> ## Documentation Index > Fetch the complete documentation index at: https://docs.onecortex.io/llms.txt > Use this file to discover all available pages before exploring further. # Call an agent over A2A > A2A, for agents calling agents, over JSON-RPC. The `A2A-Version` header picks the version: `1.0` speaks `SendMessage`, `SendStreamingMessage`, `GetTask` and `CancelTask`; no header speaks 0.3's `message/send`, `message/stream`, `tasks/get` and `tasks/cancel`. Any other version is refused with `-32009`. `SendMessage` waits until the run ends or pauses and returns the task, with the reply as one text artifact. `SendStreamingMessage` answers a stream of JSON-RPC responses: the task, status and artifact updates, and a final status. A paused run leaves the task in `TASK_STATE_INPUT_REQUIRED`; reply with a message carrying its `taskId`. A refusal of the call itself keeps its HTTP status with a JSON-RPC error body. A method's own error is a `200` with a JSON-RPC error. See [A2A](/call/a2a) for tasks, contexts, pauses and the error codes. ## OpenAPI ````yaml /openapi.yaml post /v1/agents/{agentId}/a2a openapi: 3.1.0 info: title: Onecortex API version: '1' description: >- Call a deployed agent over invoke, AG-UI or A2A, and read its public A2A Agent Card. This is the whole public API. servers: - url: https://api.onecortex.io security: - apiKey: [] paths: /v1/agents/{agentId}/a2a: post: summary: Call an agent over A2A description: > A2A, for agents calling agents, over JSON-RPC. The `A2A-Version` header picks the version: `1.0` speaks `SendMessage`, `SendStreamingMessage`, `GetTask` and `CancelTask`; no header speaks 0.3's `message/send`, `message/stream`, `tasks/get` and `tasks/cancel`. Any other version is refused with `-32009`. `SendMessage` waits until the run ends or pauses and returns the task, with the reply as one text artifact. `SendStreamingMessage` answers a stream of JSON-RPC responses: the task, status and artifact updates, and a final status. A paused run leaves the task in `TASK_STATE_INPUT_REQUIRED`; reply with a message carrying its `taskId`. A refusal of the call itself keeps its HTTP status with a JSON-RPC error body. A method's own error is a `200` with a JSON-RPC error. See [A2A](/call/a2a) for tasks, contexts, pauses and the error codes. operationId: callAgentA2a parameters: - $ref: '#/components/parameters/AgentId' - name: A2A-Version in: header required: false description: '`1.0`, or leave it out for 0.3.' schema: type: string example: '1.0' requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/JsonRpcRequest' example: jsonrpc: '2.0' id: 1 method: SendMessage params: message: messageId: m1 role: ROLE_USER parts: - text: hello responses: '200': description: A JSON-RPC response, or for a streaming method a stream of them. headers: X-Request-Id: $ref: '#/components/headers/RequestId' content: application/json: schema: $ref: '#/components/schemas/JsonRpcResponse' example: jsonrpc: '2.0' id: 1 result: task: id: tsk_01K0000000000000000000000 contextId: 01K0000000000000000000000 status: state: TASK_STATE_COMPLETED timestamp: '2026-09-29T13:14:53.344Z' artifacts: - artifactId: reply name: reply parts: - text: 'You said: hello' text/event-stream: schema: type: string description: > One `data:` frame per JSON-RPC response, each holding one of `task`, `statusUpdate`, `artifactUpdate` in its `result`. ```text data: {"jsonrpc":"2.0","id":1,"result":{"task":{"id":"tsk_01K...","contextId":"01K...","status":{"state":"TASK_STATE_WORKING"}}}} data: {"jsonrpc":"2.0","id":1,"result":{"artifactUpdate":{"taskId":"tsk_01K...","contextId":"01K...","artifact":{"artifactId":"reply","name":"reply","parts":[{"text":"You said: hello"}]}}}} data: {"jsonrpc":"2.0","id":1,"result":{"statusUpdate":{"taskId":"tsk_01K...","contextId":"01K...","status":{"state":"TASK_STATE_COMPLETED"}}}} ``` '401': description: The API key is missing, unknown, revoked or expired. content: application/json: schema: $ref: '#/components/schemas/JsonRpcResponse' example: jsonrpc: '2.0' id: null error: code: -32600 message: A valid Onecortex API key is required. '404': description: >- No such agent in your organization, or the key is scoped to another agent. content: application/json: schema: $ref: '#/components/schemas/JsonRpcResponse' example: jsonrpc: '2.0' id: null error: code: -32600 message: That agent does not exist. '429': description: Over a rate limit. Wait for `Retry-After`, then retry. headers: Retry-After: description: Whole seconds to wait. schema: type: integer content: application/json: schema: $ref: '#/components/schemas/JsonRpcResponse' x-codeSamples: - lang: python label: Python source: > import os, httpx response = httpx.post( "https://api.onecortex.io/v1/agents/agt_.../a2a", headers={"Authorization": f"Bearer {os.environ['ONECORTEX_API_KEY']}", "A2A-Version": "1.0"}, json={ "jsonrpc": "2.0", "id": 1, "method": "SendMessage", "params": {"message": {"messageId": "m1", "role": "ROLE_USER", "parts": [{"text": "hello"}]}}, }, timeout=120, ) task = response.json()["result"]["task"] print(task["status"]["state"], task["artifacts"][0]["parts"][0]["text"]) - lang: typescript label: TypeScript source: > const response = await fetch('https://api.onecortex.io/v1/agents/agt_.../a2a', { method: 'POST', headers: { Authorization: `Bearer ${process.env.ONECORTEX_API_KEY}`, 'A2A-Version': '1.0', 'Content-Type': 'application/json', }, body: JSON.stringify({ jsonrpc: '2.0', id: 1, method: 'SendMessage', params: { message: { messageId: 'm1', role: 'ROLE_USER', parts: [{ text: 'hello' }] } }, }), }) const { result } = await response.json() console.log(result.task.status.state, result.task.artifacts[0].parts[0].text) - lang: bash label: curl source: | curl https://api.onecortex.io/v1/agents/agt_.../a2a \ -H "Authorization: Bearer $ONECORTEX_API_KEY" \ -H "A2A-Version: 1.0" \ -H "Content-Type: application/json" \ -d '{"jsonrpc":"2.0","id":1,"method":"SendMessage","params":{"message":{"messageId":"m1","role":"ROLE_USER","parts":[{"text":"hello"}]}}}' components: parameters: AgentId: name: agentId in: path required: true description: The agent's ID, from its page in the dashboard. schema: type: string example: agt_01K0000000000000000000000 schemas: JsonRpcRequest: type: object required: - jsonrpc - method properties: jsonrpc: const: '2.0' id: oneOf: - type: string - type: integer method: type: string example: SendMessage params: type: object additionalProperties: true JsonRpcResponse: type: object required: - jsonrpc properties: jsonrpc: const: '2.0' id: oneOf: - type: string - type: integer - type: 'null' result: type: object additionalProperties: true description: For SendMessage on 1.0, an object holding `task`. error: type: object properties: code: type: integer description: >- -32001 task not found, -32002 not cancelable, -32004 unsupported, -32005 content type not supported, -32009 version not supported, -32600, -32601, -32602, -32603. message: type: string headers: RequestId: description: The request's ID, `req_...`. Quote it to support. schema: type: string securitySchemes: apiKey: type: http scheme: bearer description: An Onecortex API key, `oc_live_...`, from API keys in the dashboard. ````