Choose OAuth token exchange when an authorization server should approve each handoff; consider a signed capability design when agents need to pass along locally verifiable, narrowly bounded authority. They are not competing token formats: OAuth 2.0 Token Exchange (RFC 8693) defines a protocol for requesting a new security token, while “signed capability token” describes a family of credentials whose profiles determine what authority they carry and how it can be narrowed. For agent systems, the key question is where each delegation decision is made—and what the tool receiving a request must verify.
What each approach means
OAuth 2.0 Token Exchange
RFC 8693 defines an HTTP- and JSON-based Security Token Service protocol. A client sends a token request to an authorization server’s token endpoint using the token-exchange grant type. The request can identify a subject token and its type, a target resource or audience, requested scope, and—optionally—an actor token to describe the party acting on the subject’s behalf.
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The authorization server validates the presented tokens and applies its own policy before issuing a new security token. That token may be a narrower access token for a downstream service, or another security-token type. RFC 8693 is an IETF Standards Track RFC, published in January 2020 as a Proposed Standard. It defines the exchange protocol, not a universal token format or a complete authorization policy.
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A signed capability token is not one universal protocol. It is a credential that represents authority to perform specified actions; a verifier checks the signature against a configured issuer or trust anchor. Particular designs define the token format, who may hold or delegate it, what constraints it can express, and how a verifier decides whether a derived credential is within its parent’s authority. UCAN is one published specification example.
For agent delegation, the Attenuating Authorization Tokens (AAT) design is a more specific example: its June 2026 revision -01 Internet-Draft proposes signed JWTs for tool-level capabilities and argument constraints, with offline derivation and chain verification. It is a work in progress, not an adopted standard or an established deployment pattern.
How the delegation paths differ
| Decision point | OAuth token exchange (RFC 8693) | Signed capability approach (AAT draft example) |
|---|---|---|
| Who approves the next credential? | The authorization server applies policy to 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. |
| How is delegation represented? | The subject and optional actor identify delegation context; RFC 8693 defines an act claim for actor information. |
Capability claims and chain links are designed to carry and verify delegated authority across hops. |
| How specific can authority be? | The request can select a resource or audience and scope; the issued token’s actual content depends on its profile and deployment. | The AAT draft proposes task-scoped tool permissions and argument constraints. |
| What happens at each hop? | 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 mechanism. |
| What is the maturity? | RFC 8693 is an IETF Standards Track RFC (Proposed Standard). | AAT is an Internet-Draft that may change, not a finalized interoperable standard. |
These are architectural tendencies, not guarantees. RFC 8693 explicitly leaves token syntax, semantics, security properties, and trust models outside its scope. A deployment can combine OAuth-issued credentials with capability-style claims, but it must specify how identity, authorization policy, verification, and revocation fit together.
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Which approach fits an agent system?
Prefer token exchange when authorization should be centralized
Token exchange fits a design in which a central authorization server should assess each request for a downstream credential. It can apply target-specific policy, use existing OAuth integration, and issue a token tailored to the next resource. The trade-off is operational: the exchange path depends on the token endpoint being reachable whenever an exchange is requested.
Consider capabilities when authority must travel with the task
A capability design may fit multi-hop agent workflows where each recipient needs to verify bounded authority locally, without contacting an authorization server for every handoff. That can shift the enforcement point closer to the tool, but it does not eliminate infrastructure or policy requirements: verifiers need trustworthy issuer keys, precise attenuation rules, and a way to handle token status or revocation.
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Use a hybrid only with explicit boundaries
OAuth can provide an initial credential while a capability profile governs narrower tool-level delegation, but the two layers must agree about the principal, permitted audience, authority ceiling, and revocation behavior. Do not assume that an OAuth actor history enforces narrowing, or that a signed capability automatically inherits the authorization server’s policy. Define which component is authoritative at each step.
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- Choose who authorizes each hop. Decide whether the authorization server must approve every delegated token request or whether a holder can derive a credential within authority already granted.
- Set an authority ceiling. Specify the target service or resource, allowed tools or operations, argument constraints, data boundaries, and whether a downstream party may delegate again.
- Define and enforce attenuation. The verifier needs a profile-specific way to establish that a child credential is no broader than its parent. A chain or
acthistory by itself does not prove permissions narrowed. - Bind the credential to its presenter where needed. Consider client authentication and proof-of-possession or sender-constrained tokens when the threat model warrants them. A bearer token that leaks can be replayed by whoever obtains it.
- Specify lifetime and revocation. Set token lifetime, cancellation behavior, issuer-key rotation, compromised-key response, and treatment of an already issued offline credential. RFC 8693 notes that revocation propagation is not a general property of token exchange.
- Make the enforcement checks concrete. Depending on the profile, validate issuer and key, signature algorithm, token type, audience, expiry, scope or capability constraints, delegation depth, parent linkage, and replay or nonce requirements.
- Match the design to availability needs. Server-mediated exchange requires authorization-server availability for the exchange. Offline verification avoids that per-hop call but makes trust-anchor distribution, policy correctness, and status handling central responsibilities.
RFC 9700, the current IETF OAuth 2.0 Security Best Current Practice, recommends asymmetric client-authentication methods such as mutual TLS or signed JWTs in relevant deployments and discusses sender-constrained-token security. Apply that guidance alongside the chosen token profile and threat model; RFC 9700 does not define an agent-delegation capability format.
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