What Is a Capability Token, and How Does It Limit a Home AI Agent?
A capability token grants possession-based authority to a specific resource and operation, limiting an agent by withholding every capability it was not given.
A household agent may need to read one calendar, switch one room's lights, or upload a chosen backup without receiving a permanent administrator credential. A capability token can bind resource, action, expiry, budget, and context into a verifiable grant. The tool gateway checks that grant on every call, so prompt text alone cannot expand the agent's authority.
The Token Combines an Object Reference With Authority
A capability names or resolves a protected object and conveys permitted operations to whoever legitimately possesses it. Unlike a broad API key followed by an internal ACL lookup, the grant itself is deliberately narrow and unforgeable.
Object-capability research describes object capability model as encoding access rights in individual objects to restrict interactions. File descriptors are a familiar local example: they authorize access to an already opened object without granting arbitrary filesystem naming authority.
A capability can coexist with identity and policy. Identity answers who is involved; the capability answers which concrete authority was delegated for this operation. This distinction remains visible during later household testing.
Attenuation Narrows Delegation Without Expanding It
A parent service can derive a child token restricted to a smaller path, read-only method, shorter expiry, lower budget, or particular request context. The child can delegate less authority but cannot remove caveats or mint broader rights.
Macaroons introduce attenuated authorization that support contextual caveats and decentralized delegation. Verification checks the signature chain and every caveat before the resource server honors a request. The intermediate result must remain inspectable before automation follows.
This is useful for agents because a planner can receive a session capability and pass a task-specific derivative to one tool subprocess. The subprocess never needs the root credential in its prompt or environment. That boundary should be measured separately under realistic operating conditions.
Possession, Delegation, and Revocation Remain Security Boundaries
Anyone who steals a bearer capability may exercise it until expiry or revocation. Tokens need protected transport and storage, an audience or channel binding where appropriate, short lifetimes, and audit identifiers. The practical consequence appears when several sources compete for limited context.
A capability-security analysis explains the distinction between capability authority and identity-centered controls. The model reduces ambient authority but does not judge whether an agent's requested action reflects the user's true intent. This dependency should remain explicit in the final interface.
The failure boundary is an overbroad, replayable, or leaked token. Capability checks cannot prevent harmful use that falls inside the grant, so consequential actions still need target validation, approval, rate limits, and observable revocation. The result must therefore be checked against the original evidence.
Inventory Authority by Token, Tool, and Request
For every tool call, record issuer, subject or holder, audience, resource, operation, path scope, budget, expiry, delegation chain, caveats, revocation handle, approval requirement, and audit ID. This distinction remains visible during later household testing.
Compare the grant with capability-based security. Test path traversal, changed device ID, method substitution, replay, expired token, stolen child token, offline verifier, revoked parent, and an allowed but high-impact request. The intermediate result must remain inspectable before automation follows.
Pass only when each tool receives the minimum grant and cannot amplify it. Treat capability success as authorization, not proof of user intent, semantic safety, or correct tool output. That boundary should be measured separately under realistic operating conditions.
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