diff --git a/src/chatgpt/app-server-shim/launcher.ts b/src/chatgpt/app-server-shim/launcher.ts
index 629633373ef..53142ddafbf 100644
--- a/src/chatgpt/app-server-shim/launcher.ts
+++ b/src/chatgpt/app-server-shim/launcher.ts
@@ -1,6 +1,6 @@
 import { accessSync, chmodSync, constants, mkdirSync, renameSync, statSync, unlinkSync, writeFileSync } from "node:fs";
 import { randomBytes } from "node:crypto";
-import { join } from "node:path";
+import { join, posix } from "node:path";
 import { getConfigDir } from "../../config/paths";
 import { selfLaunchArgv } from "../../lib/self-launch-argv";
 
@@ -32,7 +32,7 @@ export function resolveChatgptCodexBinary(
   isExecutable: (path: string) => boolean = isExecutableFile,
 ): string | null {
   for (const layout of BUNDLED_CODEX_LAYOUTS) {
-    const candidate = join(bundleRoot, ...layout);
+    const candidate = posix.join(bundleRoot, ...layout); // macOS bundle paths are POSIX on every host
     if (isExecutable(candidate)) return candidate;
   }
   return null;
diff --git a/tests/clients/desktop-app-server-shim-launcher.test.ts b/tests/clients/desktop-app-server-shim-launcher.test.ts
index 4baaaa1d9fe..fa16a29364a 100644
--- a/tests/clients/desktop-app-server-shim-launcher.test.ts
+++ b/tests/clients/desktop-app-server-shim-launcher.test.ts
@@ -62,6 +62,16 @@ describe("experimental app-server launcher", () => {
     expect(resolveChatgptCodexBinary(root, () => false)).toBeNull();
   });
 
+  test("every probed bundle path stays POSIX, so a Windows host builds the same candidates", () => {
+    const probed: string[] = [];
+    resolveChatgptCodexBinary("/Users/example/Applications/ChatGPT.app", path => { probed.push(path); return false; });
+    expect(probed.length).toBeGreaterThan(0);
+    for (const path of probed) {
+      expect(path).not.toContain("\\");
+      expect(path.startsWith("/Users/example/Applications/ChatGPT.app/Contents/")).toBe(true);
+    }
+  });
+
   test("the writer embeds the binary it is given", () => withDir(dir => {
     const real = "/Volumes/Apps/ChatGPT.app/Contents/Resources/codex-cli/CodexCLI.app/Contents/MacOS/codex";
     const path = writeChatgptShimLauncher(dir, real);