Anthropic /v1/messages: stop_sequences not enforced + stop_reason never "stop_sequence"

[raullenchai]

Summary

After PR #462 ("forward stop to engine"), Anthropic /v1/messages accepts stop_sequences but the engine does NOT actually truncate at the requested sequence. Additionally, the response always returns stop_reason: "end_turn" and stop_sequence: null — even when the model spontaneously emits the requested sequence in its output.

Repro

curl -X POST http://127.0.0.1:8501/v1/messages \
  -H "Authorization: Bearer SEKRET" -H "Content-Type: application/json" \
  -H "anthropic-version: 2023-06-01" \
  -d '{"model":"unsloth/Qwen3.6-27B-MLX-8bit","messages":[{"role":"user","content":"List letters A through F, separated by spaces."}],"stop_sequences":["STOP"],"max_tokens":50}'

Expected: response truncated at STOP, stop_reason: "stop_sequence", stop_sequence: "STOP".
Actual: model emits "A B C STOP D E F" (continues PAST the literal STOP it just generated), stop_reason: "end_turn", stop_sequence: null.

Severity

HIGH — two stacked breakages:

1. Enforcement: stop_sequences from client are ignored by the engine. Routes/anthropic.py:_resolved_sampling_kwargs forwards stop (fix(anthropic): forward stop_sequences to engine on /v1/messages #462) but the engine's streaming loop / sampler isn't actually checking against the list per-token.
2. Reporting: even when the model emits a stop_sequence by chance, the response builder never populates stop_reason="stop_sequence" / stop_sequence="STOP" — memory note flagged this as Phase 2 follow-up to fix(anthropic): forward stop_sequences to engine on /v1/messages #462.

Likely scope

• Engine: check request.stop against generated text per-token (or per-N-tokens) and terminate generation when match found
• Anthropic route: when termination was caused by stop, map finish_reason="stop" + matching sequence → stop_reason: "stop_sequence" + stop_sequence: "<matched>"

Same "param declared but not enforced" family as #459/#464/#465/#468 but at the engine layer.

Surfaced by iter15 onboarding sweep (hybrid agent).