diff --git a/scripts/e2e/lib/codex-app-server-fixture.mjs b/scripts/e2e/lib/codex-app-server-fixture.mjs
index ff90494bafb6..89665c8cb406 100644
--- a/scripts/e2e/lib/codex-app-server-fixture.mjs
+++ b/scripts/e2e/lib/codex-app-server-fixture.mjs
@@ -24,6 +24,7 @@ export function createFakeThreadStartResponse({ params, sessionId, threadId, ver
       forkedFromId: null,
       preview: "",
       ephemeral: false,
+      projectId: params?.projectId ?? null,
       modelProvider: "openai",
       createdAt: now,
       updatedAt: now,
diff --git a/test/scripts/codex-app-server-fixture.test.ts b/test/scripts/codex-app-server-fixture.test.ts
new file mode 100644
index 000000000000..9e404d23aa37
--- /dev/null
+++ b/test/scripts/codex-app-server-fixture.test.ts
@@ -0,0 +1,18 @@
+import { describe, expect, it } from "vitest";
+import { createFakeThreadStartResponse } from "../../scripts/e2e/lib/codex-app-server-fixture.mjs";
+
+describe("createFakeThreadStartResponse", () => {
+  it.each([
+    { expected: null, params: {} },
+    { expected: "project-1", params: { projectId: "project-1" } },
+  ])("returns the protocol-required projectId as $expected", ({ expected, params }) => {
+    const response = createFakeThreadStartResponse({
+      params,
+      sessionId: "session-1",
+      threadId: "thread-1",
+      version: "0.149.1",
+    });
+
+    expect(response.thread.projectId).toBe(expected);
+  });
+});