[jjxyxsjr]

Problem and behavior

The stream-readiness watchdog reports one undifferentiated timeout: "no non-ping SSE event within the window". But two very different failure modes hide behind that message — a connection that never delivered any byte (dead path / silent upstream) versus first bytes (e.g. pings) flowing while content stalls before the first non-ping event (slow-but-alive upstream). When triaging TTFT incidents, operators currently cannot tell a network-side death from a provider-side stall without packet capture.

Changes

open-sse/utils/streamReadiness.ts — pure telemetry, no behavioral change:

• The readiness result now carries anyByteAt: a timestamp when any byte (pings included) was observed, or an explicit null when the window expired with zero bytes ever — confirmed silence, not "unknown".
• A response-header carrier exposes the same signal to the SSE layer; the timeout log lines distinguish the two shapes ("zero bytes ever" vs "stalled after first byte").
• All state is per-call scoped (no module-level timestamps), so concurrent streams cannot contaminate each other's signal.

Validation

tests/unit/stream-readiness-zero-byte-signal.test.ts (new, 5 tests) pins: null on confirmed zero bytes; a timestamp once any byte arrives; the header carrier round-trip; per-call isolation between concurrent streams; and the default-path zero-change property. Adjacent stream-readiness suites (55 tests) green · tsc exit 0 · repo npm run lint exit 0 · changelog fragment 18586-zero-byte-readiness-signal.md included.

Limitation: telemetry only — no retry/timeout decision consumes the signal in this PR; wiring it into policy (e.g. zero-byte = terminate-and-never-resend vs stall = rotate) is deliberately left for a follow-up once the observability lands.

Notes

• Default path is byte-identical: the fields are additive on the result object and the log wording change is the only visible delta.
• No new env, no dependencies, no schema, no UI.
• Base: release/v3.8.53 (active release branch); aware of the 3.8.55 v4-feature retarget policy.