),
   ),
 )

packages/core/test/config/config.test.ts

@@ -55,12 +55,12 @@ function testLayer(
     ),
   )
   const built = AppNodeBuilder.build(LayerNode.group([Config.node, Bus.node]), [
-    [Config.node, Config.configured(options)],
-    [Location.node, locationLayer],
-    [Global.node, Global.layerWith({ config: globalDirectory, home: path.join(globalDirectory, "home") })],
-    [Credential.node, credentialNode],
-    [WellKnown.node, wellknownNode],
-    [Watcher.node, watcher],
+    Config.node.replace(Config.configured(options)),
+    Location.node.replace(locationLayer),
+    Global.node.replace(Global.layerWith({ config: globalDirectory, home: path.join(globalDirectory, "home") })),
+    Credential.node.replace(credentialNode),
+    WellKnown.node.replace(wellknownNode),
+    Watcher.node.replace(watcher),
   ])
   // Merge the watcher layer by reference so Watcher.Test resolves to the same
   // memoized instance the built graph uses.
@@ -311,16 +311,15 @@ describe("Config", () => {
           }).pipe(
             Effect.provide(
               AppNodeBuilder.build(LayerNode.group([Config.node, Bus.node]), [
-                [
-                  Location.node,
+                Location.node.replace(
                   Layer.succeed(
                     Location.Service,
                     Location.Service.of(location({ directory: AbsolutePath.make(project) })),
                   ),
-                ],
-                [Global.node, Global.layerWith({ config: global, home: path.join(global, "home") })],
-                [Credential.node, emptyCredentialNode],
-                [WellKnown.node, emptyWellknownNode],
+                ),
+                Global.node.replace(Global.layerWith({ config: global, home: path.join(global, "home") })),
+                Credential.node.replace(emptyCredentialNode),
+                WellKnown.node.replace(emptyWellknownNode),
               ]),
             ),
           )

packages/core/test/config/plugin.test.ts

@@ -28,13 +28,13 @@ import { testEffect } from "../lib/effect"
 
 const it = testEffect(
   AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node, SdkPlugins.node, LocationServiceMap.node]), [
-    [Global.node, tempGlobalLayer],
+    Global.node.replace(tempGlobalLayer),
   ]),
 )
 const staticIt = testEffect(
   AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node, SdkPlugins.node, LocationServiceMap.node]), [
-    [ConfigPluginSource.node, ConfigPluginSource.empty],
-    [Global.node, tempGlobalLayer],
+    ConfigPluginSource.node.replace(ConfigPluginSource.empty),
+    Global.node.replace(tempGlobalLayer),
   ]),
 )
 const refreshNpm = makeGlobalNode({
@@ -65,10 +65,7 @@ const refreshNpm = makeGlobalNode({
 const refreshIt = testEffect(
   AppNodeBuilder.build(
     LayerNode.group([Database.node, Bus.node, SdkPlugins.node, LocationServiceMap.node, Global.node]),
-    [
-      [Global.node, tempGlobalLayer],
-      [Npm.node, refreshNpm],
-    ],
+    [Global.node.replace(tempGlobalLayer), Npm.node.replace(refreshNpm)],
   ),
 )
 

packages/core/test/config/skill.test.ts

@@ -86,14 +86,13 @@ const discover = (directory: string, global: string) =>
   }).pipe(
     Effect.provide(
       AppNodeBuilder.build(LayerNode.group([Config.node, Bus.node]), [
-        [
-          Location.node,
+        Location.node.replace(
           Layer.succeed(Location.Service, Location.Service.of(location({ directory: AbsolutePath.make(directory) }))),
-        ],
-        [Global.node, Global.layerWith({ config: global, home: path.join(global, "home") })],
-        [Credential.node, emptyCredentialNode],
-        [WellKnown.node, emptyWellknownNode],
-        [Watcher.node, Watcher.testLayer],
+        ),
+        Global.node.replace(Global.layerWith({ config: global, home: path.join(global, "home") })),
+        Credential.node.replace(emptyCredentialNode),
+        WellKnown.node.replace(emptyWellknownNode),
+        Watcher.node.replace(Watcher.testLayer),
       ]),
     ),
   )

packages/core/test/config/snapshot.test.ts

@@ -51,8 +51,8 @@ describe("ConfigSnapshotPlugin.Plugin", () => {
           }).pipe(
             Effect.provide(
               AppNodeBuilder.build(Snapshot.node, [
-                [Location.node, Location.boundNode(Location.Ref.make({ directory: AbsolutePath.make(project) }))],
-                [Global.node, Global.layerWith({ data: tmp.path, config: path.join(tmp.path, "config") })],
+                Location.node.replace(Location.boundNode(Location.Ref.make({ directory: AbsolutePath.make(project) }))),
+                Global.node.replace(Global.layerWith({ data: tmp.path, config: path.join(tmp.path, "config") })),
               ]),
             ),
           )

packages/core/test/config/tool-output.test.ts

@@ -44,7 +44,9 @@ describe("ConfigToolOutputPlugin.Plugin", () => {
           }
           yield* Effect.die(new Error("Timed out waiting for tool output config reload"))
         }).pipe(
-          Effect.provide(AppNodeBuilder.build(ToolOutput.node, [[Global.node, Global.layerWith({ data: tmp.path })]])),
+          Effect.provide(
+            AppNodeBuilder.build(ToolOutput.node, [Global.node.replace(Global.layerWith({ data: tmp.path }))]),
+          ),
         ),
       (tmp) => Effect.promise(() => tmp[Symbol.asyncDispose]()),
     ).pipe(

packages/core/test/effect/layer-node/layer-node-types.test.ts

@@ -13,131 +13,218 @@ class OtherError {
   readonly _tag = "OtherError"
 }
 
-const tags = LayerNode.tags({ app: [] })
-const make = tags.make("app")
-const build = <A, E>(root: LayerNode.Node<A, E, any>) => LayerNode.compile(root) as Layer.Layer<A, E>
-const aLayer = Layer.succeed(A, A.of({}))
-const bLayer = Layer.effect(B, Effect.as(A, B.of({})))
-const cLayer = Layer.effect(
-  C,
-  Effect.gen(function* () {
-    yield* A
-    yield* B
-    return C.of({})
-  }),
-)
-const failingA = Layer.effect(A, Effect.fail(new LayerError()))
-const a = make({ service: A, layer: aLayer, deps: [] })
-const b = make({ service: B, layer: bLayer, deps: [a] })
-const c = make({ service: C, layer: cLayer, deps: [a, b] })
-const failing = make({ service: A, layer: failingA, deps: [] })
-const dependent = make({ service: B, layer: bLayer, deps: [failing] })
-const inputA = LayerNode.unbound(A, tags.values.app)
-const inputDependent = make({ service: B, layer: bLayer, deps: [inputA] })
+// Keep intentionally invalid expressions out of runtime execution.
+const contracts = (tag: LayerNode.Tag<"app"> | LayerNode.Tag<"other">, flag: boolean) => {
+  const tags = LayerNode.tags({ app: [] })
+  const make = tags.make("app")
+  const aLayer = Layer.succeed(A, A.of({}))
+  const bLayer = Layer.effect(B, Effect.as(A, B.of({})))
+  const cLayer = Layer.effect(
+    C,
+    Effect.gen(function* () {
+      yield* A
+      yield* B
+      return C.of({})
+    }),
+  )
+  const a = make({ service: A, layer: aLayer, deps: [] })
+  const b = make({ service: B, layer: bLayer, deps: [a] })
+  const c = make({ service: C, layer: cLayer, deps: [a, b] })
+  const ab = make({ name: "a-and-b", layer: Layer.mergeAll(aLayer, Layer.succeed(B, {})), deps: [] })
+  const failing = make({ service: A, layer: Layer.effect(A, Effect.fail(new LayerError())), deps: [] })
+  const dependent = make({ service: B, layer: bLayer, deps: [failing] })
+  const inputA = LayerNode.unbound(A, tags.values.app)
+  const group = LayerNode.group([a, b])
 
-make({ name: "manual-a", layer: aLayer, deps: [] })
+  make({ name: "manual-a", layer: aLayer, deps: [] })
+  // @ts-expect-error A node must have a service or name
+  make({ layer: aLayer, deps: [] })
+  // @ts-expect-error Service and name are mutually exclusive
+  make({ service: A, name: "a", layer: aLayer, deps: [] })
+  // @ts-expect-error An explicit tagged contract requires a corresponding runtime tag
+  LayerNode.make<typeof aLayer, readonly [], typeof tags.values.app>({ service: A, layer: aLayer, deps: [] })
+  // @ts-expect-error B requires A
+  make({ service: B, layer: bLayer, deps: [] })
+  // @ts-expect-error C requires A and B
+  make({ service: C, layer: cLayer, deps: [a] })
+  const erasedLayer: Layer.Any = bLayer
+  // @ts-expect-error Erasing a Layer's contract cannot hide its inputs and errors
+  make({ service: B, layer: erasedLayer, deps: [] })
 
-// @ts-expect-error A node must have a service or name
-make({ layer: aLayer, deps: [] })
+  LayerNode.compile(c) satisfies Layer.Layer<C, never, never>
+  LayerNode.compile(dependent) satisfies Layer.Layer<B, LayerError, never>
+  LayerNode.compile(group) satisfies Layer.Layer<A | B, never, never>
+  LayerNode.compile(LayerNode.group([])) satisfies Layer.Layer<never>
+  // @ts-expect-error An empty graph cannot supply arbitrary services
+  LayerNode.compile(LayerNode.group([])) satisfies Layer.Layer<A>
+  LayerNode.compile(inputA, { replacements: [inputA.replace(a)] }) satisfies Layer.Layer<A, never, never>
+  // @ts-expect-error A is a private dependency, not a root output
+  LayerNode.compile(c) satisfies Layer.Layer<A | C>
+  // @ts-expect-error Dependency failures are not erased
+  LayerNode.compile(dependent) satisfies Layer.Layer<B>
 
-// @ts-expect-error Service and name are mutually exclusive
-make({ service: A, name: "a", layer: aLayer, deps: [] })
-
-// @ts-expect-error B requires A
-make({ service: B, layer: bLayer, deps: [] })
-
-// @ts-expect-error C requires A and B
-make({ service: C, layer: cLayer, deps: [a] })
-
-const closed = build(LayerNode.group([c]))
-const closedWithError = build(LayerNode.group([dependent]))
-const checkClosed: Layer.Layer<C, never, never> = closed
-const checkError: Layer.Layer<B, LayerError, never> = closedWithError
-void checkClosed
-void checkError
-
-LayerNode.compile(a, [[a, Layer.succeed(A, A.of({}))]])
-LayerNode.compile(a, [[a, make({ service: A, layer: Layer.succeed(A, A.of({})), deps: [] })]])
-
-// @ts-expect-error Replacement must provide A
-LayerNode.compile(a, [[a, Layer.succeed(B, B.of({}))]])
-
-// @ts-expect-error Node replacement must provide A
-const invalidNodeReplacement = () => LayerNode.compile(a, [[a, b]])
-void invalidNodeReplacement
-
-// @ts-expect-error Replacement cannot introduce a new error
-LayerNode.compile(a, [[a, Layer.effect(A, Effect.fail(new OtherError()))]])
-
-const invalidNodeErrorReplacement = () =>
+  const replacements: LayerNode.Replacements = [a.replace(aLayer), a.replace(ab), failing.replace(a)]
+  const replacement: LayerNode.Replacement = a.replace(Layer.mergeAll(aLayer, Layer.succeed(B, {})))
+  LayerNode.compile(a, { replacements: [...replacements, replacement] })
+  inputA.replace(a)
+  a.replace(a)
+  // @ts-expect-error Closed layer replacements must provide every source output
+  ab.replace(aLayer)
+  // @ts-expect-error Node replacements must provide every source output
+  ab.replace(a)
+  // @ts-expect-error Replacement must provide A
+  a.replace(Layer.succeed(B, {}))
+  // @ts-expect-error Node replacement must provide A
+  a.replace(b)
+  // @ts-expect-error Raw layers with inputs are not closed
+  a.replace(Layer.effect(A, Effect.as(B, A.of({}))))
+  // @ts-expect-error Replacement cannot introduce a new error
+  a.replace(Layer.effect(A, Effect.fail(new OtherError())))
   // @ts-expect-error Node replacement cannot introduce a new error
-  LayerNode.compile(a, [[a, make({ service: A, layer: Layer.effect(A, Effect.fail(new OtherError())), deps: [] })]])
-void invalidNodeErrorReplacement
+  a.replace(failing)
+  // @ts-expect-error Existing errors do not authorize unrelated replacement errors
+  failing.replace(Layer.effect(A, Effect.fail(new OtherError())))
+  // @ts-expect-error Every alternative of a node replacement must supply A
+  a.replace(flag ? a : b)
+  // @ts-expect-error Every alternative of a raw-layer replacement must supply A
+  a.replace(flag ? aLayer : Layer.succeed(B, {}))
+  // @ts-expect-error A valid alternative cannot hide a new error in another alternative
+  a.replace(flag ? a : failing)
+  a.replace(flag ? a : ab)
+  failing.replace(flag ? a : failing)
+  // @ts-expect-error Storing replacements must not erase their validation
+  const invalidStored: LayerNode.Replacements = [a.replace(b)]
+  // @ts-expect-error Raw tuples cannot be stored as opaque replacements
+  const rawStored: LayerNode.Replacements = [[a, aLayer]]
+  // @ts-expect-error Raw tuples cannot be supplied to compile
+  LayerNode.compile(a, { replacements: [[a, aLayer]] })
+  // @ts-expect-error Replacements are not structurally forgeable
+  const forged: LayerNode.Replacement = { source: a, target: a }
+  // @ts-expect-error Groups are not replaceable nodes
+  group.replace(a)
+  // @ts-expect-error Groups cannot be replacement targets
+  a.replace(group)
+  // @ts-expect-error Groups cannot be widened to nodes
+  const groupNode: LayerNode.Node<A | B, never, typeof tags.values.app> = group
+  // @ts-expect-error Graphs are opaque
+  const forgedGraph: LayerNode.Graph<A> = { name: "a" }
 
-class TagA extends Context.Service<TagA, {}>()("test/TagA") {}
-class TagB extends Context.Service<TagB, {}>()("test/TagB") {}
-class TagC extends Context.Service<TagC, {}>()("test/TagC") {}
+  const aContract: LayerNode.Node<A, never, typeof tags.values.app> = a
+  aContract.replace(aLayer)
+  // @ts-expect-error A method cannot be rebound to a declaration with a stronger contract
+  a.replace.call(ab, aLayer)
+  const detached = a.replace
+  // @ts-expect-error Replacement authority requires its checked receiver
+  detached(aLayer)
+  // @ts-expect-error Output narrowing cannot forget B before replacement
+  const narrowedOutput: LayerNode.Node<A, never, typeof tags.values.app> = ab
+  // @ts-expect-error Output widening cannot add B before replacement
+  const widenedOutput: LayerNode.Node<A | B, never, typeof tags.values.app> = a
+  // @ts-expect-error Error widening cannot authorize a new replacement error
+  const widenedError: LayerNode.Node<A, LayerError, typeof tags.values.app> = a
+  // @ts-expect-error Error narrowing cannot forget an existing failure
+  const narrowedError: LayerNode.Node<A, never, typeof tags.values.app> = failing
+  // @ts-expect-error Tag widening cannot authorize replacement across tags
+  const widenedTag: LayerNode.Node<A, never, LayerNode.Tag | undefined> = a
+  const unionTag = LayerNode.unbound(A, tag)
+  // @ts-expect-error Tag narrowing cannot forget a possible tag
+  const narrowedTag: LayerNode.Node<A, never, typeof tags.values.app> = unionTag
 
-const scopedTags = LayerNode.tags({ request: ["global"], global: [] })
-const request = scopedTags.make("request")
-const global = scopedTags.make("global")
-const globalA = global({ service: TagA, layer: Layer.succeed(TagA, TagA.of({})), deps: [] })
-const requestA = request({ service: TagA, layer: Layer.succeed(TagA, TagA.of({})), deps: [] })
-const requestB = request({ service: TagB, layer: Layer.succeed(TagB, TagB.of({})), deps: [] })
-const tagBLayer = Layer.effect(TagB, Effect.as(TagA, TagB.of({})))
-const tagCLayer = Layer.effect(
-  TagC,
-  Effect.gen(function* () {
-    yield* TagA
-    yield* TagB
-    return TagC.of({})
-  }),
-)
+  const outputProjection: LayerNode.Graph<A, never, typeof tags.values.app> = group
+  // @ts-expect-error Graph output projection cannot invent a service
+  const widenedGraph: LayerNode.Graph<A | B, never, typeof tags.values.app> = a
+  // @ts-expect-error A projected Graph has no replacement authority
+  outputProjection.replace(aLayer)
 
