Soulbound Tokens (SBTs)

Updated 28 October 2025

Soulbound Tokens (SBTs) are non-transferable cryptographic tokens that attach persistent, verifiable social and compliance attributes to blockchain identities.
They integrate with zero-knowledge proofs, smart contracts, and self-sovereign identity frameworks to enable privacy-preserving, on-chain verification and access control.
Key challenges include early-stage specification maturity, public default privacy settings, and issues with revocation and interoperability compared to traditional identity systems.

Soulbound Tokens (SBTs) are non-transferable cryptographic tokens, typically implemented as specialized NFTs, designed to encode persistent, verifiable attributes or social attestations onto blockchain-linked identities. Emerging from proposals for composable, trust-rich primitives in decentralized identity and governance systems, SBTs function as immutable records of reputation, status, compliance, and authorization, with claims verified directly on-chain or within related cryptographic frameworks. Their technical and social architectures support applications in dispute resolution, inheritance, governance of autonomous systems, and identity-centric access and recovery, manifesting diverse tradeoffs in privacy, abstraction, composability, and social incentive design.

1. Fundamental Properties and Conceptual Foundations

SBTs are defined as non-transferable, cryptographically-bound tokens permanently assigned to specific user wallets or agent identities—rendered “soulbound.” Once issued, an SBT can neither be sold nor traded, making it a persistent, personal attestation mapped onto a blockchain address. Early implementations equate to non-transferrable NFTs, but evolving proposals envisage flexible, privacy-preserving tokens programmable for selective disclosure, community-driven revocability, and composability with smart contracts for on-chain verification, censorship resistance, and correct execution (Gangemi et al., 2023, Goldston et al., 2023, Chaffer et al., 22 Dec 2024, Jain et al., 2022).

Property	SBTs	Notes
Transferability	No	Records persist with initial recipient
Issuer/Verifiability	App/DAO/auditor, on-chain	Verification by smart contract or ZKP
Privacy	Default public (improving)	ZKP integration under active research
Revocation/Recovery	App/DAO/social coordination	Early-stage technical specification

SBTs lack a universal technical specification; their instantiation, control logic, and allowed operations vary across platforms and research implementations. The “composability” offered by on-chain SBTs enables native, machine-verifiable access control and governance—contrasted with off-chain credentials such as Verifiable Credentials (VCs) or Decentralized Identifiers (DIDs), which require oracles for blockchain use and suffer from incomplete trust-minimization (Jain et al., 2022).

2. SBT Issuance, Management, and Formal Mechanisms

SBT issuance is triggered either by automated, rule-based conditions (reputation thresholds, completion of compliance audits), manual actions by designated authorities (DAOs, testators, auditors), or consensus actions among groups (social recovery, collective attestation).

For dispute resolution: SBTs are issued to users and judges based on honest participation or compliance. Positive SBTs denote trustworthy behavior; negative SBTs signal misconduct or non-compliance. Issuance and revocation may occur according to formal mechanisms—e.g., if a judge’s reputation score $S_j$ falls below threshold $\varepsilon_{-}$ , remove the judge and append a negative SBT; conversely, scores above $\varepsilon_{+}$ award positive SBTs and governance privileges. SBTs thus act as immutable records for participatory rights and penalization (Gangemi et al., 2023).
For inheritance: Testators mint SBTs to executors, guardians, and beneficiaries, marking their inclusion and authority in digital estate plans. Only wallets holding appropriate SBTs can participate in recovery protocols embedded in substrate pallets (e.g., Polkadot/Kusama social recovery) (Goldston et al., 2023).
For AI governance: Auditors/DAOs issue “compliance SBTs” to agent SSIs upon passing audits, risk classification, and ongoing performance. Revocation or downgrading of SBT status follows audit failures or detected anomalies, driving dynamic, automated access control via smart contract logic (Chaffer et al., 22 Dec 2024).

Exemplary Pseudocode for SBT Issuance and Revocation

contract EthosSBT {
    mapping(address => mapping(bytes32 => bool)) public agentSBTs;
    event SBTIssued(address indexed agent, bytes32 indexed sbtType);
    event SBTRevoked(address indexed agent, bytes32 indexed sbtType);

    function issueSBT(address agent, bytes32 sbtType) public onlyAuditor {
        require(!agentSBTs[agent][sbtType], "Already issued");
        agentSBTs[agent][sbtType] = true;
        emit SBTIssued(agent, sbtType);
    }

    function revokeSBT(address agent, bytes32 sbtType) public onlyAuditor {
        require(agentSBTs[agent][sbtType], "No SBT to revoke");
        agentSBTs[agent][sbtType] = false;
        emit SBTRevoked(agent, sbtType);
    }
}

Actions may be governed by risk scores:

\text{Risk Score} = w_1 \cdot \text{Autonomy} + w_2 \cdot \text{Decision-Making Complexity} + w_3 \cdot \text{Adaptability} + w_4 \cdot \text{Impact Potential}

SBT issuance or revocation is conditioned on thresholds.

3. Integration with Zero-Knowledge Protocols, Privacy, and Identity

SBTs interoperate with a variety of privacy-preserving and anti-Sybil infrastructures, notably zero-knowledge proofs (ZKP), Semaphore, MACI, and self-sovereign identity (SSI) frameworks.

