Get next message to process

Returns the single oldest message that needs processing. ## What It Returns The oldest message that is NOT processed, including: - New messages (no delivery status yet) - Delivered messages (acknowledged but not started) - Processing messages (stuck/crashed - supports crash recovery) - Failed messages (available for retry) Returns **204 No Content** if there are no messages to process. ## Workflow This is the primary endpoint for agent reasoning loops: 1. `GET /messages/next` → Get next work item 2. `POST /messages/{id}/processing` → **Required:** Mark as processing 3. Process the message (reasoning loop, tool calls, etc.) 4. `POST /messages/{id}/processed` → Mark as done, OR `POST /messages/{id}/failed` → Mark as failed with error message 5. Loop back to step 1 ## Delivery Semantics (at-least-once) Delivery is **at-least-once**. The same message can be returned more than once: after a crash or reconnect (`processing` messages are re-served for recovery), or when multiple clients poll with the same API key. Make your processing **idempotent** — deduplicate by message `id` when a repeated run would have side effects. ## Crash Recovery If your agent crashes while processing, the message stays in `processing` state. When restarted, calling `/next` will return that same stuck message (oldest first), allowing the agent to reclaim and retry it. ## Difference from GET /messages - `GET /messages` returns **all** actionable messages (for batch processing or queue inspection) - `GET /messages/next` returns **one** message (for sequential processing loops) Both use the same filter logic: everything that is NOT processed.