refactor(core): simplify system context into a belief model (#34917)

AGENTS.md

@@ -159,4 +159,5 @@ const table = sqliteTable("session", {
 - Keep local Session drains process-local until clustering is implemented. `SessionRunCoordinator` joins explicit same-Session resumes, coalesces prompt wakeups, and allows different Sessions to run concurrently. Advisory wakes drain eligible durable inbox rows only; post-crash continuation recovery requires a separate explicit design before it may retry provider work. A drain has no durable identity or transcript boundary.
 - Keep delivery vocabulary explicit. Prompts steer by default and promote at the next safe provider-turn boundary while the current drain requires continuation. An explicit `queue` input remains pending until the Session would otherwise become idle; promote one queued input at that boundary, then reevaluate continuation before promoting another. Promoting any new user input resets the selected agent's provider-turn allowance; a batch of steers resets it once.
 - Keep EventV2 replay owner claims separate from clustered Session execution ownership.
-- Keep the System Context algebra, registry, and built-ins in `src/system-context`; keep Context Source producers with their observed domains, and keep Session History selection plus Context Epoch persistence Session-owned.
+- Keep the System Context algebra and built-ins in `src/system-context`; keep Context Source producers with their observed domains, and keep Session History selection plus Context Checkpoint persistence Session-owned. The runner composes all context producers explicitly in `loadSystemContext`; there is no context registry.
+- The durable Applied record is what the model was last told, per source. Reconcile narrates drift as chronological System updates and never rewrites the baseline; only completed compaction rebaselines, and move or committed revert resets the checkpoint. Unavailable sources keep the model's prior belief, blocking only a session's first baseline.

packages/core/src/instruction-context.ts

@@ -1,6 +1,6 @@
 export * as InstructionContext from "./instruction-context"
 
-import { Array, Effect, Layer, Schema } from "effect"
+import { Array, Context, Effect, Layer, Schema } from "effect"
 import { isAbsolute, join, relative, sep } from "path"
 import { FSUtil } from "./fs-util"
 import { Flag } from "./flag/flag"
@@ -8,7 +8,6 @@ import { Global } from "./global"
 import { Location } from "./location"
 import { AbsolutePath } from "./schema"
 import { SystemContext } from "./system-context/index"
-import { SystemContextRegistry } from "./system-context/registry"
 import { makeLocationNode } from "./effect/app-node"
 
 class File extends Schema.Class<File>("InstructionContext.File")({
@@ -19,12 +18,18 @@ class File extends Schema.Class<File>("InstructionContext.File")({
 const Files = Schema.Array(File)
 const key = SystemContext.Key.make("core/instructions")
 
-const layer = Layer.effectDiscard(
+export interface Interface {
+  readonly load: () => Effect.Effect<SystemContext.SystemContext>
+}
+
+export class Service extends Context.Service<Service, Interface>()("@opencode/v2/InstructionContext") {}
+
+const layer = Layer.effect(
+  Service,
   Effect.gen(function* () {
     const fs = yield* FSUtil.Service
     const global = yield* Global.Service
     const location = yield* Location.Service
-    const registry = yield* SystemContextRegistry.Service
 
     const source = (value: ReadonlyArray<File> | SystemContext.Unavailable) =>
       SystemContext.make({
@@ -71,28 +76,24 @@ const layer = Layer.effectDiscard(
       return files.filter((file): file is File => file !== undefined)
     })
 
-    yield* registry.register({
-      key,
-      load: observe().pipe(
-        Effect.map((files) =>
-          files === SystemContext.unavailable
-            ? source(files)
-            : files.length === 0
-              ? SystemContext.empty
-              : source(files),
+    return Service.of({
+      load: () =>
+        observe().pipe(
+          Effect.map((files) =>
+            files === SystemContext.unavailable
+              ? source(files)
+              : files.length === 0
+                ? SystemContext.empty
+                : source(files),
+          ),
+          Effect.catch(() => Effect.succeed(source(SystemContext.unavailable))),
+          Effect.catchDefect(() => Effect.succeed(source(SystemContext.unavailable))),
         ),
-        Effect.catch(() => Effect.succeed(source(SystemContext.unavailable))),
-        Effect.catchDefect(() => Effect.succeed(source(SystemContext.unavailable))),
-      ),
     })
   }),
 )
 
-export const node = makeLocationNode({
-  name: "instruction-context",
-  layer,
-  deps: [FSUtil.node, Global.node, Location.node, SystemContextRegistry.node],
-})
+export const node = makeLocationNode({ service: Service, layer, deps: [FSUtil.node, Global.node, Location.node] })
 
 function render(files: ReadonlyArray<File>) {
   return files.map((file) => `Instructions from: ${file.path}\n${file.content}`).join("\n\n")

packages/core/src/location-services.ts

@@ -36,8 +36,8 @@ import { SessionTodo } from "./session/todo"
 import { SkillV2 } from "./skill"
 import { SkillGuidance } from "./skill/guidance"
 import { Snapshot } from "./snapshot"
+import { InstructionContext } from "./instruction-context"
 import { SystemContextBuiltIns } from "./system-context/builtins"
-import { SystemContextRegistry } from "./system-context/registry"
 import { SessionInstructions } from "./session/instructions"
 import { BuiltInTools } from "./tool/builtins"
 import { McpTool } from "./tool/mcp"
@@ -68,8 +68,8 @@ export const locationServices = LayerNode.group([
   Pty.node,
   Shell.node,
   SkillV2.node,
-  SystemContextRegistry.node,
   SystemContextBuiltIns.node,
+  InstructionContext.node,
   LocationMutation.node,
   FileMutation.node,
   MCP.node,

packages/core/src/mcp/guidance.ts

@@ -14,16 +14,40 @@ const Summary = Schema.Struct({
 })
 type Summary = typeof Summary.Type
 
+const entries = (servers: ReadonlyArray<Summary>) =>
+  servers.flatMap((server) => [
+    `  <server name="${server.server}">`,
+    ...server.instructions.split("\n").map((line) => `    ${line}`),
+    "  </server>",
+  ])
+
 const render = (servers: ReadonlyArray<Summary>) =>
-  [
-    "<mcp_instructions>",
-    ...servers.flatMap((server) => [
-      `  <server name="${server.server}">`,
-      ...server.instructions.split("\n").map((line) => `    ${line}`),
-      "  </server>",
-    ]),
-    "</mcp_instructions>",
+  ["<mcp_instructions>", ...entries(servers), "</mcp_instructions>"].join("\n")
+
+const update = (previous: ReadonlyArray<Summary>, current: ReadonlyArray<Summary>) => {
+  const diff = SystemContext.diffByKey(
+    previous,
+    current,
+    (server) => server.server,
+    (before, after) => before.instructions !== after.instructions,
+  )
+  // Additions and removals render as small deltas; anything else restates the full list.
+  if (diff.changed.length > 0 || (diff.added.length === 0 && diff.removed.length === 0))
+    return [
+      "The available MCP server instructions have changed. This list supersedes the previous one.",
+      render(current),
+    ].join("\n")
+  return [
+    ...(diff.added.length === 0
+      ? []
+      : ["New MCP server instructions are available in addition to those previously listed:", ...entries(diff.added)]),
+    ...(diff.removed.length === 0
+      ? []
+      : [
+          `Instructions for the following MCP servers are no longer available: ${diff.removed.map((server) => server.server).join(", ")}.`,
+        ]),
   ].join("\n")
+}
 
 export interface Interface {
   readonly load: (agent: AgentV2.Selection) => Effect.Effect<SystemContext.SystemContext>
@@ -50,7 +74,8 @@ export const layer = Layer.effect(
             return (
               owned.length === 0 ||
               owned.some(
-                (tool) => PermissionV2.evaluate(McpTool.name(tool.server, tool.name), "*", agent.permissions).effect !== "deny",
+                (tool) =>
+                  PermissionV2.evaluate(McpTool.name(tool.server, tool.name), "*", agent.permissions).effect !== "deny",
               )
             )
           })
@@ -61,11 +86,7 @@ export const layer = Layer.effect(
           codec: Schema.toCodecJson(Schema.Array(Summary)),
           load: Effect.succeed(visible),
           baseline: render,
-          update: (_previous, current) =>
-            [
-              "The available MCP server instructions have changed. This list supersedes the previous one.",
-              render(current),
-            ].join("\n"),
+          update,
           removed: () => "MCP server instructions are no longer available.",
         })
       }),

packages/core/src/reference/guidance.ts

@@ -11,20 +11,48 @@ const Summary = Schema.Struct({
   description: Schema.String.pipe(Schema.optional),
 })
 
+const entries = (references: ReadonlyArray<typeof Summary.Type>) =>
+  references.flatMap((reference) => [
+    "  <reference>",
+    `    <name>${reference.name}</name>`,
+    `    <path>${reference.path}</path>`,
+    ...(reference.description === undefined ? [] : [`    <description>${reference.description}</description>`]),
+    "  </reference>",
+  ])
+
 const render = (references: ReadonlyArray<typeof Summary.Type>) =>
   [
     "Project references provide additional directories that can be accessed when relevant.",
     "<available_references>",
-    ...references.flatMap((reference) => [
-      "  <reference>",
-      `    <name>${reference.name}</name>`,
-      `    <path>${reference.path}</path>`,
-      ...(reference.description === undefined ? [] : [`    <description>${reference.description}</description>`]),
-      "  </reference>",
-    ]),
+    ...entries(references),
     "</available_references>",
   ].join("\n")
 
+const update = (previous: ReadonlyArray<typeof Summary.Type>, current: ReadonlyArray<typeof Summary.Type>) => {
+  const diff = SystemContext.diffByKey(
+    previous,
+    current,
+    (reference) => reference.name,
+    (before, after) => before.path !== after.path || before.description !== after.description,
+  )
+  // Additions and removals render as small deltas; anything else restates the full list.
+  if (diff.changed.length > 0 || (diff.added.length === 0 && diff.removed.length === 0))
+    return [
+      "The available project references have changed. This list supersedes the previous reference list.",
+      render(current),
+    ].join("\n")
+  return [
+    ...(diff.added.length === 0
+      ? []
+      : ["New project references are available in addition to those previously listed:", ...entries(diff.added)]),
+    ...(diff.removed.length === 0
+      ? []
+      : [
+          `The following project references are no longer available and must not be used: ${diff.removed.map((reference) => reference.name).join(", ")}.`,
+        ]),
+  ].join("\n")
+}
+
 export interface Interface {
   readonly load: () => Effect.Effect<SystemContext.SystemContext>
 }
@@ -52,11 +80,7 @@ const layer = Layer.effect(
           codec: Schema.toCodecJson(Schema.Array(Summary)),
           load: Effect.succeed(available),
           baseline: render,
-          update: (_previous, current) =>
-            [
-              "The available project references have changed. This list supersedes the previous reference list.",
-              render(current),
-            ].join("\n"),
+          update,
           removed: () => "Project reference guidance is no longer available. Do not use previously listed references.",
         })
       }),

packages/core/src/session.ts

@@ -108,7 +108,7 @@ export class OperationUnavailableError extends Schema.TaggedErrorClass<Operation
   },
 ) {}
 
-export { ContextSnapshotDecodeError, MessageDecodeError } from "./session/error"
+export { MessageDecodeError } from "./session/error"
 
 export class PromptConflictError extends Schema.TaggedErrorClass<PromptConflictError>()("Session.PromptConflictError", {
   sessionID: SessionSchema.ID,
@@ -466,7 +466,9 @@ const layer = Layer.effect(
           text: skill.content,
         })
         if (input.resume !== false)
-          yield* execution.resume(input.sessionID).pipe(Effect.ignore, Effect.forkIn(scope, { startImmediately: true }), Effect.asVoid)
+          yield* execution
+            .resume(input.sessionID)
+            .pipe(Effect.ignore, Effect.forkIn(scope, { startImmediately: true }), Effect.asVoid)
       }),
       switchAgent: Effect.fn("V2Session.switchAgent")(function* (input) {
         yield* result.get(input.sessionID)
@@ -550,7 +552,9 @@ const layer = Layer.effect(
           description: input.description,
           metadata: input.metadata,
         })
-        yield* execution.resume(input.sessionID).pipe(Effect.ignore, Effect.forkIn(scope, { startImmediately: true }), Effect.asVoid)
+        yield* execution
+          .resume(input.sessionID)
+          .pipe(Effect.ignore, Effect.forkIn(scope, { startImmediately: true }), Effect.asVoid)
       }),
       interrupt: Effect.fn("V2Session.interrupt")((sessionID) =>
         Effect.uninterruptible(execution.interrupt(sessionID)),

