AsyncAPI channel · Letta · Letta Agent Message Streaming (HTTP + SSE)

/agents/{agent_id}/messages

Agent message SSE stream. The client opens this channel by issuing `POST /agents/{agent_id}/messages` with `Content-Type: application/json` and a JSON body containing `stream_steps: true` (add `stream_tokens: true` to additionally stream partial assistant-message tokens). The server responds with `Content-Type: text/event-stream` and emits an ordered sequence of `data:` lines, each carrying one JS

Open source AsyncAPI → All Letta channels Letta provider profile
Provider: Letta AsyncAPI: v2.6.0 Spec: Letta Agent Message Streaming (HTTP + SSE) Operations: 1 Messages: 7

Channel address

/agents/{agent_id}/messages

Parameters

Operations

streamAgentMessages
Subscribe to streamed agent turn events (SSE).

Messages

ReasoningMessage
The agent's internal chain-of-thought for the current step, when reasoning is enabled and visible.
Content-Type: application/json
ToolCallMessage
A tool invocation the agent is making as part of this step.
Content-Type: application/json
ToolReturnMessage
The result returned by a tool the agent called.
Content-Type: application/json
AssistantMessage
The agent's user-facing reply content for this turn. Streamed token-by-token when stream_tokens is set.
Content-Type: application/json
LettaPing
A periodic keep-alive event so the SSE connection does not time out during long-running turns.
Content-Type: application/json
UsageStatistics
Optional final chunk reporting token and step usage for the completed turn.
Content-Type: application/json
StreamDone
The literal SSE event `data: [DONE]` that marks end of stream. Not JSON; the payload is the string `[DONE]`.
Content-Type: text/plain

About AsyncAPI

The AsyncAPI specification describes event-driven APIs the way OpenAPI describes request/response APIs. A channel is the named pipe — a webhook URL, a Kafka topic, a WebSocket route, an MQTT subject — that producers and consumers publish or subscribe to. Each channel carries one or more messages with structured payloads, and an operation declares whether a given party sends or receives on that channel.

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