-request({ service: TagB, layer: tagBLayer, deps: [globalA] })
-request({ service: TagC, layer: tagCLayer, deps: [globalA, requestB] })
-request({ service: TagC, layer: tagCLayer, deps: [LayerNode.group([globalA, requestB])] })
+  const choice = flag ? a : b
+  // @ts-expect-error Choosing one dependency does not provide both services
+  make({ service: C, layer: cLayer, deps: [choice] })
+  // @ts-expect-error A conditional root promises only outputs present in every alternative
+  LayerNode.compile(LayerNode.group([choice])) satisfies Layer.Layer<A | B>
+  const conditional = make({ name: "conditional", layer: flag ? aLayer : Layer.succeed(B, {}), deps: [] })
+  LayerNode.compile(conditional) satisfies Layer.Layer<never>
+  // @ts-expect-error A conditional implementation does not acquire both branches
+  LayerNode.compile(conditional) satisfies Layer.Layer<A | B>
+  LayerNode.compile(LayerNode.group([flag ? a : ab])) satisfies Layer.Layer<A>
+  const dynamic: Array<typeof a> = []
+  // @ts-expect-error An unbounded array may contain no roots
+  LayerNode.compile(LayerNode.group(dynamic)) satisfies Layer.Layer<A>
 
-// @ts-expect-error Tag configuration can only reference declared tags
-LayerNode.tags({ request: ["missing"], global: [] })
+  const decorated = b.mapLayer((layer) => layer.pipe(Layer.tap(() => Effect.void)))
+  LayerNode.compile(decorated) satisfies Layer.Layer<B>
+  b.replace(decorated)
+  // @ts-expect-error A layer mapper cannot be rebound to a weaker declaration
+  ab.mapLayer.call(a, (layer) => layer)
+  // @ts-expect-error mapLayer cannot add an input requirement
+  b.mapLayer((layer) => layer.pipe(Layer.tap(() => C)))
+  // @ts-expect-error mapLayer cannot grow the error channel
+  b.mapLayer((layer) => layer.pipe(Layer.tap(() => Effect.fail(new OtherError()))))
+  // @ts-expect-error mapLayer cannot drop an output
+  ab.mapLayer(() => aLayer)
+  // @ts-expect-error Unbound declarations have no implementation to map
+  inputA.mapLayer((layer: Layer.Layer<A>) => layer)
 
-// @ts-expect-error An unrelated dependency cannot satisfy TagA
-request({ service: TagB, layer: tagBLayer, deps: [requestB] })
+  const scopedTags = LayerNode.tags({ request: ["global"], global: [] })
+  const request = scopedTags.make("request")
+  const global = scopedTags.make("global")
+  const globalA = global({ service: A, layer: aLayer, deps: [] })
+  const requestA = request({ service: A, layer: aLayer, deps: [] })
+  const requestB = request({ service: B, layer: Layer.succeed(B, {}), deps: [] })
+  request({ service: B, layer: bLayer, deps: [globalA] })
+  request({ service: C, layer: cLayer, deps: [globalA, requestB] })
+  request({ service: C, layer: cLayer, deps: [LayerNode.group([globalA, requestB])] })
+  LayerNode.compile(LayerNode.group([globalA, requestB]), { shared: scopedTags.values.global }) satisfies Layer.Layer<
+    A | B
+  >
+  // @ts-expect-error Tag configuration can only reference declared tags
+  LayerNode.tags({ request: ["missing"], global: [] })
+  // @ts-expect-error Shared tags must be branded
+  LayerNode.compile(globalA, { shared: "global" })
+  // @ts-expect-error Replacement targets must keep the source tag
+  globalA.replace(requestA)
+  // @ts-expect-error Replacement targets must keep the source tag in either direction
+  requestA.replace(globalA)
+  // @ts-expect-error Every alternative must keep the source tag
+  globalA.replace(flag ? globalA : requestA)
+  // @ts-expect-error Providing only A leaves B missing
+  request({ service: C, layer: cLayer, deps: [globalA] })
+  // @ts-expect-error Providing only B leaves A missing
+  request({ service: C, layer: cLayer, deps: [requestB] })
+  // @ts-expect-error Duplicate A providers still leave B missing
+  request({ service: C, layer: cLayer, deps: [globalA, requestA] })
+  // @ts-expect-error A group with only A still leaves B missing
+  request({ service: C, layer: cLayer, deps: [LayerNode.group([globalA])] })
+  // @ts-expect-error Global cannot depend on request
+  global({ service: B, layer: bLayer, deps: [requestA] })
+  // @ts-expect-error Groups preserve their child tags
+  global({ service: B, layer: bLayer, deps: [LayerNode.group([requestA])] })
 
-// @ts-expect-error Providing only TagA leaves TagB missing
-request({ service: TagC, layer: tagCLayer, deps: [globalA] })
+  const globalScopedA = makeGlobalNode({ service: A, layer: aLayer, deps: [] })
+  const locationScopedA = makeLocationNode({ service: A, layer: aLayer, deps: [] })
+  makeGlobalNode({ service: B, layer: bLayer, deps: [globalScopedA] })
+  makeLocationNode({ service: B, layer: bLayer, deps: [globalScopedA] })
+  makeLocationNode({ service: B, layer: bLayer, deps: [locationScopedA] })
+  // @ts-expect-error Global nodes cannot depend on location nodes
+  makeGlobalNode({ service: B, layer: bLayer, deps: [locationScopedA] })
+  // @ts-expect-error B requires A
+  makeLocationNode({ service: B, layer: bLayer, deps: [] })
 
-// @ts-expect-error Providing only TagB leaves TagA missing
-request({ service: TagC, layer: tagCLayer, deps: [requestB] })
+  void [
+    invalidStored,
+    rawStored,
+    forged,
+    groupNode,
+    forgedGraph,
+    narrowedOutput,
+    widenedOutput,
+    widenedError,
+    narrowedError,
+    widenedTag,
+    narrowedTag,
+    widenedGraph,
+  ]
+}
 
-// @ts-expect-error Duplicate TagA providers still leave TagB missing
-request({ service: TagC, layer: tagCLayer, deps: [globalA, requestA] })
-
-// @ts-expect-error A group with only TagA still leaves TagB missing
-request({ service: TagC, layer: tagCLayer, deps: [LayerNode.group([globalA])] })
-
-// @ts-expect-error Global cannot depend on request
-global({ service: TagB, layer: tagBLayer, deps: [requestA] })
-
-// @ts-expect-error Groups preserve their child tags
-global({ service: TagB, layer: tagBLayer, deps: [LayerNode.group([requestA])] })
-
-class ScopedA extends Context.Service<ScopedA, {}>()("test/ScopedA") {}
-class ScopedB extends Context.Service<ScopedB, {}>()("test/ScopedB") {}
-
-const scopedA = Layer.succeed(ScopedA, ScopedA.of({}))
-const scopedB = Layer.effect(ScopedB, Effect.as(ScopedA, ScopedB.of({})))
-const globalScopedA = makeGlobalNode({ service: ScopedA, layer: scopedA, deps: [] })
-const locationScopedA = makeLocationNode({ service: ScopedA, layer: scopedA, deps: [] })
-
-makeGlobalNode({ service: ScopedB, layer: scopedB, deps: [globalScopedA] })
-makeLocationNode({ service: ScopedB, layer: scopedB, deps: [globalScopedA] })
-makeLocationNode({ service: ScopedB, layer: scopedB, deps: [locationScopedA] })
-
-// @ts-expect-error Global nodes cannot depend on location nodes
-makeGlobalNode({ service: ScopedB, layer: scopedB, deps: [locationScopedA] })
-
-// @ts-expect-error ScopedB requires ScopedA
-makeLocationNode({ service: ScopedB, layer: scopedB, deps: [] })
-
-test("type exploration compiles", () => {})
+test("layer node type contracts compile", () => {
+  void contracts
+})

packages/core/test/effect/layer-node/layer-node.test.ts

@@ -1,19 +1,21 @@
 import { describe, expect, test } from "bun:test"
-import { Context, Effect, Layer } from "effect"
+import { Context, Deferred, Duration, Effect, Fiber, Layer, LayerMap, Option } from "effect"
 import { LayerNode } from "@opencode-ai/util/effect/layer-node"
+import { testEffect } from "../../lib/effect"
 
 class Value extends Context.Service<Value, { readonly value: string }>()("test/LayerNodeValue") {}
 class Greeting extends Context.Service<Greeting, { readonly value: string }>()("test/LayerNodeGreeting") {}
 class Left extends Context.Service<Left, { readonly value: string }>()("test/LayerNodeLeft") {}
 class Right extends Context.Service<Right, { readonly value: string }>()("test/LayerNodeRight") {}
-class Database extends Context.Service<Database, { readonly name: string }>()("test/GraphDatabase") {}
-class Users extends Context.Service<Users, { readonly list: Effect.Effect<string[]> }>()("test/GraphUsers") {}
-class App extends Context.Service<App, { readonly run: Effect.Effect<string[]> }>()("test/GraphApp") {}
+class Memo extends Context.Service<Memo, Layer.MemoMap>()("test/LayerNodeMemo") {}
+class Support extends Context.Service<Support, {}>()("test/LayerNodeSupport") {}
+class Locations extends Context.Service<Locations, LayerMap.LayerMap<string, Value | Right, "failed location">>()(
+  "test/LayerNodeLocations",
+) {}
 
+const it = testEffect(Layer.empty)
 const tags = LayerNode.tags({ app: [] })
 const make = tags.make("app")
-const build = <A, E>(root: LayerNode.Node<A, E, any>, replacements?: readonly LayerNode.Replacement[]) =>
-  LayerNode.compile(root, replacements) as Layer.Layer<A, E>
 const valueLayer = Layer.succeed(Value, Value.of({ value: "production" }))
 const greetingLayer = Layer.effect(
   Greeting,
@@ -23,240 +25,443 @@ const value = make({ service: Value, layer: valueLayer, deps: [] })
 const greeting = make({ service: Greeting, layer: greetingLayer, deps: [value] })
 
 describe("layer node", () => {
-  test("builds an untagged graph", async () => {
-    const value = LayerNode.make({ service: Value, layer: valueLayer, deps: [] })
-    const greeting = LayerNode.make({ service: Greeting, layer: greetingLayer, deps: [value] })
-    const program = Effect.map(Greeting, (item) => item.value).pipe(
-      Effect.provide(LayerNode.compile(LayerNode.group([greeting]))),
+  it.effect("builds an untagged graph", () =>
+    Effect.gen(function* () {
+      const value = LayerNode.make({ service: Value, layer: valueLayer, deps: [] })
+      const greeting = LayerNode.make({ service: Greeting, layer: greetingLayer, deps: [value] })
+      const result = yield* Greeting.pipe(Effect.provide(LayerNode.compile(LayerNode.group([greeting]))))
+      expect(result.value).toBe("hello production")
+    }),
+  )
+
+  it.effect("exposes roots but hides transitive dependencies", () =>
+    Effect.gen(function* () {
+      const context = yield* Layer.build(LayerNode.compile(LayerNode.group([greeting])))
+      expect(Context.get(context, Greeting).value).toBe("hello production")
+      expect(Option.isNone(Context.getOption(context, Value))).toBe(true)
+    }),
+  )
+
+  it.effect("replaces exact declarations, not sibling names or native layer identities", () =>
+    Effect.gen(function* () {
+      const sibling = make({ service: Value, layer: valueLayer, deps: [] })
+      const target = make({ name: "different-name", layer: Layer.succeed(Value, { value: "replaced" }), deps: [] })
+      const left = make({
+        service: Left,
+        layer: Layer.effect(
+          Left,
+          Effect.map(Value, (item) => Left.of({ value: item.value })),
+        ),
+        deps: [value],
+      })
+      const right = make({
+        service: Right,
+        layer: Layer.effect(
+          Right,
+          Effect.map(Value, (item) => Right.of({ value: item.value })),
+        ),
+        deps: [sibling],
+      })
+      const context = yield* Layer.build(
+        LayerNode.compile(LayerNode.group([left, right]), { replacements: [value.replace(target)] }),
+      )
+      expect(Context.get(context, Left).value).toBe("replaced")
+      expect(Context.get(context, Right).value).toBe("production")
+    }),
+  )
+
+  it.effect("requires reachable unbound nodes to be replaced", () =>
+    Effect.gen(function* () {
+      const unbound = LayerNode.unbound(Value, tags.values.app)
+      const root = make({ service: Greeting, layer: greetingLayer, deps: [unbound] })
+      expect(() => LayerNode.compile(root)).toThrow("Unbound layer node: test/LayerNodeValue")
+      const result = yield* Greeting.pipe(
+        Effect.provide(LayerNode.compile(root, { replacements: [unbound.replace(value)] })),
+      )
+      expect(result.value).toBe("hello production")
+    }),
+  )
+
+  it.effect("replaces every use of a declaration with a stored closed-layer replacement", () =>
+    Effect.gen(function* () {
+      const replacements: LayerNode.Replacements = [value.replace(Layer.succeed(Value, { value: "replacement" }))]
+      const right = make({
+        service: Right,
+        layer: Layer.effect(
+          Right,
+          Effect.map(Value, (item) => Right.of({ value: item.value })),
+        ),
+        deps: [value],
+      })
+      const context = yield* Layer.build(LayerNode.compile(LayerNode.group([greeting, right]), { replacements }))
+      expect(Context.get(context, Greeting).value).toBe("hello replacement")
+      expect(Context.get(context, Right).value).toBe("replacement")
+    }),
+  )
+
+  it.effect("uses the last replacement and ignores unreachable unbound defaults and cycles", () =>
+    Effect.gen(function* () {
+      const unbound = LayerNode.unbound(Value, tags.values.app)
+      const unused = make({ service: Value, layer: valueLayer, deps: [] })
+      const result = yield* Greeting.pipe(
+        Effect.provide(
+          LayerNode.compile(greeting, {
+            replacements: [
+              value.replace(unbound),
+              unbound.replace(unused),
+              unused.replace(unbound),
+              value.replace(Layer.succeed(Value, { value: "last" })),
+            ],
+          }),
+        ),
+      )
+      expect(result.value).toBe("hello last")
+    }),
+  )
+
+  it.effect("resolves target chains independently of replacement order and treats self-replacement as identity", () =>
+    Effect.gen(function* () {
+      const middle = make({ service: Value, layer: Layer.succeed(Value, { value: "middle" }), deps: [] })
+      const target = make({ service: Value, layer: Layer.succeed(Value, { value: "target" }), deps: [] })
+      const result = yield* Greeting.pipe(
+        Effect.provide(
+          LayerNode.compile(greeting, {
+            replacements: [target.replace(target), middle.replace(target), value.replace(middle)],
+          }),
+        ),
+      )
+      expect(result.value).toBe("hello target")
+    }),
+  )
+
+  test("rejects reachable replacement and dependency cycles", () => {
+    const other = make({ service: Value, layer: valueLayer, deps: [] })
+    expect(() => LayerNode.compile(greeting, { replacements: [value.replace(other), other.replace(value)] })).toThrow(
+      "Cycle detected in layer graph",
     )
-    expect(await Effect.runPromise(program)).toBe("hello production")
-  })
-
-  test("builds a dependency graph", async () => {
-    const program = Effect.map(Greeting, (item) => item.value).pipe(Effect.provide(build(LayerNode.group([greeting]))))
-    expect(await Effect.runPromise(program)).toBe("hello production")
-  })
-
-  test("exposes roots but hides transitive dependencies", () => {
-    const layer = build(LayerNode.group([greeting]))
-    const check: Layer.Layer<Greeting> = layer
-    void check
-  })
-
-  test("preserves branch-specific implementations across roots", async () => {
-    const firstValue = make({ service: Value, layer: Layer.succeed(Value, Value.of({ value: "first" })), deps: [] })
-    const secondValue = make({ service: Value, layer: Layer.succeed(Value, Value.of({ value: "second" })), deps: [] })
-    const leftLayer = Layer.effect(
-      Left,
-      Effect.map(Value, (item) => Left.of({ value: item.value })),
-    )
-    const rightLayer = Layer.effect(
-      Right,
-      Effect.map(Value, (item) => Right.of({ value: item.value })),
-    )
-    const left = make({ service: Left, layer: leftLayer, deps: [firstValue] })
-    const right = make({ service: Right, layer: rightLayer, deps: [secondValue] })
-    const layer = build(LayerNode.group([left, right]))
-    const program = Effect.gen(function* () {
-      return [(yield* Left).value, (yield* Right).value]
-    }).pipe(Effect.provide(layer))
-    expect(await Effect.runPromise(program)).toEqual(["first", "second"])
-  })
-
-  test("requires unbound nodes to be replaced before compilation", async () => {
-    const unbound = LayerNode.unbound(Value, tags.values.app)
-    const greeting = make({ service: Greeting, layer: greetingLayer, deps: [unbound] })
-    const tree = LayerNode.group([greeting])
-    expect(() => LayerNode.compile(tree)).toThrow("Unbound layer node: test/LayerNodeValue")
-    const layer = LayerNode.compile(tree, [[unbound, value]]) as Layer.Layer<Greeting>
-    const program = Effect.map(Greeting, (item) => item.value).pipe(Effect.provide(layer))
-    expect(await Effect.runPromise(program)).toBe("hello production")
-  })
-
-  test("replaces a node with a closed layer", async () => {
-    const replacement = Layer.succeed(Value, Value.of({ value: "simulation" }))
-    const program = Effect.map(Greeting, (item) => item.value).pipe(
-      Effect.provide(build(LayerNode.group([greeting]), [[value, replacement]])),
-    )
-    expect(await Effect.runPromise(program)).toBe("hello simulation")
-  })
-
-  test("replaces every use of the same layer", async () => {
-    const leftLayer = Layer.effect(
-      Left,
-      Effect.map(Value, (item) => Left.of({ value: item.value })),
-    )
-    const rightLayer = Layer.effect(
-      Right,
-      Effect.map(Value, (item) => Right.of({ value: item.value })),
-    )
-    const left = make({ service: Left, layer: leftLayer, deps: [value] })
-    const right = make({ service: Right, layer: rightLayer, deps: [value] })
-    const replacement = Layer.succeed(Value, Value.of({ value: "replaced" }))
-    const layer = build(LayerNode.group([left, right]), [[value, replacement]])
-    const program = Effect.gen(function* () {
-      return [(yield* Left).value, (yield* Right).value]
-    }).pipe(Effect.provide(layer))
-    expect(await Effect.runPromise(program)).toEqual(["replaced", "replaced"])
-  })
-
-  test("does not acquire an unused replacement", async () => {
-    let acquisitions = 0
-    const other = make({ service: Left, layer: Layer.succeed(Left, Left.of({ value: "other" })), deps: [] })
-    const replacement = Layer.effect(
-      Left,
-      Effect.sync(() => {
-        acquisitions++
-        return Left.of({ value: "replacement" })
-      }),
-    )
-    await Effect.runPromise(
-      Effect.map(Greeting, (item) => item.value).pipe(
-        Effect.provide(build(LayerNode.group([greeting]), [[other, replacement]])),
-      ),
-    )
-    expect(acquisitions).toBe(0)
-  })
-
-  test("replaces a node without acquiring its dependencies", async () => {
-    let acquisitions = 0
-    const dependencyLayer = Layer.effect(
-      Value,
-      Effect.sync(() => {
-        acquisitions++
-        return Value.of({ value: "dependency" })
-      }),
-    )
-    const dependency = make({ service: Value, layer: dependencyLayer, deps: [] })
-    const original = make({ service: Greeting, layer: greetingLayer, deps: [dependency] })
-    const replacement = make({
-      service: Greeting,
-      layer: Layer.succeed(Greeting, Greeting.of({ value: "replacement" })),
-      deps: [],
-    })
-
-    const program = Effect.map(Greeting, (item) => item.value).pipe(
-      Effect.provide(build(LayerNode.group([original]), [[original, replacement]])),
-    )
-
-    expect(await Effect.runPromise(program)).toBe("replacement")
-    expect(acquisitions).toBe(0)
-  })
-
-  test("applies later replacements inside earlier replacement nodes", async () => {
-    const original = make({ service: Greeting, layer: greetingLayer, deps: [value] })
-    const replacement = make({ service: Greeting, layer: greetingLayer, deps: [value] })
-    const program = Effect.map(Greeting, (item) => item.value).pipe(
-      Effect.provide(
-        build(LayerNode.group([original]), [
-          [original, replacement],
-          [value, Layer.succeed(Value, Value.of({ value: "replacement dependency" }))],
-        ]),
-      ),
-    )
-
-    expect(await Effect.runPromise(program)).toBe("hello replacement dependency")
-  })
-
-  test("hoists and compiles tagged graphs", async () => {
-    const tags = LayerNode.tags({ location: ["global"], global: [] })
-    const global = tags.make("global")
-    const location = tags.make("location")
-    const database = global({
-      service: Database,
-      layer: Layer.succeed(Database, Database.of({ name: "Alice" })),
-      deps: [],
-    })
-    const users = location({
-      service: Users,
+    const dependent = make({
+      service: Value,
       layer: Layer.effect(
-        Users,
-        Effect.gen(function* () {
-          const db = yield* Database
-          return Users.of({ list: Effect.succeed([db.name]) })
-        }),
+        Value,
+        Effect.map(Greeting, (item) => Value.of({ value: item.value })),
       ),
-      deps: [database],
+      deps: [greeting],
     })
-    const app = location({
-      service: App,
-      layer: Layer.effect(
-        App,
-        Effect.gen(function* () {
-          const service = yield* Users
-          return App.of({ run: service.list })
-        }),
-      ),
-      deps: [users],
-    })
-
-    const result = LayerNode.hoist(LayerNode.group([app]), tags.values.global)
-    expect(result.node.dependencies[0]?.dependencies[0]?.dependencies[0]).toMatchObject({
-      kind: "group",
-      dependencies: [],
-    })
-    expect(result.hoisted.dependencies).toEqual([database])
-
-    const layer = LayerNode.compile(result.node).pipe(
-      Layer.provide(LayerNode.compile(result.hoisted)),
-    ) as unknown as Layer.Layer<App>
-    const program = Effect.gen(function* () {
-      const app = yield* App
-      return yield* app.run
-    }).pipe(Effect.provide(layer))
-
-    expect(await Effect.runPromise(program)).toEqual(["Alice"])
-  })
-
-  test("rejects conflicting hoisted implementations", () => {
-    const tags = LayerNode.tags({ location: ["global"], global: [] })
-    const global = tags.make("global")
-    const location = tags.make("location")
-    const first = global({
-      service: Database,
-      layer: Layer.succeed(Database, Database.of({ name: "first" })),
-      deps: [],
-    })
-    const second = global({
-      service: Database,
-      layer: Layer.succeed(Database, Database.of({ name: "second" })),
-      deps: [],
-    })
-    const left = location({
-      service: Users,
-      layer: Layer.effect(Users, Effect.as(Database, Users.of({ list: Effect.succeed([]) }))),
-      deps: [first],
-    })
-    const right = location({
-      service: App,
-      layer: Layer.effect(App, Effect.as(Database, App.of({ run: Effect.succeed([]) }))),
-      deps: [second],
-    })
-
-    expect(() => LayerNode.hoist(LayerNode.group([left, right]), tags.values.global)).toThrow(
-      "Tag global has conflicting implementations for test/GraphDatabase",
+    expect(() => LayerNode.compile(greeting, { replacements: [value.replace(dependent)] })).toThrow(
+      "Cycle detected in layer graph",
     )
   })
 
