Loop Engineering: Fixing a Guardrail That Fired Wrong
How to fix a misfiring guardrail check in an agent loop without deleting it—why weakening checks silently disables real catches.
A recurring failure mode in agentic development loops: a deterministic guardrail—a grep-based check blocking `import mock` in production code—fires on a file that is actually clean. The banned string appears inside a docstring explaining the rule, not inside a real import statement. The natural response is to delete or weaken the check, which silences the false alarm but also silently removes the real violation it was catching elsewhere in the same run.
The piece frames this as a general lesson for engineers building generate-check-retry loops: a check's silence can mean either "no problem exists" or "the check is blind," and the verdict alone cannot tell you which. A false positive is loud and self-correcting—it interrupts you until fixed. A check quietly weakened to stop the noise fails silently, forever after. The right fix is to sharpen the pattern, such as anchoring a regex to the start of a line, rather than removing the guardrail—preserving the original catch while eliminating the noise.
The broader takeaway: treat every misfire as diagnostic data about your instrument's precision, never as evidence that the underlying risk doesn't exist. The principle extends beyond grep checks to linters, model-graded evaluators, and any deterministic gate embedded in an agent's development loop.