packages/core/src/session/context-checkpoint.ts

@@ -0,0 +1,131 @@
+export * as SessionContextCheckpoint from "./context-checkpoint"
+
+import { eq } from "drizzle-orm"
+import { DateTime, Effect, Option, Schema } from "effect"
+import type { Database } from "../database/database"
+import { EventV2 } from "../event"
+import { SystemContext } from "../system-context/index"
+import { SessionEvent } from "./event"
+import { SessionHistory } from "./history"
+import { SessionMessage } from "./message"
+import { SessionSchema } from "./schema"
+import { SessionContextCheckpointTable } from "./sql"
+
+type DatabaseService = Database.Interface["db"]
+
+const decodeApplied = Schema.decodeUnknownOption(SystemContext.Applied)
+
+/**
+ * Loads or creates the session's durable context checkpoint, narrating any
+ * drift since the model was last told as a chronological update. Completed
+ * compaction rebaselines; nothing else rewrites the baseline. Runs before
+ * input promotion so a blocked first turn leaves pending inputs untouched.
+ */
+export const prepare = Effect.fn("SessionContextCheckpoint.prepare")(function* (
+  db: DatabaseService,
+  events: EventV2.Interface,
+  context: Effect.Effect<SystemContext.SystemContext>,
+  sessionID: SessionSchema.ID,
+) {
+  const [value, stored, compaction] = yield* Effect.all(
+    [context, find(db, sessionID), SessionHistory.latestCompaction(db, sessionID)],
+    { concurrency: "unbounded" },
+  )
+  if (!stored) {
+    const baseline = yield* SystemContext.initialize(value)
+    const baselineSeq = yield* insert(db, sessionID, baseline)
+    return { baseline: baseline.text, baselineSeq }
+  }
+
+  // The applied record is comparison state only; an undecodable one heals by
+  // treating every source as new, re-announcing baselines as updates.
+  const applied = Option.getOrElse(decodeApplied(stored.snapshot), () => ({}))
+  if (compaction !== undefined && compaction.seq > stored.baseline_seq) {
+    const baseline = yield* SystemContext.rebaseline(value, applied)
+    yield* rewrite(db, sessionID, compaction.seq, baseline)
+    return { baseline: baseline.text, baselineSeq: compaction.seq }
+  }
+  const result = yield* SystemContext.reconcile(value, applied)
+  if (result._tag === "Unchanged") return { baseline: stored.baseline, baselineSeq: stored.baseline_seq }
+
+  yield* events.publish(
+    SessionEvent.ContextUpdated,
+    { sessionID, messageID: SessionMessage.ID.create(), timestamp: yield* DateTime.now, text: result.text },
+    { commit: () => advance(db, sessionID, result.applied).pipe(Effect.orDie) },
+  )
+  return { baseline: stored.baseline, baselineSeq: stored.baseline_seq }
+})
+
+export const reset = Effect.fn("SessionContextCheckpoint.reset")(function* (
+  db: DatabaseService,
+  sessionID: SessionSchema.ID,
+) {
+  yield* db
+    .delete(SessionContextCheckpointTable)
+    .where(eq(SessionContextCheckpointTable.session_id, sessionID))
+    .run()
+    .pipe(Effect.orDie)
+})
+
+const find = Effect.fnUntraced(function* (db: DatabaseService, sessionID: SessionSchema.ID) {
+  return yield* db
+    .select()
+    .from(SessionContextCheckpointTable)
+    .where(eq(SessionContextCheckpointTable.session_id, sessionID))
+    .get()
+    .pipe(Effect.orDie)
+})
+
+const insert = Effect.fnUntraced(function* (
+  db: DatabaseService,
+  sessionID: SessionSchema.ID,
+  baseline: SystemContext.Baseline,
+) {
+  const baselineSeq = yield* EventV2.latestSequence(db, sessionID)
+  yield* db
+    .insert(SessionContextCheckpointTable)
+    .values({
+      session_id: sessionID,
+      baseline: baseline.text,
+      snapshot: baseline.applied,
+      baseline_seq: baselineSeq,
+    })
+    .run()
+    .pipe(Effect.orDie)
+  return baselineSeq
+})
+
+const rewrite = Effect.fnUntraced(function* (
+  db: DatabaseService,
+  sessionID: SessionSchema.ID,
+  baselineSeq: number,
+  baseline: SystemContext.Baseline,
+) {
+  const updated = yield* db
+    .update(SessionContextCheckpointTable)
+    .set({
+      baseline: baseline.text,
+      snapshot: baseline.applied,
+      baseline_seq: baselineSeq,
+    })
+    .where(eq(SessionContextCheckpointTable.session_id, sessionID))
+    .returning({ sessionID: SessionContextCheckpointTable.session_id })
+    .get()
+    .pipe(Effect.orDie)
+  if (!updated) return yield* Effect.die("Context checkpoint not found")
+})
+
+const advance = Effect.fnUntraced(function* (
+  db: DatabaseService,
+  sessionID: SessionSchema.ID,
+  applied: SystemContext.Applied,
+) {
+  const updated = yield* db
+    .update(SessionContextCheckpointTable)
+    .set({ snapshot: applied })
+    .where(eq(SessionContextCheckpointTable.session_id, sessionID))
+    .returning({ sessionID: SessionContextCheckpointTable.session_id })
+    .get()
+    .pipe(Effect.orDie)
+  if (!updated) return yield* Effect.die("Context checkpoint not found")
+})

packages/core/src/session/context-epoch.ts

@@ -1,174 +0,0 @@
-export * as SessionContextEpoch from "./context-epoch"
-
-import { eq } from "drizzle-orm"
-import { DateTime, Effect, Schema } from "effect"
-import type { Database } from "../database/database"
-import { EventV2 } from "../event"
-import { SystemContext } from "../system-context/index"
-import { ContextSnapshotDecodeError } from "./error"
-import { SessionEvent } from "./event"
-import { SessionHistory } from "./history"
-import { SessionInput } from "./input"
-import { SessionMessage } from "./message"
-import { SessionSchema } from "./schema"
-import { SessionContextEpochTable } from "./sql"
-
-type DatabaseService = Database.Interface["db"]
-
-interface Prepared {
-  readonly baseline: string
-  readonly baselineSeq: number
-}
-
-export function initialize(
-  db: DatabaseService,
-  context: Effect.Effect<SystemContext.SystemContext>,
-  sessionID: SessionSchema.ID,
-): Effect.Effect<Prepared | undefined, SystemContext.InitializationBlocked> {
-  return initializeOnce(db, context, sessionID).pipe(Effect.withSpan("SessionContextEpoch.initialize"))
-}
-
-export function prepare(
-  db: DatabaseService,
-  events: EventV2.Interface,
-  context: Effect.Effect<SystemContext.SystemContext>,
-  sessionID: SessionSchema.ID,
-): Effect.Effect<Prepared, SystemContext.InitializationBlocked | ContextSnapshotDecodeError> {
-  return prepareOnce(db, events, context, sessionID).pipe(Effect.withSpan("SessionContextEpoch.prepare"))
-}
-
-const prepareOnce = Effect.fnUntraced(function* (
-  db: DatabaseService,
-  events: EventV2.Interface,
-  context: Effect.Effect<SystemContext.SystemContext>,
-  sessionID: SessionSchema.ID,
-) {
-  const [value, stored, compaction] = yield* Effect.all(
-    [context, find(db, sessionID), SessionHistory.latestCompaction(db, sessionID)],
-    { concurrency: "unbounded" },
-  )
-  if (!stored) {
-    const generation = yield* SystemContext.initialize(value)
-    const baselineSeq = yield* insert(db, sessionID, generation)
-    return { baseline: generation.baseline, baselineSeq }
-  }
-
-  const snapshot = yield* Schema.decodeUnknownEffect(SystemContext.Snapshot)(stored.snapshot).pipe(
-    Effect.mapError((error) => new ContextSnapshotDecodeError({ sessionID, details: String(error) })),
-  )
-  const replacementSeq = compaction !== undefined && compaction.seq > stored.baseline_seq ? compaction.seq : undefined
-  const result = replacementSeq
-    ? yield* SystemContext.replace(value, snapshot)
-    : yield* SystemContext.reconcile(value, snapshot)
-  if (result._tag === "Unchanged" || result._tag === "ReplacementBlocked") {
-    return { baseline: stored.baseline, baselineSeq: stored.baseline_seq }
-  }
-  if (result._tag === "ReplacementReady") {
-    const baselineSeq = replacementSeq ?? (yield* EventV2.latestSequence(db, sessionID))
-    yield* replace(db, sessionID, baselineSeq, result.generation)
-    return { baseline: result.generation.baseline, baselineSeq }
-  }
-
-  yield* events.publish(
-    SessionEvent.ContextUpdated,
-    { sessionID, messageID: SessionMessage.ID.create(), timestamp: yield* DateTime.now, text: result.text },
-    { commit: () => advance(db, sessionID, result.snapshot).pipe(Effect.orDie) },
-  )
-  return { baseline: stored.baseline, baselineSeq: stored.baseline_seq }
-})
-
-const initializeOnce = Effect.fnUntraced(function* (
-  db: DatabaseService,
-  context: Effect.Effect<SystemContext.SystemContext>,
-  sessionID: SessionSchema.ID,
-) {
-  if (yield* exists(db, sessionID)) return
-  const generation = yield* context.pipe(Effect.flatMap(SystemContext.initialize))
-  const baselineSeq = yield* insert(db, sessionID, generation)
-  return { baseline: generation.baseline, baselineSeq }
-})
-
-const exists = Effect.fn("SessionContextEpoch.exists")(function* (db: DatabaseService, sessionID: SessionSchema.ID) {
-  return (
-    (yield* db
-      .select({ sessionID: SessionContextEpochTable.session_id })
-      .from(SessionContextEpochTable)
-      .where(eq(SessionContextEpochTable.session_id, sessionID))
-      .get()
-      .pipe(Effect.orDie)) !== undefined
-  )
-})
-
-const find = Effect.fn("SessionContextEpoch.find")(function* (db: DatabaseService, sessionID: SessionSchema.ID) {
-  return yield* db
-    .select()
-    .from(SessionContextEpochTable)
-    .where(eq(SessionContextEpochTable.session_id, sessionID))
-    .get()
-    .pipe(Effect.orDie)
-})
-
-export const reset = Effect.fn("SessionContextEpoch.reset")(function* (
-  db: DatabaseService,
-  sessionID: SessionSchema.ID,
-) {
-  yield* db
-    .delete(SessionContextEpochTable)
-    .where(eq(SessionContextEpochTable.session_id, sessionID))
-    .run()
-    .pipe(Effect.orDie)
-})
-
-const insert = Effect.fnUntraced(function* (
-  db: DatabaseService,
-  sessionID: SessionSchema.ID,
-  generation: SystemContext.Generation,
-) {
-  const baselineSeq = yield* EventV2.latestSequence(db, sessionID)
-  yield* db
-    .insert(SessionContextEpochTable)
-    .values({
-      session_id: sessionID,
-      baseline: generation.baseline,
-      snapshot: generation.snapshot,
-      baseline_seq: baselineSeq,
-    })
-    .run()
-    .pipe(Effect.orDie)
-  return baselineSeq
-})
-
-const replace = Effect.fnUntraced(function* (
-  db: DatabaseService,
-  sessionID: SessionSchema.ID,
-  baselineSeq: number,
-  generation: SystemContext.Generation,
-) {
-  const updated = yield* db
-    .update(SessionContextEpochTable)
-    .set({
-      baseline: generation.baseline,
-      snapshot: generation.snapshot,
-      baseline_seq: baselineSeq,
-    })
-    .where(eq(SessionContextEpochTable.session_id, sessionID))
-    .returning({ sessionID: SessionContextEpochTable.session_id })
-    .get()
-    .pipe(Effect.orDie)
-  if (!updated) return yield* Effect.die("Context Epoch not found")
-})
-
-const advance = Effect.fnUntraced(function* (
-  db: DatabaseService,
-  sessionID: SessionSchema.ID,
-  snapshot: SystemContext.Snapshot,
-) {
-  const updated = yield* db
-    .update(SessionContextEpochTable)
-    .set({ snapshot })
-    .where(eq(SessionContextEpochTable.session_id, sessionID))
-    .returning({ sessionID: SessionContextEpochTable.session_id })
-    .get()
-    .pipe(Effect.orDie)
-  if (!updated) return yield* Effect.die("Context Epoch not found")
-})

packages/core/src/session/error.ts

@@ -10,15 +10,3 @@ export class MessageDecodeError extends Schema.TaggedErrorClass<MessageDecodeErr
     return `Failed to decode message ${this.messageID} in session ${this.sessionID}`
   }
 }
-
-export class ContextSnapshotDecodeError extends Schema.TaggedErrorClass<ContextSnapshotDecodeError>()(
-  "Session.ContextSnapshotDecodeError",
-  {
-    sessionID: SessionSchema.ID,
-    details: Schema.String,
-  },
-) {
-  override get message() {
-    return `Failed to decode context snapshot for session ${this.sessionID}: ${this.details}`
-  }
-}

packages/core/src/session/history.ts

@@ -4,7 +4,7 @@ import { Database } from "../database/database"
 import { MessageDecodeError } from "./error"
 import { SessionMessage } from "./message"
 import { SessionSchema } from "./schema"
-import { SessionContextEpochTable, SessionMessageTable } from "./sql"
+import { SessionContextCheckpointTable, SessionMessageTable } from "./sql"
 
 type DatabaseService = Database.Interface["db"]
 
@@ -33,6 +33,9 @@ const messageRows = Effect.fnUntraced(function* (
     .where(
       and(
         eq(SessionMessageTable.session_id, sessionID),
+        // Keep system updates visible in the gap between a completed compaction
+        // and the next prepared turn's rebaseline, when their content is not yet
+        // folded into a new baseline.
         compaction
           ? or(
               gte(SessionMessageTable.seq, compaction.seq),
@@ -67,9 +70,9 @@ export const load = Effect.fn("SessionHistory.load")(function* (db: DatabaseServ
   const [epoch, compaction] = yield* Effect.all(
     [
       db
-        .select({ baselineSeq: SessionContextEpochTable.baseline_seq })
-        .from(SessionContextEpochTable)
-        .where(eq(SessionContextEpochTable.session_id, sessionID))
+        .select({ baselineSeq: SessionContextCheckpointTable.baseline_seq })
+        .from(SessionContextCheckpointTable)
+        .where(eq(SessionContextCheckpointTable.session_id, sessionID))
         .get()
         .pipe(Effect.orDie),
       latestCompaction(db, sessionID),
@@ -79,14 +82,6 @@ export const load = Effect.fn("SessionHistory.load")(function* (db: DatabaseServ
   return yield* Effect.forEach(yield* messageRows(db, sessionID, compaction, epoch?.baselineSeq), decodeMessageRow)
 })
 
-export const loadForRunner = Effect.fn("SessionHistory.loadForRunner")(function* (
-  db: DatabaseService,
-  sessionID: SessionSchema.ID,
-  baselineSeq: number,
-) {
-  return (yield* entriesForRunner(db, sessionID, baselineSeq)).map((entry) => entry.message)
-})
-
 export const entriesForRunner = Effect.fn("SessionHistory.entriesForRunner")(function* (
   db: DatabaseService,
   sessionID: SessionSchema.ID,

packages/core/src/session/projector.ts

@@ -12,8 +12,15 @@ import { SessionMessage } from "./message"
 import { SessionMessageUpdater } from "./message-updater"
 import { SessionInput } from "./input"
 import { WorkspaceV2 } from "../workspace"
-import { SessionContextEpoch } from "./context-epoch"
-import { MessageTable, PartTable, SessionInputTable, SessionMessageTable, SessionTable } from "./sql"
+import { SessionContextCheckpoint } from "./context-checkpoint"
+import {
+  MessageTable,
+  PartTable,
+  SessionContextCheckpointTable,
+  SessionInputTable,
+  SessionMessageTable,
+  SessionTable,
+} from "./sql"
 import type { DeepMutable } from "../schema"
 import { Slug } from "../util/slug"
 
@@ -156,12 +163,16 @@ const projectFork = Effect.fn("SessionProjector.projectFork")(function* (
         .select({ seq: SessionMessageTable.seq })
         .from(SessionMessageTable)
         .where(
-          and(eq(SessionMessageTable.session_id, event.data.parentID), eq(SessionMessageTable.id, event.data.messageID)),
+          and(
+            eq(SessionMessageTable.session_id, event.data.parentID),
+            eq(SessionMessageTable.id, event.data.messageID),
+          ),
         )
         .get()
         .pipe(Effect.orDie)
     : undefined
-  if (event.data.messageID && !boundary) return yield* Effect.die(`Fork boundary message not found: ${event.data.messageID}`)
+  if (event.data.messageID && !boundary)
+    return yield* Effect.die(`Fork boundary message not found: ${event.data.messageID}`)
   const copied = yield* db
     .select({ seq: SessionMessageTable.seq })
     .from(SessionMessageTable)
@@ -206,6 +217,23 @@ const projectFork = Effect.fn("SessionProjector.projectFork")(function* (
     .pipe(Effect.orDie)
   if (!stored) return yield* Effect.die(new SessionAlreadyProjected())
 
