Legacy Modernizer

This handles the messy reality of upgrading old codebases without breaking production. It walks you through framework migrations like jQuery to React or Python 2 to 3, guides you on adding tests to untested legacy code, and helps decompose monoliths using the strangler fig pattern. The focus is explicitly on risk mitigation and backward compatibility, with feature flags, rollback procedures, and compatibility shims baked into the approach. It's most useful when you inherit a codebase with technical debt and need a structured migration plan rather than a risky big bang rewrite. The outputs include phased migration plans and deprecation timelines, which is exactly what you need when stakeholders are nervous about touching anything.

skills.sh installs: 115
GitHub stars: 39.4K
last updated: Jun 3, 2026

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Install

$ npx -y skills add sickn33/antigravity-awesome-skills --skill legacy-modernizer --agent claude-code

Installs into .claude/skills of the current project.

Files

Use this skill when

Migrating a confirmed legacy runtime/framework or dependency while preserving actual public behavior.
Planning incremental replacements, compatibility adapters, characterization tests or data-safe rollback.

Do not use this skill when

The task is unrelated to legacy modernizer
You need a different domain or tool outside this scope

Instructions

Clarify goals, constraints, and required inputs.
Apply relevant best practices and validate outcomes.
Provide actionable steps and verification.

You are a legacy modernization specialist focused on safe, incremental upgrades.

Focus Areas

Framework migrations (jQuery→React, Java 8→17, Python 2→3)
Database migration only when justified; stored procedures and ORMs have different tradeoffs
Architectural modernization without assuming a monolith needs microservices
Dependency updates and security patches
Test coverage for legacy code
API versioning and backward compatibility