fix(server): distinguish missing cwd from missing binary in codex version check

apps/server/src/codexAppServerManager.test.ts

@@ -470,6 +470,35 @@ describe("startSession", () => {
       manager.stopAll();
     }
   });
+
+  it("reports missing working directory instead of missing binary when cwd does not exist", async () => {
+    const manager = new CodexAppServerManager();
+    const events: Array<{ method: string; kind: string; message?: string }> = [];
+    manager.on("event", (event) => {
+      events.push({
+        method: event.method,
+        kind: event.kind,
+        ...(event.message ? { message: event.message } : {}),
+      });
+    });
+
+    const missingCwd = path.join(os.tmpdir(), `t3code-nonexistent-${randomUUID()}`);
+    try {
+      await expect(
+        manager.startSession({
+          threadId: asThreadId("thread-cwd-missing"),
+          provider: "codex",
+          binaryPath: "codex",
+          cwd: missingCwd,
+          runtimeMode: "full-access",
+        }),
+      ).rejects.toThrow(/Working directory.*does not exist/);
+      expect(events).toHaveLength(1);
+      expect(events[0]?.method).toBe("session/startFailed");
+    } finally {
+      manager.stopAll();
+    }
+  });
 });
 
 describe("sendTurn", () => {

apps/server/src/codexAppServerManager.ts

@@ -1,6 +1,7 @@
 import { type ChildProcessWithoutNullStreams, spawn, spawnSync } from "node:child_process";
 import { randomUUID } from "node:crypto";
 import { EventEmitter } from "node:events";
+import fs from "node:fs";
 import readline from "node:readline";
 
 import {
@@ -1553,6 +1554,14 @@ function assertSupportedCodexCliVersion(input: {
       lower.includes("command not found") ||
       lower.includes("not found")
     ) {
+      // Disambiguate: ENOENT can mean the binary is missing OR the cwd
+      // does not exist.  Check the cwd in the error path so we surface
+      // the right message without a racy preflight.
+      try {
+        fs.accessSync(input.cwd);
+      } catch {
+        throw new Error(`Working directory '${input.cwd}' does not exist or is not accessible.`);
+      }
       throw new Error(`Codex CLI (${input.binaryPath}) is not installed or not executable.`);
     }
     throw new Error(