The unit of agent security risk is the combination of capabilities held simultaneously, not the quality of the model or the prompt. Simon Willison's 'Lethal Trifecta' formalizes this: private-data access + exposure to untrusted content + ability to communicate outbound is categorically unacceptable, while any two are merely risky.
It converts a vague 'is this agent safe?' question into a discrete capability audit that can be checked before deployment and enforced structurally rather than behaviorally.
Any tool-using system should be reviewed by capability combination, not per-tool: the danger emerges from composition (read secrets + read untrusted input + write externally), which applies to RAG pipelines, MCP servers, browser agents, and CI bots alike.
Any single capability is manageable, two is risky, but all three combined is never acceptable.
AI is good at following instructions but bad at deciding whose instructions to trust.