The true-streaming SSE passthrough (Body::from_stream, no full-response
buffering, with chunk-observation hooks) was cortex-only. helexa-router
(#69) needs the same mechanism to proxy a chat-completions/messages
stream verbatim to a selected cortex. Extract it once.
New `crates/helexa-stream` owns the *mechanism* (kept HTTP-free
cortex-core untouched — it would have forced axum/reqwest/futures onto
every cortex-core consumer):
- `forward_streaming(client, url, headers, body, observer)` — POST and
stream the response back chunk-for-chunk; status-agnostic, so a
non-2xx (e.g. cortex 429) is passed through with status+headers
intact (the #69 backpressure-passthrough requirement).
- `ChunkObserver` trait + `ObservedStream` wrapper — feeds each chunk to
the observer, calls `finish` exactly once on clean end or on drop
(client disconnect).
- `BodyTail` (bounded tail accumulator) + `last_count_for` (trailing
OpenAI `usage` extraction) — the reusable pieces an observer uses.
cortex keeps its *policy*: `proxy.rs` now supplies a `CortexMetrics`
observer (per-request token metrics + per-principal reservation settle),
its logging contract, and the error envelope, driving the shared
mechanism. `proxy::last_count_for` is re-exported so `handlers`/
`anthropic_sse` call sites are unchanged. No behaviour change — the
existing cortex `streaming.rs` tests pass as-is.
helexa-stream tests prove chunk-for-chunk incremental delivery, observer
finish-once, usage extraction, and non-2xx passthrough.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01F6o3ddqmYNh9kzdwq6eowh