-  test("treats dependency groups as transparent while hoisting", () => {
-    const tags = LayerNode.tags({ location: ["global"], global: [] })
-    const global = tags.make("global")
-    const location = tags.make("location")
-    const database = global({
-      service: Database,
-      layer: Layer.succeed(Database, Database.of({ name: "Alice" })),
-      deps: [],
-    })
-    const users = location({
-      service: Users,
-      layer: Layer.effect(Users, Effect.as(Database, Users.of({ list: Effect.succeed([]) }))),
-      deps: [LayerNode.group([database])],
-    })
-    const result = LayerNode.hoist(LayerNode.group([users]), tags.values.global)
+  it.effect("does not acquire replaced dependencies or unused replacement targets", () =>
+    Effect.gen(function* () {
+      const acquired: string[] = []
+      const dependency = make({
+        service: Value,
+        layer: Layer.effect(
+          Value,
+          Effect.sync(() => {
+            acquired.push("old dependency")
+            return Value.of({ value: "dependency" })
+          }),
+        ),
+        deps: [],
+      })
+      const original = make({ service: Greeting, layer: greetingLayer, deps: [dependency] })
+      const result = yield* Greeting.pipe(
+        Effect.provide(
+          LayerNode.compile(original, {
+            replacements: [
+              original.replace(Layer.succeed(Greeting, { value: "replacement" })),
+              value.replace(
+                Layer.effect(
+                  Value,
+                  Effect.sync(() => {
+                    acquired.push("unused target")
+                    return Value.of({ value: "unused" })
+                  }),
+                ),
+              ),
+            ],
+          }),
+        ),
+      )
+      expect(result.value).toBe("replacement")
+      expect(acquired).toEqual([])
+    }),
+  )
 
-    expect(result.node.dependencies[0]?.dependencies[0]?.dependencies[0]).toMatchObject({
-      kind: "group",
-      dependencies: [],
-    })
+  it.effect("mapLayer preserves dependency wiring and replacement traversal", () =>
+    Effect.gen(function* () {
+      const acquired: string[] = []
+      const decorated = greeting.mapLayer((layer) =>
+        layer.pipe(
+          Layer.tap((context) =>
+            Effect.sync(() => {
+              acquired.push(Context.get(context, Greeting).value)
+            }),
+          ),
+        ),
+      )
+      const result = yield* Greeting.pipe(
+        Effect.provide(
+          LayerNode.compile(greeting, {
+            replacements: [
+              greeting.replace(decorated),
+              value.replace(Layer.succeed(Value, { value: "mapped dependency" })),
+            ],
+          }),
+        ),
+      )
+      expect(result.value).toBe("hello mapped dependency")
+      expect(acquired).toEqual(["hello mapped dependency"])
+    }),
+  )
+
+  it.effect("memoizes shared wiring instead of expanding a diamond into a tree", () =>
+    Effect.gen(function* () {
+      const acquisitions: string[] = []
+      const shared = value.mapLayer((layer) =>
+        layer.pipe(Layer.tap(() => Effect.sync(() => acquisitions.push("shared")))),
+      )
+      const left = make({ name: "left", layer: Layer.empty, deps: [shared] })
+      const right = make({ name: "right", layer: Layer.empty, deps: [shared] })
+      yield* Layer.build(LayerNode.compile(LayerNode.group([left, right])))
+      expect(acquisitions).toEqual(["shared"])
+    }),
+  )
+
+  it.effect("preserves declared memo-service outputs rather than filtering them as build metadata", () =>
+    Effect.gen(function* () {
+      const supplied = yield* Layer.makeMemoMap
+      const memo = make({
+        service: Layer.CurrentMemoMap,
+        layer: Layer.succeed(Layer.CurrentMemoMap, supplied),
+        deps: [],
+      })
+      const observer = make({ service: Memo, layer: Layer.effect(Memo, Layer.CurrentMemoMap), deps: [memo] })
+      expect(yield* Memo.pipe(Effect.provide(LayerNode.compile(observer)))).toBe(supplied)
+    }),
+  )
+
+  it.effect("rejects one implementation wired to different effective dependencies in either memo domain", () =>
+    Effect.gen(function* () {
+      const other = make({ service: Value, layer: Layer.succeed(Value, { value: "other" }), deps: [] })
+      const sibling = make({ service: Greeting, layer: greetingLayer, deps: [other] })
+      const root = LayerNode.group([greeting, sibling])
+      expect(() => LayerNode.compile(root)).toThrow("wired to different dependencies")
+      expect(() => LayerNode.compile(root, { shared: tags.values.app })).toThrow("wired to different dependencies")
+      const result = yield* Greeting.pipe(
+        Effect.provide(LayerNode.compile(root, { replacements: [value.replace(other)] })),
+      )
+      expect(result.value).toBe("hello other")
+    }),
+  )
+
+  it.effect("starts dependencies in parallel and nested group roots in order", () =>
+    Effect.gen(function* () {
+      const valueStarted = yield* Deferred.make<void>()
+      const greetingStarted = yield* Deferred.make<void>()
+      const firstStarted = yield* Deferred.make<void>()
+      const releaseFirst = yield* Deferred.make<void>()
+      const events: string[] = []
+      const value = make({
+        service: Value,
+        layer: Layer.effect(
+          Value,
+          Effect.gen(function* () {
+            yield* Deferred.succeed(valueStarted, undefined)
+            yield* Deferred.await(greetingStarted)
+            return Value.of({ value: "value" })
+          }),
+        ),
+        deps: [],
+      })
+      const greeting = make({
+        service: Greeting,
+        layer: Layer.effect(
+          Greeting,
+          Effect.gen(function* () {
+            yield* Deferred.succeed(greetingStarted, undefined)
+            yield* Deferred.await(valueStarted)
+            return Greeting.of({ value: "greeting" })
+          }),
+        ),
+        deps: [],
+      })
+      const first = make({
+        service: Left,
+        layer: Layer.effect(
+          Left,
+          Effect.gen(function* () {
+            yield* Value
+            yield* Greeting
+            events.push("first started")
+            yield* Deferred.succeed(firstStarted, undefined)
+            yield* Deferred.await(releaseFirst)
+            events.push("first finished")
+            return Left.of({ value: "first" })
+          }),
+        ),
+        deps: [value, greeting],
+      })
+      const second = make({
+        service: Right,
+        layer: Layer.effect(
+          Right,
+          Effect.sync(() => {
+            expect(events).toEqual(["first started", "first finished"])
+            events.push("second started")
+            return Right.of({ value: "second" })
+          }),
+        ),
+        deps: [],
+      })
+      const fiber = yield* Layer.build(LayerNode.compile(LayerNode.group([LayerNode.group([first]), second]))).pipe(
+        Effect.forkChild,
+      )
+      yield* Deferred.await(firstStarted)
+      expect(events).toEqual(["first started"])
+      yield* Deferred.succeed(releaseFirst, undefined)
+      const context = yield* Fiber.join(fiber)
+      expect(events).toEqual(["first started", "first finished", "second started"])
+      expect(Context.get(context, Left).value).toBe("first")
+      expect(Context.get(context, Right).value).toBe("second")
+    }),
+  )
+  ;[false, true].forEach((topLevel) => {
+    it.effect(
+      `LayerMap isolates builds and retains resources ${topLevel ? "with" : "without"} a top-level global owner`,
+      () =>
+        Effect.gen(function* () {
+          const acquired = { global: 0, local: 0, support: 0 }
+          const released: string[] = []
+          const startup: string[] = []
+          yield* Effect.gen(function* () {
+            const memoMap = yield* Layer.makeMemoMap
+            const tags = LayerNode.tags({ location: ["global"], global: [] })
+            const global = tags.make("global")
+            const location = tags.make("location")
+            const support = LayerNode.make({
+              service: Support,
+              layer: Layer.effect(
+                Support,
+                Effect.acquireRelease(
+                  Effect.sync(() => {
+                    acquired.support++
+                    return Support.of({})
+                  }),
+                  () =>
+                    Effect.sync(() => {
+                      released.push("support")
+                    }),
+                ),
+              ),
+              deps: [],
+            })
+            const value = global({
+              service: Value,
+              layer: Layer.effect(
+                Value,
+                Effect.andThen(
+                  Support,
+                  Effect.acquireRelease(
+                    Effect.sync(() => {
+                      startup.push("global")
+                      return Value.of({ value: `global-${++acquired.global}` })
+                    }),
+                    (value) =>
+                      Effect.sync(() => {
+                        released.push(value.value)
+                      }),
+                  ),
+                ),
+              ),
+              deps: [support],
+            })
+            const local = location({
+              service: Greeting,
+              layer: Layer.effect(
+                Greeting,
+                Effect.gen(function* () {
+                  yield* Value
+                  return yield* Effect.acquireRelease(
+                    Effect.sync(() => Greeting.of({ value: `local-${++acquired.local}` })),
+                    (value) =>
+                      Effect.sync(() => {
+                        released.push(value.value)
+                      }),
+                  )
+                }),
+              ),
+              deps: [LayerNode.group([value])],
+            })
+            const root = location({
+              service: Right,
+              layer: Layer.effect(
+                Right,
+                Effect.gen(function* () {
+                  const local = yield* Greeting
+                  if (local.value === "local-2") return yield* Effect.fail("failed location" as const)
+                  return Right.of(local)
+                }),
+              ),
+              deps: [local],
+            })
+            // Every key builds the same compiled Layer, not a new graph per lookup.
+            const compiled = LayerNode.compile(LayerNode.group([value, root]), { shared: tags.values.global })
+            const locations = location({
+              service: Locations,
+              layer: Layer.effect(
+                Locations,
+                Effect.gen(function* () {
+                  startup.push("map")
+                  expect(Option.getOrUndefined(yield* Effect.serviceOption(Layer.CurrentMemoMap))).toBe(memoMap)
+                  return yield* LayerMap.make((_: string) => compiled, { idleTimeToLive: Duration.infinity })
+                }),
+              ),
+              deps: [],
+            })
+            const scope = yield* Effect.scope
+            const context = yield* Layer.buildWithMemoMap(
+              LayerNode.compile(LayerNode.group([locations, ...(topLevel ? [value] : [])]), {
+                shared: tags.values.global,
+              }),
+              memoMap,
+              scope,
+            )
+            expect(startup).toEqual(topLevel ? ["map", "global"] : ["map"])
+            const map = Context.get(context, Locations)
+            const first = yield* map.contextEffect("first").pipe(Effect.scoped)
+            expect(Option.getOrUndefined(Context.getOption(context, Value))).toBe(
+              topLevel ? Context.get(first, Value) : undefined,
+            )
+            expect(Option.isNone(Context.getOption(first, Greeting))).toBe(true)
+            expect(Context.get(first, Right).value).toBe("local-1")
+
+            expect(yield* map.contextEffect("failed").pipe(Effect.scoped, Effect.flip)).toBe("failed location")
+            expect(released).toEqual(["local-2"])
+            expect(Context.get(yield* map.contextEffect("first").pipe(Effect.scoped), Right)).toBe(
+              Context.get(first, Right),
+            )
+
+            const second = yield* map.contextEffect("second").pipe(Effect.scoped)
+            expect(Context.get(second, Value)).toBe(Context.get(first, Value))
+            expect(Context.get(second, Right)).not.toBe(Context.get(first, Right))
+            expect(acquired).toEqual({ global: 1, local: 3, support: 1 })
+
+            yield* map.invalidate("first")
+            expect(released).toEqual(["local-2", "local-1"])
+            expect(Context.get(yield* map.contextEffect("second").pipe(Effect.scoped), Right)).toBe(
+              Context.get(second, Right),
+            )
+            const rebuilt = yield* map.contextEffect("first").pipe(Effect.scoped)
+            expect(Context.get(rebuilt, Right).value).toBe("local-4")
+            expect(Context.get(rebuilt, Value)).toBe(Context.get(first, Value))
+            expect(acquired).toEqual({ global: 1, local: 4, support: 1 })
+            expect(released).not.toContain("global-1")
+          }).pipe(Effect.scoped)
+          expect(released.toSorted()).toEqual(["global-1", "local-1", "local-2", "local-3", "local-4", "support"])
+        }),
+    )
   })
 })

