custom header problem

[2803media]

Bug Description

Custom HTTP headers (HTTP-Referer, X-Title) are not transmitted when using fetch() in Node.js v22.21.0 (undici 6.22.0), but work correctly with curl and in Node.js v20.19.0 (undici 6.21.1).

Environment

• Node.js version: v22.21.0
• undici version: 6.22.0
• OS: Linux (Debian-based)
• Working versions: Node.js v20.19.0 (undici 6.21.1)

Reproduction

Test endpoint

We tested against both https://postman-echo.com/headers and https://openrouter.ai/api/v1/chat/completions.

Code that fails (undici 6.22.0)

const response = await fetch('https://openrouter.ai/api/v1/chat/completions', {
  method: 'POST',
  headers: {
    'Authorization': 'Bearer xxx',
    'Content-Type': 'application/json',
    'HTTP-Referer': 'https://github.com/,
    'X-Title': 'test bot'
  },
  body: JSON.stringify({ /* ... */ })
});

Result: Headers HTTP-Referer and X-Title are not received by the server.

Same code with curl (works)

curl https://openrouter.ai/api/v1/chat/completions \
  -H "Authorization: Bearer xxx" \
  -H "Content-Type: application/json" \
  -H "HTTP-Referer: https://github.com/" \
  -H "X-Title: test bot" \
  -d '{"model":"xxx","messages":[...]}'

Result: Headers are correctly received (verified in response metadata).

Verification with postman-echo

# Works with curl
curl https://postman-echo.com/headers \
  -H "HTTP-Referer: https://github.com/test" \
  -H "X-Title: Test"

# Response shows both headers correctly

Expected Behavior

Custom headers should be transmitted to the remote server, just like curl does.

Actual Behavior

Custom headers HTTP-Referer and X-Title are stripped/lost during transmission.

Workaround

Using axios instead of fetch works correctly:

const axios = require('axios');
const response = await axios.post(url, data, {
  headers: {
    'HTTP-Referer': 'https://github.com',
    'X-Title': 'test bot'
  }
});

Regression

This appears to be a regression introduced in undici 6.22.0, as it works in 6.21.1.

Additional Context

The headers work correctly:

• ✅ With curl on the same machine
• ✅ With Node.js v20.19.0 (undici 6.21.1) on macOS
• ❌ With Node.js v22.21.0 (undici 6.22.0) on Linux

This affects applications that need to identify themselves via custom headers (like OpenRouter's app tracking system).

[KhafraDev]

If this was a bug, we would expect many more people to have this issue. It would also cause multiple tests to fail. We'll need a minimal example to ascertain.