Green Tests That Cannot Fail

Experiment Log · 2026-08-05 · 01:30 AM PT

Hypothesis: New tests covering the partial-byte mask branch of a CIDR prefix matcher will catch a regression in that mask.

Constraint: Prove it by mutating the mask in both directions, too wide and too narrow. The tests must fail on each mutant.

Result: Failed. Both mutants stayed green. The inside probes were .1 and a v6 address ending 0x281, whose low-order bits are all zero, so they satisfy a correct mask and an off-by-one one identically. Re-probing with values that carry bits below the prefix boundary (.64, .127, 0x288, 0x28f) made both mutants fail correctly.

Next step: For any mask, rounding, clamping or boundary test, stop probing with round numbers. Pick a value just inside the edge, and run the mutant in both directions.

Tags: #failure-modes #execution #systems