diff --git a/packages/core/src/fs-util.ts b/packages/core/src/fs-util.ts
index eb68627a55c3..177a0e599975 100644
--- a/packages/core/src/fs-util.ts
+++ b/packages/core/src/fs-util.ts
@@ -114,7 +114,15 @@ export namespace FSUtil {
       })
 
       const ensureDir = Effect.fn("FileSystem.ensureDir")(function* (path: string) {
-        yield* fs.makeDirectory(path, { recursive: true })
+        yield* fs.makeDirectory(path, { recursive: true }).pipe(
+          // Recursive makeDirectory should be idempotent, but some platforms (observed on Windows)
+          // surface AlreadyExists when the directory already exists. Tolerate that, but only when the
+          // path is genuinely a directory so a file occupying the path still fails loudly.
+          Effect.catchIf(
+            (e) => e.reason._tag === "AlreadyExists",
+            (e) => isDir(path).pipe(Effect.flatMap((exists) => (exists ? Effect.void : Effect.fail(e)))),
+          ),
+        )
       })
 
       const writeWithDirs = Effect.fn("FileSystem.writeWithDirs")(function* (
diff --git a/packages/core/test/filesystem/filesystem.test.ts b/packages/core/test/filesystem/filesystem.test.ts
index fdce1b447644..13a54996f88e 100644
--- a/packages/core/test/filesystem/filesystem.test.ts
+++ b/packages/core/test/filesystem/filesystem.test.ts
@@ -1,14 +1,47 @@
 import { describe, test, expect } from "bun:test"
-import { Effect, FileSystem } from "effect"
+import { Effect, Exit, FileSystem, Layer } from "effect"
+import { PlatformError, SystemError } from "effect/PlatformError"
+import { NodeFileSystem } from "@effect/platform-node"
 import { LayerNodePlatform } from "@opencode-ai/core/effect/app-node-platform"
 import { LayerNode } from "@opencode-ai/core/effect/layer-node"
 import { FSUtil } from "@opencode-ai/core/fs-util"
 import { testEffect } from "../lib/effect"
+import os from "os"
 import path from "path"
+import { mkdtempSync, rmSync } from "fs"
 
 const live = LayerNode.compile(LayerNode.group([FSUtil.node, LayerNodePlatform.filesystem]))
 const { effect: it } = testEffect(live)
 
+// A FileSystem whose makeDirectory always reports AlreadyExists, emulating the platforms/runtimes
+// (observed on Windows) that surface it for an already-existing directory even with { recursive: true }.
+const alreadyExistsFilesystem = Layer.effect(
+  FileSystem.FileSystem,
+  Effect.gen(function* () {
+    const real = yield* FileSystem.FileSystem
+    return {
+      ...real,
+      makeDirectory: (dir: string) =>
+        Effect.fail(
+          new PlatformError(
+            new SystemError({
+              _tag: "AlreadyExists",
+              module: "FileSystem",
+              method: "makeDirectory",
+              pathOrDescriptor: dir,
+            }),
+          ),
+        ),
+    }
+  }),
+).pipe(Layer.provide(NodeFileSystem.layer))
+
+const { effect: itAlreadyExists } = testEffect(
+  LayerNode.compile(LayerNode.group([FSUtil.node, LayerNodePlatform.filesystem]), [
+    [LayerNodePlatform.filesystem, alreadyExistsFilesystem],
+  ]),
+)
+
 describe("FSUtil", () => {
   describe("isDir", () => {
     it(
@@ -158,6 +191,36 @@ describe("FSUtil", () => {
         expect(info.type).toBe("Directory")
       }),
     )
+
+    it(
+      "fails when a file already occupies the path",
+      Effect.gen(function* () {
+        const fs = yield* FSUtil.Service
+        const filesys = yield* FileSystem.FileSystem
+        const tmp = yield* filesys.makeTempDirectoryScoped()
+        const file = path.join(tmp, "occupied")
+        yield* filesys.writeFileString(file, "x")
+
+        // makeDirectory reports AlreadyExists for a file path too; ensureDir must not mask the collision.
+        const exit = yield* fs.ensureDir(file).pipe(Effect.exit)
+        expect(Exit.isFailure(exit)).toBe(true)
+      }),
+    )
+  })
+
+  // Regression coverage for issue #35828: on some Windows runtimes a recursive makeDirectory reports
+  // AlreadyExists for a directory that already exists, which crashed startup when `.opencode` already
+  // existed. ensureDir must treat that as success.
+  describe("ensureDir tolerates platform-reported AlreadyExists", () => {
+    itAlreadyExists(
+      "succeeds when makeDirectory reports AlreadyExists for an existing directory",
+      Effect.gen(function* () {
+        const fs = yield* FSUtil.Service
+        const dir = mkdtempSync(path.join(os.tmpdir(), "oc-ensuredir-"))
+        yield* fs.ensureDir(dir)
+        rmSync(dir, { recursive: true, force: true })
+      }),
+    )
   })
 
   describe("writeWithDirs", () => {