Unstable_DevWorker.fetch fails when request has a body and receives a 401 response

[beanow-at-crabnebula]

What versions & operating system are you using?

Problem seems introduced in #12927

On Linux (x86_64)

  "devDependencies": {
    "@types/jest": "^29.5.11",
    "@types/node": "^25.5.0",
    "jest": "^29.7.0",
    "ts-jest": "^29.4.6",
    "typescript": "^5.3.3",
    "undici": "^7.24.4",
    "wrangler": "^4.75.0"
  },
  "dependencies": {
    "hono": "^4.12.2"
  }

Please provide a link to a minimal reproduction

https://github.com/beanow-at-crabnebula/undici-401-request-body

Describe the Bug

This seemed like a potential regression in undici and I've reported upstream: nodejs/undici#4910
However with purely undici and nodejs it wasn't easily reproducible.

What seems to be from a change in nodejs/undici#4747

Any (non websocket) request that includes a request body and gets a 401 response errors.

    TypeError: fetch failed

      at Object.processResponse (node_modules/.pnpm/wrangler@4.75.0/node_modules/wrangler/wrangler-dist/cli.js:31323:21)
      at node_modules/.pnpm/wrangler@4.75.0/node_modules/wrangler/wrangler-dist/cli.js:31709:23

    Cause:
    expected non-null body source

      at makeNetworkError (node_modules/.pnpm/wrangler@4.75.0/node_modules/wrangler/wrangler-dist/cli.js:30097:36)
      at httpNetworkOrCacheFetch (node_modules/.pnpm/wrangler@4.75.0/node_modules/wrangler/wrangler-dist/cli.js:31924:20)
      at httpFetch (node_modules/.pnpm/wrangler@4.75.0/node_modules/wrangler/wrangler-dist/cli.js:31733:37)
      at mainFetch (node_modules/.pnpm/wrangler@4.75.0/node_modules/wrangler/wrangler-dist/cli.js:31495:24)

Please provide any relevant error logs

No response

[beanow-at-crabnebula]

In nodejs/undici#4910 I wasn't able to reproduce the issue with regular nodejs and undici alone.
So potentially it's a workers-only issue, or another condition exists in how this SDK invokes undici for the problem to occur.

[beanow-at-crabnebula]

This is affecting all release >= 4.75.0

[petebacondarwin]

Analysis

After digging into this, the root cause is in undici's isTraversableNavigable() function (added in #4747) which unconditionally returns true even in a Node.js context where there is no browser UI to prompt for credentials:

function isTraversableNavigable(navigable) {
  // TODO
  return true  // ← always true, even in Node.js
}

This causes the 401-retry block in httpNetworkOrCacheFetch to always execute. When a request has a body, that block immediately checks whether request.body.source is non-null, and since the body has already been consumed/streamed (source is null), it throws:

makeNetworkError('expected non-null body source')

Why didn't nodejs/undici#4761 fix this? That PR (released in v7.19.1) added return response to prevent the infinite retry loop, but that line is unreachable in this case — the body.source == null guard fires first, before the loop-prevention code is ever reached. They are two separate failure modes from the same root cause.

The correct fix is for isTraversableNavigable to return false in Node.js (there is no traversable navigable — no browser tab to prompt users for credentials). This is tracked upstream in nodejs/undici#4910, which is currently open with no fix in progress.

Workaround

While waiting for an upstream fix in undici, we are going to apply a pnpm patch to the bundled undici in workers-sdk that changes isTraversableNavigable to return false. This is a one-line change that exactly mirrors the correct upstream fix and can be removed as soon as a patched undici version is available.