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Safely modifies untested legacy code by getting it under test first: characterization tests, seam identification, sprout/wrap techniques, and pinch-point testing before any production changes. Not for refactoring code that already has test coverage (use cc-refactoring-guidance).
Passed 24 of 24 test cases with the skill, and 19 of 24 without it.
and 2 more in the full report
5 fixed19 passed either way0 still failing0 broke
Tokens, turns and time describe resource use, not a dollar price.
This benchmark measured an earlier version of the skill.
A newer attempt on gemini-3.8-flash on 10/18/2026 did not produce a publishable comparison (not comparable). The snapshot above is the previously measured run.
One of 5 test cases the skill fixed. Same model, same prompt, with and without the skill.
An existing legacy object creates its internal `Logger` during construction. A developer needs to swap this logger with a mock logger in a unit test after object construction completes. Which dependency-breaking technique provides a method to replace an already-instantiated instance variable?
| Checked | Without skill | With skill |
|---|---|---|
| Use Supersede Instance Variable | Missed | Passed |
The instructions your model reads when the skill loads.
"Legacy code is simply code without tests." - Michael Feathers
Get code under test before changing it. This is the core principle.
| Symptom | Solution |
|---|---|
| Can't get class into test harness | Decision Tree: Can't Instantiate |
| Can't run method in test harness | Decision Tree: Can't Call Method |
| Don't know what tests to write | Characterization Tests |
| Under time pressure, need to change now | Sprout/Wrap Techniques |
| Need to change many classes in one area | Pinch Point Testing |
| Class is too big / monster method | Effect Sketching → find clusters |
| Afraid I'll break something | Single-Goal Editing, Preserve Signatures |
Use this for every change to untested code.
1. IDENTIFY change points
- Where in the code do you need to make modifications?
2. FIND test points
- Where can you write tests to cover the change?
- Look for pinch points where tests cover multiple changes
3. BREAK dependencies
- Remove obstacles that prevent testing
- Use dependency-breaking techniques (catalog below)
4. WRITE characterization tests
- Document what the code DOES, not what it SHOULD do
- Process: write assertion you know will fail → let failure
tell you actual behavior → change test to match
test cases passed, vs 19 / 24 without
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