How it fits with the rest

Agent ID answers one narrow question in public — whom an agent acts for, within what. The systems below answer neighbouring questions; an agent can, and usually should, have both.

System	What it does	With Agent ID
OAuth 2.0, OpenID Connect	Authenticates a session and grants an app access to an API on someone's behalf.	Works inside one service and expires with the token. Agent ID is the public, lasting statement of whom the agent serves; a token can carry the aiid.
Microsoft Entra Agent ID	Gives agents identities in an organization's Entra directory, to manage and secure their access there.	Private to that tenant. An Entra-managed agent can hold an aiid so that people outside the organization can check it too.
A2A agent card	Describes an agent to other agents: its endpoint, skills and how to talk to it.	Says what the agent can do, not whom it answers to, and is signed by no one else. Put `"agent_id"` in the card; the record holds the principal and the scope.
W3C DIDs and Verifiable Credentials	General decentralized identifiers and signed claims about any subject.	Agent ID is narrower and simpler: one record format, plain JSON, one signature, no DID method to choose. Expressing a record as a Verifiable Credential is on the roadmap.
ERC-8004 (Ethereum)	Proposed on-chain registries of agent identity, reputation and validation.	Needs a wallet and a chain. Agent ID needs a URL and a signature; a record may still be anchored to any chain (records 1 and 2 are anchored on Polygon).

What Agent ID does not do

It does not log anyone in and gives no access to anything.
It does not rate or certify behaviour.
It does not make the principal liable for what the agent does outside its declared scope — that is the point of declaring it.