ERC-5192 (Soulbound Tokens)

ERC-5192 adds a single view function, locked(uint256 tokenId), that returns a boolean. If locked is true, all ERC-721 transfer functions such as transferFrom or safeTransferFrom must revert for that token. The standard emits Locked and Unlocked events when state is set or changed, and supports ERC-165 interface detection, so an indexer can ask whether a contract implements it. It is deliberately minimal: it does not define how tokens are minted, burned or managed, and it keeps compatibility with all existing ERC-721 tooling for reading and display.

It is the technical basis for on-chain credentials, identity and reputation: academic degrees, certifications, memberships, proof of attendance and reputation badges. It addresses the need for an identity asset that cannot be sold or rented, which avoids markets for purchased credentials. Its conceptual origin is the paper Decentralized Society: Finding Web3's Soul, by Weyl, Ohlhaver and Buterin in 2022, which introduced soulbound tokens as a foundation for verifiable identity.

The standard, titled Minimal Soulbound NFTs, is in Final status, and its reference implementation is maintained by attestate on GitHub. It is the canonical way to declare non-transferability through the locked() function, and it connects naturally with attestation systems and schemes like POAP for proof of attendance. Note that many soulbound tokens enforce non-transferability ad hoc without declaring it via ERC-5192, so attributing concrete adoption requires checking contract by contract.