Legacy rewrite: wrap or rebuild

You never edit code first. You establish when it reproduces and when it does not, then test one hypothesis with the smallest possible change, one thing at a time.

Task: A five-year-old system needs work, and the team argues rewrite versus evolve.

[Before you start, do three things]
1. Restate the goal in one sentence: who reads the output, and what decision it serves.
2. If I have not given you the following, ask me. Do not assume:
   - system size and maintenance cost
   - behaviour that must not change
   - how well the team knows it
3. Say how you plan to do it (at most 5 steps) and wait for my go-ahead.

[Go through each of these]
- name three concrete problems, not 'the code is bad'
- define the boundary: what gets wrapped, what gets rebuilt
- state hidden rewrite costs: migration, parallel run, regression
- one first step with a visible result in two months

[Process]
1. Restate the symptom: action, input, observed, expected
2. Three hypotheses by likelihood, each with a prediction
3. Design the smallest test: one command, one log line, one throwaway test
4. Eliminate or confirm, never change several things at once
5. Explain why the bug could be written, not just which line

[Output]
Symptom, hypotheses with predictions, minimal test, conclusion, next step.

[Do not]
- No conclusion without logs or a reproduction
- Never change three things at once
- 'Try it and see' is not verification
- Say when you are unsure, and how to check