Semaphore binds SBT-issuance to uniqueness and personhood, mitigating Sybil attacks and enforcing “one-person-one-vote” (Gangemi et al., 2023).
MACI introduces encrypted quadratic voting, deterring collusion and bribery in dispute resolution and governance. SBTs are awarded only to users with eligible, authenticated identities (Gangemi et al., 2023).
ZKPs allow selective disclosure of SBT claims (compliance, reputation) without exposing full PII, facilitating privacy-preserving SBT minting and verification (Chaffer et al., 22 Dec 2024, Goldston et al., 2023). This architecture maintains publicly accessible SBT proofs while shielding sensitive claim data.

Identity systems (SSI, DIDs) are enhanced or extended by SBTs, as SBTs can be appended directly to on-chain identity documents for native composability with smart contracts. Experiments in “ZK-SBTs” attempt further privacy improvements, although scalability and robustness remain subjects of current research (Jain et al., 2022).

4. Applications Across Governance, Inheritance, and Autonomous Agents

SBTs underpin diverse use cases, functioning as social, reputation, or compliance primitives tightly coupled to blockchain state.

Domain	Role of SBTs	Technical Mechanism/Context
Dispute resolution (Ethereum)	Award/penalize trustworthy behavior; grant governance	Quadratic voting, revocable by app
Digital inheritance	Attest authority/role; authorize asset recovery	Social recovery pallet, m-of-n vouching
AI agent oversight (ETHOS)	Encode compliance/classification; restrict access	Risk SBTs, automated audit/revocation
Decentralized identity	Attest credentials, affiliations; programmable privacy	On-chain composability with smart contracts

Inheritance Protocol

SBTs designate executors, guardians, and beneficiaries in estate plans.
The social recovery pallet orchestrates multi-party approval for account recovery, requiring SBT-held identities to initiate asset transfer (Goldston et al., 2023).

Autonomous Agent Governance

Agents receive “compliance SBTs” post-audit; operational freedom or suspension is dictated by SBT status as enforced by smart contract logic and real-time oracle data (Chaffer et al., 22 Dec 2024).

Dispute Resolution

Positive/negative SBTs mediate the allocation of governance power, ensuring only consistent, trustworthy contributors participate in decision-making. Sybil resistance and privacy are tightly enforced (Gangemi et al., 2023).

5. Comparative Analysis of SBTs Versus Other Identity Instruments

SBTs are contrasted with DIDs/VCs along several dimensions:

Dimension	SBTs	DIDs / VCs
On-chain comp.	High	Low/moderate (needs oracles)
Abstraction	Low/moderate	High
Privacy	Low (improving)	High (theoretically)
Consent control	Weak (now), improvable	Theoretically strong
Standard maturity	Low	Higher (esp. VCs, DIDs)
Recovery/revocation	Early-stage	Early-stage

SBT provision allows direct, censorship-resistant verification of credentials or status on-chain, a property not generally achievable with VCs or DIDs without intermediary oracles. Conversely, DIDs/VCs offer better abstraction and cross-system interoperability at the cost of composable enforcement. Both suffer potential “scarlet letter” issues, as negative, non-consensual attestations may be forcibly anchored to subjects, with limited recourse for recovery or deletion (Jain et al., 2022).

6. Key Advantages, Challenges, and Open Research Trajectories

Advantages:

Non-transferability yields a durable social or compliance footprint immune to financial manipulation or bribery.
On-chain composability enables trustless, tamper-proof governance and access control.
Sybil resistance and privacy can be strengthened through ZKP and authenticated identity integrations.
SBTs facilitate inclusive participation unconstrained by financial staking requirements.

Challenges and Limitations:

Specification gaps and underspecified standards affect portability and robustness.
Privacy defaults are public and permanent; advanced privacy-preserving SBTs are experimental and non-standard.
Revocation, recovery, and consent control remain technically immature, exposing risks from arbitrary or non-consensual “scarlet letters.”
Interoperability and legal recognition (e.g., in estate planning) require further evolution.
Systemic complexity, scaling concerns (oracle, ZKP costs), and vulnerability in centralized trust assumptions (e.g., MACI’s coordinator role) present ongoing design challenges (Gangemi et al., 2023, Goldston et al., 2023, Chaffer et al., 22 Dec 2024, Jain et al., 2022).

A plausible implication is that SBTs will continue to serve as composable primitives for blockchain-based reputation, compliance, and authority, provided robust frameworks for privacy, consent, revocation, and social recovery emerge. Research trajectories include integrating SBTs with advanced zero-knowledge architectures, developing interoperable standards, and combining best practices from both abstraction-dominant VCs/DIDs and composability-focused on-chain identity systems.

7. Synthesis and Future Prospects

A pluralistic approach, treating SBTs and DIDs/VCs as complementary, best accommodates the diversification and complexity of identity, trust, and governance in decentralized systems. SBTs enable machine-verifiable, tamper-resistant social and compliance logic but demand technical innovation to counter privacy, recovery, and reputational challenges. DIDs/VCs remain better suited to abstract, cross-protocol identity management, while SBTs leverage blockchain composability for robust, automated on-chain enforcement. The abstraction-versus-composability dialectic is expected to drive ongoing experimentation and standardization, shaping future architectures for Web3 identity and governance (Jain et al., 2022).