What is Agent card?

In A2A, an Agent Card is the self-description that an agent publishes about itself. A client agent reads it to judge whether the remote agent suits a task, to build requests correctly, and to know how to authenticate.

The A2A specification lists required fields: name, description, supportedInterfaces, version, capabilities, defaultInputModes, defaultOutputModes, and skills. Optional fields include provider, documentationUrl, securitySchemes, securityRequirements, signatures, and iconUrl. The first entry in supportedInterfaces is the preferred one, capabilities cover items like streaming and push notifications, and each skill is an AgentSkill object.

For discovery, the specification gives a well-known path on the agent's domain, https://{server_domain}/.well-known/agent-card.json, which a client fetches with an HTTP GET. A discovery topic page also describes curated registries and direct configuration as other options, and says the specification does not define a standard registry API.

Cards may be digitally signed with JSON Web Signature after canonicalization. The A2A discovery guide says a card containing sensitive data must be protected with authentication, and the specification describes an authenticated extended card for a fuller version.