Most of what lands in a self custody wallet is there to be moved. You swap tokens, send them, sell an NFT, bridge an asset to another chain. So it can be confusing the first time you see an entry in your wallet that a marketplace will not list and a swap screen will not touch. In many cases that is not a bug. It is a soulbound token, and it was built that way on purpose.
What "soulbound" means
A soulbound token, usually shortened to SBT, is a token that is permanently tied to the wallet it was issued to. Under the hood it is often built on the same NFT standards covered in our guide to NFTs on Base, ERC-721 or ERC-1155, but with the transfer function disabled or blocked in the contract. Once it lands in your wallet, it stays there. It cannot be sold, sent to a friend, or moved to a new wallet along with the rest of your holdings.
The term comes from a 2022 paper and blog post by Ethereum co-founder Vitalik Buterin and collaborators, who borrowed the "soulbound" idea from items in online games that bind to a character and can never be traded away. The proposal was to use the same non-transferability for real world credentials: a university degree, a professional certification, a record of having completed a course, or proof that a specific wallet did a specific thing at a specific time. A credential that can be sold to someone else is not a very useful credential, so making it non-transferable was the point rather than a limitation.
Attestations: the data behind the badge
A closely related idea is the onchain attestation, which is a signed statement that a claim is true, recorded on a blockchain instead of a token contract. The most widely used implementation is the Ethereum Attestation Service, or EAS, an open source, free to use protocol for issuing and reading these statements. EAS is deployed as a standard part of the OP Stack, the same technology stack Base is built on, which means it is natively available on Base without any extra setup by an app. A wallet address, an app, or an organization can attest that something is true about another address, for example that it passed a KYC check, completed a task, or holds a particular role, and anyone can look that attestation up later.
An attestation is not always wrapped in an NFT you see in your wallet. Sometimes it is just a database entry on chain, queried directly by the app that cares about it. A soulbound token is one common way to represent an attestation visually, so you have something to look at, but the two ideas are distinct: attestations are the underlying data, soulbound tokens are one packaging for that data.
Where this shows up in practice
A few examples worth knowing:
- Event attendance badges. POAP, short for Proof of Attendance Protocol, has issued badges for conference and event attendance since 2019 and is the most recognized example of this category. It is worth a small correction here: POAPs are technically built as ordinary, transferable ERC-721 NFTs, not strict soulbound tokens, so they can in principle be moved or sold on a marketplace. The POAP team has acknowledged this is a mismatch with the "proof of attendance" idea and has documented ways an app can add its own non-transferability check. In practice, most people never move a POAP, but do not assume a badge is airtight proof that the current wallet holder was the one who actually attended.
- Verification and reputation marks. Social apps built around wallets, including several tied to Farcaster, use attestations or soulbound-style tokens to mark that an account passed some verification step, without needing a centralized database to store that fact.
- Onchain credentials. Some grant programs, DAOs, and communities issue non-transferable tokens to record that a wallet completed a specific contribution, so eligibility for future rounds cannot simply be bought on the open market by whoever holds the most tokens.
Why non-transferability is the whole point
A transferable token can be bought, so it only proves that a wallet currently holds it, not that the wallet did whatever the token claims to represent. A soulbound token or attestation is trying to answer a different question: what has this specific address actually done or verified, independent of what it can currently afford to buy. That is useful for things like sybil resistance, which is the problem covered in our guide to sybil attacks and airdrop farming, where a single person spinning up many wallets tries to look like many independent participants. A credential that cannot be transferred, and ideally cannot be issued twice to the same underlying person, is harder to farm than a token anyone can simply purchase.
What this means for your wallet
If a swap screen will not let you select a token, or a marketplace shows it as unlisted, that is not necessarily an error. Check the contract on a block explorer, the same habit covered in our guide to using a block explorer, and look for a transfer restriction in the contract code or a description from the issuing app. It is a very different situation from a token that will not sell because of a fee-on-transfer or honeypot mechanism designed to trap value. A soulbound token restricting transfers is doing exactly what it was built to do.
One more habit is worth carrying over from everywhere else in this space: an attestation or badge is only as trustworthy as the entity that issued it. Seeing a token labeled "verified" or "member" in your wallet is not the same as independently confirming who issued it and what checking process, if any, stood behind it. Treat a credential from an app or organization you recognize differently than one from an address you cannot identify, the same way you would treat any other unsolicited item that shows up uninvited, a topic our guide to unsolicited tokens and airdrop scams covers in more detail.
Soulbound tokens and attestations are still a young corner of the ecosystem, and standards are likely to keep evolving. But the core idea is simple and worth holding onto: not every entry in your wallet is an asset waiting to be sold. Some of it is a record, built to stay exactly where it is.