If you run deny-by-default tool guards on AI agents, your refusal is a security decision — not a logging afterthought.
I watched one source mutate a malformed tool call ~1,400 times against a production agent in a weekend. Every identical BLOCKED response was feedback for the attacker's automated search: same input shape → same refusal → "colder," changed shape → changed response → "warmer."
A Keysight paper (arXiv:2606.20470) quantifies it: deterministic detect-and-block lets attack success rate approach 1 as the query budget grows, because predictable refusals feed model-guided search. Their detect-and-misdirect approach cuts the ASR upper bound by up to ~2 orders of magnitude.
The cheap version of the fix, in pseudocode:
