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OAuth token exchange and signed capability tokens solve different parts of agent delegation. With OAuth 2.0 Token Exchange (RFC 8693), an authorization server approves a request and issues a token for a downstream context. A capability token can carry bounded authority that a verifier checks—and some designs let a holder derive a narrower credential without contacting an authorization server at each hop. Choose based on who should authorize each delegation, how finely permissions must be constrained, and what the final tool can verify.
How OAuth token exchange works
OAuth 2.0 Token Exchange, specified in IETF RFC 8693, defines how a client requests a security token from an authorization server’s token endpoint. The request uses the token-exchange grant type and can present a subject token, identify the token type, specify a target resource or audience and scope, and optionally include an actor token to describe who is acting.
The authorization server validates the presented tokens and applies its own policy before issuing a token. That result may be a narrower access token for a downstream service or another security-token type. RFC 8693 also defines an act claim for actor information, which can help represent delegation context.
- The agent or client sends a token-exchange request to the authorization server, identifying the subject and requested downstream context.
- The server checks the tokens and decides whether its policy permits the request.
- If permitted, the server returns a token for the requested use; the downstream service validates it according to its token profile and local policy.
The protocol standardizes the exchange request and response mechanics, not a universal token format or complete authorization policy. The token’s syntax, semantics, security properties, and trust model depend on the profile and deployment.
How signed capability tokens work
A signed capability token is a broad design category, not one standardized protocol. It represents authority to perform specified actions, with a signature that lets a verifier check the credential’s integrity and authenticity under a configured trust model. Specifications differ in how they express permissions, bind a token to a holder, constrain delegation, handle revocation, and require verifiers to validate a chain. UCAN is one published specification example.
For agents, the Attenuating Authorization Tokens (AAT) design is a more specific example. Its June 2026 Internet-Draft proposes signed JWT credentials for task-scoped tool permissions and argument constraints, along with offline derivation of narrower tokens and chain verification. AAT is a proposal, not an adopted or finalized interoperable standard; its details may change.
A signature alone does not make a credential safe or prove that a delegated permission is narrower than its parent. It prevents undetected alteration of signed data, but least privilege, replay protection, presenter identity, and trust in the issuer all require additional rules and enforcement.
Key differences for agent delegation
| Decision axis | OAuth token exchange (RFC 8693) | Signed capability approach (AAT draft example) |
|---|---|---|
| Where authorization happens | The authorization server evaluates the exchange request before issuing a token. | A holder may derive a narrower token under the proposal’s rules; the enforcement point verifies the chain against a root trust anchor. |
| Delegation context | The subject token and optional actor token identify the exchange context; RFC 8693 defines the act claim for actor information. |
Capability claims and chain links are designed to represent and verify delegated authority across hops. |
| Permission detail | A request can select a resource or audience and scope; actual token contents and policy are profile- and deployment-specific. | The AAT draft proposes task-scoped tool permissions and argument constraints. |
| Per-hop online dependency | Each exchange request requires an interaction with a token endpoint. | The draft proposes offline derivation and chain verification; deployments still need key distribution and a chosen status or revocation approach. |
| Operational fit | Useful when a central authorization server should mediate issuance, target-specific policy, and existing OAuth integration. | Potentially useful when multi-hop agents need locally verifiable, narrowed authority and should not contact an authorization server at each hop. |
| Maturity | RFC 8693 is an IETF Standards Track RFC, published as a Proposed Standard. | AAT is a June 2026 Internet-Draft, not a finalized interoperable standard. |
These are architectural tendencies, not guarantees. RFC 8693 does not require a particular token syntax or ensure that a returned token contains only the permissions a particular deployment intends. Likewise, a capability chain is only as restrictive as its attenuation rules and the verifier’s enforcement of them.
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How to choose between them
Choose token exchange when a server should approve each handoff
Use the exchange model when policy must be evaluated centrally for each requested downstream context—for example, when an authorization server needs to consider the subject, acting party, target audience, and requested scope before issuing a credential. It also fits systems already organized around OAuth token endpoints and centrally managed issuance. The trade-off is that an exchange depends on the authorization server being available for that request.
Consider a capability design when authority must travel with the task
A capability approach may fit a multi-hop workflow in which each agent needs a verifiable, restricted set of tool permissions and per-hop authorization-server calls are undesirable. This depends on the specific format: verify that it defines how children are constrained by parents, how chains are checked, and how keys and credential status are managed. AAT describes one proposal for this pattern, but its Internet-Draft status means it should not be treated as a settled standard or assumed deployment norm.
Combine them only with an explicit security model
The approaches are not mutually exclusive. An authorization server can issue credentials that carry capability-style claims, or a system can use OAuth for initial issuance and a capability profile for later delegation. A combination still needs a clear account of which component makes policy decisions, what each verifier trusts, how permissions narrow, and how revocation or expiry affects already-issued credentials.
Security questions to settle before implementation
Who approves each hop?
Decide whether every delegated token request must return to an authorization server, or whether a holder may derive a child credential within authority already granted. This is the central operational boundary between online mediation and local derivation.
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What is the maximum authority?
Define the allowed resource or audience, tools and operations, argument constraints, data boundaries, and whether a recipient can delegate onward. A scope label by itself may not express the limits an agent task requires.
How is narrowing proved?
Specify how a verifier establishes that a child credential grants no more than its parent. An act history or a chain link records context; neither alone proves that permissions were reduced. The token profile must define the comparison and the enforcement point must apply it.
How is the credential bound to its presenter?
Determine whether a bearer token is acceptable or whether client authentication or proof-of-possession is needed. A bearer credential can be replayed by anyone who obtains it. The IETF’s OAuth 2.0 Security Best Current Practice, RFC 9700, discusses sender-constrained tokens and recommends asymmetric client authentication methods such as mutual TLS or signed JWTs in relevant deployments; apply that guidance in light of the chosen profile and threat model.
What happens when authority must be withdrawn?
Set token lifetimes, cancellation behavior, issuer-key rotation procedures, and a response to compromised keys. Decide how the system handles an offline credential that has already been issued. RFC 8693 does not make revocation propagation a general property of token exchange, and offline verification likewise does not provide status checking automatically.
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What does each verifier check?
Define required checks for issuer and key, signature algorithm, token type, audience, expiry, scope or capability constraints, delegation depth, parent linkage, and replay or nonce protection where the profile requires it. A valid signature is one check, not the full authorization decision.
What availability can the workflow tolerate?
Server-mediated exchange relies on authorization-server availability when an exchange is requested. Offline verification can avoid that per-hop call, but shifts operational responsibility to trust-anchor distribution, policy correctness, and handling credential status.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the standards do—and do not—settle
RFC 8693 is the adopted protocol standard for requesting and obtaining security tokens from OAuth authorization servers, including tokens used for impersonation and delegation. It does not prescribe a universal authorization policy, token representation, or deployment trust model. RFC 9700 supplies current OAuth security best-practice guidance, but does not define an agent-delegation capability format. AAT, by contrast, addresses agent task permissions in a June 2026 Internet-Draft; its proposed format and rules should be evaluated as draft work rather than relied on as finalized interoperability requirements.
There is no universal winner: decide first whether the system needs centralized approval for every handoff or locally verifiable, constrained authority between handoffs. Then validate that the selected profile can enforce the required audience, permissions, presenter binding, attenuation, expiry, and revocation behavior.
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