fix(browser): avoid Safari ReadableStream upload with fetch wrappers#3706
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dustinbyrne wants to merge 2 commits into
Open
fix(browser): avoid Safari ReadableStream upload with fetch wrappers#3706dustinbyrne wants to merge 2 commits into
dustinbyrne wants to merge 2 commits into
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dustinbyrne
commented
May 29, 2026
| addTracingHeaders(hostnames, distinctId, sessionManager, req.url, headers) | ||
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| return originalFetch(req) | ||
| return originalFetch(url, { ...(init || {}), headers }) |
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Does anyone foresee any issues with spreading init like this?
dustinbyrne
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TueHaulund,
arnohillen,
fasyy612,
ksvat and
nicowaltz
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Prompt To Fix All With AIFix the following 1 code review issue. Work through them one at a time, proposing concise fixes.
---
### Issue 1 of 1
packages/browser/src/extensions/replay/external/network-plugin.ts:591-595
**ReadableStream body silently lost when body recording is enabled**
`_tryReadBody` calls `req.clone()`, which tees the underlying ReadableStream and replaces `req.body` with the first tee branch. Because `init.body` is the same `ReadableStream` object that `new Request(url, init)` stored internally, after the tee `init.body` points to the now-consumed original stream. For URL-string inputs the new path calls `originalFetch(url, init)`, so a locked/disturbed stream is passed as `init.body` — the browser's native fetch will throw or send an empty body. Before this PR `originalFetch(req)` was used, where `req.body` was already the live tee branch and the transmission succeeded. The regression is triggered only when `recordBody: true` is set and the caller passes a `ReadableStream` as the body, but in that scenario the request body is silently dropped.
Reviews (1): Last reviewed commit: "fix(browser): avoid Safari ReadableStrea..." | Re-trigger Greptile |
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Size Change: +1.97 kB (+0.01%) Total Size: 16.5 MB
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dustinbyrne
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arnohillen,
fasyy612,
ksvat and
nicowaltz
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dustinbyrne
requested review from
a team,
TueHaulund and
nicowaltz
and removed request for
a team
dustinbyrne
requested review from
arnohillen,
fasyy612 and
ksvat
and removed request for
a team,
TueHaulund,
arnohillen,
fasyy612,
ksvat and
nicowaltz
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Problem
Safari can throw
NotSupportedError: ReadableStream uploading is not supportedwhen PostHog fetch wrappers pass an internally-createdRequestobject to downstream fetch wrappers/interceptors. Those downstream wrappers can forwardrequest.body, which is aReadableStream, even when the app originally calledfetch(url, { method: 'POST', body: 'some string' }).Changes
Requestobjects downstream for originalfetch(url, init)calls in tracing headers and replay network capture wrappers.Requestfor originalfetch(Request)calls, since creating the clone can consume the original request body.Request.body.Validated locally:
pnpm --dir packages/browser typecheckpnpm --dir packages/browser exec eslint src/entrypoints/tracing-headers.ts src/extensions/replay/external/network-plugin.ts src/__tests__/extensions/replay/external/fetch-wrapper-invariants.test.ts playwright/mocked/fetch-post-body.spec.tscd packages/browser && pnpm exec jest src/__tests__/tracing-headers.test.ts src/__tests__/extensions/replay/external/fetch-wrapper-invariants.test.ts --runInBandcd packages/browser && pnpm exec playwright test playwright/mocked/fetch-post-body.spec.ts --project webkit --project chromium --project firefox --reporter=lineRelease info Sub-libraries affected
Libraries affected
Checklist
If releasing new changes
pnpm changesetto generate a changeset file