From b0ca5520a1a03f6227efa0441cd185995ac82e68 Mon Sep 17 00:00:00 2001
From: James Long <longster@gmail.com>
Date: Thu, 2 Jul 2026 16:45:09 -0400
Subject: [PATCH] feat(server): add lazy simulation layer replacements (#35024)

---
 packages/server/src/routes.ts           | 33 +++++++++++++++++--------
 packages/server/src/simulation/index.ts | 14 +++++++++++
 2 files changed, 37 insertions(+), 10 deletions(-)
 create mode 100644 packages/server/src/simulation/index.ts

diff --git a/packages/server/src/routes.ts b/packages/server/src/routes.ts
index 4d5fe537d75..0cb76573deb 100644
--- a/packages/server/src/routes.ts
+++ b/packages/server/src/routes.ts
@@ -16,7 +16,7 @@ import { SdkPlugins } from "@opencode-ai/core/plugin/sdk"
 import { ToolOutputStore } from "@opencode-ai/core/tool-output-store"
 import { HttpRouter, HttpServer } from "effect/unstable/http"
 import { HttpApiBuilder } from "effect/unstable/httpapi"
-import { Layer, Option } from "effect"
+import { Effect, Layer, Option } from "effect"
 import { Api } from "./api"
 import { ServerAuth } from "./auth"
 import { handlers } from "./handlers"
@@ -58,15 +58,24 @@ function makeRoutes<AuthError, AuthServices>(
   sdkPlugins?: SdkPlugins.Store,
 ) {
   const pluginRuntimeCell = PluginRuntime.makeCell()
-  const serviceLayer = AppNodeBuilder.build(
-    applicationServices,
-    [
-      [SessionExecution.node, SessionExecutionLocal.node],
-      [PluginRuntime.node, PluginRuntime.layerWithCell(pluginRuntimeCell)],
-      [PluginRuntime.providerNode, PluginRuntime.providerNodeWithCell(pluginRuntimeCell)],
-      ...(sdkPlugins ? [[SdkPlugins.node, SdkPlugins.layerWithStore(sdkPlugins)] as const] : []),
-    ],
-  )
+  const replacements: LayerNode.Replacements = [
+    [SessionExecution.node, SessionExecutionLocal.node],
+    [PluginRuntime.node, PluginRuntime.layerWithCell(pluginRuntimeCell)],
+    [PluginRuntime.providerNode, PluginRuntime.providerNodeWithCell(pluginRuntimeCell)],
+    ...(sdkPlugins ? [[SdkPlugins.node, SdkPlugins.layerWithStore(sdkPlugins)] as const] : []),
+  ]
+  // Simulation replacements are loaded via dynamic import so the simulation
+  // module is never eagerly loaded. Layer.unwrap defers both the import and
+  // the app-node build to layer-build time; when simulation is off the branch
+  // is byte-for-byte identical to a plain AppNodeBuilder.build call.
+  const serviceLayer = simulationEnabled()
+    ? Layer.unwrap(
+        Effect.gen(function* () {
+          const { simulationReplacements } = yield* Effect.promise(() => import("./simulation"))
+          return AppNodeBuilder.build(applicationServices, [...replacements, ...simulationReplacements])
+        }),
+      )
+    : AppNodeBuilder.build(applicationServices, replacements)
 
   return HttpApiBuilder.layer(Api, { openapiPath: "/openapi.json" }).pipe(
     Layer.provide(handlers),
@@ -79,6 +88,10 @@ function makeRoutes<AuthError, AuthServices>(
   )
 }
 
+function simulationEnabled() {
+  return !!process.env.OPENCODE_SIMULATION
+}
+
 export const routes = createRoutes()
 
 export const webHandler = () =>
diff --git a/packages/server/src/simulation/index.ts b/packages/server/src/simulation/index.ts
new file mode 100644
index 00000000000..3f499a5bf81
--- /dev/null
+++ b/packages/server/src/simulation/index.ts
@@ -0,0 +1,14 @@
+import { LayerNode } from "@opencode-ai/core/effect/layer-node"
+
+/**
+ * Layer replacements applied when the server is built in simulation mode.
+ *
+ * Empty for now; simulation-mode implementations will populate this with
+ * replacement nodes/layers that swap real services for simulated ones (e.g.
+ * a fake filesystem). The server merges these into the app node build when
+ * `OPENCODE_SIMULATION` is enabled, via a dynamic import so this module is
+ * never loaded eagerly.
+ */
+export const simulationReplacements: LayerNode.Replacements = []
+
+export * as Simulation from "./index"