[VIPKaiser]

Summary

When a client aborts a streamed /v1/chat/completions, OmniRoute never cancels the upstream
request. A local upstream (oMLX) keeps prefilling/decoding the abandoned request and blocks the
next one.

Observed (oMLX server log, single-slot local backend):

• Client aborted at T; the upstream still decoded 1041 more tokens for the dead request.
• A request aborted 435 ms in still ran a full 34 s prefill, and the next request queued behind it.

Three independent breaks, all fixed here:

1. open-sse/handlers/chatCore/upstreamTimeouts.ts executeWithUpstreamStartTimeout: links the
   client signal to the fetch signal, but its finally removed that listener as soon as the
   Promise.race settled (headers received). The timeout is meant to bound time-to-headers only;
   a later client abort no longer reached the fetch. The abort link now stays for the fetch
   lifetime on success; only the timer is cleared. It is still dropped when the attempt fails.
   (The once listener lives on the per-request signal, so it is bounded.)
2. open-sse/utils/streamHandler.ts createDisconnectAwareStream().pull(): on
   !isConnected() it closed the client-facing stream but never cancelled the upstream reader
   (only cancel() did). It now cancels the reader/writer with client_disconnected first,
   except after a client terminal marker or while a completed tool handoff drain owns the reader.
3. open-sse/services/combo/validateQuality.ts validateResponseQuality peek loop: await reader.read() was not abort-aware; a stalled prefill held the combo attempt until the first
   byte. It now takes an optional signal (passed from runtimeUnits.ts), races each read against
   it, cancels the peek reader on abort and returns { valid: true } (a client abort is not a
   provider failure, so no failover/cooldown; the combo loop's existing signal.aborted check then
   returns 499).

Usage accounting

No change to conventions. handleDisconnect already finalizes the pending call as 499
client_disconnected before either abort propagates, and that is the recorded outcome. A cancelled
upstream never delivers its final usage chunk, so an aborted stream records no upstream usage; I did
not add partial-token estimation. Non-aborted streams take the same code path as before (covered by
tests). Happy to add estimation from tokens seen if maintainers want it.

Tests Added Or Updated

• tests/unit/client-abort-propagates-upstream.test.ts (new, 8 tests):
  • Real HTTP fake upstream sending headers + chunks: abort after headers, assert the upstream
    socket closes early (fails without fix 1).
  • Non-aborted wrapper result unchanged; failed attempt does not keep the abort link.
  • pipeWithDisconnect: disconnect cancels the upstream body; a normal stream is forwarded
    intact and not cancelled.
  • validateResponseQuality: exits promptly and cancels the reader on abort (stalled body), returns
    immediately on an already-aborted signal, unchanged for a normal stream with a signal.
• 4 of 8 fail on the base without the source changes; all pass with them.
• Ran: new test, stream-handler*, stream-disconnect*, codex-tool-handoff*,
  streaming-empty*, combo* (389 tests, 0 fail), npm run typecheck:core,
  npm run check:open-sse-typecheck, eslint + prettier on touched files.

Reviewer Notes

• Fix 2 is belt-and-braces once fix 1 lands (abort already flows through streamController.signal),
  but covers executors that ignore the signal.
• Cancelling a Response.clone() tee branch does not by itself abort the upstream; that is why
  fix 1 is the load-bearing change. Peek cancellation intentionally is not awaited.
• Changelog fragment added: changelog.d/fixes/0000-propagate-client-abort-upstream.md
  (rename to the PR number).

[diegosouzapw]

Thanks @VIPKaiser — this closes a real gap: previously a client abort that arrived after
headers (mid-stream, or during the combo quality-peek) never reached the upstream fetch, so the
provider kept generating for a client that was already gone. I probed the new test suite
(tests/unit/client-abort-propagates-upstream.test.ts) against your head commit with a real
node:http fake upstream: all 8 cases pass, covering the timeout-wrapper abort link, the
disconnect-aware stream pipe cancellation, and the combo quality-peek abort race. Clean, well
scoped, well commented. Looks merge-ready to me.