AI agent skills for working with legacy code
Legacy skills are for the situation where the code works, nobody knows why, and the tests do not exist. The method they encode — characterize current behaviour, change one thing, verify — is old and unglamorous and it is what keeps a migration from becoming a rewrite. Look for one that handles the archaeology explicitly: reading commit history and issue threads to recover the reason a strange branch is there.
People land here searching for “claude code legacy code skill”, “agent skill for migration”, “codemod skill”.
270 working with legacy code skills Refine in search →
-
openai Bundle Next UpgradeUpgrade Next.js to the latest version following official migration guides and codemods. Use when upgrading Next.js versions, running codemods, or migrating between major releases.
23.3k -
jeffallan Bundle Legacy ModernizerDesigns incremental migration strategies, identifies service boundaries, produces dependency maps and migration roadmaps, and generates API facade designs for aging codebases.
10.4k -
hoangnguyen0403 Bundle Nextjs UpgradeNext.js version migrations using official guides and codemods. Use when migrating a Next.js project to a new major version using codemods.
Audited 542 -
sickn33 Skill Legacy ModernizerRefactor legacy codebases, migrate outdated frameworks, and implement gradual modernization. Handles technical debt, dependency updates, and backward compatibility.
45.1k -
project-chip Skill Code Driven Cluster MigrationStep-by-step procedure for migrating a legacy Ember-based Matter server cluster in src/app/clusters/ to the code-driven pattern. Use this skill when converting an existing cluster. All rules from code-driven-cluster-development still apply; this skill adds the migration-specific workflow on top.
Audited -
ferroxlabs Bundle Migrate Legacy CodebaseOrchestrates the systematic modernization of a legacy codebase by chaining four engineering skills into a structured migration pipeline. Covers technical debt assessment, incremental refactoring, integration testing, and safe deployment of modernized components. Use when the user needs to modernize a legacy system incrementally without breaking existing functionality. Do NOT use for greenfield rewrites, simple library upgrades, or projects where the legacy system can be replaced entirely.
Audited 37 -
vinhnx Bundle CodemodUse Codemod CLI whenever the user wants to migrate, upgrade, update, or refactor a codebase in a repeatable way. This includes framework migrations, library upgrades, version bump migrations, API surface changes, deprecations, and large-scale mechanical edits. First search the Codemod Registry for an existing package, prefer deterministic codemods before open-ended AI rewrites, run dry-runs before apply, and create a codemod package only when no suitable package exists.
Audited -
pwdev-solucoes Bundle Flow CompatRoutes legacy PWDEV Code command intents to their canonical PWDEV Flow skills, preserving history when adopting an existing .planning workspace.
Audited 2 -
github Bundle React18 Legacy ContextMigrates React legacy context API (contextTypes, childContextTypes, getChildContext) to the modern createContext API across all provider and consumer files.
Audited 36.2k - github Skill Winui3 Migration Guide
Maps legacy UWP APIs to correct Windows App SDK equivalents with before/after code snippets for migrating to WinUI 3.
Audited 36.2k -
dokhacgiakhoa Bundle Legacy ModernizerRefactor legacy codebases, migrate outdated frameworks, and implement gradual modernization.
505 -
comeonoliver Skill Nextjs UpgradeNext.js version migrations using official guides and codemods. Use when migrating a Next.js project to a new major version using codemods. (triggers: package.json, next upgrade, migration guide, codemod)
61 - sickn33 Skill Framework Migration Legacy Modernize
Orchestrate a comprehensive legacy system modernization using the strangler fig pattern, enabling gradual replacement of outdated components while maintaining continuous business operations through ex
45.1k - ferroxlabs Bundle Modernize Legacy System
Systematic legacy system modernization workflow covering code review and assessment, technical debt analysis, strategic refactoring, test coverage improvement, containerization, and CI/CD pipeline setup for a safe, incremental transition from legacy to modern architecture. Use when the user wants to modernize legacy system or needs a structured multi-step process for this goal. Do NOT use when the request is a single-step task or requires professional advice beyond educational guidance.
Audited 37 -
pproenca Bundle Nuqs Codemod RunnerMigrates a nuqs codebase off pre-v2.5 patterns — deprecated `throttleMs`, hand-rolled setTimeout/useState debounce around `useQueryState`, unchecked `parseAsJson` casts, unversioned `nuqs/adapters/react-router` imports, or `ParserBuilder<T>` type references. Scans the repo, produces a dry-run report, asks for confirmation, applies AST codemods, then runs `tsc --noEmit` + the user's lint as a gate. Trigger even if the user only mentions "upgrade nuqs" or "migrate to nuqs v2.5+" without naming the specific patterns — they almost always exist in pre-v2.5 codebases.
