/api/v1/environments/{id}/conversations/{conversationId}/messages

Body must be EXACTLY {content} (everything else — attribution, idempotency, budget —

is server- or header-bound); content is capped at 16000 chars. Idempotency-Key is

required and the reply is asynchronous: the turn runs on the agent turn port and the

reply is fetched by polling the GET face. The response includes a durable runId

(run_…) for status, event replay and cancellation. The fixed per-message price is

charged BEFORE dispatch; a replay hit (deduplicated:true) neither charges nor

dispatches again and returns the original message/run ids. Anonymous principals: 403

capability_denied.