Act one · the insider

An agent is a reasoning engine in a loop, with tools.
the modelreasons, decides the next step a toolshell, API, browser, mail what it seesresults, files, web pages againuntil it decides it is done

The lethal trifecta

private data
untrusted content
a way to send outward
All three, and a document it reads can turn it against you.
Anthropic, Building effective agents (December 2024) · Simon Willison, The lethal trifecta for AI agents (June 2025)