[diegosouzapw]

Closes #17825

Root cause

open-sse/utils/sseHeartbeat.ts built the synthetic response.in_progress keepalive once, as a module constant with a stub response ({id, status}). The same stub was used for the early/startup keepalive (src/app/api/v1/responses/route.ts) and for mid-stream heartbeats. Spec-validating clients (Vercel AI SDK @ai-sdk/openai) require response.id, response.created_at and response.model on response.in_progress and abort the whole stream with AI_TypeValidationError.

Fix

• New open-sse/utils/responsesKeepaliveFrame.ts: buildResponsesInProgressFrame() builds a complete Response (object, created_at, model, output, error, incomplete_details, background, parallel_tool_calls, tool_choice, tools), plus createResponsesInProgressTracker().
• sseHeartbeat.ts: OPENAI_RESPONSES_IN_PROGRESS_FRAME is now the complete fallback frame (neutral model). For the Responses shape the transform tracks id / created_at / model / sequence_number from passing response.created / response.in_progress lines (substring guard, split-chunk tolerant, never throws) and heartbeats reuse them.
• responses/route.ts: per-request startup and application keepalive frame (unique resp_keepalive_<uuid>, request-time created_at, request model).
• Keepalive stays a typed event (Codex CLI needs a parser-visible one, [BUG] Codex CLI 0.130.0 Responses API: stream disconnected before completion (~5s) #2544).

Regression test

tests/unit/responses-keepalive-spec-valid-17825.test.ts (4 cases).

RED on unfixed code:

node --import tsx/esm --test --test-force-exit tests/unit/responses-keepalive-spec-valid-17825.test.ts
✖ #17825: startup/keepalive response.in_progress frame is a complete Response event
✖ #17825: mid-stream heartbeat reuses the real response id/created_at/model
ℹ tests 2 / pass 0 / fail 2

GREEN after the fix: all cases in the file pass. Old tests that pinned the stub byte-for-byte (sse-heartbeat, early-stream-keepalive, sseHeartbeat, responses-route-early-keepalive-wiring) were realigned to the complete shape without removing assertions (the route wiring test now matches the per-request keepaliveFrame).

Gates

eslint (with suppressions), prettier, typecheck:core, check:open-sse-typecheck, check-file-size, check-complexity, check-cognitive-complexity, check-changelog-integrity all OK. Pre-commit hooks passed.

Note: sseHeartbeat.test.ts and terminal-heartbeat-cleanup-16258.test.ts use real 10-35 ms timers and flaked intermittently on the base as well while the host was at load average 50-100.

⚠️ base-red inherited: #17219 — Docs sync strict (docs/i18n/am/README.md migration count), not touched by this PR