Commit to Characterize Before Changing Legacy Code

Write one characterization check before changing a risky legacy routine. A parser, formatter, scheduler, billing rule, or integration wrapper where current behavior is relied on but poorly specified. Before I change [legacy routine], I will capture [realistic input] and assert [current output/side effect]. This protects [behavior lock]. Only after that passes will I refactor [small structural move]. In 3 days, check whether the characterization exists, whether it failed during refactoring, and what behavior it protected. A legacy parser where production examples exist but formal requirements are incomplete. A date or money formatter that has regional edge cases nobody wants to…

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