+  // The fork inherits the parent's transcript, so it inherits the context
+  // checkpoint that transcript was built against: copied message seqs keep
+  // folding at the same baseline horizon.
+  const checkpoint = yield* db
+    .select()
+    .from(SessionContextCheckpointTable)
+    .where(eq(SessionContextCheckpointTable.session_id, event.data.parentID))
+    .get()
+    .pipe(Effect.orDie)
+  if (checkpoint) {
+    yield* db
+      .insert(SessionContextCheckpointTable)
+      .values({ ...checkpoint, session_id: event.data.sessionID })
+      .run()
+      .pipe(Effect.orDie)
+  }
+
   const usage = emptyUsage()
   let cursor = -1
   while (true) {
@@ -452,7 +480,7 @@ const layer = Layer.effectDiscard(
           .where(eq(SessionTable.id, event.data.sessionID))
           .run()
           .pipe(Effect.orDie)
-        yield* SessionContextEpoch.reset(db, event.data.sessionID)
+        yield* SessionContextCheckpoint.reset(db, event.data.sessionID)
       }),
     )
     yield* events.project(SessionV1.Event.Deleted, (event) =>
@@ -666,7 +694,7 @@ const layer = Layer.effectDiscard(
           .where(eq(SessionTable.id, event.data.sessionID))
           .run()
           .pipe(Effect.orDie)
-        yield* SessionContextEpoch.reset(db, event.data.sessionID)
+        yield* SessionContextCheckpoint.reset(db, event.data.sessionID)
       }),
     )
   }),

packages/core/src/session/runner/index.ts

@@ -3,7 +3,7 @@ export * as SessionRunner from "./index"
 import type { LLMError } from "@opencode-ai/llm"
 import { Context, Effect } from "effect"
 import { SessionSchema } from "../schema"
-import type { ContextSnapshotDecodeError, MessageDecodeError } from "../error"
+import type { MessageDecodeError } from "../error"
 import { SessionRunnerModel } from "./model"
 import type { SystemContext } from "../../system-context/index"
 import type { ToolOutputStore } from "../../tool-output-store"
@@ -12,7 +12,6 @@ export type RunError =
   | LLMError
   | SessionRunnerModel.Error
   | MessageDecodeError
-  | ContextSnapshotDecodeError
   | SystemContext.InitializationBlocked
   | ToolOutputStore.Error
 

packages/core/src/session/runner/llm.ts

@@ -18,13 +18,14 @@ import { ModelV2 } from "../../model"
 import { ProviderV2 } from "../../provider"
 import { QuestionV2 } from "../../question"
 import { SystemContext } from "../../system-context/index"
-import { SystemContextRegistry } from "../../system-context/registry"
+import { SystemContextBuiltIns } from "../../system-context/builtins"
+import { InstructionContext } from "../../instruction-context"
 import { SkillGuidance } from "../../skill/guidance"
 import { ReferenceGuidance } from "../../reference/guidance"
 import { McpGuidance } from "../../mcp/guidance"
 import { ToolRegistry } from "../../tool/registry"
 import { ToolOutputStore } from "../../tool-output-store"
-import { SessionContextEpoch } from "../context-epoch"
+import { SessionContextCheckpoint } from "../context-checkpoint"
 import { SessionCompaction } from "../compaction"
 import { SessionEvent } from "../event"
 import { SessionHistory } from "../history"
@@ -102,7 +103,8 @@ const layer = Layer.effect(
     const models = yield* SessionRunnerModel.Service
     const store = yield* SessionStore.Service
     const location = yield* Location.Service
-    const systemContext = yield* SystemContextRegistry.Service
+    const builtins = yield* SystemContextBuiltIns.Service
+    const instructions = yield* InstructionContext.Service
     const skillGuidance = yield* SkillGuidance.Service
     const referenceGuidance = yield* ReferenceGuidance.Service
     const mcpGuidance = yield* McpGuidance.Service
@@ -170,9 +172,16 @@ const layer = Layer.effect(
       new TurnTransitionError({ _tag: "ContinueAfterOverflowCompaction", step })
 
     const loadSystemContext = (agent: AgentV2.Selection) =>
-      Effect.all([systemContext.load(), skillGuidance.load(agent), referenceGuidance.load(), mcpGuidance.load(agent)], {
-        concurrency: "unbounded",
-      }).pipe(Effect.map(SystemContext.combine))
+      Effect.all(
+        [
+          builtins.load(),
+          instructions.load(),
+          skillGuidance.load(agent),
+          referenceGuidance.load(),
+          mcpGuidance.load(agent),
+        ],
+        { concurrency: "unbounded" },
+      ).pipe(Effect.map(SystemContext.combine))
 
     const runTurnAttempt = Effect.fn("SessionRunner.runTurn")(function* (
       sessionID: SessionSchema.ID,
@@ -184,7 +193,9 @@ const layer = Layer.effect(
       if (session.location.directory !== location.directory || session.location.workspaceID !== location.workspaceID)
         return yield* Effect.interrupt
       const agent = yield* agents.select(session.agent)
-      const initialized = yield* SessionContextEpoch.initialize(db, loadSystemContext(agent), session.id)
+      // Establish what the model knows before admitting what the user said, so
+      // a blocked first turn leaves pending inputs untouched.
+      const checkpoint = yield* SessionContextCheckpoint.prepare(db, events, loadSystemContext(agent), session.id)
       const toolFibers = yield* FiberSet.make<void, ToolOutputStore.Error>()
       let needsContinuation = false
       let currentStep = step
@@ -198,10 +209,8 @@ const layer = Layer.effect(
         }
         if (promoted > 0) currentStep = 1
       }
-      const system =
-        initialized ?? (yield* SessionContextEpoch.prepare(db, events, loadSystemContext(agent), session.id))
       const model = yield* models.resolve(session)
-      const entries = yield* SessionHistory.entriesForRunner(db, session.id, system.baselineSeq)
+      const entries = yield* SessionHistory.entriesForRunner(db, session.id, checkpoint.baselineSeq)
       const context = entries.map((entry) => entry.message)
       const isLastStep = agent.info?.steps !== undefined && currentStep >= agent.info.steps
       const toolMaterialization = isLastStep
@@ -211,7 +220,10 @@ const layer = Layer.effect(
       const request = LLM.request({
         model,
         providerOptions: { openai: { promptCacheKey } },
-        system: [agent.info?.system ? agent.info.system : SessionRunnerSystemPrompt.provider(model), system.baseline]
+        system: [
+          agent.info?.system ? agent.info.system : SessionRunnerSystemPrompt.provider(model),
+          checkpoint.baseline,
+        ]
           .filter((part): part is string => part !== undefined && part.length > 0)
           .map(SystemPart.make),
         messages: [...toLLMMessages(context, model), ...(isLastStep ? [Message.assistant(MAX_STEPS_PROMPT)] : [])],
@@ -462,7 +474,8 @@ export const node = makeLocationNode({
     SessionRunnerModel.node,
     SessionStore.node,
     Location.node,
-    SystemContextRegistry.node,
+    SystemContextBuiltIns.node,
+    InstructionContext.node,
     SkillGuidance.node,
     ReferenceGuidance.node,
     McpGuidance.node,

packages/core/src/session/runner/model.ts

@@ -103,7 +103,9 @@ const withDefaults = (model: ModelV2.Info, route: AnyRoute) => {
   })
 }
 
-const providerOptions = (model: ModelV2.Info) => {
+const providerOptions = (
+  model: ModelV2.Info,
+): { readonly [key: string]: { readonly [key: string]: unknown } } | undefined => {
   if (Object.keys(model.request.settings).length === 0) return undefined
   if (model.api.type !== "aisdk") return undefined
   if (model.api.package === "@ai-sdk/openai") return { openai: model.request.settings }

packages/core/src/session/sql.ts

@@ -165,12 +165,12 @@ export const SessionInputTable = sqliteTable(
   ],
 )
 
-export const SessionContextEpochTable = sqliteTable("session_context_epoch", {
+export const SessionContextCheckpointTable = sqliteTable("session_context_epoch", {
   session_id: text()
     .$type<SessionSchema.ID>()
     .primaryKey()
     .references(() => SessionTable.id, { onDelete: "cascade" }),
   baseline: text().notNull(),
-  snapshot: text({ mode: "json" }).notNull().$type<SystemContext.Snapshot>(),
+  snapshot: text({ mode: "json" }).notNull().$type<SystemContext.Applied>(),
   baseline_seq: integer().notNull(),
 })

packages/core/src/session/store.ts

@@ -14,10 +14,6 @@ import { fromRow } from "./info"
 export interface Interface {
   readonly get: (sessionID: SessionSchema.ID) => Effect.Effect<SessionSchema.Info | undefined>
   readonly context: (sessionID: SessionSchema.ID) => Effect.Effect<SessionMessage.Message[], MessageDecodeError>
-  readonly runnerContext: (
-    sessionID: SessionSchema.ID,
-    baselineSeq: number,
-  ) => Effect.Effect<SessionMessage.Message[], MessageDecodeError>
   readonly message: (
     messageID: SessionMessage.ID,
   ) => Effect.Effect<{ readonly sessionID: SessionSchema.ID; readonly message: SessionMessage.Message } | undefined>
@@ -39,9 +35,6 @@ const layer = Layer.effect(
       context: Effect.fn("SessionStore.context")(function* (sessionID) {
         return yield* SessionHistory.load(db, sessionID)
       }),
-      runnerContext: Effect.fn("SessionStore.runnerContext")(function* (sessionID, baselineSeq) {
-        return yield* SessionHistory.loadForRunner(db, sessionID, baselineSeq)
-      }),
       message: Effect.fn("SessionStore.message")(function* (messageID) {
         const row = yield* db
           .select()

packages/core/src/skill/guidance.ts

@@ -13,23 +13,47 @@ const Summary = Schema.Struct({
 })
 type Summary = typeof Summary.Type
 
+const entries = (skills: ReadonlyArray<Summary>) =>
+  skills.flatMap((skill) => [
+    "  <skill>",
+    `    <name>${skill.name}</name>`,
+    `    <description>${skill.description}</description>`,
+    "  </skill>",
+  ])
+
 const render = (skills: ReadonlyArray<Summary>) =>
   [
     "Skills provide specialized instructions and workflows for specific tasks.",
     "Use the skill tool to load a skill when a task matches its description.",
     ...(skills.length === 0
       ? ["No skills are currently available."]
+      : ["<available_skills>", ...entries(skills), "</available_skills>"]),
+  ].join("\n")
+
+const update = (previous: ReadonlyArray<Summary>, current: ReadonlyArray<Summary>) => {
+  const diff = SystemContext.diffByKey(
+    previous,
+    current,
+    (skill) => skill.name,
+    (before, after) => before.description !== after.description,
+  )
+  // Additions and removals render as small deltas; anything else restates the full list.
+  if (diff.changed.length > 0 || (diff.added.length === 0 && diff.removed.length === 0))
+    return [
+      "The available skills have changed. This list supersedes the previous available skills list.",
+      render(current),
+    ].join("\n")
+  return [
+    ...(diff.added.length === 0
+      ? []
+      : ["New skills are available in addition to those previously listed:", ...entries(diff.added)]),
+    ...(diff.removed.length === 0
+      ? []
       : [
-          "<available_skills>",
-          ...skills.flatMap((skill) => [
-            "  <skill>",
-            `    <name>${skill.name}</name>`,
-            `    <description>${skill.description}</description>`,
-            "  </skill>",
-          ]),
-          "</available_skills>",
+          `The following skills are no longer available and must not be used: ${diff.removed.map((skill) => skill.name).join(", ")}.`,
         ]),
   ].join("\n")
+}
 
 export interface Interface {
   readonly load: (agent: AgentV2.Selection) => Effect.Effect<SystemContext.SystemContext>
@@ -61,11 +85,7 @@ const layer = Layer.effect(
           codec: Schema.toCodecJson(Schema.Array(Summary)),
           load: Effect.succeed(available),
           baseline: render,
-          update: (_previous, current) =>
-            [
-              "The available skills have changed. This list supersedes the previous available skills list.",
-              render(current),
-            ].join("\n"),
+          update,
           removed: () => "Skill guidance is no longer available. Do not use any previously listed skill.",
         })
       }),

packages/core/src/system-context/builtins.ts

@@ -1,18 +1,20 @@
 export * as SystemContextBuiltIns from "./builtins"
 
 import { makeLocationNode } from "../effect/app-node"
-import { DateTime, Effect, Layer, Schema } from "effect"
+import { Context, DateTime, Effect, Layer, Schema } from "effect"
 import { Location } from "../location"
 import { SystemContext } from "./index"
-import { InstructionContext } from "../instruction-context"
-import { SystemContextRegistry } from "./registry"
-import { FSUtil } from "../fs-util"
-import { Global } from "../global"
 
-const builtIns = Layer.effectDiscard(
+export interface Interface {
+  readonly load: () => Effect.Effect<SystemContext.SystemContext>
+}
+
+export class Service extends Context.Service<Service, Interface>()("@opencode/v2/SystemContextBuiltIns") {}
+
+const layer = Layer.effect(
+  Service,
   Effect.gen(function* () {
     const location = yield* Location.Service
-    const registry = yield* SystemContextRegistry.Service
     const environment = [
       "<env>",
       `  Working directory: ${location.directory}`,
@@ -39,12 +41,8 @@ const builtIns = Layer.effectDiscard(
       }),
     ])
 
-    yield* registry.register({ key: SystemContext.Key.make("core/builtins"), load: Effect.succeed(context) })
+    return Service.of({ load: () => Effect.succeed(context) })
   }),
 )
 
-export const node = makeLocationNode({
-  name: "system-context-builtins",
-  layer: builtIns,
-  deps: [Location.node, SystemContextRegistry.node, InstructionContext.node, FSUtil.node, Global.node],
-})
+export const node = makeLocationNode({ service: Service, layer, deps: [Location.node] })

packages/core/src/system-context/index.ts

@@ -7,13 +7,18 @@ import { Effect, Option, Schema } from "effect"
  *
  * `Source<A>` describes how to observe, compare, and render one value. `make`
  * closes over `A`, producing an opaque `SystemContext` that composes uniformly
- * with contexts built from other value types. Interpreters observe the composed
- * context once, then produce a durable structured
- * `Snapshot` alongside the exact model-visible baseline or update text.
+ * with contexts built from other value types.
+ *
+ * The durable `Applied` record tracks what the model was last told, per source:
+ * it is the model's current belief. Interpreters uphold one invariant —
+ * `reconcile` never rewrites the baseline; it only narrates drift as update
+ * text. Only `rebaseline` (compaction) and `initialize` (first turn) produce
+ * baseline text.
  *
  * Returning `unavailable` means observation failed temporarily. It differs from
- * removing a source from the context: refresh preserves the admitted snapshot,
- * and replacement waits rather than silently constructing an incomplete baseline.
+ * removing a source from the context: the model's prior belief stands.
+ * `reconcile` retains the applied value silently, and `rebaseline` restates the
+ * belief by rendering the last-applied value instead of a live observation.
  *
  * @module
  */
@@ -45,39 +50,30 @@ export interface SystemContext {
   readonly [ContextTypeId]: ReadonlyArray<PackedSource>
 }
 
