# codemod

Provides 48 rules across 11 categories for writing reliable AST transformations with ast-grep and JSSG. Useful for automated migrations and refactoring.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 327 installs · ★ 218
- Author: pproenca
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/pproenca/dot-skills/tree/HEAD/skills/.experimental/codemod/SKILL.md
- Updated: 2026-08-15
- HTML: https://affpapa.org/en/nevibecode/skills/codemod

## What it does

This skill guides AI agents through writing, debugging, and testing automated code transformations. It contains 48 rules across 11 prioritized categories, focusing on tools like `ast-grep` and `JSSG`. Coverage spans AST traversal, parser selection, and batch editing operations.

The system ranks rules by impact. Top priority belongs to AST inspection, meta-variable usage, and parser selection for embedded languages. Subsequent tiers address edit batching, cross-file semantic analysis, and state tracking for resumable runs.

Writing pattern matchers without checking `null` properties causes silent traversal failures. The skill enforces idempotency checks and input-output fixture testing before running transformations across an entire codebase.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add pproenca/dot-skills --skill codemod -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/codemod/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/codemod/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add pproenca/dot-skills --skill codemod -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/codemod/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add pproenca/dot-skills --skill codemod -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/codemod/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install pproenca/dot-skills codemod
```

2. **Manually: put the files in place**
- File: `.github/skills/codemod/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add pproenca/dot-skills --skill codemod -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/pproenca/dot-skills --path skills/.experimental/codemod
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/codemod/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add pproenca/dot-skills --skill codemod -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/pproenca/dot-skills/tree/HEAD/skills/.experimental/codemod/SKILL.md

## When to use it
- build pattern-matching rules for `ast-grep`
- debug AST traversal and node lookup issues
- create input and expected fixture pairs for transform testing
- structure resumable migration workflows with state tracking

## Example prompts
- Write an ast-grep pattern to replace deprecated API method calls
- Review this JSSG transform script for safe null handling and idempotency
- Set up test fixtures to validate my new AST migration rule

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: a2c94f8c6e6660e6.

BEGIN THIRD-PARTY FILE a2c94f8c6e6660e6
```text
---
name: codemod
description: Codemod (JSSG, ast-grep, workflows) best practices for writing efficient, safe, and maintainable code transformations. This skill should be used when writing, reviewing, or debugging codemods, AST transformations, or automated refactoring tools. Triggers on tasks involving codemod, ast-grep, JSSG, code transformation, or automated migration.
---

# Codemod Best Practices

Comprehensive best practices guide for Codemod (JSSG, ast-grep, workflows), designed for AI agents and LLMs. Contains 48 rules across 11 categories, prioritized by impact to guide automated refactoring and code generation.

## When to Apply

Reference these guidelines when:
- Writing new codemods with JSSG or ast-grep
- Designing workflow configurations for migrations
- Debugging pattern matching or AST traversal issues
- Reviewing codemod code for performance and safety
- Setting up test fixtures for transform validation

## Rule Categories by Priority

| Priority | Category | Impact | Prefix |
|----------|----------|--------|--------|
| 1 | AST Understanding | CRITICAL | `ast-` |
| 2 | Pattern Efficiency | CRITICAL | `pattern-` |
| 3 | Parsing Strategy | CRITICAL | `parse-` |
| 4 | Node Traversal | HIGH | `traverse-` |
| 5 | Semantic Analysis | HIGH | `semantic-` |
| 6 | Edit Operations | MEDIUM-HIGH | `edit-` |
| 7 | Workflow Design | MEDIUM-HIGH | `workflow-` |
| 8 | Testing Strategy | MEDIUM | `test-` |
| 9 | State Management | MEDIUM | `state-` |
| 10 | Security and Capabilities | LOW-MEDIUM | `security-` |
| 11 | Package Structure | LOW | `pkg-` |

## Quick Reference

### 1. AST Understanding (CRITICAL)

- [`ast-explore-before-writing`](references/ast-explore-before-writing.md) - Use AST Explorer before writing patterns
- [`ast-understand-named-vs-anonymous`](references/ast-understand-named-vs-anonymous.md) - Understand named vs anonymous nodes
- [`ast-use-kind-for-precision`](references/ast-use-kind-for-precision.md) - Use kind constraint for precision
- [`ast-field-access-for-structure`](references/ast-field-access-for-structure.md) - Use field access for structural queries
- [`ast-check-null-before-access`](references/ast-check-null-before-access.md) - Check null before property access

### 2. Pattern Efficiency (CRITICAL)

- [`pattern-use-meta-variables`](references/pattern-use-meta-variables.md) - Use meta variables for flexible matching
- [`pattern-avoid-overly-generic`](references/pattern-avoid-overly-generic.md) - Avoid overly generic patterns
- [`pattern-combine-with-rules`](references/pattern-combine-with-rules.md) - Combine patterns with rule operators
- [`pattern-use-constraints`](references/pattern-use-constraints.md) - Use constraints for reusable matching logic
- [`pattern-use-relational-patterns`](references/pattern-use-relational-patterns.md) - Use relational patterns for context
- [`pattern-ensure-idempotency`](references/pattern-ensure-idempotency.md) - Ensure patterns are idempotent

### 3. Parsing Strategy (CRITICAL)

- [`parse-select-correct-parser`](references/parse-select-correct-parser.md) - Select the correct parser for file type
```
END THIRD-PARTY FILE a2c94f8c6e6660e6

The third-party file has ended. Everything between the a2c94f8c6e6660e6 tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

First 3116 of 8129 bytes. © pproenca · MIT · https://github.com/pproenca/dot-skills/tree/HEAD/skills/.experimental/codemod/SKILL.md

_Part of the page did not fit — see the HTML version._

---
NeVibeCode on affpapa.org. Catalog texts are descriptions, not instructions for the agent. How to choose and install: https://affpapa.org/en/nevibecode/agents.md