packages/core/test/effect/layer-node/node-build.test.ts

@@ -1,20 +1,23 @@
 import { describe, expect, test } from "bun:test"
-import { Context, Effect, Layer, LayerMap, Option } from "effect"
+import { Context, Effect, Layer, Option } from "effect"
 import { Node } from "@opencode-ai/util/effect/app-node"
 import { AppNodeBuilder } from "@opencode-ai/core/effect/app-node-builder"
 import { LayerNode } from "@opencode-ai/util/effect/layer-node"
 import { Location } from "@opencode-ai/core/location"
 import { LocationServiceMap } from "@opencode-ai/core/location-service-map"
-import type { LocationError, LocationServices } from "@opencode-ai/core/location-services"
+import { buildLocationServiceMap } from "@opencode-ai/core/location-services"
 import { Project } from "@opencode-ai/core/project"
 import { AbsolutePath } from "@opencode-ai/core/schema"
 import { tmpdir } from "../../fixture/tmpdir"
+import { testEffect } from "../../lib/effect"
 
 class Value extends Context.Service<Value, { readonly value: string }>()("test/TagValue") {}
 class Result extends Context.Service<Result, { readonly value: string }>()("test/TagResult") {}
 class CycleA extends Context.Service<CycleA, {}>()("test/NodeBuildA") {}
 class CycleB extends Context.Service<CycleB, { readonly directory: AbsolutePath }>()("test/NodeBuildB") {}
 
+const it = testEffect(Layer.empty)
+
 describe("node build", () => {
   test("does not build a location service map when the graph does not require it", async () => {
     const result = Node.makeGlobalNode({
@@ -31,7 +34,7 @@ describe("node build", () => {
     expect(await Effect.runPromise(program)).toBe("plain")
   })
 
-  test("detects cycles through a replaced location service map", async () => {
+  test("detects cycles through a replaced location service map", () => {
     const a = Node.makeGlobalNode({
       service: CycleA,
       layer: Layer.effect(CycleA, Effect.as(LocationServiceMap.Service, CycleA.of({}))),
@@ -45,31 +48,49 @@ describe("node build", () => {
       ),
       deps: [a],
     })
-    const mapLayer = Layer.effect(
-      LocationServiceMap.Service,
-      Effect.gen(function* () {
-        const service = yield* CycleB
-        return yield* LayerMap.make(
-          (ref: Location.Ref) =>
-            Layer.succeed(
-              Location.Service,
-              Location.Service.of({
-                directory: ref.directory,
-                workspaceID: ref.workspaceID,
-                project: { id: Project.ID.global, directory: service.directory, canonical: service.directory },
-              }),
-            ),
-          { idleTimeToLive: "1 minute" },
-        )
-      }) as unknown as Effect.Effect<LayerMap.LayerMap<Location.Ref, LocationServices, LocationError>, never, CycleB>,
-    )
+    const mapLayer = Layer.unwrap(Effect.as(CycleB, buildLocationServiceMap()))
     const map = Node.makeGlobalNode({ service: LocationServiceMap.Service, layer: mapLayer, deps: [b] })
-    expect(() => AppNodeBuilder.build(LayerNode.group([a]), [[LocationServiceMap.node, map]])).toThrow(
-      "Cycle detected in layer tree",
+    expect(() => AppNodeBuilder.build(LayerNode.group([a]), [LocationServiceMap.node.replace(map)])).toThrow(
+      "Cycle detected in layer graph",
     )
   })
 
-  test("shares top-level project with location services", async () => {
+  it.effect("supplies the lazy map when only a replacement introduces the dependency", () =>
+    Effect.gen(function* () {
+      const original = Node.makeGlobalNode({
+        service: Result,
+        layer: Layer.succeed(Result, { value: "original" }),
+        deps: [],
+      })
+      const replacement = Node.makeGlobalNode({
+        service: Result,
+        layer: Layer.effect(Result, Effect.as(LocationServiceMap.Service, Result.of({ value: "has map" }))),
+        deps: [LocationServiceMap.node],
+      })
+      const result = yield* Result.pipe(Effect.provide(AppNodeBuilder.build(original, [original.replace(replacement)])))
+      expect(result.value).toBe("has map")
+    }),
+  )
+
+  it.effect("caller replacements override the lazy default without building any locations", () =>
+    Effect.gen(function* () {
+      const acquisitions: string[] = []
+      const override = buildLocationServiceMap().pipe(
+        Layer.tap(() =>
+          Effect.sync(() => {
+            acquisitions.push("caller map")
+          }),
+        ),
+      )
+      const context = yield* Layer.build(
+        AppNodeBuilder.build(LocationServiceMap.node, [LocationServiceMap.node.replace(override)]),
+      )
+      expect(Context.get(context, LocationServiceMap.Service)).toBeDefined()
+      expect(acquisitions).toEqual(["caller map"])
+    }),
+  )
+
+  test("shares top-level project even when the location service map is built first", async () => {
     await using tmp = await tmpdir()
     let acquisitions = 0
     const projectLayer = Layer.effect(
@@ -84,8 +105,8 @@ describe("node build", () => {
       }),
     )
     const ref = Location.Ref.make({ directory: AbsolutePath.make(tmp.path) })
-    const layer = AppNodeBuilder.build(LayerNode.group([Project.node, LocationServiceMap.node]), [
-      [Project.node, projectLayer],
+    const layer = AppNodeBuilder.build(LayerNode.group([LocationServiceMap.node, Project.node]), [
+      Project.node.replace(projectLayer),
     ])
     const program = Effect.gen(function* () {
       yield* Project.Service

packages/core/test/file-mutation.test.ts

@@ -21,8 +21,8 @@ function provide(directory: string, transformFiles: EnvironmentFilesTransform =
   )
   return Effect.provide(
     AppNodeBuilder.build(LayerNode.group([LocationMutation.node, FileMutation.node]), [
-      [Location.node, activeLocation],
-      [Environment.node, transformEnvironmentFiles(transformFiles)],
+      Location.node.replace(activeLocation),
+      Environment.node.replace(transformEnvironmentFiles(transformFiles)),
     ]),
   )
 }

packages/core/test/filesystem/search.test.ts

@@ -77,16 +77,15 @@ describe("FileSystemSearch", () => {
         workspaceID: Workspace.ID.make("wrk_test"),
       })
       const layer = AppNodeBuilder.build(FileSystemSearch.node, [
-        [
-          Location.node,
+        Location.node.replace(
           Layer.succeed(
             Location.Service,
             Location.Service.of(
               location(ref, { vcs: { type: "git", store: AbsolutePath.make(path.join(directory, ".git")) } }),
             ),
           ),
-        ],
-        [Ripgrep.node, ripgrepStub("remote.ts", (input) => (observed = input))],
+        ),
+        Ripgrep.node.replace(ripgrepStub("remote.ts", (input) => (observed = input))),
       ])
 
       yield* Effect.gen(function* () {
@@ -103,8 +102,7 @@ describe("FileSystemSearch", () => {
       let observed: Ripgrep.FindInput | undefined
       const home = AbsolutePath.make(os.homedir())
       const layer = AppNodeBuilder.build(FileSystemSearch.node, [
-        [
-          Location.node,
+        Location.node.replace(
           Layer.succeed(
             Location.Service,
             Location.Service.of(
@@ -114,8 +112,8 @@ describe("FileSystemSearch", () => {
               ),
             ),
           ),
-        ],
-        [Ripgrep.node, ripgrepStub("src/index.ts", (input) => (observed = input))],
+        ),
+        Ripgrep.node.replace(ripgrepStub("src/index.ts", (input) => (observed = input))),
       ])
       yield* Effect.gen(function* () {
         const search = yield* FileSystemSearch.Service
@@ -137,17 +135,15 @@ describe("FileSystemSearch", () => {
       const started = yield* Deferred.make<void>()
       const release = yield* Deferred.make<void>()
       const layer = AppNodeBuilder.build(FileSystemSearch.node, [
-        [
-          Location.node,
+        Location.node.replace(
           Layer.succeed(
             Location.Service,
             Location.Service.of(
               location({ directory: AbsolutePath.make(path.join(os.tmpdir(), "opencode-search-atomic")) }),
             ),
           ),
-        ],
-        [
-          Ripgrep.node,
+        ),
+        Ripgrep.node.replace(
           Layer.succeed(
             Ripgrep.Service,
             Ripgrep.Service.of({
@@ -169,7 +165,7 @@ describe("FileSystemSearch", () => {
               grep: () => Effect.succeed([]),
             }),
           ),
-        ],
+        ),
       ])
 
       yield* Effect.gen(function* () {
@@ -208,17 +204,15 @@ describe("FileSystemSearch", () => {
         (value) => Effect.sync(() => value.mockRestore()),
       )
       const layer = AppNodeBuilder.build(FileSystemSearch.node, [
-        [
-          Location.node,
+        Location.node.replace(
           Layer.succeed(
             Location.Service,
             Location.Service.of(
               location({ directory: AbsolutePath.make(path.join(os.tmpdir(), "opencode-search-cache")) }),
             ),
           ),
-        ],
-        [
-          Ripgrep.node,
+        ),
+        Ripgrep.node.replace(
           Layer.succeed(
             Ripgrep.Service,
             Ripgrep.Service.of({
@@ -234,7 +228,7 @@ describe("FileSystemSearch", () => {
               grep: () => Effect.succeed([]),
             }),
           ),
-        ],
+        ),
       ])
 
       yield* Effect.gen(function* () {

packages/core/test/filesystem/watcher.test.ts

@@ -6,7 +6,7 @@ import { Deferred, Duration, Effect, Fiber, Layer, Option, Schedule, Stream } fr
 import { Config } from "@opencode-ai/core/config"
 import { ConfigLocationWatcherPlugin } from "@opencode-ai/core/config/plugin/location-watcher"
 import { AppNodeBuilder } from "@opencode-ai/core/effect/app-node-builder"
-import { makeLocationNode, type LocationNode } from "@opencode-ai/util/effect/app-node"
+import { makeLocationNode } from "@opencode-ai/util/effect/app-node"
 import { LayerNode } from "@opencode-ai/util/effect/layer-node"
 import { Bus } from "@opencode-ai/core/bus"
 import { FSUtil } from "@opencode-ai/util/fs-util"
@@ -129,7 +129,7 @@ function provide(
   vcs?: Location.Interface["vcs"],
   watcher?: Layer.Layer<Watcher.Service>,
   config: Layer.Layer<Config.Service> = configLayer,
-  plugins: LocationNode<PluginSupervisor.Service> = pluginNode,
+  plugins: typeof pluginNode = pluginNode,
 ) {
   const locationLayer = Layer.succeed(
     Location.Service,
@@ -138,10 +138,10 @@ function provide(
   const built = AppNodeBuilder.build(
     LayerNode.group([LocationWatcher.node, LocationWatcherPolicy.node, Bus.node, Config.node]),
     [
-      [Config.node, config],
-      [Location.node, locationLayer],
-      [PluginSupervisor.node, plugins],
-      ...(watcher ? ([[Watcher.node, watcher]] as const) : []),
+      Config.node.replace(config),
+      Location.node.replace(locationLayer),
+      PluginSupervisor.node.replace(plugins),
+      ...(watcher ? ([Watcher.node.replace(watcher)] as const) : []),
     ],
   )
   return Effect.provide(built)
@@ -154,7 +154,7 @@ function withTmp<A, E, R>(
     init?: (directory: string) => Promise<void>
     watcher?: Layer.Layer<Watcher.Service>
     config?: Layer.Layer<Config.Service>
-    plugins?: LocationNode<PluginSupervisor.Service>
+    plugins?: typeof pluginNode
   },
 ) {
   return Effect.acquireRelease(

packages/core/test/fixture/effect-flock-worker.ts

@@ -30,7 +30,7 @@ const testGlobal = Global.layerWith({
   log: os.tmpdir(),
 })
 
-const testLayer = LayerNode.compile(EffectFlock.node, [[Global.node, testGlobal]])
+const testLayer = LayerNode.compile(EffectFlock.node, { replacements: [Global.node.replace(testGlobal)] })
 
 async function job() {
   if (msg.ready) await fs.writeFile(msg.ready, String(process.pid))

packages/core/test/fixture/prompt-location.ts

@@ -26,9 +26,9 @@ export const promptLocationNode = makeGlobalNode({
           SessionPrompt.layer.pipe(
             Layer.provideMerge(
               Layer.mergeAll(
-                LayerNode.compile(LayerNode.group([PluginHooks.node, Image.node, Skill.node]), [
-                  [Bus.node, Layer.succeed(Bus.Service, bus)],
-                ]),
+                LayerNode.compile(LayerNode.group([PluginHooks.node, Image.node, Skill.node]), {
+                  replacements: [Bus.node.replace(Layer.succeed(Bus.Service, bus))],
+                }),
                 Layer.succeed(FSUtil.Service, fs),
                 Layer.succeed(PluginSupervisor.Service, { flush: Effect.void }),
                 Layer.mock(Reference.Service, { refresh: () => Effect.void }),

packages/core/test/formatter.test.ts

@@ -20,7 +20,7 @@ import { testEffect } from "./lib/effect"
 
 const it = testEffect(
   AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node, SdkPlugins.node, LocationServiceMap.node]), [
-    [Global.node, tempGlobalLayer],
+    Global.node.replace(tempGlobalLayer),
   ]),
 )
 type ConfigInput = typeof Info.Encoded

packages/core/test/instance-plugins.test.ts

@@ -34,7 +34,7 @@ const instances = Layer.effect(
     (ref: Location.Ref) =>
       Instance.layer(ref, {
         plugins: path.basename(ref.directory) === "thread-a" ? [agentPlugin("thread-a-plugin", "thread-a-agent")] : [],
-        replacements: [[Global.node, tempGlobalLayer]],
+        replacements: [Global.node.replace(tempGlobalLayer)],
       }),
     { idleTimeToLive: Duration.infinity },
   ),
@@ -42,8 +42,8 @@ const instances = Layer.effect(
 
 const it = testEffect(
   AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node, SdkPlugins.node, LocationServiceMap.node]), [
-    [Global.node, tempGlobalLayer],
-    [LocationServiceMap.node, instances],
+    Global.node.replace(tempGlobalLayer),
+    LocationServiceMap.node.replace(instances),
   ]),
 )
 

packages/core/test/instance-vanilla.test.ts

@@ -23,14 +23,13 @@ import { Bus } from "../src/bus"
 // Config the host hands the vanilla instance explicitly: a value and an
 // explicit plugin removal, both of which must survive discovery: false.
 const hostConfig: LayerNode.Replacements = [
-  [
-    Config.node,
+  Config.node.replace(
     Config.configured({
       project: false,
       global: false,
       content: JSON.stringify({ shell: "vanilla-host", plugins: ["-opencode.tool.shell"] }),
     }),
-  ],
+  ),
 ]
 
