Some fetch operations fail with testProxy enabled

Link to the code that reproduces this issue

https://github.com/dustinsoftware/next-testmode-issue-repro

To Reproduce

USE_TEST_PROXY=1 BUCKET_NAME=bucket-name npm run dev

Current vs. Expected behavior

Having the testProxy mode enabled in next config results in some fetch operations failing when the app is run in dev mode and in standalone mode. This is similar to https://github.com/vercel/next.js/issues/66238

Impacted fetch calls fail with the following stack:

{"level":50,"time":1755810957931,"pid":10975,"hostname":"xxx","name":"console","err":{"type":"TypeError","message":"fetch failed: expect header not supported","stack":"TypeError: fetch failed\n    at node:internal/deps/undici/undici:13510:13\n    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)\n    at async /Users/dustin.masters/Code/next-aws-sdk-minimal-repro/.next/standalone/node_modules/next/dist/experimental/testmode/httpget.js:16:26\n    at async s.push.done (/Users/dustin.masters/Code/next-aws-sdk-minimal-repro/.next/standalone/node_modules/next/dist/compiled/@mswjs/interceptors/ClientRequest/index.js:1:15030)\ncaused by: NotSupportedError: expect header not supported\n    at processHeader (node:internal/deps/undici/undici:2174:15)\n    at new Request (node:internal/deps/undici/undici:1998:15)\n    at [dispatch] (node:internal/deps/undici/undici:7802:25)\n    at Intercept (node:internal/deps/undici/undici:7535:20)\n    at [Intercepted Dispatch] (node:internal/deps/undici/undici:572:16)\n    at Client.dispatch (node:internal/deps/undici/undici:588:44)\n    at [dispatch] (node:internal/deps/undici/undici:819:32)\n    at [Intercepted Dispatch] (node:internal/deps/undici/undici:565:33)\n    at Pool.dispatch (node:internal/deps/undici/undici:588:44)\n    at [dispatch] (node:internal/deps/undici/undici:8230:27)","$metadata":{"attempts":1,"totalRetryDelay":0}},"msg":"fetch failed"}

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.6.0: Mon Jul 14 11:30:40 PDT 2025; root:xnu-11417.140.69~1/RELEASE_ARM64_T6041
  Available memory (MB): 131072
  Available CPU cores: 16
Binaries:
  Node: 22.16.0
  npm: 10.9.2
  Yarn: 4.9.1-git.20250411.hash-1908ee79f
  pnpm: N/A
Relevant Packages:
  next: 15.5.1-canary.3 // Latest available version is detected (15.5.1-canary.3).
  eslint-config-next: N/A
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.9.2
Next.js Config:
  output: standalone

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

Runtime

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

next dev (local), next start (local), next build (local)

Additional context

There is a detailed readme in the repo with some more background.

We have a workaround (use an env var to control testProxy) but it was surprising to me that requests made not during a playwright run are impacted.

Run the linked reproduction with USE_TEST_PROXY=1 and inspect the fetch path shown in .next/standalone/node_modules/next/dist/experimental/testmode/httpget.js, along with the compiled @mswjs/interceptors ClientRequest stack. Confirm the failure in next dev and standalone mode, then verify that fetches succeed outside Playwright while testProxy remains enabled.