This paper argues that prompt-based guardrails are brittle because agents inheriting human credentials can bypass intent-based limits. The authors propose Out-of-Band Policy Enforcement (OBPE), which operates at a trusted boundary outside the agent's reasoning loop. OBPE authorizes typed operations, narrows queries before backend execution, and filters or masks response data based on semantic gating.
- Prompt instructions are insufficient for security; agents can override reasoning limits.
- OBPE enforces policy at the tool boundary, separate from the LLM.
- Pre-execution query narrowing reduces data exposure in model context.
- Semantic gating allows denial or holding of calls based on argument values.
- Response filtering masks sensitive fields before data returns to the agent.