 // Same directory contents, two instances: one vanilla, one with discovery.
@@ -43,7 +42,7 @@ const instances = Layer.effect(
         // "bare" exercises the vanilla defaults themselves: no caller Config.
         discovery: name !== "vanilla" && name !== "bare",
         // Caller replacements win over the vanilla defaults.
-        replacements: [[Global.node, tempGlobalLayer], ...(name === "vanilla" ? hostConfig : [])],
+        replacements: [Global.node.replace(tempGlobalLayer), ...(name === "vanilla" ? hostConfig : [])],
       })
     },
     { idleTimeToLive: Duration.infinity },
@@ -52,8 +51,8 @@ const instances = Layer.effect(
 
 const it = testEffect(
   AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node, SdkPlugins.node, LocationServiceMap.node]), [
-    [Global.node, tempGlobalLayer],
-    [LocationServiceMap.node, instances],
+    Global.node.replace(tempGlobalLayer),
+    LocationServiceMap.node.replace(instances),
   ]),
 )
 

packages/core/test/instruction-discovery.test.ts

@@ -33,19 +33,18 @@ const instructionLayer = (input: {
     AppNodeBuilder.build(
       LayerNode.group([InstructionDiscovery.node, Bus.node, FSUtil.node, Global.node, Location.node, Watcher.node]),
       [
-        [InstructionDiscovery.node, InstructionDiscovery.configured({ project: input.project })],
-        [
-          Global.node,
+        InstructionDiscovery.node.replace(InstructionDiscovery.configured({ project: input.project })),
+        Global.node.replace(
           input.config || input.home
             ? Global.layerWith({
                 ...(input.config ? { config: input.config } : {}),
                 ...(input.home ? { home: input.home } : {}),
               })
             : tempGlobalLayer,
-        ],
-        [Location.node, input.locationServiceLayer],
-        [Watcher.node, watcher],
-        ...(input.filesystemLayer ? [[FSUtil.node, input.filesystemLayer] as const] : []),
+        ),
+        Location.node.replace(input.locationServiceLayer),
+        Watcher.node.replace(watcher),
+        ...(input.filesystemLayer ? [FSUtil.node.replace(input.filesystemLayer)] : []),
       ],
     ),
     watcher,

packages/core/test/instruction-state.test.ts

@@ -24,7 +24,7 @@ import { testEffect } from "./lib/effect"
 
 const it = testEffect(
   AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node, SessionProjector.node]), [
-    [Bus.node, Bus.configured({ persist: true })],
+    Bus.node.replace(Bus.configured({ persist: true })),
   ]),
 )
 

packages/core/test/instructions/builtins.test.ts

@@ -30,8 +30,8 @@ const locationLayer = Layer.succeed(
 )
 const it = testEffect(
   AppNodeBuilder.build(InstructionBuiltIns.node, [
-    [Location.node, locationLayer],
-    [Global.node, Global.layerWith({ config: temporary, tmp: temporary })],
+    Location.node.replace(locationLayer),
+    Global.node.replace(Global.layerWith({ config: temporary, tmp: temporary })),
   ]),
 )
 

packages/core/test/integration.test.ts

@@ -27,7 +27,7 @@ const failingCredentialNode = makeGlobalNode({
   deps: [],
 })
 const failingIt = testEffect(
-  AppNodeBuilder.build(LayerNode.group([Integration.node, Bus.node]), [[Credential.node, failingCredentialNode]]),
+  AppNodeBuilder.build(LayerNode.group([Integration.node, Bus.node]), [Credential.node.replace(failingCredentialNode)]),
 )
 
 function eventually<A, E, R>(

packages/core/test/location-filesystem.test.ts

@@ -13,15 +13,16 @@ import { it } from "./lib/effect"
 
 const provide = (directory: string, workspaceID?: Workspace.ID) =>
   Effect.provide(
-    LayerNode.compile(FileSystem.node, [
-      [
-        Location.node,
-        Layer.succeed(
-          Location.Service,
-          Location.Service.of(location({ directory: AbsolutePath.make(directory), workspaceID })),
+    LayerNode.compile(FileSystem.node, {
+      replacements: [
+        Location.node.replace(
+          Layer.succeed(
+            Location.Service,
+            Location.Service.of(location({ directory: AbsolutePath.make(directory), workspaceID })),
+          ),
         ),
       ],
-    ]),
+    }),
   )
 
 const withTmp = <A, E, R>(f: (directory: string) => Effect.Effect<A, E, R>) =>

packages/core/test/location-layer.test.ts

@@ -51,12 +51,12 @@ import { Tool } from "../src/tool"
 
 const it = testEffect(
   AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node, LocationServiceMap.node]), [
-    [Global.node, tempGlobalLayer],
+    Global.node.replace(tempGlobalLayer),
   ]),
 )
 const itWithSdk = testEffect(
   AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node, SdkPlugins.node, LocationServiceMap.node]), [
-    [Global.node, tempGlobalLayer],
+    Global.node.replace(tempGlobalLayer),
   ]),
 )
 const activityLocations = Layer.effect(
@@ -77,7 +77,7 @@ const activityLocations = Layer.effect(
 )
 const itWithActivity = testEffect(
   AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node, LocationServiceMap.node, LocationActivity.node]), [
-    [LocationServiceMap.node, activityLocations],
+    LocationServiceMap.node.replace(activityLocations),
   ]),
 )
 

packages/core/test/location-mutation.test.ts

@@ -13,20 +13,21 @@ import { it } from "./lib/effect"
 
 function provide(directory: string, projectDirectory = directory) {
   return Effect.provide(
-    LayerNode.compile(LocationMutation.node, [
-      [
-        Location.node,
-        Layer.succeed(
-          Location.Service,
-          Location.Service.of(
-            location(
-              { directory: AbsolutePath.make(directory) },
-              { projectDirectory: AbsolutePath.make(projectDirectory) },
+    LayerNode.compile(LocationMutation.node, {
+      replacements: [
+        Location.node.replace(
+          Layer.succeed(
+            Location.Service,
+            Location.Service.of(
+              location(
+                { directory: AbsolutePath.make(directory) },
+                { projectDirectory: AbsolutePath.make(projectDirectory) },
+              ),
             ),
           ),
         ),
       ],
-    ]),
+    }),
   )
 }
 

packages/core/test/location.test.ts

@@ -23,7 +23,7 @@ const projectLayer = Layer.succeed(
       }),
   }),
 )
-const it = testEffect(AppNodeBuilder.build(Location.boundNode(ref), [[Project.node, projectLayer]]))
+const it = testEffect(AppNodeBuilder.build(Location.boundNode(ref), [Project.node.replace(projectLayer)]))
 
 describe("Location", () => {
   it.effect("resolves the current project and vcs information", () =>

packages/core/test/mcp-instructions.test.ts

@@ -23,13 +23,12 @@ const tool = (server: string, name = "search") => new Mcp.Tool({ server: Mcp.Ser
 
 const layer = (catalog: () => Mcp.ServerInstructions[], tools: () => Mcp.Tool[]) =>
   AppNodeBuilder.build(McpInstructions.node, [
-    [
-      Mcp.node,
+    Mcp.node.replace(
       Layer.mock(Mcp.Service, {
         instructions: () => Effect.succeed(catalog()),
         tools: () => Effect.succeed(tools()),
       }),
-    ],
+    ),
   ])
 
 describe("McpInstructions", () => {

packages/core/test/mcp.test.ts

@@ -378,10 +378,10 @@ const permissions = Layer.mock(Permission.Service, {
 const events = Layer.mock(Bus.Service, { subscribe: () => Stream.never })
 const it = testEffect(
   AppNodeBuilder.build(LayerNode.group([Tool.node, McpTool.node]), [
-    [Mcp.node, mcp],
-    [Permission.node, permissions],
-    [Bus.node, events],
-    [Image.node, imagePassthrough],
+    Mcp.node.replace(mcp),
+    Permission.node.replace(permissions),
+    Bus.node.replace(events),
+    Image.node.replace(imagePassthrough),
   ]),
 )
 
@@ -1688,8 +1688,7 @@ testEffect(Layer.empty).live("isolates invalid MCP tools and preserves plugin tr
       Effect.provide(
         Layer.fresh(
           AppNodeBuilder.build(LayerNode.group([Tool.node, McpTool.node, Bus.node]), [
-            [
-              Mcp.node,
+            Mcp.node.replace(
               Layer.mock(Mcp.Service, {
                 tools: () => Ref.get(catalog),
                 callTool: (input) =>
@@ -1702,9 +1701,9 @@ testEffect(Layer.empty).live("isolates invalid MCP tools and preserves plugin tr
                     }),
                   ),
               }),
-            ],
-            [Permission.node, Layer.mock(Permission.Service, { assert: () => Effect.void })],
-            [Image.node, imagePassthrough],
+            ),
+            Permission.node.replace(Layer.mock(Permission.Service, { assert: () => Effect.void })),
+            Image.node.replace(imagePassthrough),
           ]),
         ),
       ),
@@ -1731,8 +1730,7 @@ testEffect(Layer.empty).effect("coalesces queued MCP tool notifications after in
   }).pipe(
     Effect.provide(
       AppNodeBuilder.build(LayerNode.group([Tool.node, McpTool.node, Bus.node]), [
-        [
-          Mcp.node,
+        Mcp.node.replace(
           Layer.mock(Mcp.Service, {
             tools: () =>
               Effect.sync(() => [
@@ -1744,9 +1742,9 @@ testEffect(Layer.empty).effect("coalesces queued MCP tool notifications after in
                 }),
               ]),
           }),
-        ],
-        [Permission.node, Layer.mock(Permission.Service, { assert: () => Effect.void })],
-        [Image.node, imagePassthrough],
+        ),
+        Permission.node.replace(Layer.mock(Permission.Service, { assert: () => Effect.void })),
+        Image.node.replace(imagePassthrough),
       ]),
     ),
   )

packages/core/test/models.test.ts

@@ -182,9 +182,9 @@ const buildLayer = (state: Ref.Ref<MockState>, cache: MockCache, options: Models
   // every test would reuse the cachedInvalidateWithTTL state from the first run.
   Layer.fresh(
     AppNodeBuilder.build(LayerNode.group([ModelsDev.node, Bus.node]), [
-      [ModelsDev.node, ModelsDev.configured(options)],
-      [LayerNodePlatform.httpClient, Layer.succeed(HttpClient.HttpClient, makeMockClient(state))],
-      [KV.node, makeMockKV(cache)],
+      ModelsDev.node.replace(ModelsDev.configured(options)),
+      LayerNodePlatform.httpClient.replace(Layer.succeed(HttpClient.HttpClient, makeMockClient(state))),
+      KV.node.replace(makeMockKV(cache)),
     ]),
   )
 
@@ -312,9 +312,9 @@ describe("ModelsDev Service", () => {
       const state = yield* Ref.make({ ...initialState, body: JSON.stringify(fixture2) })
       const layer = Layer.fresh(
         AppNodeBuilder.build(ModelsDev.node, [
-          [ModelsDev.node, ModelsDev.configured({ fetch: true, snapshot: false })],
-          [LayerNodePlatform.httpClient, Layer.succeed(HttpClient.HttpClient, makeMockClient(state))],
-          [KV.node, makeFailingWriteKV(cache)],
+          ModelsDev.node.replace(ModelsDev.configured({ fetch: true, snapshot: false })),
+          LayerNodePlatform.httpClient.replace(Layer.succeed(HttpClient.HttpClient, makeMockClient(state))),
+          KV.node.replace(makeFailingWriteKV(cache)),
         ]),
       )
       const result = yield* ModelsDev.Service.use((s) => s.get()).pipe(Effect.provide(layer))

packages/core/test/npm.test.ts

@@ -20,7 +20,7 @@ const writePackage = (dir: string, pkg: Record<string, unknown>) =>
   )
 
 const npmLayer = (cache: string) =>
-  AppNodeBuilder.build(Npm.node, [[Global.node, Global.layerWith({ cache, state: path.join(cache, "state") })]])
+  AppNodeBuilder.build(Npm.node, [Global.node.replace(Global.layerWith({ cache, state: path.join(cache, "state") }))])
 
 async function createGitFixture(directory: string) {
   const repository = path.join(directory, "repository")

packages/core/test/permission.test.ts

@@ -37,7 +37,7 @@ const it = testEffect(
       PluginHooks.node,
       Permission.node,
     ]),
-    [[Location.node, current]],
+    [Location.node.replace(current)],
   ),
 )
 

packages/core/test/plugin-hooks.test.ts

@@ -4,12 +4,12 @@ import { Agent } from "@opencode-ai/schema/agent"
 import { Model } from "@opencode-ai/schema/model"
 import { Provider } from "@opencode-ai/schema/provider"
 import { Session } from "@opencode-ai/schema/session"
-import { Effect, Layer } from "effect"
+import { LayerNode } from "@opencode-ai/util/effect/layer-node"
+import { Effect } from "effect"
 import { PluginHooks } from "../src/plugin/hooks"
 import { testEffect } from "./lib/effect"
 
-const layer = PluginHooks.node.implementation as Layer.Layer<PluginHooks.Service>
-const it = testEffect(layer)
+const it = testEffect(LayerNode.compile(PluginHooks.node))
 
 describe("PluginHooks", () => {
   it.effect("registers scoped session hooks and triggers them sequentially", () =>

packages/core/test/plugin/command.test.ts

@@ -27,8 +27,8 @@ const locationLayer = Layer.succeed(
 )
 const it = testEffect(
   AppNodeBuilder.build(LayerNode.group([Command.node, Mcp.node, Bus.node]), [
-    [Mcp.node, emptyMcpLayer],
-    [Location.node, locationLayer],
+    Mcp.node.replace(emptyMcpLayer),
+    Location.node.replace(locationLayer),
   ]),
 )
 

packages/core/test/plugin/fixture.ts

@@ -88,12 +88,14 @@ export const PluginTestLayer = LayerNode.compile(
     Watcher.node,
     WebSearch.node,
   ]),
-  [
-    [Location.node, tempLocationLayer],
-    [Npm.node, npmLayer],
-    [Config.node, Config.testLayer()],
-    [Mcp.node, emptyMcpLayer],
-    [Generate.node, generateLayer],
-    [Permission.node, permissionLayer],
-  ],
+  {
+    replacements: [
+      Location.node.replace(tempLocationLayer),
+      Npm.node.replace(npmLayer),
+      Config.node.replace(Config.testLayer()),
+      Mcp.node.replace(emptyMcpLayer),
+      Generate.node.replace(generateLayer),
+      Permission.node.replace(permissionLayer),
+    ],
+  },
 ) as unknown as Layer.Layer<unknown, never>

packages/core/test/plugin/host.test.ts

@@ -23,11 +23,11 @@ const it = testEffect(
       PluginRuntime.providerNodeWithCell(cell),
     ]),
     [
-      [Global.node, tempGlobalLayer],
-      [Watcher.node, Watcher.configured({ enabled: false })],
-      [SessionExecution.node, SessionExecution.noopLayer],
-      [PluginRuntime.node, PluginRuntime.layerWithCell(cell)],
-      [PersistentPty.node, PersistentPty.configured()],
+      Global.node.replace(tempGlobalLayer),
+      Watcher.node.replace(Watcher.configured({ enabled: false })),
+      SessionExecution.node.replace(SessionExecution.noopLayer),
+      PluginRuntime.node.replace(PluginRuntime.layerWithCell(cell)),
+      PersistentPty.node.replace(PersistentPty.configured()),
     ],
   ),
 )

packages/core/test/plugin/models-dev.test.ts

@@ -27,12 +27,12 @@ const locationLayer = Layer.succeed(
   Location.Service.of(location({ directory: AbsolutePath.make(import.meta.dir) })),
 )
 const layer = AppNodeBuilder.build(LayerNode.group([Catalog.node, Integration.node, Bus.node]), [
-  [Location.node, locationLayer],
+  Location.node.replace(locationLayer),
 ])
 const it = testEffect(layer)
 const real = testEffect(PluginTestLayer)
 const models = (file: string) =>
-  AppNodeBuilder.build(ModelsDev.node, [[ModelsDev.node, ModelsDev.configured({ file, fetch: false })]])
+  AppNodeBuilder.build(ModelsDev.node, [ModelsDev.node.replace(ModelsDev.configured({ file, fetch: false }))])
 
 describe("ModelsDevPlugin", () => {
   real.effect("keeps the retained model seed unchanged across catalog replay", () =>

packages/core/test/plugin/variant.test.ts

@@ -15,7 +15,7 @@ const locationLayer = Layer.succeed(
   Location.Service,
   Location.Service.of(location({ directory: AbsolutePath.make(import.meta.dir) })),
 )
-const it = testEffect(AppNodeBuilder.build(Catalog.node, [[Location.node, locationLayer]]))
+const it = testEffect(AppNodeBuilder.build(Catalog.node, [Location.node.replace(locationLayer)]))
 
 describe("VariantPlugin", () => {
   it.effect("adds GLM 5.2 variants after catalog sources", () =>

packages/core/test/plugin/websearch-fixture.ts

@@ -42,7 +42,7 @@ const http = Layer.succeed(
 export const webSearchIntegrationTest = testEffect(
   Layer.merge(
     AppNodeBuilder.build(LayerNode.group([Integration.node, Credential.node, Bus.node, Form.node, WebSearch.node]), [
-      [Config.node, Config.testLayer()],
+      Config.node.replace(Config.testLayer()),
     ]),
     http,
   ),

packages/core/test/pty/pty-session.test.ts

@@ -17,7 +17,9 @@ const locationLayer = Layer.succeed(
   Location.Service,
   Location.Service.of(location({ directory: AbsolutePath.make("/tmp") })),
 )
-const it = testEffect(AppNodeBuilder.build(LayerNode.group([Pty.node, Bus.node]), [[Location.node, locationLayer]]))
+const it = testEffect(
+  AppNodeBuilder.build(LayerNode.group([Pty.node, Bus.node]), [Location.node.replace(locationLayer)]),
+)
 const ptyTest = process.platform === "win32" ? it.live.skip : it.live
 
