soulbound token
A soulbound token is a token that cannot be transferred once you hold it — it is bound to your wallet (your 'soul'). The name borrows from video games, where a soulbound item is one you can equip but never trade away. Because it cannot be bought, sold, or moved, an SBT is unsuited to representing money or collectibles and instead suited to representing things that should not be tradable: credentials, memberships, attendance, reputation, and identity.
Technically an SBT is usually a token (often based on ERC-721) whose transfer functions are disabled — the contract overrides transferFrom and safeTransferFrom to revert, allowing only mint (issuing it to you) and sometimes burn (you discarding it). Standards have crystallized this: EIP-5192 defines a minimal locked() flag and a Locked event for non-transferable ERC-721s, and EIP-5114 describes badges bound at mint. The idea was popularized by Vitalik Buterin, Glen Weyl and Puja Ohlhaver's 'Decentralized Society' paper.
Soulbound tokens matter because transferability is the enemy of identity. A diploma you can sell is worthless as proof you studied; a vote you can sell enables bribery; a 'unique human' badge you can transfer enables Sybil attacks. By making the token non-transferable, an SBT ties a claim to a specific actor, enabling use cases like on-chain credentials, proof-of-attendance (POAPs are a related idea), reputation for undercollateralized lending, and Sybil-resistant governance. The open hard problems are key loss (your soul is your private key) and privacy (public credentials are public).
function _update(address to, uint256 id, address auth) internal override returns (address) {
address from = _ownerOf(id);
require(from == address(0) || to == address(0), "Soulbound: non-transferable");
return super._update(to, id, auth); // allow mint (from==0) and burn (to==0) only
}Disabling transfers by allowing only mint and burn.
Soulbound does not mean unforgeable or private — anyone can read your SBTs on-chain, and a stolen private key takes your 'soul' with it. Non-transferability solves trading, not key custody or confidentiality.