Playwright tests with the next fixture always fail (using experimental testProxy)

[GRA0007]

Link to the code that reproduces this issue

https://github.com/GRA0007/next-test-proxy-repro

To Reproduce

1. Set up a test with Playwright that uses the NextFixture to mock a request (following instructions from here)
2. Run the tests, notice that if the NextFixture is used, the test fails with the error TypeError: Cannot destructure property 'complete' of 'undefined' as it is undefined.

Current vs. Expected behavior

I expect mocking a request to successfully mock the request without causing an error. All the tests I've set up in my reproduction should pass.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.0.0: Mon Aug 12 20:51:54 PDT 2024; root:xnu-11215.1.10~2/RELEASE_ARM64_T6000
  Available memory (MB): 16384
  Available CPU cores: 10
Binaries:
  Node: 20.11.1
  npm: 10.2.4
  Yarn: 1.22.21
  pnpm: 10.6.5
Relevant Packages:
  next: 15.2.3 // Latest available version is detected (15.2.3).
  eslint-config-next: N/A
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.8.2
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Testing

Which stage(s) are affected? (Select all that apply)

next dev (local), next start (local)

Additional context

I initially started making this reproduction because I notice that the testProxy didn't work with GET route handlers, only with pages, but it seems something else is going on because I can't get any of the mocked tests to work.

[cedricss]

Same here. It looks like this issue occurs with @playwright/test version 1.51.
Downgrading to 1.50.1 works around the problem.

Also, see this recent comment about the "Test Step improvements" update: microsoft/playwright#27059 (comment)