docs(test-driven-development): add guidance for modifying behavior with existing tests#2023
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docs(test-driven-development): add guidance for modifying behavior with existing tests#2023wrcapitalcorp wants to merge 1 commit into
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…th existing tests The red-green-refactor cycle documents writing new behavior from an empty test file and gives little guidance for changing behavior that is already covered by tests. The only note on that case, "Other tests fail? Fix now," can be read as making a failing test pass without first determining whether the code, rather than the test, is at fault. This adds a "Modifying Existing Behavior" section covering three points: inventory the existing tests before changing code; update the existing test to the new contract first, in the same commit; and, for each additional failing test, decide whether it is a genuine regression (fix the code) or an intended behavior change (update the test). The "Other tests fail?" line under Verify GREEN is updated to state this decision inline and link to the new section. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Summary
Adds a "Modifying Existing Behavior" section to the
test-driven-developmentskill and clarifies the existing "Other tests fail?" note under Verify GREEN. Documentation only, one file.Motivation
The red-green-refactor cycle in this skill is written for new behavior, beginning from an empty test file. It offers little guidance for changing behavior that is already covered by tests, which is a common and time-consuming case in practice. The only sentence that addresses it is:
Taken literally, "fix now" can be read as license to make a failing test pass without first determining whether the code, rather than the test, is at fault. That inverts the purpose of the test: a test that fails on an unintended change is doing its job, and editing it to pass removes that signal.
Change
Adds a focused section covering the modification case:
It also notes a brittleness signal: a refactor that changes no observable behavior yet breaks many tests indicates tests coupled to implementation rather than behavior.
The "Other tests fail?" line under Verify GREEN is updated to state this decision inline and link to the new section.
Notes
The change is intentionally scoped to a single section so it continues the existing red-green narrative rather than introducing a separate skill. If the maintainers would prefer a standalone companion skill to match the library's composition style, I would be glad to restructure it accordingly.