> ## Documentation Index > Fetch the complete documentation index at: https://docs.langchain.com/llms.txt > Use this file to discover all available pages before exploring further. # Protocol v2 Event Stream (SSE) > Open a connection-scoped SSE event stream for a thread. The request body is a `ProtocolEventStreamRequest` carrying channel and namespace filters; the server replies with `Content-Type: text/event-stream` and pushes matching `ProtocolEvent` frames for the lifetime of the connection. Closing the connection unsubscribes — no state is persisted server-side. Reconnect: clients pass the last `seq` they received as `since` in the body. Buffered events with `seq > since` are replayed before the stream goes live. The endpoint is POST-only, so browser-native `EventSource` auto-resume (`Last-Event-ID`) does not apply — clients drive resume explicitly via the body. ## OpenAPI ````yaml /langsmith/agent-server-openapi.json post /threads/{thread_id}/stream/events openapi: 3.1.0 info: title: LangSmith Deployment version: 0.1.0 servers: [] security: [] tags: - name: Assistants description: An assistant is a configured instance of a graph. - name: Threads description: A thread contains the accumulated outputs of a group of runs. - name: Thread Runs description: >- A run is an invocation of a graph / assistant on a thread. It updates the state of the thread. - name: Stateless Runs description: >- A run is an invocation of a graph / assistant, with no state or memory persistence. - name: Crons description: >- A cron is a periodic run that recurs on a given schedule. The repeats can be isolated, or share state in a thread - name: Store description: >- Store is an API for managing persistent key-value store (long-term memory) that is available from any thread. - name: A2A description: >- Agent-to-Agent Protocol related endpoints for exposing assistants as A2A-compliant agents. - name: MCP description: >- Model Context Protocol related endpoints for exposing an agent as an MCP server. - name: System description: System endpoints for health checks, metrics, and server information. - name: Streaming description: >- Thread-centric streaming endpoints. Provides a structured command/event surface over SSE+HTTP; WebSocket is also supported at `/threads/{thread_id}/stream/events` (not documented here — OpenAPI 3.1 does not describe WebSocket). paths: /threads/{thread_id}/stream/events: post: tags: - Streaming summary: Protocol v2 Event Stream (SSE) description: >- Open a connection-scoped SSE event stream for a thread. The request body is a `ProtocolEventStreamRequest` carrying channel and namespace filters; the server replies with `Content-Type: text/event-stream` and pushes matching `ProtocolEvent` frames for the lifetime of the connection. Closing the connection unsubscribes — no state is persisted server-side. Reconnect: clients pass the last `seq` they received as `since` in the body. Buffered events with `seq > since` are replayed before the stream goes live. The endpoint is POST-only, so browser-native `EventSource` auto-resume (`Last-Event-ID`) does not apply — clients drive resume explicitly via the body. operationId: protocol_thread_events_post parameters: - name: thread_id in: path required: true schema: type: string format: uuid title: Thread Id description: The ID of the thread to observe. requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/ProtocolEventStreamRequest' responses: '200': description: SSE event stream content: text/event-stream: schema: type: string description: >- Stream of `ProtocolEvent` frames. Each SSE frame uses the event's `method` as the `event:` field and a JSON-encoded `ProtocolEvent` as the `data:` field; `id:` is set to the event's `seq`. **Example frame:** id: 42 event: messages data: {"type":"event","event_id":"42","seq":42,"method":"messages","params":{"namespace":[],"timestamp":1700000000000,"data":{...}}} '400': description: Invalid request body content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' components: schemas: ProtocolEventStreamRequest: type: object title: ProtocolEventStreamRequest description: >- Body of `POST /threads/{thread_id}/stream/events`. Filters the connection-scoped SSE stream. required: - channels properties: channels: type: array items: $ref: '#/components/schemas/ProtocolChannel' minItems: 1 description: Channels to subscribe to for the lifetime of this connection. namespaces: type: array items: $ref: '#/components/schemas/ProtocolNamespace' description: >- Prefix-match filter. An event matches if its namespace starts with any of these prefixes. Omit for all namespaces. depth: type: integer minimum: 0 description: Max depth below the namespace prefix. Omit for unbounded depth. since: type: integer minimum: 0 description: >- Replay events with `seq` greater than this value before streaming live events. additionalProperties: true ErrorResponse: type: object required: - detail properties: detail: type: string description: Human-readable error message title: ErrorResponse description: Error response returned from the server ProtocolChannel: type: string title: Protocol Channel description: >- Subscribable event channel. One of the eight fixed channels or a user-defined `custom:` namespaced custom channel emitted by a stream transformer. `debug` and `checkpoints` were removed from the protocol in `@langchain/protocol@0.0.10` — task-level debug info now flows on the `tasks` channel, and checkpoint pointers ride on `values` events as an attached `checkpoint` envelope. anyOf: - type: string enum: - values - updates - messages - tools - lifecycle - input - tasks - custom - type: string pattern: ^custom:.+$ description: User-defined namespaced custom channel. ProtocolNamespace: type: array items: type: string title: Protocol Namespace description: >- Hierarchical path identifying a position in the agent tree. Empty array denotes the root agent; each segment names a nested subgraph. ````