POST requests returning non-2xx status crash with "fetch failed" on Vite 8

[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?