The OpenAI-compatible API server only enforced a hardcoded cap of 10
concurrent runs on /v1/runs, leaving /v1/chat/completions and
/v1/responses unbounded — a request flood could exhaust CPU, memory,
and upstream LLM quota (#7483).
- Add gateway.api_server.max_concurrent_runs (config.yaml, default 10,
0 disables). No env var.
- Shared concurrency gate across all three agent-serving endpoints,
counting both the chat/responses in-flight counter and the /v1/runs
stream set. Returns OpenAI-style 429 + Retry-After when at the cap.
- Remove the dead hardcoded _MAX_CONCURRENT_RUNS class attribute.
Closes#7483.