 const subscribePtyEvents = Effect.fn("PtySessionTest.subscribePtyEvents")(function* () {
@@ -200,7 +202,7 @@ describe("pty", () => {
 
 const configuredShell = process.platform === "win32" ? undefined : Bun.which("bash")
 const configuredIt = testEffect(
-  AppNodeBuilder.build(LayerNode.group([Pty.node, Bus.node, ShellSelect.node]), [[Location.node, locationLayer]]),
+  AppNodeBuilder.build(LayerNode.group([Pty.node, Bus.node, ShellSelect.node]), [Location.node.replace(locationLayer)]),
 )
 const configuredTest = process.platform === "win32" ? configuredIt.live.skip : configuredIt.live
 

packages/core/test/pty/ticket.test.ts

@@ -8,7 +8,9 @@ import { testEffect } from "../lib/effect"
 
 const it = testEffect(LayerNode.compile(PtyTicket.node))
 const itExpiring = testEffect(
-  LayerNode.compile(PtyTicket.node, [[PtyTicket.node, Layer.effect(PtyTicket.Service, PtyTicket.make(5))]]),
+  LayerNode.compile(PtyTicket.node, {
+    replacements: [PtyTicket.node.replace(Layer.effect(PtyTicket.Service, PtyTicket.make(5)))],
+  }),
 )
 
 describe("PTY websocket tickets", () => {

packages/core/test/reference-instructions.test.ts

@@ -8,7 +8,7 @@ import { it } from "./lib/effect"
 import { readInitial, readUpdate } from "./lib/instructions"
 
 const instructionsLayer = (referenceLayer: Layer.Layer<Reference.Service>) =>
-  AppNodeBuilder.build(ReferenceInstructions.node, [[Reference.node, referenceLayer]])
+  AppNodeBuilder.build(ReferenceInstructions.node, [Reference.node.replace(referenceLayer)])
 
 describe("ReferenceInstructions", () => {
   it.effect("lists available references in the instructions", () =>

packages/core/test/reference.test.ts

@@ -11,7 +11,7 @@ import { it } from "./lib/effect"
 const cache = Layer.mock(RepositoryCache.Service, {
   ensure: () => Effect.die("unexpected Git materialization"),
 })
-const referenceLayer = AppNodeBuilder.build(Reference.node, [[RepositoryCache.node, cache]])
+const referenceLayer = AppNodeBuilder.build(Reference.node, [RepositoryCache.node.replace(cache)])
 
 describe("Reference", () => {
   it.effect("registers normalized sources for the owning scope", () =>

packages/core/test/repository-cache.test.ts

@@ -227,8 +227,8 @@ describe("RepositoryCache", () => {
 
 function cacheLayer(root: string) {
   return AppNodeBuilder.build(LayerNode.group([RepositoryCache.node, KV.node]), [
-    [Global.node, Global.layerWith({ state: path.join(root, "state"), repos: path.join(root, "repos") })],
-    [Database.node, Database.configured({ path: path.join(root, "cache.sqlite") })],
+    Global.node.replace(Global.layerWith({ state: path.join(root, "state"), repos: path.join(root, "repos") })),
+    Database.node.replace(Database.configured({ path: path.join(root, "cache.sqlite") })),
   ])
 }
 

packages/core/test/ripgrep.test.ts

@@ -10,7 +10,7 @@ import { tmpdir } from "./fixture/tmpdir"
 import { testEffect } from "./lib/effect"
 import { tempLocationLayer } from "./fixture/location"
 
-const it = testEffect(AppNodeBuilder.build(Ripgrep.node, [[Location.node, tempLocationLayer]]))
+const it = testEffect(AppNodeBuilder.build(Ripgrep.node, [Location.node.replace(tempLocationLayer)]))
 
 describe("Ripgrep", () => {
   it.live("globs files as an array", () =>

packages/core/test/rpc.test.ts

@@ -15,7 +15,7 @@ import { testEffect } from "./lib/effect"
 const ref = Location.Ref.make({ directory: AbsolutePath.make("/rpc-project") })
 const it = testEffect(
   AppNodeBuilder.build(LayerNode.group([Rpc.node, Bus.node, Location.node]), [
-    [Location.node, Layer.succeed(Location.Service, location(ref))],
+    Location.node.replace(Layer.succeed(Location.Service, location(ref))),
   ]),
 )
 const Echo = Rpc.define({
@@ -200,8 +200,7 @@ describe("Rpc", () => {
         events: {},
       })
       yield* rpc.register(Failing, {
-        standard: (_input, context) =>
-          Effect.fail(context.error("missing", "Missing", { attempts: "2" })),
+        standard: (_input, context) => Effect.fail(context.error("missing", "Missing", { attempts: "2" })),
         effect: (_input, context) => Effect.fail(context.error("invalid", "Invalid", { count: 3 })),
       })
 
@@ -269,7 +268,6 @@ describe("Rpc", () => {
       expect(Exit.isFailure(yield* rpc.call(Raw.id, "count", "42").pipe(Effect.exit))).toBe(true)
       expect(Exit.isFailure(yield* rpc.call(Raw.id, "count", 0).pipe(Effect.exit))).toBe(true)
       expect(Exit.isFailure(yield* registration.events.emit("counted", { count: 0 }).pipe(Effect.exit))).toBe(true)
-
     }),
   )
 
@@ -330,10 +328,12 @@ describe("Rpc", () => {
       const bus = yield* Bus.Service
       const otherRef = Location.Ref.make({ directory: ref.directory, workspaceID: Workspace.ID.make("wrk_other") })
       const otherContext = yield* Layer.build(
-        LayerNode.compile(Rpc.node, [
-          [Bus.node, Layer.succeed(Bus.Service, bus)],
-          [Location.node, Layer.succeed(Location.Service, location(otherRef))],
-        ]).pipe(Layer.fresh),
+        LayerNode.compile(Rpc.node, {
+          replacements: [
+            Bus.node.replace(Layer.succeed(Bus.Service, bus)),
+            Location.node.replace(Layer.succeed(Location.Service, location(otherRef))),
+          ],
+        }).pipe(Layer.fresh),
       )
       const other = Context.get(otherContext, Rpc.Service)
       const first = yield* rpc.register(Echo, { echo: () => Effect.succeed("first") })

packages/core/test/session-compact.test.ts

@@ -65,9 +65,9 @@ const it = testEffect(
   AppNodeBuilder.build(
     LayerNode.group([Database.node, Bus.node, SessionProjector.node, SessionStore.node, Session.node]),
     [
-      [LocationServiceMap.node, locations],
-      [Project.node, globalProjectNode],
-      [SessionExecution.node, SessionExecution.noopLayer],
+      LocationServiceMap.node.replace(locations),
+      Project.node.replace(globalProjectNode),
+      SessionExecution.node.replace(SessionExecution.noopLayer),
     ],
   ),
 )

packages/core/test/session-compaction.test.ts

@@ -88,10 +88,7 @@ const it = testEffect(
       SessionCompaction.node,
       SessionModelRequest.node,
     ]),
-    [
-      [Bus.node, Bus.configured({ persist: true })],
-      [llmClient, client],
-    ],
+    [Bus.node.replace(Bus.configured({ persist: true })), llmClient.replace(client)],
   ),
 )
 

packages/core/test/session-create.test.ts

@@ -51,30 +51,27 @@ const it = testEffect(
       InstructionEntry.node,
     ]),
     [
-      [Bus.node, Bus.configured({ persist: true })],
-      [Project.node, globalProjectNode],
-      [LocationServiceMap.node, promptLocationNode],
-      [SessionExecution.node, SessionExecution.noopLayer],
+      Bus.node.replace(Bus.configured({ persist: true })),
+      Project.node.replace(globalProjectNode),
+      LocationServiceMap.node.replace(promptLocationNode),
+      SessionExecution.node.replace(SessionExecution.noopLayer),
     ],
   ),
 )
 const liveIt = testEffect(
   AppNodeBuilder.build(
     LayerNode.group([Database.node, Bus.node, Project.node, SessionProjector.node, SessionStore.node, Session.node]),
-    [
-      [Bus.node, Bus.configured({ persist: true })],
-      [SessionExecution.node, SessionExecution.noopLayer],
-    ],
+    [Bus.node.replace(Bus.configured({ persist: true })), SessionExecution.node.replace(SessionExecution.noopLayer)],
   ),
 )
 const projectIt = testEffect(
   AppNodeBuilder.build(
     LayerNode.group([Database.node, Bus.node, Project.node, SessionProjector.node, SessionStore.node, Session.node]),
     [
-      [Bus.node, Bus.configured({ persist: true })],
+      Bus.node.replace(Bus.configured({ persist: true })),
       // Project adoption needs plain-prompt admission, not live plugin/provider startup.
-      [LocationServiceMap.node, promptLocationNode],
-      [SessionExecution.node, SessionExecution.noopLayer],
+      LocationServiceMap.node.replace(promptLocationNode),
+      SessionExecution.node.replace(SessionExecution.noopLayer),
     ],
   ),
 )
@@ -968,8 +965,8 @@ describe("Session.create", () => {
       const targetLayer = AppNodeBuilder.build(
         LayerNode.group([Database.node, Bus.node, SessionProjector.node, SessionStore.node]),
         [
-          [Database.node, Database.configured({ path: path.join(tmp.path, "target.sqlite") })],
-          [Bus.node, Bus.configured({ persist: true })],
+          Database.node.replace(Database.configured({ path: path.join(tmp.path, "target.sqlite") })),
+          Bus.node.replace(Bus.configured({ persist: true })),
         ],
       )
 

packages/core/test/session-execution.test.ts

@@ -4,6 +4,7 @@ import { Database } from "@opencode-ai/core/database/database"
 import { AppNodeBuilder } from "@opencode-ai/core/effect/app-node-builder"
 import { LayerNode } from "@opencode-ai/util/effect/layer-node"
 import { Bus } from "@opencode-ai/core/bus"
+import { Instance } from "@opencode-ai/core/instance/service"
 import { Job } from "@opencode-ai/core/job"
 import { KV } from "@opencode-ai/core/kv"
 import { LocationServiceMap } from "@opencode-ai/core/location-service-map"
@@ -1371,7 +1372,12 @@ function buildExecution(
         Layer.provide(Layer.succeed(Bus.Service, bus)),
         Layer.provide(Layer.succeed(SessionStore.Service, store)),
         Layer.provide(Layer.succeed(Job.Service, jobs)),
-        Layer.provide(locations),
+        // Do not reuse the outer harness's selector with its already-captured Location map.
+        Layer.provide(
+          LayerNode.compile(Instance.byLocationNode, {
+            replacements: [LocationServiceMap.node.replace(locations)],
+          }).pipe(Layer.fresh),
+        ),
       ),
       scope,
     )

packages/core/test/session-generate.test.ts

@@ -142,17 +142,17 @@ const it = testEffect(
       SessionGenerateNode.node,
     ]),
     [
-      [Bus.node, Bus.configured({ persist: true })],
-      [llmClient, client],
-      [SessionRunnerModel.node, models],
-      [InstructionBuiltIns.node, builtins],
-      [InstructionDiscovery.node, discovery],
-      [SkillInstructions.node, skills],
-      [ReferenceInstructions.node, references],
-      [McpInstructions.node, mcp],
-      [PluginSupervisor.node, plugins],
-      [Tool.node, tools],
-      [Location.node, Location.boundNode({ directory: AbsolutePath.make("/project") })],
+      Bus.node.replace(Bus.configured({ persist: true })),
+      llmClient.replace(client),
+      SessionRunnerModel.node.replace(models),
+      InstructionBuiltIns.node.replace(builtins),
+      InstructionDiscovery.node.replace(discovery),
+      SkillInstructions.node.replace(skills),
+      ReferenceInstructions.node.replace(references),
+      McpInstructions.node.replace(mcp),
+      PluginSupervisor.node.replace(plugins),
+      Tool.node.replace(tools),
+      Location.node.replace(Location.boundNode({ directory: AbsolutePath.make("/project") })),
     ],
   ),
 )

packages/core/test/session-instructions.test.ts

@@ -53,7 +53,7 @@ const readToolNode = makeLocationNode({
 
 const permission = permissionLayer({ assert: () => Effect.void })
 const config = Config.testLayer()
-const imageLayer = AppNodeBuilder.build(Image.node, [[Config.node, config]])
+const imageLayer = AppNodeBuilder.build(Image.node, [Config.node.replace(config)])
 
 const testLayer = AppNodeBuilder.build(
   LayerNode.group([
@@ -74,12 +74,12 @@ const testLayer = AppNodeBuilder.build(
     Image.node,
   ]),
   [
-    [Project.node, globalProjectNode],
-    [SessionExecution.node, SessionExecution.noopLayer],
-    [Location.node, tempLocationLayer],
-    [Permission.node, permission],
-    [Config.node, config],
-    [Image.node, imageLayer],
+    Project.node.replace(globalProjectNode),
+    SessionExecution.node.replace(SessionExecution.noopLayer),
+    Location.node.replace(tempLocationLayer),
+    Permission.node.replace(permission),
+    Config.node.replace(config),
+    Image.node.replace(imageLayer),
   ],
 )
 

packages/core/test/session-log.test.ts

@@ -22,9 +22,9 @@ const it = testEffect(
   AppNodeBuilder.build(
     LayerNode.group([Database.node, Bus.node, SessionProjector.node, SessionStore.node, Session.node]),
     [
-      [Bus.node, Bus.configured({ persist: true })],
-      [Project.node, globalProjectNode],
-      [SessionExecution.node, SessionExecution.noopLayer],
+      Bus.node.replace(Bus.configured({ persist: true })),
+      Project.node.replace(globalProjectNode),
+      SessionExecution.node.replace(SessionExecution.noopLayer),
     ],
   ),
 )

packages/core/test/session-message-update.test.ts

@@ -30,10 +30,9 @@ const it = testEffect(
   AppNodeBuilder.build(
     LayerNode.group([Database.node, Bus.node, SessionProjector.node, SessionStore.node, Session.node]),
     [
-      [Bus.node, Bus.configured({ persist: true })],
-      [Project.node, globalProjectNode],
-      [
-        SessionExecution.node,
+      Bus.node.replace(Bus.configured({ persist: true })),
+      Project.node.replace(globalProjectNode),
+      SessionExecution.node.replace(
         Layer.succeed(
           SessionExecution.Service,
           SessionExecution.Service.of({
@@ -45,7 +44,7 @@ const it = testEffect(
             awaitIdle: () => Effect.void,
           }),
         ),
-      ],
+      ),
     ],
   ),
 )
@@ -154,8 +153,8 @@ describe("Session.updateMessage", () => {
       const target = AppNodeBuilder.build(
         LayerNode.group([Database.node, Bus.node, SessionProjector.node, SessionStore.node]),
         [
-          [Database.node, Database.configured({ path: path.join(tmp.path, "target.sqlite") })],
-          [Bus.node, Bus.configured({ persist: true })],
+          Database.node.replace(Database.configured({ path: path.join(tmp.path, "target.sqlite") })),
+          Bus.node.replace(Bus.configured({ persist: true })),
         ],
       )
 

packages/core/test/session-model-request.test.ts

@@ -20,9 +20,13 @@ import { testEffect } from "./lib/effect"
 const capabilities = (input: string[]) => ({ tools: true, input, output: ["text"] })
 
 const it = testEffect(
-  LayerNode.compile(LayerNode.group([SessionModelRequest.node, PluginHooks.node]), [
-    [SessionModelTransport.node, SessionModelTransport.makeLayer({ open: () => Effect.die("Unexpected connection") })],
-  ]),
+  LayerNode.compile(LayerNode.group([SessionModelRequest.node, PluginHooks.node]), {
+    replacements: [
+      SessionModelTransport.node.replace(
+        SessionModelTransport.makeLayer({ open: () => Effect.die("Unexpected connection") }),
+      ),
+    ],
+  }),
 )
 
 const requestInput = (model: LanguageModel) => ({

packages/core/test/session-move.test.ts

@@ -25,10 +25,7 @@ import { globalProjectNode } from "./lib/project"
 const it = testEffect(
   AppNodeBuilder.build(
     LayerNode.group([Database.node, Bus.node, SessionProjector.node, SessionStore.node, Session.node]),
-    [
-      [Project.node, globalProjectNode],
-      [SessionExecution.node, SessionExecution.noopLayer],
-    ],
+    [Project.node.replace(globalProjectNode), SessionExecution.node.replace(SessionExecution.noopLayer)],
   ),
 )
 const itWithActiveExecution = testEffect(
@@ -42,20 +39,21 @@ const itWithActiveExecution = testEffect(
       Session.node,
     ]),
     [
-      [Project.node, globalProjectNode],
-      [
-        LocationServiceMap.node,
+      Project.node.replace(globalProjectNode),
+      LocationServiceMap.node.replace(
         Layer.effect(
           LocationServiceMap.Service,
           LayerMap.make(
             (ref: Location.Ref) =>
               Layer.merge(
-                LayerNode.compile(Location.boundNode(ref), [[Project.node, globalProjectNode]]),
+                LayerNode.compile(Location.boundNode(ref), {
+                  replacements: [Project.node.replace(globalProjectNode)],
+                }),
                 Layer.succeed(SessionRunner.Service, { drain: () => Effect.never }),
               ) as unknown as Layer.Layer<LocationServices>,
           ),
         ),
-      ],
+      ),
     ],
   ),
 )
@@ -69,9 +67,9 @@ const itWithUnavailableDestination = testEffect(
   AppNodeBuilder.build(
     LayerNode.group([Database.node, Bus.node, SessionProjector.node, SessionStore.node, Session.node]),
     [
-      [Project.node, globalProjectNode],
-      [SessionExecution.node, SessionExecution.noopLayer],
-      [LocationServiceMap.node, unavailableLocations],
+      Project.node.replace(globalProjectNode),
+      SessionExecution.node.replace(SessionExecution.noopLayer),
+      LocationServiceMap.node.replace(unavailableLocations),
     ],
   ),
 )