-/** Durable comparison state for one admitted source. */
-export const SourceSnapshot = Schema.Struct({
+/** The value last applied to the model for one admitted source. */
+export const AppliedSource = Schema.Struct({
   value: Schema.Json,
   removed: Schema.optional(Schema.NonEmptyString),
 })
-export type SourceSnapshot = typeof SourceSnapshot.Type
+export type AppliedSource = typeof AppliedSource.Type
 
-/** Durable structured comparison state for one active context generation. */
-export const Snapshot = Schema.Record(Key, SourceSnapshot)
-export type Snapshot = Readonly<Record<string, SourceSnapshot>>
+/** Durable record of what the model currently believes, per source. */
+export const Applied = Schema.Record(Key, AppliedSource)
+export type Applied = Readonly<Record<string, AppliedSource>>
 
-export interface Generation {
-  readonly baseline: string
-  readonly snapshot: Snapshot
+/** A rendered baseline together with the applied values it was rendered from. */
+export interface Baseline {
+  readonly text: string
+  readonly applied: Applied
 }
 
 export interface Updated {
   readonly _tag: "Updated"
   readonly text: string
-  readonly snapshot: Snapshot
+  readonly applied: Applied
 }
 
-export interface ReplacementReady {
-  readonly _tag: "ReplacementReady"
-  readonly generation: Generation
-}
-
-export interface ReplacementBlocked {
-  readonly _tag: "ReplacementBlocked"
-}
-
-export type ReplacementResult = ReplacementReady | ReplacementBlocked
-export type ReconcileResult = { readonly _tag: "Unchanged" } | Updated | ReplacementResult
+export type ReconcileResult = { readonly _tag: "Unchanged" } | Updated
 
 export class InitializationBlocked extends Schema.TaggedErrorClass<InitializationBlocked>()(
   "SystemContext.InitializationBlocked",
@@ -98,36 +94,24 @@ export class DuplicateKeyError extends Schema.TaggedErrorClass<DuplicateKeyError
 
 interface PackedSource {
   readonly key: Key
-  readonly load: Effect.Effect<Loaded | Unavailable>
+  readonly load: Effect.Effect<Observed | Unavailable>
+  /** Restates the model's belief from a last-applied value when the source cannot be observed. */
+  readonly recall: (stored: AppliedSource) => string | undefined
 }
 
-interface Loaded {
-  readonly baseline: () => Rendered
-  readonly compare: (previous: Schema.Json) => Compared
+interface Observed {
+  readonly applied: AppliedSource
+  readonly baseline: () => string
+  /** `undefined` means unchanged. An undecodable previous value re-renders the baseline (treat-as-new). */
+  readonly update: (previous: AppliedSource) => string | undefined
 }
 
-interface Rendered {
-  readonly text: string
-  readonly snapshot: SourceSnapshot
-}
-
-type Compared =
-  | { readonly _tag: "Incompatible" }
-  | { readonly _tag: "Unchanged" }
-  | { readonly _tag: "Updated"; readonly render: () => Rendered }
-
-interface AvailableEntry extends Loaded {
-  readonly _tag: "Available"
+interface Entry {
   readonly key: Key
+  readonly recall: PackedSource["recall"]
+  readonly observed: Observed | Unavailable
 }
 
-interface UnavailableEntry {
-  readonly _tag: "Unavailable"
-  readonly key: Key
-}
-
-type Entry = AvailableEntry | UnavailableEntry
-
 /** The identity context. */
 export const empty = context([])
 
@@ -136,42 +120,65 @@ export function make<A>(source: Source<A>): SystemContext {
   const decode = Schema.decodeUnknownOption(source.codec)
   const encode = Schema.encodeSync(source.codec)
   const equivalent = Schema.toEquivalence(source.codec)
+  const baseline = (value: A) => requireText(source.key, "baseline", source.baseline(value))
   return context([
     {
       key: source.key,
+      recall: (stored) =>
+        Option.match(decode(stored.value), {
+          onNone: () => undefined,
+          onSome: baseline,
+        }),
       load: source.load.pipe(
         Effect.map((value) => {
           if (isUnavailable(value)) return value
-          const snapshot = (): SourceSnapshot => ({
-            value: encode(value),
-            ...(source.removed ? { removed: requireText(source.key, "removal", source.removed(value)) } : {}),
-          })
           return {
-            baseline: (): Rendered => ({
-              text: requireText(source.key, "baseline", source.baseline(value)),
-              snapshot: snapshot(),
-            }),
-            compare: (previous): Compared =>
-              Option.match(decode(previous), {
-                onNone: (): Compared => ({ _tag: "Incompatible" }),
-                onSome: (decoded): Compared =>
+            applied: {
+              value: encode(value),
+              ...(source.removed ? { removed: requireText(source.key, "removal", source.removed(value)) } : {}),
+            },
+            baseline: () => baseline(value),
+            update: (previous) =>
+              Option.match(decode(previous.value), {
+                onNone: () => baseline(value),
+                onSome: (decoded) =>
                   equivalent(decoded, value)
-                    ? { _tag: "Unchanged" }
-                    : {
-                        _tag: "Updated",
-                        render: () => ({
-                          text: requireText(source.key, "update", source.update(decoded, value)),
-                          snapshot: snapshot(),
-                        }),
-                      },
+                    ? undefined
+                    : requireText(source.key, "update", source.update(decoded, value)),
               }),
-          }
+          } satisfies Observed
         }),
       ),
     },
   ])
 }
 
+/**
+ * Keyed three-way diff for list-shaped sources rendering delta updates.
+ * `changed` compares two values sharing a key; entries equal under it are dropped.
+ */
+export function diffByKey<A>(
+  previous: ReadonlyArray<A>,
+  current: ReadonlyArray<A>,
+  key: (value: A) => string,
+  changed: (previous: A, current: A) => boolean,
+): {
+  readonly added: ReadonlyArray<A>
+  readonly removed: ReadonlyArray<A>
+  readonly changed: ReadonlyArray<{ readonly previous: A; readonly current: A }>
+} {
+  const currentKeys = new Set(current.map(key))
+  const previousByKey = new Map(previous.map((value) => [key(value), value] as const))
+  return {
+    added: current.filter((value) => !previousByKey.has(key(value))),
+    removed: previous.filter((value) => !currentKeys.has(key(value))),
+    changed: current.flatMap((value) => {
+      const before = previousByKey.get(key(value))
+      return before === undefined || !changed(before, value) ? [] : [{ previous: before, current: value }]
+    }),
+  }
+}
+
 /** Combines contexts in order and rejects duplicate source keys immediately. */
 export function combine(values: ReadonlyArray<SystemContext>): SystemContext {
   const sources = values.flatMap((value) => value[ContextTypeId])
@@ -183,111 +190,91 @@ const observe = (value: SystemContext) =>
   Effect.forEach(
     value[ContextTypeId],
     (source) =>
-      source.load.pipe(
-        Effect.map(
-          (result): Entry =>
-            result === unavailable
-              ? { _tag: "Unavailable", key: source.key }
-              : { _tag: "Available", key: source.key, ...result },
-        ),
-      ),
+      source.load.pipe(Effect.map((observed): Entry => ({ key: source.key, recall: source.recall, observed }))),
     { concurrency: "unbounded" },
   )
 
-/** Creates the immutable baseline and durable snapshot for a new generation. */
-export function initialize(value: SystemContext): Effect.Effect<Generation, InitializationBlocked> {
+/** Creates the first baseline. Blocks rather than admit a baseline missing an unobservable source. */
+export function initialize(value: SystemContext): Effect.Effect<Baseline, InitializationBlocked> {
   return observe(value).pipe(
     Effect.flatMap((entries) => {
-      const unavailable = entries.flatMap((entry) => (entry._tag === "Unavailable" ? [entry.key] : []))
-      if (unavailable.length > 0) return new InitializationBlocked({ keys: unavailable })
-      return Effect.succeed(initializeObservation(entries))
+      const blocked = entries.flatMap((entry) => (entry.observed === unavailable ? [entry.key] : []))
+      if (blocked.length > 0) return new InitializationBlocked({ keys: blocked })
+      const parts: string[] = []
+      const applied: Record<string, AppliedSource> = {}
+      for (const entry of entries) {
+        if (entry.observed === unavailable) continue
+        parts.push(entry.observed.baseline())
+        applied[entry.key] = entry.observed.applied
+      }
+      return Effect.succeed({ text: render(parts), applied })
     }),
   )
 }
 
-function initializeObservation(entries: ReadonlyArray<Entry>): Generation {
-  const available = entries.filter((entry): entry is AvailableEntry => entry._tag === "Available")
-  const rendered = available.map((entry) => [entry.key, entry.baseline()] as const)
-  return {
-    baseline: render(rendered.map(([, result]) => result.text)),
-    snapshot: Object.fromEntries(rendered.map(([key, result]) => [key, result.snapshot])),
-  }
-}
-
-/** Reconciles current source values with one active generation. */
-export function reconcile(value: SystemContext, previous: Snapshot): Effect.Effect<ReconcileResult> {
+/** Narrates drift between current source values and the model's beliefs. Never rewrites the baseline. */
+export function reconcile(value: SystemContext, previous: Applied): Effect.Effect<ReconcileResult> {
   return observe(value).pipe(
     Effect.map((entries): ReconcileResult => {
-      const result = reconcileObservation(entries, previous)
-      if (result._tag === "Unchanged" || result._tag === "Updated") return result
-      return replaceObservation(entries, previous)
+      const updates: string[] = []
+      const applied: Record<string, AppliedSource> = {}
+      for (const entry of entries) {
+        const stored = get(previous, entry.key)
+        if (entry.observed === unavailable) {
+          // The prior belief stands while the source cannot be observed.
+          if (stored) applied[entry.key] = stored
+          continue
+        }
+        if (!stored) {
+          updates.push(entry.observed.baseline())
+          applied[entry.key] = entry.observed.applied
+          continue
+        }
+        const text = entry.observed.update(stored)
+        if (text === undefined) {
+          applied[entry.key] = stored
+          continue
+        }
+        updates.push(text)
+        applied[entry.key] = entry.observed.applied
+      }
+      const keys = new Set<string>(entries.map((entry) => entry.key))
+      for (const key of Object.keys(previous).sort()) {
+        if (keys.has(key)) continue
+        const removed = previous[key].removed
+        // An unannounced removal retains the belief; it clears at the next rebaseline.
+        if (removed === undefined) applied[key] = previous[key]
+        else updates.push(removed)
+      }
+      if (updates.length === 0) return { _tag: "Unchanged" }
+      return { _tag: "Updated", text: render(updates), applied }
     }),
   )
 }
 
-function reconcileObservation(
-  entries: ReadonlyArray<Entry>,
-  previous: Snapshot,
-): { readonly _tag: "Unchanged" } | Updated | { readonly _tag: "Replace" } {
-  const keys = new Set(entries.map((entry) => entry.key))
-  const comparisons = new Map<Key, Compared>()
-  for (const entry of entries) {
-    if (entry._tag === "Unavailable") continue
-    const stored = getSnapshot(previous, entry.key)
-    if (!stored) continue
-    const compared = entry.compare(stored.value)
-    if (compared._tag === "Incompatible") return { _tag: "Replace" }
-    comparisons.set(entry.key, compared)
-  }
-  for (const key of Object.keys(previous).sort()) {
-    if (keys.has(Key.make(key))) continue
-    if (previous[key].removed === undefined) return { _tag: "Replace" }
-  }
-
-  const snapshot: Record<string, SourceSnapshot> = {}
-  const updates: string[] = []
-  for (const entry of entries) {
-    const stored = getSnapshot(previous, entry.key)
-    if (entry._tag === "Unavailable") {
-      if (stored) snapshot[entry.key] = stored
-      continue
-    }
-    if (!stored) {
-      const rendered = entry.baseline()
-      updates.push(rendered.text)
-      snapshot[entry.key] = rendered.snapshot
-      continue
-    }
-    const compared = comparisons.get(entry.key)
-    if (!compared || compared._tag === "Incompatible")
-      throw new Error(`Missing comparison for system context source ${entry.key}`)
-    if (compared._tag === "Unchanged") {
-      snapshot[entry.key] = stored
-      continue
-    }
-    const rendered = compared.render()
-    updates.push(rendered.text)
-    snapshot[entry.key] = rendered.snapshot
-  }
-  for (const key of Object.keys(previous).sort()) {
-    if (keys.has(Key.make(key))) continue
-    const removed = previous[key].removed
-    if (removed === undefined) throw new Error(`Missing removal rendering for system context source ${key}`)
-    updates.push(removed)
-  }
-  if (updates.length === 0) return { _tag: "Unchanged" }
-  return { _tag: "Updated", text: render(updates), snapshot }
-}
-
-/** Creates a complete replacement generation or blocks while admitted context is unavailable. */
-export function replace(value: SystemContext, previous: Snapshot): Effect.Effect<ReplacementResult> {
-  return observe(value).pipe(Effect.map((entries) => replaceObservation(entries, previous)))
-}
-
-function replaceObservation(entries: ReadonlyArray<Entry>, previous: Snapshot): ReplacementResult {
-  if (entries.some((entry) => entry._tag === "Unavailable" && getSnapshot(previous, entry.key) !== undefined))
-    return { _tag: "ReplacementBlocked" }
-  return { _tag: "ReplacementReady", generation: initializeObservation(entries) }
+/** Rebuilds the baseline, restating unobservable sources from the model's last-applied beliefs. */
+export function rebaseline(value: SystemContext, previous: Applied): Effect.Effect<Baseline> {
+  return observe(value).pipe(
+    Effect.map((entries): Baseline => {
+      const parts: string[] = []
+      const applied: Record<string, AppliedSource> = {}
+      for (const entry of entries) {
+        if (entry.observed !== unavailable) {
+          parts.push(entry.observed.baseline())
+          applied[entry.key] = entry.observed.applied
+          continue
+        }
+        const stored = get(previous, entry.key)
+        if (!stored) continue
+        const text = entry.recall(stored)
+        // An undecodable belief cannot be restated; the source re-announces when observable again.
+        if (text === undefined) continue
+        parts.push(text)
+        applied[entry.key] = stored
+      }
+      return { text: render(parts), applied }
+    }),
+  )
 }
 
 function context(sources: ReadonlyArray<PackedSource>): SystemContext {
@@ -298,8 +285,8 @@ function render(parts: ReadonlyArray<string>) {
   return parts.join("\n\n")
 }
 
