fix(sse): rebuild deadline-wrapped request from public accessors

[TrippyEngineer]

Summary

• Every request to /v1/chat/completions, /v1/messages and /v1/responses failed with HTTP 500 on release/v3.8.51:
  TypeError: Cannot read private member #state from an object whose class did not declare it at withDeadlineSignal (open-sse/utils/earlyStreamKeepalive.ts).
• Cause: Next.js hands App Router handlers that have no dynamic export a Proxy around the NextRequest (proxyNextRequest in next/dist/server/route-modules/app-route/module.js). withDeadlineSignal() (added in fix(sse): bound slow keepalive path with absolute last-resort deadline #14808) wrapped it with new Request(request, { signal, headers }); the Request constructor reads the input's private #state, which cannot be reached through a Proxy.
• Fix: rebuild the wrapped request from its public accessors (url, method, headers, redirect, body stream with duplex: "half"), the same pattern rebuildRequest() in chatBodyAdmission.ts already uses. The deadline token header and combined signal are unchanged.

Related Issues

• Related to fix(sse): bound slow keepalive path with absolute last-resort deadline #14808 (regression introduced there)

Validation

• Change type: routing
• Focused tests: node --import tsx/esm --test tests/unit/early-stream-keepalive.test.ts — 26/26 pass
• eslint + prettier on changed files (lint-staged pre-commit, with repo suppressions)
• npm run typecheck:core — clean
• Production-code changes include a new automated test in this PR
• Live check: npm run dev, POST /v1/chat/completions went from HTTP 500 to reaching combo routing

Tests Added Or Updated

• tests/unit/early-stream-keepalive.test.ts
  • withDeadlineSignal accepts a Proxy-wrapped request (Next.js proxyNextRequest) — fails on the base with the exact production error, passes with the fix; also asserts method/url/headers/body are preserved, the deadline controller still resolves, and a client abort still reaches the wrapped signal.
  • withDeadlineSignal keeps a body-less GET request valid.

Coverage Notes

• The single changed function (withDeadlineSignal) is covered by the new tests plus the existing deadline tests in the same file.

Reviewer Notes

• Reproduced on Windows 11, Node 24.19.0, Next 16.3.5 (npm run dev, Turbopack). The Proxy is applied for dynamic = undefined/"auto", so production builds are affected too.

[diegosouzapw]

Thanks @TrippyEngineer for the clear root-cause write-up on the #14808 regression. Next's proxyNextRequest wrapping plus new Request(request, init) reading #state is exactly what was sending those routes to 500.

#14886 (merged 2026-09-28) fixed this on release/v3.8.51. withDeadlineSignal in open-sse/utils/earlyStreamKeepalive.ts now rebuilds the request from its public accessors (url, method, headers, body, with duplex: "half") and keeps the deadline token header and the combined signal. tests/unit/early-stream-keepalive.test.ts covers the Proxy-wrapped case. Several near-identical fixes arrived within hours of each other and only one could land, so I'm closing this as covered.

If your extra assertions (such as redirect preservation or client-abort propagation through the wrapper) test something the tip suite doesn't, a small test-only PR would be welcome. Thank you!