Actor specialization for a non-human, automated, delegated, or AI-assisted actor. An Agent Actor means the event source attributes the action to automation operating on behalf of another actor rather than to the principal acting directly. This distinction is fundamental for safe automation: routine operations may be delegated to agents, while sensitive operations may require direct principal action. A Timeline Provider or event source is not expected to understand every Blue document that may later consume the entry, and it is not responsible for deciding whether the message is valid for a particular document. Its role is to authenticate and authorize the party that appends to the timeline under the provider's own rules, record the actor attribution it can verify, preserve the entry, assign ordering information, and provide completeness/finality guarantees. A provider may verify that an agent has permission to append to a timeline or act for an account under a delegated credential, but Blue documents independently enforce their own Actor Policy and operation rules. The `onBehalfOf` field identifies the principal or actor whose authority is being used. Concrete agent types should add fields that make the delegation auditable: agent identity, session reference, delegation document, allowed scope, issuing provider, revocation context, or model/runtime details where appropriate. The goal is not to pretend that the agent is a human; the goal is to make automation explicit, attributable, auditable, and policy-checkable.