Pages Router: gssp `res` mutated after return collapses to empty `pageProps`

[james-elicx]

This issue was created by an agent analysing CI failures from the Next.js Deploy Suite (vinext main vs Next.js v16.2.6, 2026-05-22).

Problem

When a getServerSideProps function mutates res (for example by setting headers) after it has returned, vinext renders the page with pageProps: {} instead of the actual returned props. The asynchronous mutation appears to race with prop serialisation and overwrite the response.

Pages Router gssp that mutates res after returning the props object loses the props

Estimated Impact

~2 test failures across the deploy suite.

Affected Test Suites

• test/e2e/getserversideprops/test/index.test.ts (2 failures)

Recommendation

1. Reproduce first in vinext's own test suite. Add a Pages Router page where gssp calls setTimeout(() => res.setHeader(...)) after returning. Assert the response still contains the originally-returned pageProps.

2. Snapshot props before allowing res mutation. Once gssp returns, the props payload should be committed to the response body / __NEXT_DATA__ before any subsequent async res mutation can alter the headers.

3. Treat post-return mutation as a no-op for the body. Mirror Next.js behaviour where headers set after the response has begun are dropped, but the rendered body remains intact.

[james-elicx]

Triage update (agent investigation): this is already fixed on main by #1568 (fix(pages-router): await Promise-shaped gssp props, 2026-05-26), which added await Promise.resolve(result.props) to both gssp dispatch paths (pages-page-data.ts:503 shared by prod-server + Workers entry, and dev-server.ts:722). The res-mutation framing in the title is a red herring — the res.setHeader shim never sets headersSent/writableEnded, so a late setHeader never short-circuits the already-resolved props. The only mechanism that produced empty pageProps was the unawaited Promise, now fixed. Verified by porting the upstream promise/mutate-res*.js fixtures: both render hello res with pageProps:{text:res} on current main (and fail when the await is reverted). Closing as completed — recommend a Deploy Suite re-run to confirm green, since the original failing suite (test/e2e/getserversideprops) runs in real deploy mode.

Actioned as part of the milestone CI-failure audit.