[!todo] Seed note. A starting point, not a finished note yet.
An Agent Card is the discovery primitive of Google’s A2A protocol: a JSON document served at /.well-known/agent-card.json that tells other agents what this agent can do, what inputs and outputs it accepts, how to authenticate, and which protocol bindings (JSON-RPC, gRPC, REST) it speaks. It is the agent-to-agent equivalent of an OpenAPI spec or a service registry entry, and it is what makes cross-vendor delegation possible without prior coordination: a client agent fetches the card, reads the capabilities, and starts a Task. The interesting friction is that an Agent Card and an MCP tool definition are both “here is a capability and how to call it,” which is where the clean MCP/A2A boundary starts to blur. Seeded from MCP vs A2A.