Starts a chat agent session in a conversation.

Agent must be active and use an active LLM. The agent cannot access past messages unless the conversation_history placeholder is included in the prompt. Be sure to include it if past messages are important for the use case.

Accepts an optional schedule_at parameter to schedule the chat session instead of placing it immediately, but attention, the API responds with 202 Accepted instead of 201 Created in this case. Scheduled accepted responses include data.id, the stable ID that the eventual chat agent session will use when the scheduled job creates it. Acceptance does not guarantee downstream chat agent session creation succeeds.