A2A Client MCP Server

Instructions

Implementation Reference

• index.ts:202-244 (handler)

  Handler for the 'a2a_send_task_subscribe' MCP tool. Selects an A2A client, sends a task with subscription for streaming updates, collects up to maxUpdates events, and returns the task ID and updates as JSON.

  case "a2a_send_task_subscribe": {
    const { message, taskId, agentId, maxUpdates = 10 } = args as {
      message: string;
      taskId?: string;
      agentId?: string;
      maxUpdates?: number;
    };
    
    const client = agentId ? agentManager.getClientById(agentId) : agentManager.getAllClients().values().next().value;
    
    if (!client) {
      throw new Error(`No available agent${agentId ? ` with ID ${agentId}` : ''}`);
    }
  
    const id = taskId || crypto.randomUUID();
    const stream = client.sendTaskSubscribe({
      id,
      message: {
        role: "user",
        parts: [{ text: message }],
      },
    });
  
    const updates = [];
    let count = 0;
    
    for await (const event of stream) {
      updates.push(event);
      count++;
      if (count >= maxUpdates) break;
      
      if (event.final) break;
    }
  
    return {
      content: [
        {
          type: "text",
          text: JSON.stringify({ taskId: id, updates }, null, 2),
        },
      ],
    };
  }

• index.ts:94-114 (schema)

  Input schema for the a2a_send_task_subscribe tool, validating message (required string), optional taskId, agentId, and maxUpdates (number).

  inputSchema: {
    type: "object",
    properties: {
      message: {
        type: "string",
        description: "Message to send to the agent",
      },
      taskId: {
        type: "string",
        description: "Optional task ID. If not provided, a new UUID will be generated",
      },
      agentId: {
        type: "string",
        description: "Optional agent ID. If not provided, the first available agent will be used",
      },
      maxUpdates: {
        type: "number",
        description: "Maximum number of updates to receive (default: 10)",
      },
    },
    required: ["message"],

• index.ts:91-116 (registration)

  Registration of the a2a_send_task_subscribe tool in the ListToolsRequestSchema response, including name, description, and input schema.

  {
    name: "a2a_send_task_subscribe",
    description: "Send a task and subscribe to updates (streaming)",
    inputSchema: {
      type: "object",
      properties: {
        message: {
          type: "string",
          description: "Message to send to the agent",
        },
        taskId: {
          type: "string",
          description: "Optional task ID. If not provided, a new UUID will be generated",
        },
        agentId: {
          type: "string",
          description: "Optional agent ID. If not provided, the first available agent will be used",
        },
        maxUpdates: {
          type: "number",
          description: "Maximum number of updates to receive (default: 10)",
        },
      },
      required: ["message"],
    },
  },

• a2a-client.ts:391-409 (helper)

  Supporting utility method A2AClient.sendTaskSubscribe that initiates the streaming JSON-RPC request to 'tasks/sendSubscribe' for real-time task updates.

  sendTaskSubscribe(
    params: TaskSendParams
  ): AsyncIterable<TaskStatusUpdateEvent | TaskArtifactUpdateEvent> {
    const streamGenerator = async function* (
      this: A2AClient
    ): AsyncIterable<TaskStatusUpdateEvent | TaskArtifactUpdateEvent> {
      const httpResponse = await this._makeHttpRequest(
        "tasks/sendSubscribe",
        params,
        "text/event-stream"
      );
      yield* this._handleStreamingResponse<TaskStatusUpdateEvent | TaskArtifactUpdateEvent>(
        httpResponse,
        "tasks/sendSubscribe"
      );
    }.bind(this)();
  
    return streamGenerator;
  }