AST-based structural code search, security scanning, and refactoring using ast-grep
(sg CLI) with pattern matching and code transformation across 40+ languages.
Use when performing structural code search, AST-based refactoring, codemod operations,
security pattern scanning, or syntax-aware code transformations across files.
Do NOT use for simple text search (use Grep tool instead)
or full codebase exploration (use Explore agent instead).
Designed for Claude CodeReadGrepGlobBash(sg:*)Bash(ast-grep:*)mcp__context7__resolve-library-idmcp__context7__get-library-docs
AST-Grep Integration
Structural code search, lint, and transformation tool using Abstract Syntax Tree analysis.
Quick Reference
What is AST-Grep
AST-Grep (sg) is a fast, polyglot tool for structural code search and transformation. Unlike regex-based search, it understands code syntax and matches patterns based on AST structure.
When to Use
Searching for code patterns that regex cannot capture such as nested function calls
Refactoring code across multiple files with semantic awareness
Security scanning for vulnerability patterns including SQL injection and XSS
API migration and deprecation handling
Enforcing code style rules at the syntax level
Core Commands
Pattern search: Execute sg run with pattern option specifying the code pattern to find, lang option for the programming language, and the source directory path.
Security scan with rules: Execute sg scan with config option pointing to your sgconfig.yml file.
Code transformation: Execute sg run with pattern option for the code to find, rewrite option for the replacement, lang option for the language, and source directory path.
Test rules: Execute sg test to validate your rule definitions.
Pattern Syntax Basics
The dollar sign followed by a variable name such as VAR matches any single AST node and acts as a meta-variable for capturing.
The dollar sign followed by three dollar signs and a variable name such as ARGS matches zero or more nodes using variadic capture.
The double dollar sign followed by underscore matches any single node as an anonymous capture when the value is not needed.
Integration with Explore Agent
When using the Explore agent, AST-Grep is automatically prioritized for:
Class hierarchy analysis
Function signature matching
Import dependency mapping
API usage pattern detection
Meta-variables
Meta-variables capture matching AST nodes in patterns.
Single node capture uses $NAME syntax. For example, pattern const $NAME = require($PATH) captures the variable name and path.
Variadic capture uses $$$ARGS syntax. For example, pattern function $NAME($$$ARGS) captures function name and all arguments.
Anonymous single capture uses $$_ syntax when you need to match but not reference the value.
Code Transformation
Simple Rewrite
To rename a function, run sg with pattern oldFunc($ARGS), rewrite newFunc($ARGS), and lang python.
To update an API call, run sg with pattern axios.get($URL), rewrite fetch($URL), and lang typescript.
Complex Transformation with YAML Rules
Create a YAML rule file with the following structure. Set the id field to a unique rule identifier such as convert-var-to-const. Set language to the target language such as javascript. Under the rule section, specify the pattern to match such as var $NAME = $VALUE. Set the fix field to the replacement pattern such as const $NAME = $VALUE. Add a message describing the issue and set severity to warning or error.
Run sg scan with the rule option pointing to your rule file and the source directory.
Rule-Based Scanning
Configuration File
Create an sgconfig.yml file with the following sections. The ruleDirs section lists directories containing rule files such as ./rules/security and ./rules/quality. The testConfigs section specifies test file patterns. The languageGlobs section maps languages to file patterns, mapping python to .py files, typescript to .ts and .tsx files, and javascript to .js and .jsx files.
Security Rule Example
Create a security rule file for SQL injection detection. Set the id to sql-injection-risk. Set language to python and severity to error. Write a descriptive message about the vulnerability. Under the rule section, use the any operator to match multiple patterns including cursor.execute with percent formatting, cursor.execute with format method, and cursor.execute with f-string interpolation. Set the fix to show the parameterized query alternative.
Relational Rules
Inside Rule for Scoped Search
Create a rule that searches for console.log calls only inside function declarations. Set the pattern to console.log($$$ARGS) and add an inside constraint with pattern function $NAME($$$PARAMS).
Has Rule for Contains Check
Create a rule to find async functions without await. Set the pattern to async function $NAME($$$PARAMS) with a not constraint containing a has rule with pattern await $EXPR. Add message indicating async function without await.
Follows and Precedes Rules
Create a rule to detect missing error handling. Set the pattern to match error assignment $ERR := $CALL and add a not constraint with follows rule checking for if $ERR != nil error handling block.
Composite Rules
Create complex rules using the all operator to combine multiple conditions. For example, combine pattern useState($INIT) with inside constraint for function component and not precedes constraint for useEffect call.
Advanced Patterns
For comprehensive documentation including complex multi-file transformations, custom language configuration, CI/CD integration patterns, and performance optimization tips, see the following module files.
Pattern syntax reference is available in modules/pattern-syntax.md.
Security scanning rule templates are documented in modules/security-rules.md.
Common refactoring patterns are covered in modules/refactoring-patterns.md.
Language-specific patterns are detailed in modules/language-specific.md.
Context7 Integration
For latest AST-Grep documentation, follow this two-step process.
Step 1: Use mcp__context7__resolve-library-id with query ast-grep to resolve the library identifier.
Step 2: Use mcp__context7__get-library-docs with the resolved library ID to fetch current documentation.
MoAI-ADK Integration
AST-Grep is integrated into MoAI-ADK through the Tool Registry as AST_ANALYZER type in internal/hook/registry.go, PostToolUse Hook for automatic security scanning after Write/Edit operations, and Permissions with Bash(sg:) and Bash(ast-grep:) auto-allowed.
Running Scans
To scan with MoAI-ADK rules, execute sg scan with config pointing to .claude/skills/moai-tool-ast-grep/rules/sgconfig.yml.
To scan a specific directory, execute sg scan with config sgconfig.yml and the src/ directory.
For JSON output suitable for CI/CD, execute sg scan with config and json flag, redirecting to results.json.
Works Well With
moai-workflow-testing: DDD integration and test pattern detection
moai-foundation-quality: TRUST 5 compliance and code quality gates
moai-domain-backend: API pattern detection and security scanning
For additional information, consult the AST-Grep Official Documentation at ast-grep.github.io, the AST-Grep GitHub Repository at github.com/ast-grep/ast-grep, the Pattern Playground at ast-grep.github.io/playground.html, and the Rule Configuration Reference at ast-grep.github.io/reference/yaml.html.
