ast-grep Structural Code Search & Refactoring

Structural code search and refactoring using ast-grep — matches code by its AST (abstract syntax tree) rather than text patterns.

When to Use This Skill

When to Use ast-grep vs Grep/ripgrep

Decision rule: If your search pattern contains wildcards for "any expression," "any arguments," or "any function name," use ast-grep.

Find References With the Sharpest Instrument

Promoted from ~/.claude/rules/tool-use-patterns.md. Edit refuses on a file unread this session, so "read it first" is already a harness invariant. The uncovered half is checking what else depends on what you're changing, and the reflex there is grep, which cannot tell a call from a comment, a definition from a mention, or foo() from "foo" in a string. Measured across 282 local sessions: ~2,400 code-symbol searches went through grep/rg, against 6 ast-grep calls and 6 LSP calls.

Ladder, sharpest first. Falling back down it is normal; starting at the bottom for a code symbol is the miss.

Two things that make the middle rung less automatic than it looks. A bare identifier pattern matches the definition too (ast-grep --lang bash -p 'foo' returns foo() { … } alongside every call), so an unshaped pattern is just grep with extra syntax. And a language server has to be on PATH, not merely installed — bash-language-server sitting in ~/.local/share/nvim/mason/bin is invisible to the LSP tool, which silently drops shell to the middle rung.

Structural tools carry their own empty-result trap: an ast-grep pattern that fails to parse returns no matches and exit 0, indistinguishable from a clean tree. Control-test the negative against a term you know is present, in the tool you are actually using (agent-patterns-plugin:tool-result-traps).

Pattern Syntax

Essential Commands

Search for Patterns

# Find structural patterns
ast-grep -p 'console.log($$$)' --lang js
ast-grep -p 'def $FUNC($$$): $$$' --lang py
ast-grep -p 'fn $NAME($$$) -> $RET { $$$ }' --lang rs

# Search in specific directory
ast-grep -p 'import $PKG' --lang js src/

# JSON output for parsing
ast-grep -p 'pattern' --lang js --json=compact

Search and Replace

# Preview changes (default - shows matches)
ast-grep -p 'var $V = $X' -r 'const $V = $X' --lang js

# Apply changes to all files
ast-grep -p 'var $V = $X' -r 'const $V = $X' --lang js -U

# Interactive review
ast-grep -p 'oldAPI($$$ARGS)' -r 'newAPI($$$ARGS)' --lang py -i

# Convert function syntax
ast-grep -p 'function($$$ARGS) { return $EXPR }' \
         -r '($$$ARGS) => $EXPR' --lang js -U

# Update import paths
ast-grep -p "import $NAME from '@old/$PATH'" \
         -r "import $NAME from '@new/$PATH'" --lang ts -U

Language Codes

Common Patterns by Language

JavaScript/TypeScript

# Find React hooks
ast-grep -p 'const [$STATE, $SETTER] = useState($INIT)' --lang jsx

# Find async functions
ast-grep -p 'async function $NAME($$$) { $$$ }' --lang js

# Find type assertions
ast-grep -p '$EXPR as $TYPE' --lang ts

# Find empty catch blocks
ast-grep -p 'try { $$$ } catch ($E) { }' --lang js

# Find eval usage (security)
ast-grep -p 'eval($$$)' --lang js

# Find innerHTML (XSS risk)
ast-grep -p '$ELEM.innerHTML = $$$' --lang js

# Find specific imports
ast-grep -p "import { $$$IMPORTS } from '$PKG'" --lang js

Python

# Find class definitions
ast-grep -p 'class $NAME($$$BASES): $$$' --lang py

# Find decorated functions
ast-grep -p '@$DECORATOR\ndef $FUNC($$$): $$$' --lang py

# Find dangerous shell calls
ast-grep -p 'os.system($$$)' --lang py

# Find SQL concatenation (injection risk)
ast-grep -p '"SELECT * FROM " + $VAR' --lang py

Rust

# Find unsafe blocks
ast-grep -p 'unsafe { $$$ }' --lang rs

# Find impl blocks
ast-grep -p 'impl $TRAIT for $TYPE { $$$ }' --lang rs

# Find public functions
ast-grep -p 'pub fn $NAME($$$) { $$$ }' --lang rs

Go

# Find goroutines
ast-grep -p 'go $FUNC($$$)' --lang go

# Find defer statements
ast-grep -p 'defer $FUNC($$$)' --lang go

# Find error handling
ast-grep -p 'if err != nil { $$$ }' --lang go

Common Refactoring Recipes

# Replace deprecated API calls
ast-grep -p 'oldAPI.$METHOD($$$)' -r 'newAPI.$METHOD($$$)' --lang js -U

# Rename a function across files
ast-grep -p 'oldName($$$ARGS)' -r 'newName($$$ARGS)' --lang py -U

# Remove console.log statements
ast-grep -p 'console.log($$$)' -r '' --lang js -U

# Convert require to import
ast-grep -p 'const $NAME = require($PKG)' \
         -r 'import $NAME from $PKG' --lang js -i

# Add error handling wrapper
ast-grep -p 'await $EXPR' \
         -r 'await $EXPR.catch(handleError)' --lang ts -i

Command-Line Flags

YAML Rules (Scan Mode)

For reusable rules, use ast-grep scan with YAML configuration:

# Scan with config
ast-grep scan -c sgconfig.yml

# Run specific rule
ast-grep scan -r rule-name

# Initialize a rules project
ast-grep new project my-linter
ast-grep new rule no-console-log

Minimal rule file:

id: no-empty-catch
language: JavaScript
severity: warning
message: Empty catch block hides errors
rule:
  pattern: try { $$$ } catch ($E) { }
fix: |
  try { $$$ } catch ($E) { console.error($E) }

For comprehensive YAML rule syntax, constraints, transformations, and testing, see REFERENCE.md.

Agentic Optimizations

Quick Reference

# 1. Search for pattern
ast-grep -p 'pattern' --lang js src/

# 2. Preview rewrite
ast-grep -p 'old' -r 'new' --lang js

# 3. Apply rewrite
ast-grep -p 'old' -r 'new' --lang js -U

# 4. Scan with rules
ast-grep scan -c sgconfig.yml

# 5. Debug pattern
ast-grep -p 'pattern' --debug-query --lang js