[xz-dev]

Problem

/v1/responses opens an early SSE stream when upstream routing, cooldown waits, or first-byte readiness exceed the route threshold. Its immediate startup frame currently creates a complete synthetic reasoning output item containing ✨.

Although the item is now structurally closed, downstream clients can still display or persist the placeholder as reasoning, and may replay it in a later request. An SSE comment is content-neutral but does not reset clients such as Codex that apply their idle timeout to the next parsed EventSource event rather than raw bytes.

Fix

Reuse OmniRoute's existing Responses heartbeat shape from sseHeartbeat.ts:

data: {"type":"response.in_progress"}

The Responses route now supplies that frame for both:

• the immediate slow-path startup frame; and
• recurring early keepalive ticks until the real handler response is available.

response.in_progress is a parsed Responses bookkeeping event, so it keeps event-level watchdogs active without creating an output item, reasoning summary, visible text, output index, or replayable reasoning signature.

The change removes the obsolete synthetic reasoning frame and the output-index stack/test scaffolding that existed only to keep that placeholder lifecycle well formed. Fast-path responses, HTTP statuses, combo/readiness execution, thresholds, error framing, correlation logging, cancellation, and real body forwarding remain unchanged.

Related history

• fix(sse-heartbeat): shape-aware keepalives keep streams alive through stricter proxies #2233 introduced the existing shape-aware response.in_progress heartbeat.
• fix(responses): use protocol-neutral keepalive before response starts #8948 proposed comments, but comments do not satisfy parsed-event watchdogs.
• fix(sse): Gemini TPM/RPD quota classification + combo cooldown-wait resilience #8213, fix(sse): brand-neutral keepalive frames #8888, and fix(sse): close the synthetic keepalive reasoning item + harden output_index allocation #10330 introduced, neutralized, and structurally closed the synthetic reasoning placeholder respectively; this PR removes the placeholder instead.

Verification

• node --test tests/unit/responses-route-early-keepalive-wiring.test.ts — 1 passed
• node --import tsx/esm --test tests/unit/early-stream-keepalive.test.ts tests/unit/attempt-logging-early-keepalive-merge.test.ts — 22 passed
• OMNIROUTE_SSE_COMMENTS=on node --import tsx/esm --test tests/unit/sse-heartbeat.test.ts — 9 passed
• npm run check:cycles — passed, 424 files checked
• Focused ESLint — passed
• Focused Prettier check — passed
• git diff --check — passed
• Independent final review — approved with no remaining findings

npm run typecheck:core currently reports only pre-existing omniglyph dependency API drift in untouched compression modules; there are zero diagnostics in files changed by this PR.

Review notes

This is intentionally a Draft PR for maintainer review. It has not been deployed to production and has not been smoke-tested against external clients in this branch.