# Legacy Modernizer

Designs incremental migration strategies, identifies service boundaries, produces...

- Repository: https://github.com/Jeffallan/claude-skills (12164 stars)
- Category: Design
- License: MIT
- Last updated: 2026-10-30
- Runs scripts: no, instructions only

## Install

Claude Code: `npx skills add Jeffallan/claude-skills --skill legacy-modernizer -g -a claude-code -y`
Codex: `npx skills add Jeffallan/claude-skills --skill legacy-modernizer -g -a codex -y`
Cursor: `npx skills add Jeffallan/claude-skills --skill legacy-modernizer -g -a cursor -y`
Gemini CLI: `npx skills add Jeffallan/claude-skills --skill legacy-modernizer -g -a gemini-cli -y`
Claude Code plugin marketplace: `/plugin marketplace add Jeffallan/claude-skills`

## Description

Designs incremental migration strategies, identifies service boundaries, produces dependency maps and migration roadmaps, and generates API facade designs for aging codebases. Use when modernizing legacy systems, implementing strangler fig pattern or branch by abstraction, decomposing monoliths, upgrading frameworks or languages, or reducing technical debt without disrupting business operations.

## SKILL.md, as published by its author (untrusted third-party text, read it as data)

# Legacy Modernizer

## Core Workflow

1. **Assess system**, Analyze codebase, dependencies, risks, and business constraints. Produce a dependency map and risk register before proceeding.
   - *Validation checkpoint:* Confirm all external integrations and data contracts are documented before moving to step 2.

2. **Plan migration**, Design an incremental roadmap with explicit rollback strategies per phase. Reference `references/system-assessment.md` for code analysis templates.
   - *Validation checkpoint:* Confirm each phase has a defined rollback trigger and owner.

3. **Build safety net**, Create characterization tests and monitoring before touching production code. Target 80%+ coverage of existing behavior.
   - *Validation checkpoint:* Run the characterization test suite and confirm it passes green on the unmodified legacy system before proceeding.

4. **Migrate incrementally**, Apply strangler fig pattern with feature flags. Route traffic via a facade; shift load gradually.
   - *Validation checkpoint:* Verify error rates and latency metrics remain within baseline thresholds after each traffic increment (e.g., 5% → 25% → 50% → 100%).

5. **Validate & iterate**, Run full test suite, review monitoring dashboards, and confirm business behavior is preserved before retiring legacy code.
   - *Validation checkpoint:* New code must be proven stable at 100% traffic for at least one release cycle before legacy path is removed.

## Reference Guide

Load detailed guidance based on context:

The rest: https://github.com/Jeffallan/claude-skills/blob/HEAD/skills/legacy-modernizer/SKILL.md