-function getSnapshot(snapshot: Snapshot, key: Key) {
-  return Object.hasOwn(snapshot, key) ? snapshot[key] : undefined
+function get(applied: Applied, key: Key) {
+  return Object.hasOwn(applied, key) ? applied[key] : undefined
 }
 
 function isUnavailable(value: unknown): value is Unavailable {

packages/core/src/system-context/registry.ts

@@ -1,49 +0,0 @@
-export * as SystemContextRegistry from "./registry"
-
-import { Context, Effect, Layer, Ref, Scope } from "effect"
-import { SystemContext } from "./index"
-import { makeLocationNode } from "../effect/app-node"
-
-export interface Entry {
-  readonly key: SystemContext.Key
-  readonly load: Effect.Effect<SystemContext.SystemContext>
-}
-
-export interface Interface {
-  readonly register: (entry: Entry) => Effect.Effect<void, never, Scope.Scope>
-  readonly load: () => Effect.Effect<SystemContext.SystemContext>
-}
-
-export class Service extends Context.Service<Service, Interface>()("@opencode/v2/SystemContextRegistry") {}
-
-const layer = Layer.effect(
-  Service,
-  Effect.gen(function* () {
-    const entries = yield* Ref.make<ReadonlyArray<Entry>>([])
-
-    return Service.of({
-      register: Effect.fn("SystemContextRegistry.register")(function* (entry) {
-        yield* Effect.acquireRelease(
-          Ref.modify(entries, (current) => {
-            if (current.some((item) => item.key === entry.key)) return [false, current]
-            return [true, [...current, entry]]
-          }).pipe(
-            Effect.flatMap((added) =>
-              added ? Effect.void : Effect.die(`Duplicate system context entry key: ${entry.key}`),
-            ),
-            Effect.as(entry),
-          ),
-          (entry) => Ref.update(entries, (current) => current.filter((item) => item !== entry)),
-        )
-      }),
-      load: Effect.fn("SystemContextRegistry.load")(function* () {
-        const current = (yield* Ref.get(entries)).toSorted((a, b) => (a.key < b.key ? -1 : a.key > b.key ? 1 : 0))
-        return SystemContext.combine(
-          yield* Effect.forEach(current, (entry) => entry.load, { concurrency: "unbounded" }),
-        )
-      }),
-    })
-  }),
-)
-
-export const node = makeLocationNode({ service: Service, layer, deps: [] })

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

@@ -173,6 +173,7 @@ describe("ConfigAgentPlugin.Plugin", () => {
         model: { providerID: "anthropic", id: "claude-sonnet", variant: undefined },
       })
       expect(reviewer.request).toEqual({
+        settings: {},
         headers: { first: "one", shared: "last", second: "two" },
         body: { enabled: true, profile: "review", retries: 2, effort: "high" },
       })

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

@@ -10,7 +10,6 @@ import { InstructionContext } from "@opencode-ai/core/instruction-context"
 import { Location } from "@opencode-ai/core/location"
 import { AbsolutePath } from "@opencode-ai/core/schema"
 import { SystemContext } from "@opencode-ai/core/system-context"
-import { SystemContextRegistry } from "@opencode-ai/core/system-context/registry"
 import { location } from "./fixture/location"
 import { tmpdir } from "./fixture/tmpdir"
 import { testEffect } from "./lib/effect"
@@ -22,7 +21,7 @@ const instructionLayer = (input: {
   locationServiceLayer: Layer.Layer<Location.Service>
   filesystemLayer?: Layer.Layer<FSUtil.Service>
 }) =>
-  AppNodeBuilder.build(LayerNode.group([SystemContextRegistry.node, InstructionContext.node]), [
+  AppNodeBuilder.build(InstructionContext.node, [
     [Global.node, Global.layerWith({ config: input.config })],
     [Location.node, input.locationServiceLayer],
     ...(input.filesystemLayer ? [[FSUtil.node, input.filesystemLayer] as const] : []),
@@ -52,7 +51,7 @@ describe("InstructionContext", () => {
             await fs.writeFile(packageFile, "package")
           })
 
-          const load = SystemContextRegistry.Service.pipe(
+          const load = InstructionContext.Service.pipe(
             Effect.flatMap((service) => service.load()),
             Effect.provide(
               instructionLayer({
@@ -71,23 +70,23 @@ describe("InstructionContext", () => {
           )
 
           const initialized = yield* SystemContext.initialize(yield* load)
-          expect(initialized.baseline).toBe(
+          expect(initialized.text).toBe(
             [
               `Instructions from: ${globalFile}\nglobal`,
               `Instructions from: ${packageFile}\npackage`,
               `Instructions from: ${projectFile}\nproject`,
             ].join("\n\n"),
           )
-          expect(initialized.baseline).not.toContain("outside")
+          expect(initialized.text).not.toContain("outside")
 
           yield* Effect.promise(() => fs.writeFile(packageFile, "changed"))
-          expect(yield* SystemContext.reconcile(yield* load, initialized.snapshot)).toMatchObject({
+          expect(yield* SystemContext.reconcile(yield* load, initialized.applied)).toMatchObject({
             _tag: "Updated",
             text: expect.stringContaining(`Instructions from: ${packageFile}\nchanged`),
           })
 
           yield* Effect.promise(() => fs.rm(packageFile))
-          const partial = yield* SystemContext.reconcile(yield* load, initialized.snapshot)
+          const partial = yield* SystemContext.reconcile(yield* load, initialized.applied)
           expect(partial).toEqual({
             _tag: "Updated",
             text: [
@@ -95,14 +94,14 @@ describe("InstructionContext", () => {
               `Instructions from: ${globalFile}\nglobal`,
               `Instructions from: ${projectFile}\nproject`,
             ].join("\n\n"),
-            snapshot: expect.any(Object),
+            applied: expect.any(Object),
           })
 
           yield* Effect.promise(() => Promise.all([fs.rm(globalFile), fs.rm(projectFile)]))
-          expect(yield* SystemContext.reconcile(yield* load, initialized.snapshot)).toEqual({
+          expect(yield* SystemContext.reconcile(yield* load, initialized.applied)).toEqual({
             _tag: "Updated",
             text: "Previously loaded instructions no longer apply.",
-            snapshot: {},
+            applied: {},
           })
         }),
       ),
@@ -118,7 +117,7 @@ describe("InstructionContext", () => {
         Effect.gen(function* () {
           const file = path.join(tmp.path, "AGENTS.md")
           yield* Effect.promise(() => fs.writeFile(file, ""))
-          const context = yield* SystemContextRegistry.Service.pipe(
+          const context = yield* InstructionContext.Service.pipe(
             Effect.flatMap((service) => service.load()),
             Effect.provide(
               instructionLayer({
@@ -131,7 +130,7 @@ describe("InstructionContext", () => {
             ),
           )
 
-          expect((yield* SystemContext.initialize(context)).baseline).toBe(`Instructions from: ${file}\n`)
+          expect((yield* SystemContext.initialize(context)).text).toBe(`Instructions from: ${file}\n`)
         }),
       ),
     ),
@@ -147,7 +146,7 @@ describe("InstructionContext", () => {
           ),
         ),
       ).pipe(Layer.provide(LayerNode.compile(FSUtil.node)))
-      const context = yield* SystemContextRegistry.Service.pipe(
+      const context = yield* InstructionContext.Service.pipe(
         Effect.flatMap((service) => service.load()),
         Effect.provide(
           instructionLayer({
@@ -187,7 +186,7 @@ describe("InstructionContext", () => {
           ),
         ),
       ).pipe(Layer.provide(LayerNode.compile(FSUtil.node)))
-      const context = yield* SystemContextRegistry.Service.pipe(
+      const context = yield* InstructionContext.Service.pipe(
         Effect.flatMap((service) => service.load()),
         Effect.provide(
           instructionLayer({
@@ -231,7 +230,7 @@ describe("InstructionContext", () => {
         ),
       ).pipe(Layer.provide(LayerNode.compile(FSUtil.node)))
 
-      yield* SystemContextRegistry.Service.pipe(
+      yield* InstructionContext.Service.pipe(
         Effect.flatMap((service) => service.load()),
         Effect.provide(
           instructionLayer({
@@ -261,7 +260,7 @@ describe("InstructionContext", () => {
       let scanned = false
       process.env.OPENCODE_DISABLE_PROJECT_CONFIG = "1"
 
-      yield* SystemContextRegistry.Service.pipe(
+      yield* InstructionContext.Service.pipe(
         Effect.flatMap((service) => service.load()),
         Effect.provide(
           instructionLayer({
@@ -293,7 +292,7 @@ describe("InstructionContext", () => {
   it.effect("does not discover project instructions outside the canonical project root", () =>
     Effect.gen(function* () {
       let scanned = false
-      yield* SystemContextRegistry.Service.pipe(
+      yield* InstructionContext.Service.pipe(
         Effect.flatMap((service) => service.load()),
         Effect.provide(
           instructionLayer({

packages/core/test/plugin/host.ts

@@ -279,7 +279,11 @@ function agentInfo(value: AgentV2.Info) {
   return {
     ...value,
     model: value.model && { ...value.model },
-    request: { headers: { ...value.request.headers }, body: { ...value.request.body } },
+    request: {
+      settings: { ...value.request.settings },
+      headers: { ...value.request.headers },
+      body: { ...value.request.body },
+    },
     permissions: value.permissions.map((permission) => ({ ...permission })),
   }
 }
@@ -288,7 +292,11 @@ function providerInfo(value: ProviderV2.MutableInfo) {
   return {
     ...value,
     api: { ...value.api, settings: value.api.settings && { ...value.api.settings } },
-    request: { settings: { ...value.request.settings }, headers: { ...value.request.headers }, body: { ...value.request.body } },
+    request: {
+      settings: { ...value.request.settings },
+      headers: { ...value.request.headers },
+      body: { ...value.request.body },
+    },
   }
 }
 

packages/core/test/plugin/provider-amazon-bedrock.test.ts

@@ -93,7 +93,7 @@ describe("AmazonBedrockPlugin", () => {
         })
         catalog.provider.update(bedrock.id, (item) => {
           item.api = bedrock.api
-          item.request = bedrock.request
+          item.request = { settings: {}, headers: {}, body: { endpoint: "https://bedrock.example" } }
         })
       })
       yield* addPlugin()

packages/core/test/plugin/provider-anthropic.test.ts

@@ -36,7 +36,7 @@ describe("AnthropicPlugin", () => {
         })
         catalog.provider.update(item.id, (draft) => {
           draft.api = item.api
-          draft.request = item.request
+          draft.request = { settings: {}, headers: { Existing: "1" }, body: {} }
         })
       })
       yield* addPlugin()

packages/core/test/plugin/provider-azure.test.ts

@@ -87,7 +87,7 @@ describe("AzurePlugin", () => {
           })
           catalog.provider.update(azure.id, (item) => {
             item.api = azure.api
-            item.request = azure.request
+            item.request = { settings: {}, headers: {}, body: { resourceName: "from-config" } }
           })
           catalog.provider.update(ProviderV2.ID.openai, () => {})
         })
@@ -110,7 +110,7 @@ describe("AzurePlugin", () => {
           })
           catalog.provider.update(azure.id, (item) => {
             item.api = azure.api
-            item.request = azure.request
+            item.request = { settings: {}, headers: {}, body: { resourceName: "" } }
           })
         })
         yield* addPlugin()
@@ -131,7 +131,7 @@ describe("AzurePlugin", () => {
           })
           catalog.provider.update(azure.id, (item) => {
             item.api = azure.api
-            item.request = azure.request
+            item.request = { settings: {}, headers: {}, body: { resourceName: "   " } }
           })
         })
         yield* addPlugin()

packages/core/test/plugin/provider-kilo.test.ts

@@ -32,7 +32,7 @@ describe("KiloPlugin", () => {
             package: "@ai-sdk/openai-compatible",
             url: "https://api.kilo.ai/api/gateway",
           }
-          provider.request = { headers: { Existing: "value" }, body: {} }
+          provider.request = { settings: {}, headers: { Existing: "value" }, body: {} }
         })
         catalog.provider.update(ProviderV2.ID.openrouter, () => {})
       })

packages/core/test/plugin/provider-llmgateway.test.ts

@@ -39,7 +39,7 @@ describe("LLMGatewayPlugin", () => {
             package: "@ai-sdk/openai-compatible",
             url: "https://api.llmgateway.io/v1",
           }
-          provider.request = { headers: { Existing: "value" }, body: {} }
+          provider.request = { settings: {}, headers: { Existing: "value" }, body: {} }
         })
         catalog.provider.update(ProviderV2.ID.openrouter, () => {})
       })

packages/core/test/plugin/provider-nvidia.test.ts

@@ -32,7 +32,7 @@ describe("NvidiaPlugin", () => {
             package: "@ai-sdk/openai-compatible",
             url: "https://integrate.api.nvidia.com/v1",
           }
-          provider.request = { headers: { Existing: "value" }, body: {} }
+          provider.request = { settings: {}, headers: { Existing: "value" }, body: {} }
         })
         catalog.provider.update(ProviderV2.ID.openrouter, () => {})
       })
@@ -80,6 +80,7 @@ describe("NvidiaPlugin", () => {
             url: "https://integrate.api.nvidia.com/v1",
           }
           provider.request = {
+            settings: {},
             headers: { "X-BILLING-INVOKE-ORIGIN": "CustomOrigin" },
             body: { baseURL: "https://integrate.api.nvidia.com/v1" },
           }

packages/core/test/plugin/provider-opencode.test.ts

@@ -373,7 +373,7 @@ describe("OpencodePlugin", () => {
             cost: cost(1),
           })
           catalog.provider.update(provider.id, (draft) => {
-            draft.request = provider.request
+            draft.request = { settings: {}, headers: {}, body: { apiKey: "configured" } }
           })
           catalog.model.update(provider.id, model.id, (draft) => {
             draft.cost = [...model.cost]

packages/core/test/plugin/provider-openrouter.test.ts

@@ -31,7 +31,7 @@ describe("OpenRouterPlugin", () => {
       yield* catalog.transform((catalog) => {
         catalog.provider.update(ProviderV2.ID.openrouter, (provider) => {
           provider.api = { type: "aisdk", package: "@openrouter/ai-sdk-provider" }
-          provider.request = { headers: { Existing: "value" }, body: {} }
+          provider.request = { settings: {}, headers: { Existing: "value" }, body: {} }
         })
         catalog.provider.update(ProviderV2.ID.make("nvidia"), () => {})
       })

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

@@ -16,10 +16,10 @@ describe("ReferenceGuidance", () => {
       const guidance = yield* ReferenceGuidance.Service
       const generation = yield* SystemContext.initialize(yield* guidance.load())
 
-      expect(generation.baseline).toContain("<available_references>")
-      expect(generation.baseline).toContain("<name>docs</name>")
-      expect(generation.baseline).toContain("<path>/docs</path>")
-      expect(generation.baseline).toContain("<description>Use for product documentation</description>")
+      expect(generation.text).toContain("<available_references>")
+      expect(generation.text).toContain("<name>docs</name>")
+      expect(generation.text).toContain("<path>/docs</path>")
+      expect(generation.text).toContain("<description>Use for product documentation</description>")
     }).pipe(
       Effect.provide(
         guidanceLayer(
@@ -47,7 +47,7 @@ describe("ReferenceGuidance", () => {
     Effect.gen(function* () {
       const guidance = yield* ReferenceGuidance.Service
       const generation = yield* SystemContext.initialize(yield* guidance.load())
-      expect(generation.baseline).toBe("")
+      expect(generation.text).toBe("")
     }).pipe(Effect.provide(guidanceLayer(Layer.mock(Reference.Service, { list: () => Effect.succeed([]) })))),
   )
 
