Discovery and Agent Cards

Discovery resolves server capabilities before constructing a client transport.

Public and extended cards

DiscoveryClient supports:

Fetch(base_url) for the standard Agent Card.
FetchExtendedAgentCard(base_url) for the extended Agent Card endpoint.

Fetched cards are cached for kDefaultDiscoveryCacheTtl (300 seconds) unless a different TTL is supplied.

Interface resolution

AgentCardResolver::SelectPreferredInterface(card, preferred) selects a ResolvedInterface for one of:

PreferredTransport::kRest
PreferredTransport::kJsonRpc
PreferredTransport::kGrpc

The result includes the transport, URL, security requirements, and security schemes needed to configure a client.

Operational guidance

Validate and log the selected endpoint during startup.
Prefer an explicit fallback order when cards advertise multiple transports.
Treat discovery metadata as untrusted input until validated.
Refresh cached cards after deployment or capability changes.

include/a2a/client/discovery.h
include/a2a/core/agent_card/agent_card_provider.h
include/a2a/server/agent_card/agent_card_serializer.h