POST requests returning non-2xx status crash with "fetch failed"

[Plopix]

When using @cloudflare/vite-plugin with Vite dev server, any POST request handler that returns a non-2xx HTTP status code (e.g. 401, 400, 403, 404) causes the Miniflare proxy to crash with a generic fetch failed error. The actual response from the worker is lost and Vite returns a 500 to the client.

GET requests with the same non-2xx status codes work correctly.

NOTE: this is only on DEV (via vite)

Minimal reproduction:

  // app.ts (Hono, but framework doesn't matter)
  import { Hono } from 'hono';

  const app = new Hono();

  // ✅ Works — GET + 401
  app.get('/test-get-401', (c) => c.json({ error: 'Unauthorized' }, 401));

  // ✅ Works — POST + 200
  app.post('/test-post-200', (c) => c.json({ ok: true }, 200));

  // ✅ Works — POST + 201
  app.post('/test-post-201', (c) => c.json({ ok: true }, 201));

  // ❌ Crashes — POST + 401
  app.post('/test-post-401', (c) => c.json({ error: 'Unauthorized' }, 401));

Test commands:

  curl -v http://localhost:5173/test-get-401      # ✅ 401
  curl -v -X POST http://localhost:5173/test-post-200   # ✅ 200
  curl -v -X POST http://localhost:5173/test-post-201   # ✅ 201
  curl -v -X POST http://localhost:5173/test-post-401   # ❌ 500 (should be 401)

Error output in Vite terminal:

  [vite] Internal server error: fetch failed
      at Object.processResponse (node_modules/undici/lib/web/fetch/index.js:237:16)
      ...
      at async _Miniflare.dispatchFetch (node_modules/miniflare/dist/src/index.js:86755:22)
      at async file:///node_modules/@cloudflare/vite-plugin/dist/index.mjs:1601:19

Versions:

• vite: 8.0.1
• @cloudflare/vite-plugin: 1.30.0
• miniflare: 4.20260317.1
• wrangler: 4.75.0
• OS: macOS

Pinning Miniflare to this version fixes the issue:

"overrides": {
	  "miniflare": "4.20251217.0"
	},

[Plopix]

claude found something, just pasting it here, for reference, help etc.

⏺ Found it. Line 241: options.reset = true.

  This unconditionally tells undici to close the connection immediately after the request completes. Here's why that breaks POST + non-2xx:

  1. A POST request has a body stream that undici is sending to workerd
  2. Workerd decides to respond with 401 before fully consuming the request body
  3. With reset = true, undici tries to close/reset the connection immediately
  4. But the request body stream is still open/pending — undici treats this conflict as a network error (response.type = 'error'), rejecting
  the fetch promise
  5. The vite plugin catches the error and returns 500

  Why other cases work:
  - GET + 401: No request body, so reset = true just cleanly closes after the response — no conflict
  - POST + 200/201: Workerd fully consumes the body before responding, so by the time the connection resets, everything is done
  - POST + 401: Workerd short-circuits with 401 before consuming the body → body stream still open → reset causes a network error

This was added in PR #11642 ("Improve miniflare test stability") to fix keep-alive socket issues. The fix needs to preserve
that intent while not breaking POST + non-2xx.

Here's the fix — replace options.reset = true with a Connection: close header instead, which achieves the same goal (no keep-alive) but lets undici handle the request/response lifecycle properly:

# packages/miniflare/src/http/fetch.ts
 240 -      // the request is complete
 241 -      options.reset = true;
 240 +      // the request is complete.
 241 +      // Note: We use `Connection: close` header instead of `options.reset = true`
 242 +      // because `reset` forcibly closes the socket, which can cause network errors
 243 +      // when the server responds before fully consuming the request body (e.g. a
 244 +      // POST request receiving a 401 response).
 245 +      headers.set("connection", "close");

I also confirm that this pin fixes the issue

	"overrides": {
	  "miniflare": "4.20251217.0"
	},

[beanow-at-crabnebula]

Maybe related to #12967?

[petebacondarwin]

Yes I think this is the same as #12967 - we have a fix for that and will release new versions of Wrangler and vite-plugin tomorrow.

[dkarlovi]

@petebacondarwin the issue seems to still be present with latest currently available version of Wrangler and vite-plugin.

[petebacondarwin]

Yes the problem with my fix was that it only patched the bundled copy of undici and Miniflare cannot bundle that library.
So it is just using the unpatched library in your project.
You could try patching it yourself in the interim but I see that Undici has a fix in the works, which will probably be released shortly. As soon as that happens we will bump our dependency on it and the issue should be resolved.