From 816f5e526bbc5f2e8ce46d9e8d87f879275d22ef Mon Sep 17 00:00:00 2001
From: James Long <longster@gmail.com>
Date: Fri, 15 May 2026 15:33:00 -0400
Subject: [PATCH] tool for writing files

---
 .../opencode/src/cli/cmd/tui/simulation-mcp.ts     |  8 ++++++++
 .../server/routes/instance/httpapi/simulation.ts   |  9 +++++++++
 .../opencode/src/testing/simulation/service.ts     | 14 +++++++++++++-
 .../test/testing/simulation/service.test.ts        | 11 +++++++++++
 4 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/packages/opencode/src/cli/cmd/tui/simulation-mcp.ts b/packages/opencode/src/cli/cmd/tui/simulation-mcp.ts
index b89de021b07..6e433563c6b 100644
--- a/packages/opencode/src/cli/cmd/tui/simulation-mcp.ts
+++ b/packages/opencode/src/cli/cmd/tui/simulation-mcp.ts
@@ -300,6 +300,14 @@ function createServer(options: Options) {
     },
     async (input) => toolResult(await control(options, "POST", "/experimental/simulation/filesystem/seed", input)),
   )
+  server.registerTool(
+    "simulation_control_filesystem_write",
+    {
+      description: "Write one file into the backend simulated filesystem.",
+      inputSchema: z.object({ path: z.string(), content: FileContentSchema }),
+    },
+    async (input) => toolResult(await control(options, "POST", "/experimental/simulation/filesystem/write", input)),
+  )
   server.registerTool(
     "simulation_control_network_register",
     {
diff --git a/packages/opencode/src/server/routes/instance/httpapi/simulation.ts b/packages/opencode/src/server/routes/instance/httpapi/simulation.ts
index 01902f059be..b2c1027215b 100644
--- a/packages/opencode/src/server/routes/instance/httpapi/simulation.ts
+++ b/packages/opencode/src/server/routes/instance/httpapi/simulation.ts
@@ -28,6 +28,15 @@ export const simulationRoute = HttpRouter.use((router) =>
       ),
     )
 
+    yield* router.add("POST", "/experimental/simulation/filesystem/write", () =>
+      json(
+        Effect.gen(function* () {
+          const input = yield* HttpServerRequest.schemaBodyJson(Simulation.FilesystemWriteInput)
+          return yield* simulation.writeFile(input)
+        }),
+      ),
+    )
+
     yield* router.add("POST", "/experimental/simulation/network/register", () =>
       json(
         Effect.gen(function* () {
diff --git a/packages/opencode/src/testing/simulation/service.ts b/packages/opencode/src/testing/simulation/service.ts
index 36e90e159ff..ce798a8e4c7 100644
--- a/packages/opencode/src/testing/simulation/service.ts
+++ b/packages/opencode/src/testing/simulation/service.ts
@@ -12,6 +12,11 @@ export const FilesystemSeedInput = Schema.Struct({
   files: Schema.Record(Schema.String, FileContent),
 })
 
+export const FilesystemWriteInput = Schema.Struct({
+  path: Schema.String,
+  content: FileContent,
+})
+
 export const NetworkRegisterInput = Schema.Union([
   Schema.Struct({
     kind: Schema.Literal("json"),
@@ -80,6 +85,7 @@ export class SimulationLLMError extends Schema.TaggedErrorClass<SimulationLLMErr
 export interface Interface {
   readonly reset: () => Effect.Effect<void>
   readonly seedFilesystem: (input: typeof FilesystemSeedInput.Type) => Effect.Effect<{ files: string[] }, unknown>
+  readonly writeFile: (input: typeof FilesystemWriteInput.Type) => Effect.Effect<{ file: string }, unknown>
   readonly registerNetwork: (input: typeof NetworkRegisterInput.Type) => Effect.Effect<{ registered: string }, unknown>
   readonly enqueueLLM: (input: typeof LLMEnqueueInput.Type) => Effect.Effect<{ queued: number }>
   readonly nextLLM: () => Effect.Effect<LLMScript, SimulationLLMError>
@@ -131,6 +137,12 @@ export const layer = Layer.effect(
       return { files }
     })
 
+    const writeFile = Effect.fn("Simulation.writeFile")(function* (input: typeof FilesystemWriteInput.Type) {
+      yield* fs.writeWithDirs(path.isAbsolute(input.path) ? input.path : path.join("/opencode", input.path), fileContent(input.content))
+      yield* Ref.update(state, (current) => ({ ...current, files: [...current.files, input.path] }))
+      return { file: input.path }
+    })
+
     const registerNetwork = Effect.fn("Simulation.registerNetwork")(function* (input: typeof NetworkRegisterInput.Type) {
       switch (input.kind) {
         case "json":
@@ -178,7 +190,7 @@ export const layer = Layer.effect(
       }
     })
 
-    return Service.of({ reset, seedFilesystem, registerNetwork, enqueueLLM, nextLLM, snapshot })
+    return Service.of({ reset, seedFilesystem, writeFile, registerNetwork, enqueueLLM, nextLLM, snapshot })
   }),
 )
 
diff --git a/packages/opencode/test/testing/simulation/service.test.ts b/packages/opencode/test/testing/simulation/service.test.ts
index f099cc687f4..ceb1cad01b1 100644
--- a/packages/opencode/test/testing/simulation/service.test.ts
+++ b/packages/opencode/test/testing/simulation/service.test.ts
@@ -28,6 +28,17 @@ describe("Simulation", () => {
     }),
   )
 
+  it.effect("writes one file into the simulated filesystem", () =>
+    Effect.gen(function* () {
+      const simulation = yield* Simulation.Service
+      const fs = yield* AppFileSystem.Service
+
+      expect(yield* simulation.writeFile({ path: "src/generated.txt", content: "generated" })).toEqual({ file: "src/generated.txt" })
+      expect(yield* fs.readFileString("/opencode/src/generated.txt")).toBe("generated")
+      expect((yield* simulation.snapshot()).files).toContain("src/generated.txt")
+    }),
+  )
+
   it.effect("registers network responses through control state", () =>
     Effect.gen(function* () {
       const simulation = yield* Simulation.Service