Write characterization tests before refactoring an old code

The situation

The code inherited without testing is the riskiest to change. The method of characterization tests is to capture what the code does today, including its bizarreness, before transforming it. A code agent is effective in writing these tests in mass; Human decides which behaviors are bugs.

Before you start

Tool cited: Codex; the prompt also works with Claude Code and GitHub Copilot. Fill in the bracketed items: [chemin], [préciser]. Strip confidential data that is not needed and check your tool’s access terms. Features and plans can change.

The method, in three steps.

1. 01

   Gather the context

   Spot the bracketed items in the prompt ([chemin], [préciser]) and prepare what they ask for.

2. 02

   Adapt the prompt

   Copy the prompt below, replace the bracketed items, then paste it into Codex, Claude Code or GitHub Copilot.

3. 03

   Reread before using

   Reread the result, correct it if needed and keep the final decision. Caveats are detailed below.

Your starting point.

Replace the bracketed items, then adapt the proposal to your situation. This prompt is written by the iadujour desk.

Refactoring module: [chemin]. Test language and framework: [préciser].

1. Read the module and list its observable behaviors (inputs, outputs, onboard effects).
2. Write characterization tests that freeze current behavior, including limit cases. Do not change the module code.
3. Run the tests and show the result.
4. List separately behaviors that look like bugs, for human decision.
Only then, proposes a small step refactoring plan.

What deserves your attention

Useful skills

Official skills on the same task, paired by the desk: they are not part of the described case.

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