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Fix XMLHttpRequest.setRequestHeader to append duplicate headers per spec#56394

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zmunm wants to merge 2 commits intofacebook:mainfrom
zmunm:xhr-setrequestheader-append
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Fix XMLHttpRequest.setRequestHeader to append duplicate headers per spec#56394
zmunm wants to merge 2 commits intofacebook:mainfrom
zmunm:xhr-setrequestheader-append

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@zmunm zmunm commented Apr 9, 2026 •
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Summary:

XMLHttpRequest.setRequestHeader() overwrites the previous value when called multiple times with the same header name. Per the XHR Living Standard §4.5.2, duplicate headers should be appended with , .

Real-world example: Sentry JS SDK assumes append behavior
when setting the baggage header, which causes previously set baggage values to be silently dropped on React Native.

Changelog:

[GENERAL] [FIXED] - Fix XMLHttpRequest.setRequestHeader to append duplicate headers per spec

Note: This is technically a breaking change for code that relied on the previous overwrite behavior (e.g., calling setRequestHeader twice to replace a value). However, that behavior was non-compliant with the XHR spec, so such code should use a single call with the desired final value instead.

Test Plan:

Unit tests — 3 tests added in XMLHttpRequest-test.js:

  • Appends values when same header is set twice
  • Merges case-insensitively
  • Throws when called before open()

Run the snippet below in both browser DevTools and RN debugger console to compare:

const xhr = new XMLHttpRequest();
xhr.open('GET', 'https://httpbin.org/headers');
xhr.setRequestHeader('X-Test', 'value1');
xhr.setRequestHeader('X-Test', 'value2');
xhr.onload = () => {
  const headers = JSON.parse(xhr.responseText).headers;
  console.log('X-Test:', headers['X-Test']);
  // Before: "value2"
  // After:  "value1, value2"
};
xhr.send();

zmunm added 2 commits April 9, 2026 16:37
@zmunm
test: add setRequestHeader append behavior tests
40b2641 
Add tests verifying XHR spec compliance for setRequestHeader:
- Same header called multiple times should append with ', '
- Case-insensitive header name merging
- Throw when called before open()

These tests currently FAIL, exposing the spec violation.
@zmunm
fix: setRequestHeader should append per XHR spec
5f3ff21 
Comply with XHR Living Standard 4.5.2: when setRequestHeader is
called multiple times with the same header, append with ', '
instead of overwriting.
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meta-cla bot commented Apr 9, 2026

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@meta-cla meta-cla bot added the CLA Signed This label is managed by the Facebook bot. Authors need to sign the CLA before a PR can be reviewed. label Apr 9, 2026
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meta-codesync bot commented Apr 9, 2026

@fabriziocucci has imported this pull request. If you are a Meta employee, you can view this in D100144689.

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