Agent Authorization Token Structure and Lifetime

Why agent tokens differ from standard OAuth tokens

OAuth 2.0, as defined in RFC 6749, establishes access tokens as scoped, time-limited credentials representing an authorization grant, but it does not mandate a specific token format. RFC 9068 later standardized a JWT-based structure with claims such as issuer, subject, audience, expiration, issued-at, scope, and client ID. These fields were designed primarily for human-delegated OAuth flows involving a browser client and a resource server.

AI agents introduce a different pattern. An agent often acts on behalf of a human or system, invokes multiple downstream tools autonomously, and may run for extended periods without direct human interaction at each step. The standard OAuth claim set does not natively express which party originally authorized the action, what specific task the token is bound to, or how far the delegation chain extends. No IETF or NIST standard currently defines a distinct "agent authorization token" type. In practice, organizations extend RFC 9068 style JWTs with additional claims to capture this context, using patterns borrowed from RFC 8693 token exchange rather than a formal agent-specific specification.

Claims structure: identity, delegation, and scope

A well-formed agent token separates three distinct concerns: the identity of the human or system that initiated the request, the identity of the agent itself (which may be a service account or a distinct non-human principal), and the specific scope of the delegated task. RFC 8693 supports this separation through actor claims, which allow a token to represent both the original subject and the acting party, distinguishing delegation from impersonation.

Scope should be bound as narrowly as possible. RFC 8707 defines a resource parameter that restricts a token's audience to a specific target service or API, rather than issuing one broadly scoped token usable across many tools. For agents that call multiple downstream systems, this audience restriction principle argues for separate, narrowly scoped tokens per tool rather than a single token with broad reach. It is worth noting that scope is not self-enforcing under RFC 6749: the resource server, not the token itself, is responsible for validating that the presented scope actually authorizes the requested action.

Determining token lifetime for autonomous tasks

NIST SP 800-63B recommends that authenticator and token lifetimes be set according to risk rather than a fixed universal duration. Applied to agents, this means token lifetime should reflect the expected duration and risk profile of the bounded task the agent is performing, not a default value copied from human session standards.

Multi-step or long-running agent workflows create a practical tension. A token with a lifetime long enough to cover an entire multi-hour task increases the window in which a leaked or misused credential can cause damage. RFC 9700, the current OAuth security best practice, recommends short-lived access tokens paired with refresh tokens rather than single long-lived credentials. For agents, this suggests re-issuance or refresh patterns tied to task checkpoints, allowing the effective lifetime of any single token to remain short even as the overall task extends over a longer period. This also creates a natural point at which anomalous agent behavior can trigger token non-renewal, rather than waiting for a long-lived token to expire on its own.

Risks from long-lived or overly broad tokens

Two failure modes recur in agent deployments. The first is excessive token lifetime, where a credential remains valid well beyond the task it was issued for. If that token is exfiltrated from a compromised agent runtime, an attacker retains access for the full remaining lifetime, with no forced re-authentication. The second is excessive scope, where a single token grants access to more tools or resources than a given task requires, increasing the blast radius of any compromise.

Agent runtime environments are often less controlled than traditional browser-based OAuth clients, which increases the practical risk of credential leakage. RFC 9449 defines DPoP, a mechanism that binds an access token to a specific client key pair, preventing a stolen bearer token from being replayed by an unauthorized party. RFC 9700 similarly recommends sender-constraining mechanisms such as DPoP or mutual TLS to reduce the impact of token leakage. These mechanisms do not eliminate the need for short lifetimes and narrow scope, but they reduce the consequences when a token is exposed.

Core elements of an agent authorization token

Four claim categories recur across working implementations of agent tokens:

Enforcing token validity at runtime

Because scope is not self-enforcing and tokens may persist longer than intended, runtime enforcement requires deliberate practices at each stage of the agent's operation:

• Validate at every callCheck token expiration, audience, and scope at each tool invocation, not only when the token is first issued, consistent with the continuous verification principle in NIST SP 800-207.
• Separate tokens per toolIssue audience-restricted tokens scoped to individual tools or resources rather than one broad-access token, per RFC 8707.
• Log the delegation chainRecord the invoking human or system, the agent identity, and the granted scope for each token issuance to support later audit.
• Prefer refresh over long expiryUse short-lived tokens with refresh or re-issuance at task checkpoints rather than a single long-duration credential.
• Enable early revocationSupport mid-task token termination if anomalous agent behavior is detected, rather than relying solely on natural expiration.

Governance implications for security teams

Because no formal standard governs agent token structure, organizations bear direct responsibility for defining policy. This includes setting maximum token lifetime by task risk classification rather than a single default duration, consistent with the risk-based approach in NIST SP 800-63B. It also includes periodic review of the scopes granted to agent tokens, since agents are frequently extended to new tools over time and scope creep can occur without a formal review cycle.

Audit requirements are also distinct for agents. Because a token may represent a delegation chain rather than a single human actor, audit records need to capture the original authorizing party, the agent identity, and the scope granted, not just the fact that a valid token was presented. Zero Trust guidance in NIST SP 800-207 calls for continuous, per-request verification rather than reliance on long-lived implicit trust; extending this principle explicitly to non-human agent identities is an emerging governance requirement rather than a settled practice across the industry.

Runtime governance platforms, including Trussed AI, apply these principles operationally by enforcing scope and expiration checks at each tool call an agent makes, and by logging the delegation chain for audit purposes, translating the standards described here into consistent runtime policy across agent-to-tool interactions.