Agent Cards & Discovery — A2A Protocol & Google ADK

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Agent Cards & Discovery

🌐 A2A Protocol & Google ADK⏱ 10 min⭐ 100 BASE XP⌨ HANDS-ON LAB

How Agents Find Each Other

In A2A, every agent publishes an Agent Card - a JSON metadata document hosted at the standardized URI /.well-known/agent-card.json (with earlier versions and local ADK manifests also recognizing /.well-known/agent.json).

Agent Card Structure

{
  "name": "TravelBooker",
  "description": "Books flights, hotels, and rental cars",
  "version": "2.1.0",
  "url": "https://travel-agent.example.com/a2a",
  "capabilities": {
    "streaming": true,
    "pushNotifications": true,
    "stateTransitionHistory": true
  },
  "authentication": {
    "type": "oauth2",
    "authorizationUrl": "https://travel-agent.example.com/auth"
  },
  "skills": [
    {
      "id": "book_flight",
      "name": "Flight Booking",
      "description": "Search and book flights. Supports one-way and round-trip.",
      "inputSchema": { "type": "object", "properties": { "origin": {}, "destination": {}, "date": {} } }
    }
  ]
}

Discovery Flow

  1. Client agent queries /.well-known/agent-card.json at the target URL.
  2. Reads capabilities: What tasks can this agent handle? What auth does it need?
  3. Authenticates if required (OAuth 2.0, API keys, or open access).
  4. Creates a Task - sends a structured request to the remote agent.
🎯 Pro Tip: Agent Cards are like API documentation for agents. The more detailed and accurate your Agent Card, the more reliably other agents can discover and use your service.
⌨ HANDS-ON LABPublish and Query Agent Card Registry
⭐ +150 XP

Publish an Agent Card advertising domain capabilities and query the registry to locate candidate agents for delegation.

1Query Agent Card registry for legal compliance agents.
lab-sandbox — simulated environment
INFINITY LAB SANDBOX v2.6 — simulated shell
Type the command for the current objective. Helpers: "hint", "solution", "clear".
$
OBJECTIVE 1 / 1 — type "hint" if stuck
SYNAPSE VERIFICATION
QUERY 1 // 3
Where is an A2A Agent Card typically hosted?
In the system prompt
At /.well-known/agent-card.json
In a vector database
In the model weights