Replay Harness LLM Streaming Wire-Fidelity Oracle

LIFECYCLE: permanent — a deterministic, advisory structural protocol test for the LLM gateway's OpenAI-compatible streaming path. Run it with:

npm run test:replay-llm-streaming-wire-fidelity

The oracle starts a loopback-only OpenAI-compatible fake upstream that sends synthetic SSE frames across deliberately fragmented HTTP chunks. It drives the real gateway router, resolver, executor, OpenAICompatibleChatCompletionProvider.stream_chat_completion, and StreamingResponse. A test-only dependency override selects the loopback provider; production routing, provider construction, retries, schemas, and deployment configuration remain unchanged.

Contract

The oracle proves only that:

1. the provider request selects streaming and terminal usage;
2. the gateway responds with text/event-stream;
3. fragmented upstream SSE produces nonterminal client data frame(s), then one terminal usage/finish frame, then exactly one [DONE]; and
4. an inbound upstream_url is rejected before the provider path, within a bounded local deadline and with non-loopback socket egress denied.

Residual boundary

This is not provider conformance, production endpoint behavior, client compatibility, traffic capture/replay, a release gate, LC3 or timing qualification, or Phase 0B. It neither changes nor replaces the Listen Pusher or Sync Cloud Tasks gauntlets, and it makes no real provider calls.

Data safety

The fake parses request and SSE envelopes only long enough to assert protocol shape. It retains and prints only endpoint/frame labels and ordering, schema-key sets, status/error classes, a timing bucket, and request count. It never logs or persists prompt/completion/token values, SSE payloads, headers, credentials, identifiers, or provider bodies.