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refactoring-legacy-code

Skill1 starsChanged 2 months agoMITView on GitHub

Use when refactoring complex, untyped, tightly-coupled legacy code or migrating monolithic modules safely without breaking existing behavior.

The file

We split this skill into the rules it gives an agent

4 rules. About 0 are common, 4 are distinctive, and 0 are very specific to this project.

  • ≈0

    Common · estimate

    Many files like this have a rule of this kind.

  • ≈4

    Distinctive · estimate

    This author's own choices. Worth a look.

  • 0

    Very specific

    Tied to their project's names, files or tools. Probably skip.

  • Common
  • Distinctive
  • Very specific
  1. Distinctive:

    Never perform a ground-up rewrite.

    line 8
  2. Distinctive:

    Before changing ANY logic, wrap the legacy module in automated snapshot tests using real production-like inputs.

    line 11
  3. Distinctive:

    Write the new implementation alongside the legacy implementation.

    line 19
  4. Distinctive:

    Verify that all characterization tests and unit tests remain green.

    line 25

≈ “Common” is an estimate: at least 1 in 5 of the 350,495 skill files we read have a rule of the same kind, not the same words. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

Keep what fits and drop the rest. Your version, ready for your agents.
Files
SKILL.md

Legacy Code Refactoring (Strangler Fig)

Never perform a ground-up rewrite. Safely decouple and modernize legacy systems step by step:

1. Characterization Testing (Golden Master)

  • Before changing ANY logic, wrap the legacy module in automated snapshot tests using real production-like inputs.
  • Record existing outputs (including known quirks/bugs) as the baseline standard of truth.

2. Identify the Seam & Define Clean Interface

  • Identify the clean boundary where legacy code receives input and returns output.
  • Define a type-safe interface or TypeScript contract representing the modern desired API.

3. Strangler Fig Migration Pattern

  • Write the new implementation alongside the legacy implementation.
  • Route a small slice of traffic or callers through the new implementation.
  • Compare outputs in shadow mode or feature flags if the domain logic is critical (billing, auth).

4. Prune Dead Legacy Branches

  • Once 100% of callers utilize the new implementation, safely delete the legacy file.
  • Verify that all characterization tests and unit tests remain green.

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