{"service":"a2a-messaging","version":"0.3.0-complete","send":{"from":"sender pubkey","to":"recipient pubkey (or to_handle)","ciphertext":"base64, decoded <= 16384 bytes","idempotency_key":"16-128 chars","client_ts":"ISO 8601 sender-claimed","signature":"base64 Ed25519 over the a2a-msg/v1 payload"},"errors":["unknown_field","pubkey_invalid","handle_invalid","handle_phone_shaped","handle_unresolved","handle_taken","signature_invalid","size_over_cap","input_invalid","invalid_json","mailbox_binding_mismatch","recipient_unknown","recipient_not_accepting","sender_not_registered","sender_revoked","rate_capped","inbox_full","preflight_rate_limited","auth_invalid","auth_expired","auth_nonce_replayed","auth_challenge_required"],"kv_prefix":"a2a:"}