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A2A Protocol (Agent-to-Agent)

ai4j supports the Google Agent2Agent (A2A) protocol — an open standard for cross-framework agent interop. An ai4j agent can both call external A2A agents (as a client) and be exposed as an A2A service (as a server). JDK stdlib only, no new dependency.

A2A Client — call external agents​

A2AClient client = new A2AClient();
// optional auth:
// A2AClient client = new A2AClient("shared-secret-key");

AgentCard card = client.discover("https://other-agent.example.com");
String response = client.sendTask("https://other-agent.example.com", "What is 2+2?");

Wrap the client as a tool so your agent can call external agents:

A2ATool a2a = new A2ATool("https://other-agent.example.com");
Agent agent = Agents.react()
        .modelClient(modelClient).model("glm-5.1")
        .toolExecutor(a2a)
        .toolRegistry(a2aRegistry)  // defines the ask_remote_agent tool
        .build();
// agent can now call the external A2A agent as a tool

A2A Server — expose your agent​

A2AServer server = new A2AServer(
        myAgent,      // the ai4j Agent to expose
        0,            // port (0 = auto-assign)
        "my-agent",   // name (shown in AgentCard)
        "does stuff", // description
        "secret-key"  // optional shared-secret auth (null = open)
);

// External agents (LangChain, CrewAI, etc.) can now call:
//   GET  http://localhost:PORT/.well-known/agent.json  → AgentCard
//   POST http://localhost:PORT/tasks/send              → JSON-RPC task
server.close();

Auth​

Both client and server support optional shared-secret auth via X-API-Key header:

  • Client: new A2AClient("secret-key") — sends X-API-Key on all requests.
  • Server: new A2AServer(agent, port, name, desc, "secret-key") — validates X-API-Key on /tasks/send. The AgentCard endpoint is always open (discovery should be public).

Full JWT/OIDC auth is a future addition.

Where this fits​