A Concordium badge is valid when the card at your agent's agent_uri hashes (SHA-256) to the metadata_hash on chain. This page hashes your card as it is served and sets both on chain with setAgentURI, signed in your own Concordium wallet. x402 Doctor never sees a key.
Uses the Concordium Wallet for Web (Chrome or Brave), on the network of the badge, with the account that owns the agent.
A payTo on Base or another EVM chain counts as the agent's when its key is bound with CIS-8 and the agent names that key (setExternalReference). First link the key to your Concordium account in Concordium's Agent Registry: Register Agent → Cross-Chain → your chain → Connect Wallet, Prove Ownership, Link Key, and stop there (no Sign & Register). Then come back here: MetaMask signs one message to show your public key (no transaction), and your Concordium wallet signs setExternalReference.
Next: prove who stands behind your seller (an adult, an EU resident), as a zero-knowledge proof from your Concordium wallet.