Legacy Code Characterization Test Builder

The problem

Refactoring legacy code without a safety net often leads to regression bugs, especially when side effects and edge cases are undocumented. Relying on existing tests is risky when they are brittle, over-mocked, or only cover happy paths.

What it does

• Identifies and inventories observable legacy behavior including side effects, state changes, and error handling.
• Classifies behavior into contractual, accidental, and known bugs to prioritize what must stay stable.
• Maps complex input boundaries like nulls, legacy formats, and permission variants.
• Generates a refactoring safety matrix that labels known quirks instead of fixing them prematurely.
• Designs or implements characterization tests using your existing framework and repository conventions.

Why this beats prompting it yourself

Generic prompts often try to "fix" or "clean up" code during test generation, which hides current behavior. This skill strictly enforces the separation of characterization from refactoring, ensuring you capture exactly what the system does today, including its historical quirks and bugs.

Use cases

• Building a regression suite before migrating a monolith to microservices.
• Locking down behavior before replacing a core third-party dependency.
• Documenting and protecting a risky, undocumented module before cleanup.
• Capturing current API responses to ensure a rewrite maintains the contract.

Known limitations

Does not execute tests or verify passes without environment access. It will not automatically fix production bugs or guarantee 100% behavioral capture for non-deterministic systems.