Agent capabilities (A2A)

Agent capabilities are the optional protocol features an A2A agent declares in its Agent Card’s required capabilities object, so a client knows which operations it may call before it calls them.

Fields.

Field	Meaning when true
streaming	SendStreamingMessage and SubscribeToTask are available
pushNotifications	The agent can POST task updates to a client webhook
extendedAgentCard	Authenticated clients can fetch a fuller card with GetExtendedAgentCard
extensions	A list of AgentExtension entries, each with a uri, a description, a required flag and optional params

A flag that is absent counts as false.

Enforcement. Section 3.3.4 turns these flags into rules. If streaming is false or missing, streaming operations must fail with UnsupportedOperationError (JSON-RPC -32004). If pushNotifications is false or missing, all four push notification config operations must fail with PushNotificationNotSupportedError (-32003). If extendedAgentCard is false or missing, GetExtendedAgentCard must fail with UnsupportedOperationError. An agent that declares the extended card but has none configured returns ExtendedAgentCardNotConfiguredError (-32007). When an extension is marked required and the client does not declare support for it, the agent returns ExtensionSupportRequiredError (-32008). Clients should read the card first.

A live example. Emissar’s public agent card, fetched on 26 September 2026, declares:

{"capabilities": {"streaming": false, "pushNotifications": false, "extendedAgentCard": false}}

A client following that card polls with GetTask and does not open streams or register webhooks.

Version changes. In v0.3 the extended card flag was a top-level card field, supportsAuthenticatedExtendedCard, and Appendix A.2.2 records its move into capabilities as a breaking change. The v0.3 stateTransitionHistory capability is not part of v1.0.

Neighbouring terms. Capabilities describe protocol features. Skills describe what the agent can do for a caller, and securitySchemes describes how to authenticate.