@@ -55,7 +55,7 @@ describe("ReferenceGuidance", () => {
     Effect.gen(function* () {
       const guidance = yield* ReferenceGuidance.Service
       const generation = yield* SystemContext.initialize(yield* guidance.load())
-      expect(generation.baseline).toBe("")
+      expect(generation.text).toBe("")
     }).pipe(
       Effect.provide(
         guidanceLayer(
@@ -73,4 +73,41 @@ describe("ReferenceGuidance", () => {
       ),
     ),
   )
+
+  it.effect("announces added and removed references as deltas", () => {
+    const reference = (name: string, description: string) =>
+      new Reference.Info({
+        name,
+        path: AbsolutePath.make(`/${name}`),
+        description,
+        source: Reference.LocalSource.make({ type: "local", path: AbsolutePath.make(`/${name}`), description }),
+      })
+    let references = [reference("docs", "Use for product documentation")]
+    return Effect.gen(function* () {
+      const guidance = yield* ReferenceGuidance.Service
+      const initialized = yield* SystemContext.initialize(yield* guidance.load())
+
+      references = [reference("docs", "Use for product documentation"), reference("examples", "Use for examples")]
+      const added = yield* SystemContext.reconcile(yield* guidance.load(), initialized.applied)
+      expect(added).toMatchObject({
+        _tag: "Updated",
+        text: [
+          "New project references are available in addition to those previously listed:",
+          "  <reference>",
+          "    <name>examples</name>",
+          "    <path>/examples</path>",
+          "    <description>Use for examples</description>",
+          "  </reference>",
+        ].join("\n"),
+      })
+
+      references = [reference("examples", "Use for examples")]
+      expect(
+        yield* SystemContext.reconcile(yield* guidance.load(), added._tag === "Updated" ? added.applied : {}),
+      ).toMatchObject({
+        _tag: "Updated",
+        text: "The following project references are no longer available and must not be used: docs.",
+      })
+    }).pipe(Effect.provide(guidanceLayer(Layer.mock(Reference.Service, { list: () => Effect.succeed(references) }))))
+  })
 })

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

@@ -19,7 +19,12 @@ import { SessionMessageUpdater } from "@opencode-ai/core/session/message-updater
 import { SessionProjector } from "@opencode-ai/core/session/projector"
 import { SessionExecution } from "@opencode-ai/core/session/execution"
 import { SessionInput } from "@opencode-ai/core/session/input"
-import { SessionInputTable, SessionMessageTable, SessionTable } from "@opencode-ai/core/session/sql"
+import {
+  SessionContextCheckpointTable,
+  SessionInputTable,
+  SessionMessageTable,
+  SessionTable,
+} from "@opencode-ai/core/session/sql"
 import { testEffect } from "./lib/effect"
 import { Snapshot } from "@opencode-ai/core/snapshot"
 
@@ -67,6 +72,10 @@ describe("SessionProjector", () => {
         .insert(SessionMessageTable)
         .values([assistantRow(boundary, 1), assistantRow(SessionMessage.ID.make("msg_later"), 2)])
         .run()
+      yield* db
+        .insert(SessionContextCheckpointTable)
+        .values({ session_id: sessionID, baseline: "baseline", snapshot: {}, baseline_seq: 0 })
+        .run()
       const events = yield* EventV2.Service
       yield* events.publish(SessionEvent.RevertEvent.Staged, {
         sessionID,
@@ -93,6 +102,8 @@ describe("SessionProjector", () => {
       expect(
         (yield* db.select({ id: SessionMessageTable.id }).from(SessionMessageTable).all()).map((row) => row.id),
       ).toEqual([boundary])
+      // A committed revert resets the context checkpoint so the next turn re-initializes.
+      expect(yield* db.select().from(SessionContextCheckpointTable).get().pipe(Effect.orDie)).toBeUndefined()
     }),
   )
 

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

@@ -31,7 +31,8 @@ import { ToolOutputStore } from "@opencode-ai/core/tool-output-store"
 import { SessionTable } from "@opencode-ai/core/session/sql"
 import { SessionStore } from "@opencode-ai/core/session/store"
 import { Location } from "@opencode-ai/core/location"
-import { SystemContextRegistry } from "@opencode-ai/core/system-context/registry"
+import { SystemContextBuiltIns } from "@opencode-ai/core/system-context/builtins"
+import { InstructionContext } from "@opencode-ai/core/instruction-context"
 import { SystemContext } from "@opencode-ai/core/system-context"
 import { SkillGuidance } from "@opencode-ai/core/skill/guidance"
 import { ReferenceGuidance } from "@opencode-ai/core/reference/guidance"
@@ -72,7 +73,8 @@ const model = OpenAIChat.route
   })
   .model({ id: "gpt-4o-mini" })
 const models = SessionRunnerModel.layerWith(() => Effect.succeed(model))
-const systemContext = AppNodeBuilder.build(SystemContextRegistry.node)
+const systemContext = Layer.mock(SystemContextBuiltIns.Service, { load: () => Effect.succeed(SystemContext.empty) })
+const instructionContext = Layer.mock(InstructionContext.Service, { load: () => Effect.succeed(SystemContext.empty) })
 const skillGuidance = Layer.mock(SkillGuidance.Service, { load: () => Effect.succeed(SystemContext.empty) })
 const referenceGuidance = Layer.mock(ReferenceGuidance.Service, { load: () => Effect.succeed(SystemContext.empty) })
 const mcpGuidance = Layer.mock(McpGuidance.Service, { load: () => Effect.succeed(SystemContext.empty) })
@@ -81,7 +83,8 @@ const runnerLayer = AppNodeBuilder.build(SessionRunnerLLM.node, [
   [Snapshot.node, Snapshot.noopLayer],
   [LayerNodePlatform.llmClient, client],
   [SessionRunnerModel.node, models],
-  [SystemContextRegistry.node, systemContext],
+  [SystemContextBuiltIns.node, systemContext],
+  [InstructionContext.node, instructionContext],
   [Location.node, Location.boundNode({ directory: AbsolutePath.make("/project") })],
   [SkillGuidance.node, skillGuidance],
   [ReferenceGuidance.node, referenceGuidance],
@@ -116,7 +119,8 @@ const it = testEffect(
       AgentV2.node,
       ToolRegistry.node,
       SessionRunnerModel.node,
-      SystemContextRegistry.node,
+      SystemContextBuiltIns.node,
+      InstructionContext.node,
       SkillGuidance.node,
       ReferenceGuidance.node,
       Config.node,
@@ -129,7 +133,8 @@ const it = testEffect(
       [PermissionV2.node, permission],
       [ToolOutputStore.node, ToolOutputStore.nodeWithoutConfig],
       [SessionRunnerModel.node, models],
-      [SystemContextRegistry.node, systemContext],
+      [SystemContextBuiltIns.node, systemContext],
+      [InstructionContext.node, instructionContext],
       [Location.node, Location.boundNode({ directory: AbsolutePath.make("/project") })],
       [SkillGuidance.node, skillGuidance],
       [ReferenceGuidance.node, referenceGuidance],

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

@@ -25,7 +25,6 @@ import { QuestionV2 } from "@opencode-ai/core/question"
 import { AbsolutePath } from "@opencode-ai/core/schema"
 import { SessionV2 } from "@opencode-ai/core/session"
 import { Snapshot } from "@opencode-ai/core/snapshot"
-import { ContextSnapshotDecodeError } from "@opencode-ai/core/session/error"
 import { SessionEvent } from "@opencode-ai/core/session/event"
 import { SessionCompaction } from "@opencode-ai/core/session/compaction"
 import { SessionTitle } from "@opencode-ai/core/session/title"
@@ -46,14 +45,15 @@ import { Config } from "@opencode-ai/core/config"
 import { ConfigCompaction } from "@opencode-ai/core/config/compaction"
 import { Tool } from "@opencode-ai/core/tool/tool"
 import {
-  SessionContextEpochTable,
+  SessionContextCheckpointTable,
   SessionInputTable,
   SessionMessageTable,
   SessionTable,
 } from "@opencode-ai/core/session/sql"
 import { SessionStore } from "@opencode-ai/core/session/store"
 import { SystemContext } from "@opencode-ai/core/system-context"
-import { SystemContextRegistry } from "@opencode-ai/core/system-context/registry"
+import { SystemContextBuiltIns } from "@opencode-ai/core/system-context/builtins"
+import { InstructionContext } from "@opencode-ai/core/instruction-context"
 import { SkillGuidance } from "@opencode-ai/core/skill/guidance"
 import { ReferenceGuidance } from "@opencode-ai/core/reference/guidance"
 import { McpGuidance } from "@opencode-ai/core/mcp/guidance"
@@ -169,35 +169,28 @@ let systemRemoved = false
 let systemUnavailable = false
 let systemLoadHook = Effect.void
 const skillBaselines = new Map<AgentV2.ID, string>()
-const systemContext = Layer.effectDiscard(
-  SystemContextRegistry.Service.pipe(
-    Effect.flatMap((registry) =>
-      registry.register({
-        key: systemContextKey,
-        load: Effect.sync(() =>
-          SystemContext.combine(
-            systemRemoved
-              ? []
-              : [
-                  SystemContext.make({
-                    key: systemContextKey,
-                    codec: Schema.toCodecJson(Schema.String),
-                    load: systemLoadHook.pipe(
-                      Effect.andThen(
-                        Effect.sync(() => (systemUnavailable ? SystemContext.unavailable : systemBaseline)),
-                      ),
-                    ),
-                    baseline: String,
-                    update: (_previous, current) => current,
-                    removed: () => "System context source removed: test/context",
-                  }),
-                ],
-          ),
-        ),
-      }),
+const systemContext = Layer.mock(SystemContextBuiltIns.Service, {
+  load: () =>
+    Effect.sync(() =>
+      SystemContext.combine(
+        systemRemoved
+          ? []
+          : [
+              SystemContext.make({
+                key: systemContextKey,
+                codec: Schema.toCodecJson(Schema.String),
+                load: systemLoadHook.pipe(
+                  Effect.andThen(Effect.sync(() => (systemUnavailable ? SystemContext.unavailable : systemBaseline))),
+                ),
+                baseline: String,
+                update: (_previous, current) => current,
+                removed: () => "System context source removed: test/context",
+              }),
+            ],
+      ),
     ),
-  ),
-).pipe(Layer.provideMerge(AppNodeBuilder.build(SystemContextRegistry.node)))
+})
+const instructionContext = Layer.mock(InstructionContext.Service, { load: () => Effect.succeed(SystemContext.empty) })
 const skillGuidance = Layer.mock(SkillGuidance.Service, {
   load: (agent) =>
     Effect.succeed(
@@ -236,7 +229,8 @@ const runnerLayer = AppNodeBuilder.build(SessionRunnerLLM.node, [
   [Snapshot.node, Snapshot.noopLayer],
   [LayerNodePlatform.llmClient, client],
   [SessionRunnerModel.node, models],
-  [SystemContextRegistry.node, systemContext],
+  [SystemContextBuiltIns.node, systemContext],
+  [InstructionContext.node, instructionContext],
   [Location.node, Location.boundNode({ directory: AbsolutePath.make("/project") })],
   [SkillGuidance.node, skillGuidance],
   [ReferenceGuidance.node, referenceGuidance],
@@ -274,7 +268,8 @@ const it = testEffect(
       ToolRegistry.toolsNode,
       echoNode,
       SessionRunnerModel.node,
-      SystemContextRegistry.node,
+      SystemContextBuiltIns.node,
+      InstructionContext.node,
       SkillGuidance.node,
       ReferenceGuidance.node,
       Config.node,
@@ -287,7 +282,8 @@ const it = testEffect(
       [LayerNodePlatform.llmClient, client],
       [PermissionV2.node, permission],
       [SessionRunnerModel.node, models],
-      [SystemContextRegistry.node, systemContext],
+      [SystemContextBuiltIns.node, systemContext],
+      [InstructionContext.node, instructionContext],
       [Location.node, Location.boundNode({ directory: AbsolutePath.make("/project") })],
       [SkillGuidance.node, skillGuidance],
       [ReferenceGuidance.node, referenceGuidance],
@@ -699,8 +695,8 @@ describe("SessionRunnerLLM", () => {
       expect(
         yield* db
           .select()
-          .from(SessionContextEpochTable)
-          .where(eq(SessionContextEpochTable.session_id, sessionID))
+          .from(SessionContextCheckpointTable)
+          .where(eq(SessionContextCheckpointTable.session_id, sessionID))
           .get(),
       ).toBeUndefined()
 
@@ -731,8 +727,8 @@ describe("SessionRunnerLLM", () => {
       expect(
         yield* db
           .select()
-          .from(SessionContextEpochTable)
-          .where(eq(SessionContextEpochTable.session_id, sessionID))
+          .from(SessionContextCheckpointTable)
+          .where(eq(SessionContextCheckpointTable.session_id, sessionID))
           .get(),
       ).toBeUndefined()
 
@@ -745,7 +741,36 @@ describe("SessionRunnerLLM", () => {
     }),
   )
 
-  it.effect("fails gracefully when a stored context snapshot cannot be decoded", () =>
+  it.effect("copies the context checkpoint to a fork", () =>
+    Effect.gen(function* () {
+      yield* setup
+      const session = yield* SessionV2.Service
+      const { db } = yield* Database.Service
+      yield* session.prompt({ sessionID, prompt: Prompt.make({ text: "First" }), resume: false })
+      response = []
+      yield* session.resume(sessionID)
+
+      const forked = yield* session.fork({ sessionID })
+
+      const parent = yield* db
+        .select()
+        .from(SessionContextCheckpointTable)
+        .where(eq(SessionContextCheckpointTable.session_id, sessionID))
+        .get()
+        .pipe(Effect.orDie)
+      expect(parent).toBeDefined()
+      expect(
+        yield* db
+          .select()
+          .from(SessionContextCheckpointTable)
+          .where(eq(SessionContextCheckpointTable.session_id, forked.id))
+          .get()
+          .pipe(Effect.orDie),
+      ).toEqual({ ...parent!, session_id: forked.id })
+    }),
+  )
+
+  it.effect("heals an undecodable stored applied record by re-announcing context", () =>
     Effect.gen(function* () {
       yield* setup
       const session = yield* SessionV2.Service
@@ -754,19 +779,28 @@ describe("SessionRunnerLLM", () => {
       response = []
       yield* session.resume(sessionID)
       yield* db
-        .update(SessionContextEpochTable)
+        .update(SessionContextCheckpointTable)
         .set({ snapshot: { invalid: { value: "bad" } } })
-        .where(eq(SessionContextEpochTable.session_id, sessionID))
+        .where(eq(SessionContextCheckpointTable.session_id, sessionID))
         .run()
         .pipe(Effect.orDie)
       yield* session.prompt({ sessionID, prompt: Prompt.make({ text: "Second" }), resume: false })
       requests.length = 0
 
-      const exit = yield* session.resume(sessionID).pipe(Effect.exit)
+      yield* session.resume(sessionID)
 
