Definition
A bounded action or class of action that a system component can perform. In secure agent systems, a useful capability description also states the authority attached to that action: which identity may use it, on which resources, for how long, in which environment, and under what conditions.
Keeping ability and authority separate prevents a common error. A model may know how to compose a database deletion request. A tool may technically be able to execute it. Neither fact means this run is authorized to delete production data. The runtime should grant the narrowest executable authority that satisfies the task, then enforce it below the model's instruction layer.
Capability as an architectural unit
Capabilities make authority composable. Instead of handing an agent a broad credential and relying on a prompt, a system can expose a small operation such as read_invoice or refund_order with tenant, amount, identity, and time constraints. The capability can carry audit and revocation behavior that a natural-language rule cannot enforce by itself.
Distinguish it from nearby terms
A feature is product behavior available to a user. A permission is a policy decision about access. A tool is an interface the model can call. A capability joins a bounded operation with the conditions under which it can be exercised. Protocol specifications also use capability more narrowly for optional features a participant supports; context should make the sense clear.
Check your understanding
An agent needs to attach a receipt to one expense report. Should it receive the employee's full cloud-drive token? No. Give it a capability restricted to the named file, report, action, identity, and time window, then revoke or expire it after the run.