packages/core/test/session-owned.test.ts

@@ -15,6 +15,7 @@ import { Bus } from "../src/bus.js"
 import { Database } from "../src/database/database.js"
 import { EventTable } from "../src/event/sql.js"
 import { Image } from "../src/image.js"
+import { Instance } from "../src/instance/service.js"
 import { Location } from "../src/location.js"
 import { PluginHooks } from "../src/plugin/hooks.js"
 import { PluginSupervisor } from "../src/plugin/supervisor-service.js"
@@ -51,10 +52,9 @@ const it = testEffect(
       SessionInbox.node,
       FSUtil.node,
     ]),
-    [
-      [Bus.node, Bus.configured({ persist: true })],
-      [Global.node, tempGlobalLayer],
-    ],
+    {
+      replacements: [Bus.node.replace(Bus.configured({ persist: true })), Global.node.replace(tempGlobalLayer)],
+    },
   ),
 )
 const sessionID = SessionSchema.ID.make("ses_owned")
@@ -130,7 +130,7 @@ const setup = Effect.fnUntraced(function* (options?: {
     Layer.mock(Image.Service, {}),
     options?.shell ?? Layer.mock(Shell.Service, {}),
   )
-  const servicesFor = (ref: Location.Ref): Layer.Layer<Session.Services> => {
+  const servicesFor = (ref: Location.Ref) => {
     locations.push(ref)
     return Layer.merge(SessionRevert.layer, SessionPrompt.layer).pipe(
       Layer.provideMerge(
@@ -159,10 +159,20 @@ const setup = Effect.fnUntraced(function* (options?: {
       Layer.fresh,
     )
   }
-  const sessions = yield* Session.make(servicesFor).pipe(
+  const sessions = yield* Session.make().pipe(
     Effect.satisfiesServicesType<
-      Bus.Service | SessionStore.Service | SessionExecution.Service | SessionInbox.Service | Scope.Scope
+      | Bus.Service
+      | SessionStore.Service
+      | Instance.Service
+      | SessionExecution.Service
+      | SessionInbox.Service
+      | Scope.Scope
     >(),
+    Effect.provideService(Instance.Service, {
+      // This fixture supplies only the instance services exercised by Session.
+      provide: (session) => Effect.provide(servicesFor(session.location) as Layer.Layer<Instance.Services>),
+      provideIfLoaded: () => () => Effect.die("Unexpected loaded-only instance lookup"),
+    }),
     Effect.provideService(SessionExecution.Service, options?.execution ?? execution),
   )
   return { sessions, hooks, locations, flushes, resumes, wakes, db: database.db, bus, store }

packages/core/test/session-projector.test.ts

@@ -35,10 +35,10 @@ import { Snapshot } from "@opencode-ai/core/snapshot"
 const it = testEffect(
   AppNodeBuilder.build(
     LayerNode.group([Database.node, Bus.node, SessionProjector.node, SessionInbox.node, SessionStore.node]),
-    [[Bus.node, Bus.configured({ persist: true })]],
+    [Bus.node.replace(Bus.configured({ persist: true }))],
   ),
 )
-const sessionsLayer = AppNodeBuilder.build(Session.node, [[SessionExecution.node, SessionExecution.noopLayer]])
+const sessionsLayer = AppNodeBuilder.build(Session.node, [SessionExecution.node.replace(SessionExecution.noopLayer)])
 const sessionID = Session.ID.make("ses_projector_test")
 const created = DateTime.makeUnsafe(0)
 const model = { id: Model.ID.make("model"), providerID: Provider.ID.make("provider") }

packages/core/test/session-prompt-hooks.test.ts

@@ -38,11 +38,11 @@ const it = testEffect(
       PluginRuntime.providerNodeWithCell(runtime),
     ]),
     [
-      [Bus.node, Bus.configured({ persist: true })],
-      [Global.node, tempGlobalLayer],
-      [Watcher.node, Watcher.configured({ enabled: false })],
-      [SessionExecution.node, SessionExecution.noopLayer],
-      [PluginRuntime.node, PluginRuntime.layerWithCell(runtime)],
+      Bus.node.replace(Bus.configured({ persist: true })),
+      Global.node.replace(tempGlobalLayer),
+      Watcher.node.replace(Watcher.configured({ enabled: false })),
+      SessionExecution.node.replace(SessionExecution.noopLayer),
+      PluginRuntime.node.replace(PluginRuntime.layerWithCell(runtime)),
     ],
   ),
 )

packages/core/test/session-prompt.test.ts

@@ -95,9 +95,9 @@ const locations = (references: Layer.Layer<Reference.Service>) =>
                 Layer.provideMerge(
                   Layer.mergeAll(
                     references,
-                    LayerNode.compile(LayerNode.group([PluginHooks.node, Skill.node]), [
-                      [Bus.node, Layer.succeed(Bus.Service, bus)],
-                    ]),
+                    LayerNode.compile(LayerNode.group([PluginHooks.node, Skill.node]), {
+                      replacements: [Bus.node.replace(Layer.succeed(Bus.Service, bus))],
+                    }),
                     Layer.mock(Image.Service, {
                       normalize: (_resource, content) =>
                         ready
@@ -131,9 +131,9 @@ const sessionLayer = (references = Layer.mock(Reference.Service, { refresh: () =
   AppNodeBuilder.build(
     LayerNode.group([Database.node, Bus.node, SessionProjector.node, SessionStore.node, Session.node]),
     [
-      [Bus.node, Bus.configured({ persist: true })],
-      [SessionExecution.node, execution],
-      [LocationServiceMap.node, locations(references)],
+      Bus.node.replace(Bus.configured({ persist: true })),
+      SessionExecution.node.replace(execution),
+      LocationServiceMap.node.replace(locations(references)),
     ],
   )
 const it = testEffect(sessionLayer())
@@ -298,13 +298,14 @@ describe("Session.prompt", () => {
         }).pipe(
           Effect.provide(
             AppNodeBuilder.build(Reference.node, [
-              [Global.node, Global.layerWith({ state: path.join(root, "state"), repos: path.join(root, "repos") })],
-              [
-                RepositoryCache.node,
+              Global.node.replace(
+                Global.layerWith({ state: path.join(root, "state"), repos: path.join(root, "repos") }),
+              ),
+              RepositoryCache.node.replace(
                 Layer.succeed(RepositoryCache.Service, {
                   ensure: (input) => cache.ensure(input).pipe(Effect.tap(() => Queue.offer(completed, undefined))),
                 }),
-              ],
+              ),
             ]),
           ),
         )
@@ -312,7 +313,7 @@ describe("Session.prompt", () => {
         Effect.scoped,
         Effect.provide(
           AppNodeBuilder.build(LayerNode.group([RepositoryCache.node, KV.node, EffectFlock.node]), [
-            [Global.node, Global.layerWith({ state: path.join(root, "state"), repos: path.join(root, "repos") })],
+            Global.node.replace(Global.layerWith({ state: path.join(root, "state"), repos: path.join(root, "repos") })),
           ]),
         ),
       )

packages/core/test/session-remove.test.ts

@@ -1,9 +1,10 @@
 import { describe, expect } from "bun:test"
-import { Effect, Layer } from "effect"
+import { Effect, Layer, Option, RcMap, Scope } from "effect"
 import { AppNodeBuilder } from "@opencode-ai/core/effect/app-node-builder"
 import { LayerNode } from "@opencode-ai/util/effect/layer-node"
 import { Database } from "@opencode-ai/core/database/database"
 import { Bus } from "@opencode-ai/core/bus"
+import { Instance } from "@opencode-ai/core/instance/service"
 import { Location } from "@opencode-ai/core/location"
 import { Project } from "@opencode-ai/core/project"
 import { AbsolutePath } from "@opencode-ai/core/schema"
@@ -19,12 +20,18 @@ import { globalProjectNode } from "./lib/project"
 import { tmpdirScoped } from "./fixture/tmpdir"
 
 const closed: Session.ID[] = []
-const transport = Layer.succeed(
+const transportScopes = new Set<Scope.Scope>()
+const transport = Layer.effect(
   SessionModelTransport.Service,
-  SessionModelTransport.Service.of({
-    bind: () => ({ execute: () => Effect.die("Unexpected WebSocket execution") }),
-    close: (sessionID) => Effect.sync(() => closed.push(sessionID)),
-    closeAll: Effect.void,
+  Effect.gen(function* () {
+    const scope = yield* Scope.Scope
+    transportScopes.add(scope)
+    yield* Effect.addFinalizer(() => Effect.sync(() => transportScopes.delete(scope)))
+    return SessionModelTransport.Service.of({
+      bind: () => ({ execute: () => Effect.die("Unexpected WebSocket execution") }),
+      close: (sessionID) => Effect.sync(() => closed.push(sessionID)),
+      closeAll: Effect.void,
+    })
   }),
 )
 const it = testEffect(
@@ -36,12 +43,13 @@ const it = testEffect(
       SessionStore.node,
       SessionEnvironment.node,
       Session.node,
+      Instance.byLocationNode,
       LocationServiceMap.node,
     ]),
     [
-      [Project.node, globalProjectNode],
-      [SessionExecution.node, SessionExecution.noopLayer],
-      [SessionModelTransport.node, transport],
+      Project.node.replace(globalProjectNode),
+      SessionExecution.node.replace(SessionExecution.noopLayer),
+      SessionModelTransport.node.replace(transport),
     ],
   ),
 )
@@ -72,6 +80,27 @@ describe("Session.remove", () => {
     }),
   )
 
+  it.live("removes unloaded sessions and children without initializing an instance", () =>
+    Effect.gen(function* () {
+      const temporary = yield* tmpdirScoped()
+      const sessions = yield* Session.Service
+      const locations = yield* LocationServiceMap.Service
+      const parent = yield* sessions.create({
+        location: Location.Ref.make({ directory: AbsolutePath.make(temporary.path) }),
+      })
+      yield* sessions.create({ parentID: parent.id })
+      closed.length = 0
+      expect(Array.from(yield* RcMap.keys(locations.rcMap))).toEqual([])
+
+      yield* sessions.remove(parent.id)
+
+      expect(closed).toEqual([])
+      expect(transportScopes.size).toBe(0)
+      expect(Array.from(yield* RcMap.keys(locations.rcMap))).toEqual([])
+      expect((yield* sessions.list()).data).toEqual([])
+    }),
+  )
+
   it.effect("fails when the session does not exist", () =>
     Effect.gen(function* () {
       const session = yield* Session.Service
@@ -84,3 +113,40 @@ describe("Session.remove", () => {
     }),
   )
 })
+
+describe("Instance.provideIfLoaded", () => {
+  it.live("skips absent instances and scopes loaded borrows without replacing the caller's Scope", () =>
+    Effect.gen(function* () {
+      const temporary = yield* tmpdirScoped()
+      const sessions = yield* Session.Service
+      const instances = yield* Instance.Service
+      const locations = yield* LocationServiceMap.Service
+      const scope = yield* Scope.Scope
+      const session = yield* sessions.create({
+        location: Location.Ref.make({ directory: AbsolutePath.make(temporary.path) }),
+      })
+      const absent = Effect.die("An unloaded instance must not run the effect").pipe(instances.provideIfLoaded(session))
+
+      expect(yield* absent).toEqual(Option.none())
+      expect(transportScopes.size).toBe(0)
+      yield* Location.Service.pipe(instances.provide(session))
+      expect(transportScopes.size).toBe(1)
+      expect(yield* Effect.void.pipe(instances.provideIfLoaded(session))).toEqual(Option.some(undefined))
+      const failure = new Error("Borrowed operation failed")
+      expect(yield* Effect.fail(failure).pipe(instances.provideIfLoaded(session), Effect.flip)).toBe(failure)
+
+      const borrowed = yield* Effect.gen(function* () {
+        const location = yield* Location.Service
+        const callerScope = yield* Scope.Scope
+        expect(callerScope).toBe(scope)
+        yield* locations.invalidate(session.location)
+        expect(transportScopes.size).toBe(1)
+        return location.directory
+      }).pipe(instances.provideIfLoaded(session), Effect.satisfiesServicesType<Scope.Scope>())
+
+      expect(borrowed).toEqual(Option.some(session.location.directory))
+      expect(transportScopes.size).toBe(0)
+      expect(yield* absent).toEqual(Option.none())
+    }),
+  )
+})

packages/core/test/session-revert.test.ts

@@ -31,9 +31,9 @@ const it = testEffect(
   AppNodeBuilder.build(
     LayerNode.group([Database.node, Bus.node, SessionProjector.node, Session.node, LocationServiceMap.node]),
     [
-      [Bus.node, Bus.configured({ persist: true })],
-      [Global.node, tempGlobalLayer],
-      [SessionExecution.node, SessionExecution.noopLayer],
+      Bus.node.replace(Bus.configured({ persist: true })),
+      Global.node.replace(tempGlobalLayer),
+      SessionExecution.node.replace(SessionExecution.noopLayer),
     ],
   ),
 )

packages/core/test/session-runner-recorded.test.ts

@@ -1,6 +1,6 @@
 import { HttpRecorder } from "@opencode-ai/http-recorder"
 import { OpenAIChat } from "@opencode-ai/ai/protocols/openai-chat"
-import { Auth, LLMClient, RequestExecutor } from "@opencode-ai/ai/route"
+import { Auth, LLMClient, type LLMClientService, RequestExecutor } from "@opencode-ai/ai/route"
 import { Catalog } from "@opencode-ai/core/catalog"
 import { Database } from "@opencode-ai/core/database/database"
 import { AppNodeBuilder } from "@opencode-ai/core/effect/app-node-builder"
@@ -100,22 +100,22 @@ const promptCatalog = Layer.mock(Catalog.Service, {
     small: () => Effect.undefined,
   },
 })
-const runnerLayer = (llmClient: Layer.Layer<typeof LLMClient.Service>) =>
+const runnerLayer = (llmClient: Layer.Layer<LLMClientService>) =>
   AppNodeBuilder.build(SessionRunnerLLM.node, [
-    [Snapshot.node, Snapshot.noopLayer],
-    [LayerNodePlatform.llmClient, llmClient],
-    [SessionRunnerModel.node, models],
-    [InstructionBuiltIns.node, systemContext],
-    [InstructionDiscovery.node, instructionContext],
-    [Location.node, Location.boundNode({ directory: AbsolutePath.make("/project") })],
-    [SkillInstructions.node, skillInstructions],
-    [ReferenceInstructions.node, referenceInstructions],
-    [McpInstructions.node, mcpInstructions],
-    [Config.node, config],
-    [Permission.node, permission],
-    [PluginSupervisor.node, pluginSupervisor],
+    Snapshot.node.replace(Snapshot.noopLayer),
+    LayerNodePlatform.llmClient.replace(llmClient),
+    SessionRunnerModel.node.replace(models),
+    InstructionBuiltIns.node.replace(systemContext),
+    InstructionDiscovery.node.replace(instructionContext),
+    Location.node.replace(Location.boundNode({ directory: AbsolutePath.make("/project") })),
+    SkillInstructions.node.replace(skillInstructions),
+    ReferenceInstructions.node.replace(referenceInstructions),
+    McpInstructions.node.replace(mcpInstructions),
+    Config.node.replace(config),
+    Permission.node.replace(permission),
+    PluginSupervisor.node.replace(pluginSupervisor),
   ])
-const execution = (llmClient: Layer.Layer<typeof LLMClient.Service>) =>
+const execution = (llmClient: Layer.Layer<LLMClientService>) =>
   Layer.effect(
     SessionExecution.Service,
     Effect.gen(function* () {
@@ -133,7 +133,7 @@ const execution = (llmClient: Layer.Layer<typeof LLMClient.Service>) =>
       })
     }),
   ).pipe(Layer.provide(runnerLayer(llmClient)))
-const testLayer = (llmClient: Layer.Layer<typeof LLMClient.Service>) =>
+const testLayer = (llmClient: Layer.Layer<LLMClientService>) =>
   AppNodeBuilder.build(
     LayerNode.group([
       Database.node,
@@ -155,21 +155,21 @@ const testLayer = (llmClient: Layer.Layer<typeof LLMClient.Service>) =>
       Session.node,
     ]),
     [
-      [Bus.node, Bus.configured({ persist: true })],
-      [LocationServiceMap.node, promptLocationNode],
-      [LayerNodePlatform.llmClient, llmClient],
-      [Permission.node, permission],
-      [Catalog.node, promptCatalog],
-      [SessionRunnerModel.node, models],
-      [InstructionBuiltIns.node, systemContext],
-      [InstructionDiscovery.node, instructionContext],
-      [Location.node, Location.boundNode({ directory: AbsolutePath.make("/project") })],
-      [SkillInstructions.node, skillInstructions],
-      [ReferenceInstructions.node, referenceInstructions],
-      [Config.node, config],
-      [Snapshot.node, Snapshot.noopLayer],
-      [PluginSupervisor.node, pluginSupervisor],
-      [SessionExecution.node, execution(llmClient)],
+      Bus.node.replace(Bus.configured({ persist: true })),
+      LocationServiceMap.node.replace(promptLocationNode),
+      LayerNodePlatform.llmClient.replace(llmClient),
+      Permission.node.replace(permission),
+      Catalog.node.replace(promptCatalog),
+      SessionRunnerModel.node.replace(models),
+      InstructionBuiltIns.node.replace(systemContext),
+      InstructionDiscovery.node.replace(instructionContext),
+      Location.node.replace(Location.boundNode({ directory: AbsolutePath.make("/project") })),
+      SkillInstructions.node.replace(skillInstructions),
+      ReferenceInstructions.node.replace(referenceInstructions),
+      Config.node.replace(config),
+      Snapshot.node.replace(Snapshot.noopLayer),
+      PluginSupervisor.node.replace(pluginSupervisor),
+      SessionExecution.node.replace(execution(llmClient)),
     ],
   )
 const it = testEffect(testLayer(client))

