@clerk/testing: signIn() times out with concurrent Playwright workers (--workers=2+)

[joerizz23]

Description

When using @clerk/testing with Playwright and --workers=2 (or more), all tests fail with TimeoutError: page.waitForFunction: Timeout 15000ms exceeded at clerk.signIn(). With --workers=1, authentication works 100% reliably.

Environment

• @clerk/testing: 1.13.36
• @clerk/nextjs: latest
• @playwright/test: 1.57.0
• Next.js: 15.4.11
• OS: Windows 11 (also likely affects Linux/macOS)
• Auth strategy: emailAddress

Reproduction

1. Set up a Next.js app with Clerk authentication
2. Create a Playwright fixture using @clerk/testing:

import { clerkSetup, clerk } from "@clerk/testing/playwright";

// In fixture:
await clerk.signIn({
  page,
  emailAddress: testUser.email,
});

3. Run tests with --workers=1 → all tests pass
4. Run tests with --workers=2 → all tests fail

Error

TimeoutError: page.waitForFunction: Timeout 15000ms exceeded.

  at loaded (node_modules/@clerk/testing/src/playwright/helpers.ts:100:14)
  at Object.b (node_modules/@clerk/testing/src/playwright/helpers.ts:120:9)

The timeout occurs at the window.Clerk?.loaded === true && window.Clerk?.user !== null wait inside signIn().

Analysis

Each Playwright worker creates a separate browser instance. When 2+ workers call clerk.signIn() concurrently:

• setupClerkTestingToken() injects the dev-browser token into each browser
• Both browsers navigate and wait for window.Clerk.loaded && window.Clerk.user
• The wait times out on both workers

Possible causes:

1. Clerk Backend API rate-limits concurrent testing token validations from the same publishable key
2. ClerkJS client-side hydration conflicts between concurrent browser sessions
3. The dev-browser token mechanism doesn't support multiple simultaneous sessions

Expected Behavior

@clerk/testing should support concurrent Playwright workers, or the documentation should explicitly state that --workers=1 is required.

Suggested Improvements

• Support parallel test users with isolated Clerk sessions
• Provide a testing mode that doesn't require live ClerkJS hydration (e.g., a static token that bypasses the window.Clerk.loaded wait)
• Document the --workers=1 limitation if concurrent sessions aren't supported