> ## 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. # A2A JSON-RPC > Communicate with an assistant using the Agent-to-Agent (A2A) Protocol over JSON-RPC 2.0. This endpoint accepts a JSON-RPC envelope and dispatches based on `method`. **Supported Methods:** - `message/send`: Send a message and wait for the final Task result. - `message/stream`: Send a message and receive Server-Sent Events (SSE) JSON-RPC responses. - `tasks/get`: Fetch the current state of a Task by ID. - `tasks/cancel`: Request cancellation (currently not supported; returns an error). **LangGraph Mapping:** - `message.contextId` maps to LangGraph `thread_id`. **Notes:** - Only `text` and `data` parts are supported; `file` parts are not. - If `message.contextId` is omitted, a new context is created. - Text parts require the assistant input schema to include a `messages` field. ## OpenAPI ````yaml /langsmith/agent-server-openapi.json post /a2a/{assistant_id} 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. paths: /a2a/{assistant_id}: post: tags: - A2A summary: A2A JSON-RPC description: > Communicate with an assistant using the Agent-to-Agent (A2A) Protocol over JSON-RPC 2.0. This endpoint accepts a JSON-RPC envelope and dispatches based on `method`. **Supported Methods:** - `message/send`: Send a message and wait for the final Task result. - `message/stream`: Send a message and receive Server-Sent Events (SSE) JSON-RPC responses. - `tasks/get`: Fetch the current state of a Task by ID. - `tasks/cancel`: Request cancellation (currently not supported; returns an error). **LangGraph Mapping:** - `message.contextId` maps to LangGraph `thread_id`. **Notes:** - Only `text` and `data` parts are supported; `file` parts are not. - If `message.contextId` is omitted, a new context is created. - Text parts require the assistant input schema to include a `messages` field. operationId: post_a2a parameters: - name: assistant_id in: path required: true schema: type: string format: uuid description: The ID of the assistant to communicate with - name: Accept in: header required: true schema: type: string description: >- For `message/stream`, must include `text/event-stream`. For all other methods, use `application/json`. requestBody: required: true content: application/json: schema: type: object properties: jsonrpc: type: string enum: - '2.0' description: JSON-RPC version id: type: string description: Request identifier method: type: string enum: - message/send - message/stream - tasks/get - tasks/cancel description: The method to invoke params: type: object description: Method parameters; shape depends on the method. oneOf: - title: Message Send/Stream Parameters type: object properties: message: type: object properties: role: type: string enum: - user - agent description: A2A role for the message sender. parts: type: array items: oneOf: - title: Text Part type: object properties: kind: type: string enum: - text text: type: string description: Text content. required: - kind - text - title: Data Part type: object properties: kind: type: string enum: - data data: type: object description: >- Structured JSON merged into the assistant input. required: - kind - data description: Message parts (text/data only). messageId: type: string description: Client-generated message identifier. contextId: type: string description: >- Conversation context ID; maps to LangGraph thread_id. taskId: type: string description: >- Optional task ID. Currently ignored by this implementation. required: - role - parts - messageId required: - message - title: Task Get Parameters type: object properties: id: type: string description: Task (run) identifier to retrieve. contextId: type: string description: Context identifier (thread_id) for the task. historyLength: type: integer minimum: 0 maximum: 10 description: Maximum number of history messages to include. required: - id - contextId - title: Task Cancel Parameters type: object properties: id: type: string description: Task (run) identifier to cancel. contextId: type: string description: Context identifier (thread_id) for the task. required: - id - contextId required: - jsonrpc - id - method examples: message_send: summary: Send a message (synchronous) value: jsonrpc: '2.0' id: '1' method: message/send params: message: role: user parts: - kind: text text: Hello from A2A - kind: data data: locale: en-US messageId: msg-1 contextId: f5bd2a40-74b6-4f7a-b649-ea3f09890003 message_stream: summary: Send a message (streaming) value: jsonrpc: '2.0' id: '2' method: message/stream params: message: role: user parts: - kind: text text: Stream this response messageId: msg-2 contextId: f5bd2a40-74b6-4f7a-b649-ea3f09890003 tasks_get: summary: Get a task value: jsonrpc: '2.0' id: '3' method: tasks/get params: id: run-uuid contextId: f5bd2a40-74b6-4f7a-b649-ea3f09890003 historyLength: 10 tasks_cancel: summary: Cancel a task (currently unsupported) value: jsonrpc: '2.0' id: '4' method: tasks/cancel params: id: run-uuid contextId: f5bd2a40-74b6-4f7a-b649-ea3f09890003 message_stream_with_context: summary: Stream multi-turn messages using the same contextId description: >- Send a streaming message; reuse the same contextId for subsequent turns to keep conversation state. value: jsonrpc: '2.0' id: '1' method: message/stream params: message: role: user contextId: thread-123 parts: - kind: text text: Hello! messageId: msg-1 interrupt_resume_text: summary: Resume an interrupted task with a text reply description: >- When a task is in input-required state, send a normal text part with the same contextId and taskId to resume. value: jsonrpc: '2.0' id: '2' method: message/send params: message: role: user contextId: thread-123 parts: - kind: text text: Here is the info you asked for. messageId: msg-2 taskId: thread-123:run-456 interrupt_resume_data_parallel: summary: Resume parallel interrupts with a data part description: >- Advanced: if your agent supports parallel interrupts, provide a structured resume payload in a data part with the same contextId and taskId. value: jsonrpc: '2.0' id: '3' method: message/send params: message: role: user contextId: thread-123 parts: - kind: data data: resume: - id: interrupt-1 value: choice: A - id: interrupt-2 value: choice: B messageId: msg-3 taskId: thread-123:run-456 responses: '200': description: >- JSON-RPC response for non-streaming methods. For `message/stream`, the response is an SSE stream of JSON-RPC envelopes. content: application/json: schema: type: object properties: jsonrpc: type: string enum: - '2.0' id: type: string result: type: object description: Success result containing task information or task details error: type: object properties: code: type: integer message: type: string description: Error information if request failed required: - jsonrpc - id examples: task_result: summary: Task result (message/send) value: jsonrpc: '2.0' id: '1' result: kind: task id: run-uuid contextId: f5bd2a40-74b6-4f7a-b649-ea3f09890003 status: state: completed artifacts: - artifactId: artifact-uuid name: Assistant Response parts: - kind: text text: Hello back error_result: summary: Error response value: jsonrpc: '2.0' id: '1' error: code: -32602 message: 'Invalid request: Missing required parameter: contextId' text/event-stream: schema: type: string description: SSE stream of JSON-RPC response objects. examples: stream_event: summary: SSE data chunk (JSON-RPC) value: >+ data: {"jsonrpc":"2.0","id":"2","result":{"kind":"status-update","taskId":"run-uuid","contextId":"f5bd2a40-74b6-4f7a-b649-ea3f09890003","status":{"state":"working"},"final":false}} '400': description: >- Bad Request (invalid JSON-RPC, invalid params, or missing Accept header). '404': description: Assistant not found '500': description: Internal server error ````