# 2.12 — Anthropic SSE state machine (RED)

Phase: 2 — LLM surfaces
Stage: RED
Depends on: [2.11](2.11-anthropic-response-translation.md), [1.2](1.2-streaming-passthrough.md), [1.3](1.3-disconnect-propagation.md)

`POST /llm/v1/messages` with `"stream":true` must emit Anthropic SSE. Anthropic uses
NAMED events carrying content-block indices; the upstream emits flat OpenAI data-only
chunks. Write the failing tests against the event sequence before writing a translator.

- [ ] Every frame has both an `event:` line and a `data:` line; a bare `data:` frame is
      a failure on this surface
- [ ] `Content-Type` is `text/event-stream`
- [ ] Event order for a full response is exactly: `message_start`, then for each block
      `content_block_start`, one or more `content_block_delta`, `content_block_stop`,
      then `message_delta`, then `message_stop`
- [ ] `message_start` carries the message envelope with the client-sent model, `role`
      `assistant`, empty `content`, and `usage.input_tokens`
- [ ] The block carrying `reasoning_content` is index 0 with block type `thinking`, and
      its deltas are `thinking_delta`
- [ ] The block carrying `content` is index 1 with block type `text`, and its deltas
      are `text_delta`
- [ ] The end of reasoning is only knowable when `content` first arrives, so the
      arrival of the first `content` token MUST emit `content_block_stop` for index 0
      before `content_block_start` for index 1 — the two blocks never overlap
- [ ] If a response has no `reasoning_content` at all, the text block is index 0 and no
      thinking block is started; indices are assigned in emission order with no gaps
- [ ] If a response has `reasoning_content` and never any `content`, the thinking block
      is still closed before `message_delta`
- [ ] `message_delta` carries `stop_reason` and `usage.output_tokens`; upstream
      `finish_reason` `stop` becomes `end_turn` and `length` becomes `max_tokens`
- [ ] `message_stop` is the final frame and is emitted exactly once per response
- [ ] Translation is streaming and unbuffered: each upstream chunk is converted and
      flushed as it arrives, and the response is never accumulated to be inspected
- [ ] A client disconnect mid-stream cancels the upstream request immediately and
      releases the slot, rather than orphaning the generation
- [ ] An upstream failure after `message_start` terminates the stream with an error
      frame rather than a truncated but apparently successful sequence
- [ ] The OpenAI `data: [DONE]` sentinel is consumed by the translator and never
      forwarded to a `/llm` client

An orphaned generation holds one of only eight vLLM sequence slots in the cluster,
which is why disconnect cancellation is an acceptance criterion here and not only in
the proxy layer.

## Verify

```bash
curl -N -s localhost:8080/llm/v1/messages -H 'content-type: application/json' \
  -d '{"model":"reasoning","max_tokens":64,"stream":true,"messages":[{"role":"user","content":"hi"}]}' \
  | grep '^event:'
# expected: message_start, content_block_start, content_block_delta..., content_block_stop,
#           content_block_start, content_block_delta..., content_block_stop, message_delta, message_stop

curl -N -s localhost:8080/llm/v1/messages -H 'content-type: application/json' \
  -d '{"model":"reasoning","max_tokens":64,"stream":true,"messages":[{"role":"user","content":"hi"}]}' \
  | grep -n -E 'content_block_stop|"index":1' | head -3
# expected: the index 0 content_block_stop line precedes the first line mentioning index 1

timeout 1 curl -N -s localhost:8080/llm/v1/messages -H 'content-type: application/json' \
  -d '{"model":"reasoning","max_tokens":512,"stream":true,"messages":[{"role":"user","content":"long"}]}' >/dev/null
grep -c 'cancelled' /tmp/stub-reasoning.log
# expected: 1 within a second of the client going away
```