Respond to chat message (streaming)

Streams a single assistant turn as normalized SSE events with stable turn, message, and part IDs. Emits events: turn.started, turn.status, message.created, message.part.updated, message.completed, session.usage, turn.completed.

When to use this endpoint: Recommended for building custom chat UIs that need real-time streaming progress. This is the primary streaming endpoint for new integrations.

Alternatives:

• POST /chat/:id/message — synchronous, returns complete response as JSON (best for server-to-server)