Client agent

A client agent is the party in an A2A interaction that starts the exchange, sending messages and task operations to a remote agent on behalf of a user or another system.

Terminology. The specification calls this role the “A2A Client”. The project’s key-concepts guide also calls it the client agent and says it can be an application, a service or another AI agent. The role applies to one interaction at a time. An agent that is a client in one exchange can be the remote agent in another.

What a client agent does.

Discovers. It fetches the remote agent’s Agent Card, picks the first entry in supportedInterfaces that it supports, uses that entry’s URL, and copies the entry’s tenant value into every request if one is set.
Authenticates. It reads securitySchemes, obtains credentials out of band, and sends them with every request.
States its version. It sends the A2A-Version header on each request. Without it, the server assumes version 0.3.
Converses. It sends messages with role set to ROLE_USER. That role marks messages from client to server, even when the client is itself an AI agent. To continue a task it includes the taskId, and any contextId it sends must match the task’s.
Tracks work. It polls with GetTask, streams with SendStreamingMessage or SubscribeToTask, or registers a push notification webhook.
Handles interruptions. It answers TASK_STATE_INPUT_REQUIRED with more information, and resolves TASK_STATE_AUTH_REQUIRED by obtaining authorization.

A follow-up that answers an input-required task (the params of a SendMessage call):

{
  "message": {
    "messageId": "msg-2",
    "taskId": "task-123",
    "contextId": "ctx-1",
    "role": "ROLE_USER",
    "parts": [{"text": "Order 1042, placed on 3 September."}]
  }
}

Chains of agents. A client agent may itself be working on a task for someone else. If a remote agent asks it for authorization, it can pass the request up by moving its own task to TASK_STATE_AUTH_REQUIRED, which forms a chain of tasks waiting on authorization.

Neighbouring terms. The remote agent is the other side of the exchange. The Agent Card is what the client agent reads first.

Sources

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