-      expect(Exit.isFailure(exit)).toBe(true)
-      if (Exit.isFailure(exit)) expect(Cause.squash(exit.cause)).toBeInstanceOf(ContextSnapshotDecodeError)
-      expect(requests).toHaveLength(0)
+      // Comparison state was lost, so every source re-announces as new.
+      expect(requests).toHaveLength(1)
+      expect(requests[0]?.system.map((part) => part.text)).toEqual([defaultSystem, "Initial context"])
+      expect(requests[0]?.messages.map((message) => message.role)).toEqual(["user", "system", "user"])
+      expect(requests[0]?.messages.at(1)?.content).toEqual([{ type: "text", text: "Initial context" }])
+      const healed = yield* db
+        .select({ snapshot: SessionContextCheckpointTable.snapshot })
+        .from(SessionContextCheckpointTable)
+        .where(eq(SessionContextCheckpointTable.session_id, sessionID))
+        .get()
+        .pipe(Effect.orDie)
+      expect(healed?.snapshot).toEqual({ "test/context": { value: "Initial context", removed: expect.any(String) } })
     }),
   )
 
@@ -787,8 +821,8 @@ describe("SessionRunnerLLM", () => {
         [defaultSystem, "Initial context"],
         [defaultSystem, "Initial context"],
       ])
-      expect(requests[1]?.messages.map((message) => message.role)).toEqual(["user", "user", "system"])
-      expect(requests[1]?.messages.at(-1)?.content).toEqual([{ type: "text", text: "Changed context" }])
+      expect(requests[1]?.messages.map((message) => message.role)).toEqual(["user", "system", "user"])
+      expect(requests[1]?.messages.at(1)?.content).toEqual([{ type: "text", text: "Changed context" }])
       expect(yield* session.messages({ sessionID })).toHaveLength(3)
       const { db } = yield* Database.Service
       expect(
@@ -1049,8 +1083,8 @@ describe("SessionRunnerLLM", () => {
       yield* session.prompt({ sessionID, prompt: Prompt.make({ text: "Second" }), resume: false })
       yield* session.resume(sessionID)
 
-      expect(requests[1]?.messages.map((message) => message.role)).toEqual(["user", "user", "system"])
-      expect(requests[1]?.messages.at(-1)?.content).toEqual([
+      expect(requests[1]?.messages.map((message) => message.role)).toEqual(["user", "system", "user"])
+      expect(requests[1]?.messages.at(1)?.content).toEqual([
         { type: "text", text: "System context source removed: test/context" },
       ])
       expect(yield* session.messages({ sessionID })).toHaveLength(3)
@@ -1085,15 +1119,15 @@ describe("SessionRunnerLLM", () => {
         [defaultSystem, "Initial context"],
         [defaultSystem, "Initial context"],
       ])
-      expect(requests[1]?.messages.map((message) => message.role)).toEqual(["user", "user", "system"])
+      expect(requests[1]?.messages.map((message) => message.role)).toEqual(["user", "system", "user"])
       expect(requests[2]?.messages.filter((message) => message.role === "system")).toHaveLength(2)
       expect((yield* session.context(sessionID)).map((message) => message.type)).toEqual([
-        "user",
         "user",
         "system",
+        "user",
         "model-switched",
-        "user",
         "system",
+        "user",
       ])
       yield* replaySessionProjection(sessionID)
       expect(yield* session.messages({ sessionID })).toHaveLength(6)
@@ -1361,7 +1395,7 @@ describe("SessionRunnerLLM", () => {
     }),
   )
 
-  it.effect("preserves effective System updates while compaction rebaseline is blocked", () =>
+  it.effect("rebaselines after compaction from the last-applied belief while unobservable", () =>
     Effect.gen(function* () {
       yield* setup
       const session = yield* SessionV2.Service
@@ -1393,8 +1427,9 @@ describe("SessionRunnerLLM", () => {
       yield* session.prompt({ sessionID, prompt: Prompt.make({ text: "Third" }), resume: false })
       yield* session.resume(sessionID)
 
-      expect(requests.at(-1)?.system.map((part) => part.text)).toEqual([defaultSystem, "Initial context"])
-      expect(systemTexts(requests.at(-1)!)).toContain("Changed context")
+      // The rebaseline proceeds while the source is unobservable, restating the model's belief.
+      expect(requests.at(-1)?.system.map((part) => part.text)).toEqual([defaultSystem, "Changed context"])
+      expect(systemTexts(requests.at(-1)!)).not.toContain("Changed context")
     }),
   )
 

packages/core/test/skill/guidance.test.ts

@@ -53,7 +53,7 @@ describe("SkillGuidance", () => {
         .load({ id: agent.id, info: agent })
         .pipe(Effect.flatMap(SystemContext.initialize))
 
-      expect(initialized.baseline).toBe(
+      expect(initialized.text).toBe(
         [
           "Skills provide specialized instructions and workflows for specific tasks.",
           "Use the skill tool to load a skill when a task matches its description.",
@@ -65,16 +65,82 @@ describe("SkillGuidance", () => {
           "</available_skills>",
         ].join("\n"),
       )
-      expect(initialized.baseline).not.toContain("manual")
+      expect(initialized.text).not.toContain("manual")
 
       skills = []
       expect(
         yield* guidance
           .load({ id: agent.id, info: agent })
-          .pipe(Effect.flatMap((context) => SystemContext.reconcile(context, initialized.snapshot))),
+          .pipe(Effect.flatMap((context) => SystemContext.reconcile(context, initialized.applied))),
       ).toMatchObject({
         _tag: "Updated",
-        text: expect.stringContaining("No skills are currently available."),
+        text: "The following skills are no longer available and must not be used: effect.",
+      })
+    }).pipe(Effect.provide(layer(() => skills)))
+  })
+
+  it.effect("announces added and removed skills as deltas without restating the list", () => {
+    const agent = AgentV2.Info.make(AgentV2.Info.empty(build))
+    const debugging = SkillV2.Info.make({
+      name: "debugging",
+      description: "Diagnose hard bugs",
+      location: AbsolutePath.make(path.resolve("/skills/debugging/SKILL.md")),
+      content: "Debugging guidance",
+    })
+    let skills = [effect]
+    return Effect.gen(function* () {
+      const guidance = yield* SkillGuidance.Service
+      const initialized = yield* guidance
+        .load({ id: agent.id, info: agent })
+        .pipe(Effect.flatMap(SystemContext.initialize))
+
+      skills = [effect, debugging]
+      const added = yield* guidance
+        .load({ id: agent.id, info: agent })
+        .pipe(Effect.flatMap((context) => SystemContext.reconcile(context, initialized.applied)))
+      expect(added).toMatchObject({
+        _tag: "Updated",
+        text: [
+          "New skills are available in addition to those previously listed:",
+          "  <skill>",
+          "    <name>debugging</name>",
+          "    <description>Diagnose hard bugs</description>",
+          "  </skill>",
+        ].join("\n"),
+      })
+
+      skills = [debugging]
+      const removed = yield* guidance
+        .load({ id: agent.id, info: agent })
+        .pipe(
+          Effect.flatMap((context) => SystemContext.reconcile(context, added._tag === "Updated" ? added.applied : {})),
+        )
+      expect(removed).toMatchObject({
+        _tag: "Updated",
+        text: "The following skills are no longer available and must not be used: effect.",
+      })
+    }).pipe(Effect.provide(layer(() => skills)))
+  })
+
+  it.effect("restates the full skill list when a description changes", () => {
+    const agent = AgentV2.Info.make(AgentV2.Info.empty(build))
+    let skills = [effect]
+    return Effect.gen(function* () {
+      const guidance = yield* SkillGuidance.Service
+      const initialized = yield* guidance
+        .load({ id: agent.id, info: agent })
+        .pipe(Effect.flatMap(SystemContext.initialize))
+
+      skills = [SkillV2.Info.make({ ...effect, description: "Build applications with Effect v4" })]
+      expect(
+        yield* guidance
+          .load({ id: agent.id, info: agent })
+          .pipe(Effect.flatMap((context) => SystemContext.reconcile(context, initialized.applied))),
+      ).toMatchObject({
+        _tag: "Updated",
+        text: expect.stringContaining(
+          "The available skills have changed. This list supersedes the previous available skills list.",
+        ),
       })
     }).pipe(Effect.provide(layer(() => skills)))
   })
@@ -89,8 +155,8 @@ describe("SkillGuidance", () => {
       expect(
         yield* guidance.load({ id: agent.id, info: agent }).pipe(Effect.flatMap(SystemContext.initialize)),
       ).toEqual({
-        baseline: "",
-        snapshot: {},
+        text: "",
+        applied: {},
       })
     }).pipe(Effect.provide(layer(() => [effect])))
   })
@@ -108,8 +174,8 @@ describe("SkillGuidance", () => {
       expect(
         yield* guidance.load({ id: agent.id, info: agent }).pipe(Effect.flatMap(SystemContext.initialize)),
       ).toEqual({
-        baseline: "",
-        snapshot: {},
+        text: "",
+        applied: {},
       })
     }).pipe(Effect.provide(layer(() => [effect])))
   })
@@ -125,7 +191,7 @@ describe("SkillGuidance", () => {
     return Effect.gen(function* () {
       const guidance = yield* SkillGuidance.Service
       expect(
-        (yield* guidance.load({ id: agent.id, info: agent }).pipe(Effect.flatMap(SystemContext.initialize))).baseline,
+        (yield* guidance.load({ id: agent.id, info: agent }).pipe(Effect.flatMap(SystemContext.initialize))).text,
       ).toContain("<name>effect</name>")
     }).pipe(Effect.provide(layer(() => [effect])))
   })
@@ -144,8 +210,8 @@ describe("SkillGuidance", () => {
       expect(
         yield* guidance.load({ id: agent.id, info: agent }).pipe(Effect.flatMap(SystemContext.initialize)),
       ).toEqual({
-        baseline: "",
-        snapshot: {},
+        text: "",
+        applied: {},
       })
     }).pipe(Effect.provide(layer(() => [effect])))
   })

packages/core/test/system-context/builtins.test.ts

@@ -9,7 +9,7 @@ import { Global } from "@opencode-ai/core/global"
 import { AbsolutePath } from "@opencode-ai/core/schema"
 import { SystemContext } from "@opencode-ai/core/system-context"
 import { SystemContextBuiltIns } from "@opencode-ai/core/system-context/builtins"
-import { SystemContextRegistry } from "@opencode-ai/core/system-context/registry"
+import { InstructionContext } from "@opencode-ai/core/instruction-context"
 import { location } from "../fixture/location"
 import { testEffect } from "../lib/effect"
 
@@ -27,7 +27,7 @@ const locationLayer = Layer.succeed(
     ),
   ),
 )
-const builtInsNode = LayerNode.group([SystemContextBuiltIns.node, SystemContextRegistry.node])
+const builtInsNode = LayerNode.group([SystemContextBuiltIns.node, InstructionContext.node])
 const it = testEffect(
   AppNodeBuilder.build(builtInsNode, [
     [Location.node, locationLayer],
@@ -58,10 +58,10 @@ describe("SystemContextBuiltIns", () => {
   it.effect("loads location-scoped environment and host-local date context", () =>
     Effect.gen(function* () {
       yield* TestClock.setTime(timestamp)
-      const context = yield* SystemContextRegistry.Service
+      const context = yield* SystemContextBuiltIns.Service
       const initialized = yield* SystemContext.initialize(yield* context.load())
 
-      expect(initialized.baseline).toBe(
+      expect(initialized.text).toBe(
         [
           "Here is some useful information about the environment you are running in:",
           "<env>",
@@ -80,11 +80,11 @@ describe("SystemContextBuiltIns", () => {
   it.effect("reconciles the date without repeating unchanged environment context", () =>
     Effect.gen(function* () {
       yield* TestClock.setTime(timestamp)
-      const context = yield* SystemContextRegistry.Service
+      const context = yield* SystemContextBuiltIns.Service
       const initialized = yield* SystemContext.initialize(yield* context.load())
 
       yield* TestClock.setTime(timestamp + 24 * 60 * 60 * 1000)
-      const refreshed = yield* SystemContext.reconcile(yield* context.load(), initialized.snapshot)
+      const refreshed = yield* SystemContext.reconcile(yield* context.load(), initialized.applied)
 
       expect(refreshed).toMatchObject({
         _tag: "Updated",
@@ -96,20 +96,24 @@ describe("SystemContextBuiltIns", () => {
   it.effect("does not update again within the same local calendar day", () =>
     Effect.gen(function* () {
       yield* TestClock.setTime(timestamp)
-      const context = yield* SystemContextRegistry.Service
+      const context = yield* SystemContextBuiltIns.Service
       const initialized = yield* SystemContext.initialize(yield* context.load())
 
       yield* TestClock.setTime(timestamp + 60 * 60 * 1000)
-      expect(yield* SystemContext.reconcile(yield* context.load(), initialized.snapshot)).toEqual({ _tag: "Unchanged" })
+      expect(yield* SystemContext.reconcile(yield* context.load(), initialized.applied)).toEqual({ _tag: "Unchanged" })
     }),
   )
 
   itWithInstructions.effect("composes ambient instructions after built-in context", () =>
     Effect.gen(function* () {
       yield* TestClock.setTime(timestamp)
-      const context = yield* SystemContextRegistry.Service
+      const builtIns = yield* SystemContextBuiltIns.Service
+      const instructions = yield* InstructionContext.Service
+      const context = {
+        load: () => Effect.all([builtIns.load(), instructions.load()]).pipe(Effect.map(SystemContext.combine)),
+      }
 
-      expect((yield* SystemContext.initialize(yield* context.load())).baseline).toBe(
+      expect((yield* SystemContext.initialize(yield* context.load())).text).toBe(
         [
           "Here is some useful information about the environment you are running in:",
           "<env>",

packages/core/test/system-context/index.test.ts

@@ -32,11 +32,11 @@ describe("SystemContext", () => {
         removed: () => "Date removed",
       })
 
-      expect((yield* SystemContext.initialize(context)).snapshot["core/date"].value).toBe("2026-06-03T12:00:00.000Z")
+      expect((yield* SystemContext.initialize(context)).applied["core/date"].value).toBe("2026-06-03T12:00:00.000Z")
     }),
   )
 
-  it.effect("loads once and initializes a baseline with a structured snapshot", () =>
+  it.effect("loads once and initializes a baseline with the applied values", () =>
     Effect.gen(function* () {
       let loads = 0
       const context = SystemContext.combine([
@@ -55,8 +55,8 @@ describe("SystemContext", () => {
       ])
 
       expect(yield* SystemContext.initialize(context)).toEqual({
-        baseline: "Today's date is 2026-06-03.\n\nDirectory: /repo",
-        snapshot: {
+        text: "Today's date is 2026-06-03.\n\nDirectory: /repo",
+        applied: {
           "core/date": { value: "2026-06-03", removed: "The date was removed." },
           "core/location": { value: "/repo" },
         },
@@ -84,7 +84,7 @@ describe("SystemContext", () => {
       expect(yield* SystemContext.reconcile(changed, previous)).toEqual({
         _tag: "Updated",
         text: "The date changed from 2026-06-03 to 2026-06-04.",
-        snapshot: {
+        applied: {
           "core/date": { value: "2026-06-04", removed: "The date was removed." },
           "core/location": { value: "/repo", removed: "Removed: /repo" },
         },
@@ -113,19 +113,17 @@ describe("SystemContext", () => {
       expect(yield* SystemContext.reconcile(context, {})).toEqual({
         _tag: "Updated",
         text: "Available skill: effect",
-        snapshot: { "core/skills": { value: "effect" } },
+        applied: { "core/skills": { value: "effect" } },
       })
     }),
   )
 