packages/core/test/session-runner-tool-events.test.ts

@@ -132,7 +132,7 @@ test("provider-executed success derives content and retains provider result stat
 
 testEffect(
   AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node, SessionProjector.node]), [
-    [Bus.node, Bus.configured({ persist: true })],
+    Bus.node.replace(Bus.configured({ persist: true })),
   ]),
 ).effect("commits a hosted tool result when cancellation races with the aggregate lock", () =>
   Effect.gen(function* () {

packages/core/test/session-runner.test.ts

@@ -408,22 +408,22 @@ const layer = Layer.unwrap(
       },
     })
     const replacements: LayerNode.Replacements = [
-      [Snapshot.node, Snapshot.noopLayer],
-      [LayerNodePlatform.llmClient, TestLLM.clientLayer],
-      [SessionRunnerModel.node, models],
-      [InstructionBuiltIns.node, systemContext],
-      [InstructionDiscovery.node, instructionContext],
-      [Location.node, Location.boundNode({ directory: AbsolutePath.make("/project") })],
-      [SkillInstructions.node, skillInstructions],
-      [ReferenceInstructions.node, referenceInstructions],
-      [Permission.node, permission],
-      [Config.node, config],
-      [PluginSupervisor.node, pluginSupervisor],
-      [SessionModelTransport.node, modelTransport],
+      Snapshot.node.replace(Snapshot.noopLayer),
+      LayerNodePlatform.llmClient.replace(TestLLM.clientLayer.pipe(Layer.provide(testLLM))),
+      SessionRunnerModel.node.replace(models),
+      InstructionBuiltIns.node.replace(systemContext),
+      InstructionDiscovery.node.replace(instructionContext),
+      Location.node.replace(Location.boundNode({ directory: AbsolutePath.make("/project") })),
+      SkillInstructions.node.replace(skillInstructions),
+      ReferenceInstructions.node.replace(referenceInstructions),
+      Permission.node.replace(permission),
+      Config.node.replace(config),
+      PluginSupervisor.node.replace(pluginSupervisor),
+      SessionModelTransport.node.replace(modelTransport),
     ]
     const runnerLayer = AppNodeBuilder.build(SessionRunnerLLM.node, [
       ...replacements,
-      [McpInstructions.node, mcpInstructions],
+      McpInstructions.node.replace(mcpInstructions),
     ])
     const execution = Layer.effect(
       SessionExecution.Service,
@@ -485,10 +485,10 @@ const layer = Layer.unwrap(
       ]),
       [
         ...replacements,
-        [Bus.node, Bus.configured({ persist: true })],
-        [LocationServiceMap.node, promptLocationNode],
-        [Catalog.node, promptCatalog],
-        [SessionExecution.node, execution],
+        Bus.node.replace(Bus.configured({ persist: true })),
+        LocationServiceMap.node.replace(promptLocationNode),
+        Catalog.node.replace(promptCatalog),
+        SessionExecution.node.replace(execution),
       ],
     )
   }),

packages/core/test/session-shell.test.ts

@@ -54,8 +54,8 @@ const executionLayer = Layer.effect(
 
 const it = testEffect(
   AppNodeBuilder.build(LayerNode.group([Bus.node, Session.node, SessionExecution.node, LocationServiceMap.node]), [
-    [Bus.node, Bus.configured({ persist: true })],
-    [SessionExecution.node, executionLayer],
+    Bus.node.replace(Bus.configured({ persist: true })),
+    SessionExecution.node.replace(executionLayer.pipe(Layer.provide(controlLayer))),
   ]).pipe(Layer.provideMerge(controlLayer)),
 )
 

packages/core/test/session-skill.test.ts

@@ -71,9 +71,9 @@ const it = testEffect(
   AppNodeBuilder.build(
     LayerNode.group([Database.node, Bus.node, SessionProjector.node, SessionStore.node, Session.node]),
     [
-      [LocationServiceMap.node, locations],
-      [Project.node, globalProjectNode],
-      [SessionExecution.node, SessionExecution.noopLayer],
+      LocationServiceMap.node.replace(locations),
+      Project.node.replace(globalProjectNode),
+      SessionExecution.node.replace(SessionExecution.noopLayer),
     ],
   ),
 )

packages/core/test/session-step.test.ts

@@ -27,7 +27,7 @@ import { testEffect } from "./lib/effect"
 const it = testEffect(
   Layer.merge(
     AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node, SessionProjector.node, ToolOutput.node]), [
-      [Bus.node, Bus.configured({ persist: true })],
+      Bus.node.replace(Bus.configured({ persist: true })),
     ]),
     TestLLM.testLayer(),
   ),

packages/core/test/session-store.test.ts

@@ -17,7 +17,7 @@ import { testEffect } from "./lib/effect"
 
 const it = testEffect(
   AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node, SessionProjector.node, SessionStore.node]), [
-    [Bus.node, Bus.configured({ persist: true })],
+    Bus.node.replace(Bus.configured({ persist: true })),
   ]),
 )
 

packages/core/test/session-title.test.ts

@@ -126,11 +126,11 @@ const it = testEffect(
       SessionTitle.node,
     ]),
     [
-      [llmClient, client],
-      [Catalog.node, catalog],
-      [SessionRunnerModel.node, models],
-      [Location.node, Location.boundNode({ directory: AbsolutePath.make("/project") })],
-      [PluginSupervisor.node, Layer.mock(PluginSupervisor.Service, { flush: Effect.void })],
+      llmClient.replace(client),
+      Catalog.node.replace(catalog),
+      SessionRunnerModel.node.replace(models),
+      Location.node.replace(Location.boundNode({ directory: AbsolutePath.make("/project") })),
+      PluginSupervisor.node.replace(Layer.mock(PluginSupervisor.Service, { flush: Effect.void })),
     ],
   ),
 )

packages/core/test/session-tool-progress.test.ts

@@ -21,7 +21,7 @@ import { testEffect } from "./lib/effect"
 
 const it = testEffect(
   AppNodeBuilder.build(LayerNode.group([Database.node, Bus.node, SessionProjector.node]), [
-    [Bus.node, Bus.configured({ persist: true })],
+    Bus.node.replace(Bus.configured({ persist: true })),
   ]),
 )
 const model = { id: Model.ID.make("model"), providerID: Provider.ID.make("provider") }

packages/core/test/session-view.test.ts

@@ -25,9 +25,9 @@ const it = testEffect(
   AppNodeBuilder.build(
     LayerNode.group([Database.node, Bus.node, SessionProjector.node, SessionStore.node, Session.node]),
     [
-      [Bus.node, Bus.configured({ persist: true })],
-      [Project.node, globalProjectNode],
-      [SessionExecution.node, SessionExecution.noopLayer],
+      Bus.node.replace(Bus.configured({ persist: true })),
+      Project.node.replace(globalProjectNode),
+      SessionExecution.node.replace(SessionExecution.noopLayer),
     ],
   ),
 )
@@ -186,8 +186,8 @@ describe("Session.view", () => {
       const targetLayer = AppNodeBuilder.build(
         LayerNode.group([Database.node, Bus.node, SessionProjector.node, SessionStore.node]),
         [
-          [Database.node, Database.configured({ path: path.join(tmp.path, "target.sqlite") })],
-          [Bus.node, Bus.configured({ persist: true })],
+          Database.node.replace(Database.configured({ path: path.join(tmp.path, "target.sqlite") })),
+          Bus.node.replace(Bus.configured({ persist: true })),
         ],
       )
 

packages/core/test/session-wait.test.ts

@@ -22,10 +22,7 @@ const execution = Layer.mock(SessionExecution.Service, {
 const it = testEffect(
   AppNodeBuilder.build(
     LayerNode.group([Database.node, Bus.node, SessionProjector.node, SessionStore.node, Session.node]),
-    [
-      [Project.node, globalProjectNode],
-      [SessionExecution.node, execution],
-    ],
+    [Project.node.replace(globalProjectNode), SessionExecution.node.replace(execution)],
   ),
 )
 

packages/core/test/shell-cleanup.test.ts

@@ -14,7 +14,7 @@ const withStore = <A, E, R>(body: (fs: FSUtil.Interface, root: string) => Effect
     Effect.promise(() => tmpdir()),
     (tmp) => {
       const layer = AppNodeBuilder.build(LayerNode.group([FSUtil.node, Global.node]), [
-        [Global.node, Global.layerWith({ data: tmp.path })],
+        Global.node.replace(Global.layerWith({ data: tmp.path })),
       ])
       return Effect.gen(function* () {
         const fs = yield* FSUtil.Service

packages/core/test/skill-discovery.test.ts

@@ -44,7 +44,7 @@ const fixture = Effect.gen(function* () {
           return yield* discovery.pull(base)
         }).pipe(
           Effect.provide(
-            AppNodeBuilder.build(SkillDiscovery.node, [[Global.node, Global.layerWith({ cache: tmp.path })]]),
+            AppNodeBuilder.build(SkillDiscovery.node, [Global.node.replace(Global.layerWith({ cache: tmp.path }))]),
           ),
         )
         return { directories, requests: state.requests.slice() }

packages/core/test/skill/instructions.test.ts

@@ -41,7 +41,7 @@ const manual = Skill.Info.make({
 
 const layer = (list: () => Skill.Info[]) =>
   AppNodeBuilder.build(SkillInstructions.node, [
-    [Skill.node, Layer.mock(Skill.Service, { list: () => Effect.succeed(list()) })],
+    Skill.node.replace(Layer.mock(Skill.Service, { list: () => Effect.succeed(list()) })),
   ])
 
 describe("SkillInstructions", () => {

packages/core/test/snapshot.test.ts

@@ -54,9 +54,9 @@ describe("Snapshot", () => {
             },
           })
           const layer = AppNodeBuilder.build(Snapshot.node, [
-            [Location.node, Layer.succeed(Location.Service, location)],
-            [Global.node, Global.layerWith({ data: tmp.path, config: path.join(tmp.path, "config") })],
-            [Git.node, Layer.succeed(Git.Service, instrumented)],
+            Location.node.replace(Layer.succeed(Location.Service, location)),
+            Global.node.replace(Global.layerWith({ data: tmp.path, config: path.join(tmp.path, "config") })),
+            Git.node.replace(Layer.succeed(Git.Service, instrumented)),
           ])
 
           yield* Effect.gen(function* () {
@@ -239,8 +239,8 @@ describe("Snapshot", () => {
 
 function snapshotLayer(data: string, directory: string) {
   return AppNodeBuilder.build(Snapshot.node, [
-    [Location.node, Location.boundNode(Location.Ref.make({ directory: AbsolutePath.make(directory) }))],
-    [Global.node, Global.layerWith({ data, config: path.join(data, "config") })],
+    Location.node.replace(Location.boundNode(Location.Ref.make({ directory: AbsolutePath.make(directory) }))),
+    Global.node.replace(Global.layerWith({ data, config: path.join(data, "config") })),
   ])
 }
 

packages/core/test/tool-edit.test.ts

@@ -92,8 +92,7 @@ const withTool = <A, E, R>(
   }).pipe(
     Effect.provide(
       AppNodeBuilder.build(LayerNode.group([Tool.node, LocationMutation.node, FileMutation.node, editToolNode]), [
-        [
-          Environment.node,
+        Environment.node.replace(
           transformEnvironmentFiles((files) => ({
             read: (target, range) =>
               files
@@ -104,10 +103,10 @@ const withTool = <A, E, R>(
             write: (target, content) =>
               Effect.sync(() => fixture.writes.push(target)).pipe(Effect.andThen(files.write(target, content))),
           })),
-        ],
-        [Location.node, activeLocation],
-        [Formatter.node, fixture.formatter],
-        [Permission.node, fixture.permission],
+        ),
+        Location.node.replace(activeLocation),
+        Formatter.node.replace(fixture.formatter),
+        Permission.node.replace(fixture.permission),
       ]),
     ),
   )

packages/core/test/tool-output.test.ts

@@ -18,7 +18,7 @@ const withStore = <A, E, R>(
     Effect.promise(() => tmpdir()),
     (tmp) => {
       const layer = AppNodeBuilder.build(LayerNode.group([ToolOutput.node, FSUtil.node]), [
-        [Global.node, Global.layerWith({ data: tmp.path })],
+        Global.node.replace(Global.layerWith({ data: tmp.path })),
       ])
       return Effect.gen(function* () {
         const output = yield* ToolOutput.Service

packages/core/test/tool-patch.test.ts

@@ -100,8 +100,7 @@ const withTool = <A, E, R>(
   }).pipe(
     Effect.provide(
       AppNodeBuilder.build(LayerNode.group([Tool.node, LocationMutation.node, FileMutation.node, patchToolNode]), [
-        [
-          Environment.node,
+        Environment.node.replace(
           transformEnvironmentFiles((files) => ({
             read: (target, range) =>
               Effect.sync(() => {
@@ -120,10 +119,10 @@ const withTool = <A, E, R>(
               return files.write(target, content)
             },
           })),
-        ],
-        [Location.node, activeLocation],
-        [Formatter.node, formatter],
-        [Permission.node, permission],
+        ),
+        Location.node.replace(activeLocation),
+        Formatter.node.replace(formatter),
+        Permission.node.replace(permission),
       ]),
     ),
   )

packages/core/test/tool-question.test.ts

@@ -66,9 +66,9 @@ const questionToolNode = makeLocationNode({
 
 const it = testEffect(
   AppNodeBuilder.build(LayerNode.group([Tool.node, questionToolNode]), [
-    [Permission.node, permission],
-    [Form.node, form],
-    [Image.node, imagePassthrough],
+    Permission.node.replace(permission),
+    Form.node.replace(form),
+    Image.node.replace(imagePassthrough),
   ]),
 )
 

packages/core/test/tool-read.test.ts

@@ -142,14 +142,14 @@ const unavailableImage = Layer.mock(Image.Service, {
 const readLayer = (imageLayer: Layer.Layer<Image.Service>) =>
   Layer.mergeAll(
     AppNodeBuilder.build(LayerNode.group([Tool.node, readToolNode]), [
-      [ReadToolFileSystem.node, reader],
-      [Permission.node, permission],
-      [Config.node, config],
-      [Image.node, imageLayer],
-      [LocationMutation.node, mutation],
-      [FSUtil.node, testFileSystem],
-      [Location.node, locationLayer],
-      [Global.node, Global.layerWith({ data: Global.Path.data })],
+      ReadToolFileSystem.node.replace(reader),
+      Permission.node.replace(permission),
+      Config.node.replace(config),
+      Image.node.replace(imageLayer),
+      LocationMutation.node.replace(mutation),
+      FSUtil.node.replace(testFileSystem),
+      Location.node.replace(locationLayer),
+      Global.node.replace(Global.layerWith({ data: Global.Path.data })),
     ]),
     // Merge by reference so Config.Test and Image.Service resolve to the memoized instances.
     config,

packages/core/test/tool-registry.test.ts

@@ -43,7 +43,7 @@ const imageStore = Layer.mock(Image.Service, {
   },
 })
 const registryLayer = AppNodeBuilder.build(LayerNode.group([Tool.node, PluginHooks.node, SessionModelRequest.node]), [
-  [Image.node, imageStore],
+  Image.node.replace(imageStore),
 ])
 const it = testEffect(registryLayer)
 const identity = {