> ## 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 Command > Send a single protocol command scoped to a thread. The request body is a `ProtocolCommand` envelope with a `method` (e.g. `run.start`, `input.respond`, `agent.getTree`) and method-specific `params`. The response is either a `ProtocolSuccess` (with method-specific `result`) or a `ProtocolError`. Commands that create runs (`run.start`, `input.respond`) leave the run executing in the background on the worker queue. Event streaming for that run is observed via a concurrent `POST /threads/{thread_id}/stream/events` connection. WebSocket clients use the same command envelope in-band on `/threads/{thread_id}/stream/events` and additionally have access to `subscription.subscribe` / `subscription.unsubscribe` over the same connection. ## OpenAPI ````yaml /langsmith/agent-server-openapi.json post /threads/{thread_id}/commands 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}/commands: post: tags: - Streaming summary: Protocol v2 Command description: >- Send a single protocol command scoped to a thread. The request body is a `ProtocolCommand` envelope with a `method` (e.g. `run.start`, `input.respond`, `agent.getTree`) and method-specific `params`. The response is either a `ProtocolSuccess` (with method-specific `result`) or a `ProtocolError`. Commands that create runs (`run.start`, `input.respond`) leave the run executing in the background on the worker queue. Event streaming for that run is observed via a concurrent `POST /threads/{thread_id}/stream/events` connection. WebSocket clients use the same command envelope in-band on `/threads/{thread_id}/stream/events` and additionally have access to `subscription.subscribe` / `subscription.unsubscribe` over the same connection. operationId: protocol_thread_command_post parameters: - name: thread_id in: path required: true schema: type: string format: uuid title: Thread Id description: The ID of the thread the command applies to. requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/ProtocolCommand' responses: '200': description: Protocol response — either `ProtocolSuccess` or `ProtocolError`. content: application/json: schema: oneOf: - $ref: '#/components/schemas/ProtocolSuccess' - $ref: '#/components/schemas/ProtocolError' '400': description: Malformed command envelope content: application/json: schema: $ref: '#/components/schemas/ProtocolError' components: schemas: ProtocolCommand: type: object title: ProtocolCommand description: >- A client → server command envelope. Known methods: `run.start`, `input.respond`, `subscription.subscribe` (WS only), `subscription.unsubscribe` (WS only), `agent.getTree`. required: - id - method properties: id: type: integer minimum: 0 description: Client-assigned command id; echoed in the response. method: type: string description: Command method name (e.g. `run.start`). params: type: object description: Method-specific parameters. See the CDDL spec for per-method shapes. additionalProperties: true additionalProperties: true ProtocolSuccess: type: object title: ProtocolSuccess description: Successful command response. required: - type - id - result properties: type: type: string const: success id: type: integer minimum: 0 result: type: object description: Method-specific result payload. additionalProperties: true meta: $ref: '#/components/schemas/ProtocolResponseMeta' additionalProperties: true ProtocolError: type: object title: ProtocolError description: Error response for a command. required: - type - id - error - message properties: type: type: string const: error id: oneOf: - type: integer minimum: 0 - type: 'null' description: >- The `id` of the offending command, or null if the envelope was unparseable. error: type: string enum: - invalid_argument - unknown_command - unknown_error - no_such_run - no_such_subscription - no_such_namespace - no_such_interrupt - no_such_checkpoint - permission_denied - not_supported message: type: string stacktrace: type: string meta: $ref: '#/components/schemas/ProtocolResponseMeta' additionalProperties: true ProtocolResponseMeta: type: object title: ProtocolResponseMeta description: Optional metadata attached to command responses. properties: applied_through_seq: type: integer minimum: 0 description: >- Highest event `seq` observed on the session when this response was generated. additionalProperties: true ````