-  it.effect("retains admitted snapshots while a source is temporarily unavailable", () =>
+  it.effect("retains the belief while a source is temporarily unavailable", () =>
     Effect.gen(function* () {
       const previous = { "core/remote": { value: "instructions", removed: "Instructions removed" } }
       const context = stringContext({ key: "core/remote", value: SystemContext.unavailable })
 
       expect(yield* SystemContext.reconcile(context, previous)).toEqual({ _tag: "Unchanged" })
-      expect(yield* SystemContext.replace(context, previous)).toEqual({ _tag: "ReplacementBlocked" })
-      expect(yield* SystemContext.replace(context, {})).toMatchObject({ _tag: "ReplacementReady" })
     }),
   )
 
@@ -152,17 +150,29 @@ describe("SystemContext", () => {
       ).toEqual({
         _tag: "Updated",
         text: "Instructions removed; stop applying them.",
-        snapshot: {},
+        applied: {},
       })
     }),
   )
 
-  it.effect("requests replacement when a source without removal text disappears", () =>
+  it.effect("retains an unannounced removal silently", () =>
     Effect.gen(function* () {
+      expect(yield* SystemContext.reconcile(SystemContext.empty, { "core/date": { value: "2026-06-04" } })).toEqual({
+        _tag: "Unchanged",
+      })
+
+      // The retained belief survives alongside other updates.
       expect(
-        yield* SystemContext.reconcile(SystemContext.empty, { "core/date": { value: "2026-06-04" } }),
-      ).toMatchObject({
-        _tag: "ReplacementReady",
+        yield* SystemContext.reconcile(stringContext({ key: "core/skills", value: "effect" }), {
+          "core/date": { value: "2026-06-04" },
+        }),
+      ).toEqual({
+        _tag: "Updated",
+        text: "effect",
+        applied: {
+          "core/skills": { value: "effect" },
+          "core/date": { value: "2026-06-04" },
+        },
       })
     }),
   )
@@ -189,17 +199,48 @@ describe("SystemContext", () => {
     }),
   )
 
-  it.effect("requests replacement when a stored value no longer decodes", () =>
+  it.effect("re-announces the baseline when a stored value no longer decodes", () =>
     Effect.gen(function* () {
       expect(
         yield* SystemContext.reconcile(stringContext({ key: "core/date", value: "2026-06-04" }), {
           "core/date": { value: 42, removed: "Date removed" },
         }),
-      ).toMatchObject({ _tag: "ReplacementReady" })
+      ).toEqual({
+        _tag: "Updated",
+        text: "2026-06-04",
+        applied: { "core/date": { value: "2026-06-04" } },
+      })
     }),
   )
 
-  it.effect("replaces from one coherent source observation", () =>
+  it.effect("renders undecodable re-announcements alongside other updates", () =>
+    Effect.gen(function* () {
+      const context = SystemContext.combine([
+        stringContext({
+          key: "core/date",
+          value: "2026-06-04",
+          update: (before, current) => `${before} -> ${current}`,
+        }),
+        stringContext({ key: "core/location", value: "/repo" }),
+      ])
+
+      expect(
+        yield* SystemContext.reconcile(context, {
+          "core/date": { value: "2026-06-03" },
+          "core/location": { value: 42 },
+        }),
+      ).toEqual({
+        _tag: "Updated",
+        text: "2026-06-03 -> 2026-06-04\n\n/repo",
+        applied: {
+          "core/date": { value: "2026-06-04" },
+          "core/location": { value: "/repo" },
+        },
+      })
+    }),
+  )
+
+  it.effect("rebaselines from one coherent source observation", () =>
     Effect.gen(function* () {
       let loads = 0
       const context = SystemContext.make({
@@ -213,52 +254,83 @@ describe("SystemContext", () => {
         update: (_previous, current) => current,
       })
 
-      expect(yield* SystemContext.reconcile(context, { "core/date": { value: 42 } })).toMatchObject({
-        _tag: "ReplacementReady",
-        generation: { baseline: "2026-06-04" },
+      expect(yield* SystemContext.rebaseline(context, { "core/date": { value: "2026-06-03" } })).toEqual({
+        text: "2026-06-04",
+        applied: { "core/date": { value: "2026-06-04" } },
       })
       expect(loads).toBe(1)
     }),
   )
 
-  it.effect("does not render discarded updates while replacing", () =>
+  it.effect("rebaselines an unavailable source from the last-applied belief", () =>
     Effect.gen(function* () {
-      let updates = 0
       const context = SystemContext.combine([
+        stringContext({ key: "core/date", value: "2026-06-04" }),
         stringContext({
-          key: "core/date",
-          value: "2026-06-04",
-          update: () => {
-            updates++
-            return "updated"
-          },
+          key: "core/remote",
+          value: SystemContext.unavailable,
+          baseline: (value) => `Instructions: ${value}`,
         }),
-        stringContext({ key: "core/location", value: "/repo" }),
       ])
 
       expect(
-        yield* SystemContext.reconcile(context, {
-          "core/date": { value: "2026-06-03" },
-          "core/location": { value: 42 },
+        yield* SystemContext.rebaseline(context, {
+          "core/remote": { value: "contents", removed: "Instructions removed" },
         }),
-      ).toMatchObject({ _tag: "ReplacementReady" })
-      expect(updates).toBe(0)
+      ).toEqual({
+        text: "2026-06-04\n\nInstructions: contents",
+        applied: {
+          "core/date": { value: "2026-06-04" },
+          "core/remote": { value: "contents", removed: "Instructions removed" },
+        },
+      })
     }),
   )
 
-  it.effect("blocks an incompatible replacement while another admitted source is unavailable", () =>
+  it.effect("drops undecodable beliefs and removed sources at rebaseline", () =>
     Effect.gen(function* () {
-      const previous = {
-        "core/date": { value: 42, removed: "Date removed" },
-        "core/remote": { value: "instructions", removed: "Instructions removed" },
-      }
-      const context = SystemContext.combine([
-        stringContext({ key: "core/date", value: "2026-06-04" }),
-        stringContext({ key: "core/remote", value: SystemContext.unavailable }),
-      ])
+      const context = stringContext({ key: "core/remote", value: SystemContext.unavailable })
 
-      expect(yield* SystemContext.reconcile(context, previous)).toEqual({ _tag: "ReplacementBlocked" })
-      expect(yield* SystemContext.replace(context, previous)).toEqual({ _tag: "ReplacementBlocked" })
+      // Undecodable belief cannot be restated; removed source entries self-clean.
+      expect(
+        yield* SystemContext.rebaseline(context, {
+          "core/remote": { value: 42 },
+          "core/gone": { value: "gone" },
+        }),
+      ).toEqual({ text: "", applied: {} })
+    }),
+  )
+
+  it.effect("diffs list values by key with a changed comparator", () =>
+    Effect.sync(() => {
+      const previous = [
+        { name: "effect", description: "Build with Effect" },
+        { name: "debugging", description: "Diagnose bugs" },
+        { name: "retired", description: "Old" },
+      ]
+      const current = [
+        { name: "effect", description: "Build with Effect v4" },
+        { name: "debugging", description: "Diagnose bugs" },
+        { name: "writing", description: "Write prose" },
+      ]
+
+      expect(
+        SystemContext.diffByKey(
+          previous,
+          current,
+          (value) => value.name,
+          (before, after) => before.description !== after.description,
+        ),
+      ).toEqual({
+        added: [{ name: "writing", description: "Write prose" }],
+        removed: [{ name: "retired", description: "Old" }],
+        changed: [
+          {
+            previous: { name: "effect", description: "Build with Effect" },
+            current: { name: "effect", description: "Build with Effect v4" },
+          },
+        ],
+      })
     }),
   )
 
@@ -281,7 +353,7 @@ describe("SystemContext", () => {
             stringContext({ key: "core/date", value: "date" }),
             stringContext({ key: "core/location", value: "location" }),
           ]),
-        )).baseline,
+        )).text,
       ).toBe("date\n\nlocation")
     }),
   )
@@ -295,13 +367,13 @@ describe("SystemContext", () => {
     }),
   )
 
-  it.effect("requires namespaced durable snapshot keys", () =>
+  it.effect("requires namespaced applied keys", () =>
     Effect.sync(() => {
-      const decodeSnapshot = Schema.decodeUnknownSync(SystemContext.Snapshot)
+      const decodeApplied = Schema.decodeUnknownSync(SystemContext.Applied)
 
-      expect(Object.keys(decodeSnapshot({ "core/date": { value: "date" } }))).toEqual(["core/date"])
-      expect(() => decodeSnapshot({ date: { value: "date" } })).toThrow()
-      expect(() => decodeSnapshot({ "core/date": { value: "date", removed: "" } })).toThrow()
+      expect(Object.keys(decodeApplied({ "core/date": { value: "date" } }))).toEqual(["core/date"])
+      expect(() => decodeApplied({ date: { value: "date" } })).toThrow()
+      expect(() => decodeApplied({ "core/date": { value: "date", removed: "" } })).toThrow()
     }),
   )
 })

packages/core/test/system-context/registry.test.ts

@@ -1,114 +0,0 @@
-import { describe, expect } from "bun:test"
-import { Cause, Effect, Exit, Schema, Scope } from "effect"
-import { AppNodeBuilder } from "@opencode-ai/core/effect/app-node-builder"
-import { SystemContext } from "@opencode-ai/core/system-context"
-import { SystemContextRegistry } from "@opencode-ai/core/system-context/registry"
-import { testEffect } from "../lib/effect"
-
-const entry = (key: string, text: string, sourceKey = key) => ({
-  key: SystemContext.Key.make(key),
-  load: Effect.succeed(
-    SystemContext.make({
-      key: SystemContext.Key.make(sourceKey),
-      codec: Schema.toCodecJson(Schema.String),
-      load: Effect.succeed(text),
-      baseline: String,
-      update: (_previous, current) => current,
-    }),
-  ),
-})
-
-const it = testEffect(AppNodeBuilder.build(SystemContextRegistry.node))
-
-describe("SystemContextRegistry", () => {
-  it.effect("loads empty system context when there are no entries", () =>
-    Effect.gen(function* () {
-      const registry = yield* SystemContextRegistry.Service
-
-      expect(yield* SystemContext.initialize(yield* registry.load())).toEqual({ baseline: "", snapshot: {} })
-    }),
-  )
-
-  it.effect("loads scoped entries in stable key order", () =>
-    Effect.gen(function* () {
-      const registry = yield* SystemContextRegistry.Service
-      yield* registry.register(entry("test/second", "second"))
-      yield* registry.register(entry("test/first", "first"))
-
-      expect((yield* SystemContext.initialize(yield* registry.load())).baseline).toBe("first\n\nsecond")
-    }),
-  )
-
-  it.effect("re-evaluates entry producers on each load", () =>
-    Effect.gen(function* () {
-      const registry = yield* SystemContextRegistry.Service
-      let loads = 0
-      yield* registry.register({
-        key: SystemContext.Key.make("test/dynamic"),
-        load: Effect.sync(() => {
-          loads++
-          return SystemContext.empty
-        }),
-      })
-
-      yield* registry.load()
-      yield* registry.load()
-
-      expect(loads).toBe(2)
-    }),
-  )
-
-  it.effect("propagates entry producer failures", () =>
-    Effect.gen(function* () {
-      const registry = yield* SystemContextRegistry.Service
-      const failure = new Error("entry failed")
-      yield* registry.register({ key: SystemContext.Key.make("test/failure"), load: Effect.die(failure) })
-
-      const exit = yield* registry.load().pipe(Effect.exit)
-
-      expect(Exit.isFailure(exit)).toBe(true)
-      if (Exit.isFailure(exit)) expect(Cause.squash(exit.cause)).toBe(failure)
-    }),
-  )
-
-  it.effect("rejects duplicate source keys from separate entries", () =>
-    Effect.gen(function* () {
-      const registry = yield* SystemContextRegistry.Service
-      yield* registry.register(entry("test/first", "first", "test/duplicate"))
-      yield* registry.register(entry("test/second", "second", "test/duplicate"))
-
-      const exit = yield* registry.load().pipe(Effect.exit)
-
-      expect(Exit.isFailure(exit)).toBe(true)
-      if (Exit.isFailure(exit)) {
-        expect(Cause.squash(exit.cause)).toBeInstanceOf(SystemContext.DuplicateKeyError)
-        expect(Cause.squash(exit.cause)).toMatchObject({ key: SystemContext.Key.make("test/duplicate") })
-      }
-    }),
-  )
-
-  it.effect("rejects duplicate entry keys", () =>
-    Effect.gen(function* () {
-      const registry = yield* SystemContextRegistry.Service
-      yield* registry.register(entry("test/duplicate", "first"))
-
-      const exit = yield* registry.register(entry("test/duplicate", "second", "test/other")).pipe(Effect.exit)
-
-      expect(Exit.isFailure(exit)).toBe(true)
-      if (Exit.isFailure(exit)) expect(Cause.pretty(exit.cause)).toContain("Duplicate system context entry key")
-    }),
-  )
-
-  it.effect("removes an entry when its owning scope closes", () =>
-    Effect.gen(function* () {
-      const registry = yield* SystemContextRegistry.Service
-      const scope = yield* Scope.make()
-      yield* registry.register(entry("test/scoped", "scoped")).pipe(Scope.provide(scope))
-
-      expect((yield* SystemContext.initialize(yield* registry.load())).baseline).toBe("scoped")
-
-      yield* Scope.close(scope, Exit.void)
-      expect(yield* SystemContext.initialize(yield* registry.load())).toEqual({ baseline: "", snapshot: {} })
-    }),
-  )
-})

packages/llm/src/protocols/anthropic-messages.ts

@@ -508,11 +508,13 @@ const lowerThinking = Effect.fn("AnthropicMessages.lowerThinking")(function* (re
   const thinking = anthropicOptions(request)?.thinking
   if (!ProviderShared.isRecord(thinking)) return undefined
   if (thinking.type === "adaptive") {
-    const display = thinking.display
-    return {
-      type: "adaptive" as const,
-      ...(display === "summarized" || display === "omitted" ? { display } : {}),
-    }
+    const display =
+      thinking.display === "summarized"
+        ? ("summarized" as const)
+        : thinking.display === "omitted"
+          ? ("omitted" as const)
+          : undefined
+    return { type: "adaptive" as const, ...(display === undefined ? {} : { display }) }
   }
   if (thinking.type === "disabled") return { type: "disabled" as const }
   if (thinking.type !== "enabled") return undefined