Definition
Hard rules that constrain what an agent can say, do, or output — independent of the model's judgment.
In practice
Examples: a regex filter that blocks 'remove me' replies from being argued with, a hard rule that escalates anything mentioning 'lawsuit', a JSON schema validator that rejects malformed tool calls, a max-cost cap on a query. Guardrails should be deterministic, not LLM-based — you never want your safety mechanism to itself be the unreliable part.