Authenticating an agent tells you who it is, but not whether to let it transact, access data, or act on a user's behalf. As autonomous agents begin crossing organisational boundaries, the systems they reach face a binary choice: block all agent traffic or accept it without verification - neither of which scales as agent traffic grows.

TSAI (Trust Signals for Agentic Interactions) is an open source protocol that fills this gap. Built on W3C Verifiable Credentials and Decentralised Identifiers (DIDs), it carries trust signals beyond identity - reputation, economic stake, authorization, and endorsements - in cryptographically signed credentials that any system can verify offline. Independent Trust Authorities issue them and agents present them when accessing a service, while receiving systems make their own access decisions based on the signals. Credentials describe the agent, not the user, which preserves user privacy and keeps existing user authentication unchanged.

In this talk, we walk through the architecture - the four-tier trust model that scales from offline verification at low risk to real-time checks at high stakes, the credential format and lifecycle, and how TSAI composes with agent protocols like MCP and A2A. TSAI is developed by AWS and Trusted Shops, combining agent infrastructure expertise with decades of online trust certification.