AI agent authentication is not one protocol. It is a stack. An agent may need to discover a tool, prove which workload is running, authenticate an HTTP request, show that a user delegated authority, and leave enough evidence to reconstruct the action later. ARD, workload identity, Web Bot Auth, and OAuth solve different parts of that sequence. Treating any one of them as the whole answer creates an identity gap. That distinction got sharper in 2026. Google announced the open Agentic R...