How it works in AgenID
The signer removes signature, canonicalizes the rest with RFC 8785 and passes those exact bytes to Ed25519. This is PureEdDSA: no Ed25519ph and no separate pre-hash of the payload. The 64-byte result is carried as base64url without padding, and public keys appear in key documents as public_key_b64u. The browser signer at /issue generates the key pair locally and never transmits the private key. With 32-byte keys and 64-byte signatures, a key document and proof fit comfortably in a resolution envelope.
Why it matters
Anyone with the public key can check a signature offline, so a verifier needs neither an account with AgenID nor faith in its registry. The registry can store an operator's signature but cannot forge one. The specification publishes executed test vectors, and the independent conformance suite lets another implementation show it reproduces them byte for byte.
Deterministic signing also keeps testing honest. The same key and the same canonical bytes always produce the same signature, so a published vector either reproduces exactly or it does not; there is no result that is merely close.
What it does not mean
A valid signature proves the key holder signed these bytes. It does not prove the signed statements are true, it does not show the key had the right role (AgenID checks that separately), and it does not say what the agent is allowed to do.