- github Skill Generate Custom Instructions From Codebase
Analyzes differences between two project versions to generate precise migration instructions for GitHub Copilot, enabling consistent code transformations during technology migrations, refactoring, or framework upgrades.
Audited 36.2k -
autohandai-community-skills Skill Legacy Modernization OrchestratorOrchestrate multi-agent legacy modernization assessment, planning, and code-conversion workflows.
-
yigityildiz0 Bundle Legacy ModernizerModernize legacy codebases using proven migration strategies like Strangler Fig pattern. Use when upgrading frameworks, migrating to new architectures.
-
aj-geddes-useful-ai-prompts Bundle Refactor Legacy CodeModernize and improve legacy codebases while maintaining functionality. Use when you need to refactor old code, reduce technical debt, modernize deprecated patterns, or improve code maintainability without breaking existing behavior.
-
phmatray Bundle Migrate LegacyUse when upgrading, migrating, or modernizing a legacy application — outdated target framework, out-of-support runtime, obsolete APIs, old packages, or legacy idioms. Triggers on "upgrade this app", "migrate to .NET 10", "modernize this codebase", "this project targets an old framework", « mets à niveau cette app », « migre vers .NET 10 », « modernise ce code legacy », /migrate, /migrate-assess, /migrate-verify. Drives a seven-phase, gate-verified pipeline (assessment → verified production) in which RoselineMCP performs all C# analysis and transformation.
Audited -
0xharryriddle Skill Legacy ModernizerUse when legacy modernizer expertise is needed to unblock implementation decisions.
3 - pproenca Bundle Codemod React Pipeline
Guided, scripted pipeline for running JSX/TSX/React codemods safely across large legacy codebases. Use when you need to scaffold a codemod, dry-run it, validate its findings, and apply it across many files (50 to 100k+) without breaking the build. Walks the full inner/outer loop with the Codemod CLI (JSSG, ast-grep, workflows). Triggers on large-scale refactor, legacy React migration, codemod a prop/API change, migrate components across the codebase, run a codemod safely, batched/resumable codemod apply, dry-run a codemod.
- openai Bundle Migrate To Codex
Migrate Claude Code configuration, skills, agents, and MCP servers to Codex project and global files.
23.3k -
wedabro Skill Speckit MigrateLegacy Code Migrator - Reverse-engineer existing codebase to SDD standard.
Audited
How to install a working with legacy code skill
- Compare the skills. Read the working with legacy code skills below — each page shows the full SKILL.md, its safety verdict, and the capabilities it declares.
- Install it. Run npx skillmds@latest add <owner>/<name>. The CLI writes the skill into every agent directory it detects, or use --agent to pin one.
- Use it. Restart your agent. It loads the skill on demand the next time you ask for something that matches — you do not have to name the skill.
What makes a good working with legacy code skill?
Legacy skills are for the situation where the code works, nobody knows why, and the tests do not exist. The method they encode — characterize current behaviour, change one thing, verify — is old and unglamorous and it is what keeps a migration from becoming a rewrite. Look for one that handles the archaeology explicitly: reading commit history and issue threads to recover the reason a strange branch is there.
Every skill listed here is a plain SKILL.md file in the format Anthropic documents for Agent Skills, read unchanged by Cursor, OpenAI Codex and 60+ agents. Each passes a safety review before it is publicly listed; the verdict and declared capabilities are on every skill's page.
Frequently asked questions
What is an agent skill for working with legacy code?
Legacy skills are for the situation where the code works, nobody knows why, and the tests do not exist. The method they encode — characterize current behaviour, change one thing, verify — is old and unglamorous and it is what keeps a migration from becoming a rewrite. Look for one that handles the archaeology explicitly: reading commit history and issue threads to recover the reason a strange branch is there.
Can an agent safely upgrade a major dependency version?
With a test suite and an incremental skill, often yes. Without tests, a major upgrade is an unverifiable change, and skills built for this will usually insist on characterization tests before touching the version.
Which agents can use these working with legacy code skills?
Any agent that reads SKILL.md files: Claude Code, Claude.ai, Cursor, OpenAI Codex, Windsurf, OpenCode and 60+ others. The format is not vendor-specific, so the same file works everywhere — each agent just keeps its skills in a different directory, listed at /agents.
Are these working with legacy code skills free?
Yes. Searching, reading and installing skills on SkillMD is free and needs no account. Individual skills carry their own licence, shown on each skill's page.