ast-grep

ast-grep

sg (also installed as ast-grep) is an AST-aware search and rewrite tool across 25 languages. It treats your pattern as code, parses it the same way it parses your project, and matches structurally. It is the right tool whenever your question depends on code shape rather than text bytes.

This skill ships a Python wrapper at scripts/ast_grep_helper.py and platform install scripts at install.sh (POSIX) and install.ps1 (Windows). The helper adds offline pattern validation, the two-pass write trick, and binary auto-resolution. Use it as your default entry point.

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OmO native: the ast-grep MCP tools (fastest path, no CLI needed)

OmO native also registers a bundled ast-grep MCP server. Its three tools are deferred: they are absent from the resident tool list and activate automatically the first time you call them by name — no tool_search round trip needed.

• mcp__ast_grep_search({ pattern, language, paths }) — AST-shape search, the MCP equivalent of sg run -p. Prefer this over shelling out to sg for one-off lookups: no install, no PATH issues.
• mcp__ast_grep_rewrite({ pattern, rewrite, language, paths, apply }) — preview matches (apply: false, the default) or apply the rewrite (apply: true). Same two-pass discipline as sg run -r: preview first, then apply.
• mcp__ast_grep_scan({ paths }) — run project YAML rules (sgconfig.yml, rules/), the MCP equivalent of sg scan.

Same pattern language as the CLI: $VAR matches one AST node, $$$VAR matches zero or more, and the pattern is code, not regex. paths accepts a file or directory list; language is required for patterns that are ambiguous across languages.

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When to use this skill

Use it whenever the user's question is about code structure, not bytes:

• "Find every function that takes a Request parameter."
• "Rewrite every console.log(x) to logger.info(x)."
• "Strip every as any cast."
• "Replace require(...) with import across the repo."
• "Find empty catch blocks."
• "Migrate Optional[X] to X | None."
• "Apply this codemod across these 200 files."
• "Run our YAML lint rules and surface violations."

Switch to plain grep / rg when the question is text-shaped (string literal contents, comments, license headers, file names, cross-language regex). When in doubt, ask: "does the answer depend on the language's syntax tree, or just on the file's bytes?" If the former, ast-grep. If the latter, grep.

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Three things the agent must internalize

1. ast-grep is NOT regex

The wildcards are $VAR (one AST node) and $$$ (zero or more nodes). Regex syntax fails silently:

| You wrote | What ast-grep saw | What you wanted |

|---|---|---|

| foo\|bar | bitwise-or of foo and bar | run two separate searches |

| .*foo | not parseable | $$$ foo (if $$$ is a list of nodes) or use rg |

| \w+ | not parseable | $VAR to capture any identifier |

| [a-z] | character class, not parseable | switch to rg |

The full anti-pattern table is in references/pitfalls.md §1. The helper's validate subcommand catches these mechanically — call it before debugging "no matches" by hand.

2. Patterns must be valid code

The pattern itself must parse. def $FN($$$): fails because the trailing : makes it incomplete; use def $FN($$$). function $NAME without params/body fails; use function $NAME($$$) { $$$ }. Full table per language in references/pitfalls.md §2.

3. --update-all and --json are mutually exclusive (silently)

This is the single biggest gotcha when scripting. sg run -p P -r R --json --update-all returns the JSON but does not mutate files. To both preview AND apply, run two passes:

sg run -p P -r R --json=compact .   # pass 1: see what would change
sg run -p P -r R --update-all .     # pass 2: actually apply

The helper does this automatically when you call replace --apply. Read references/pitfalls.md §9.

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The helper script — scripts/ast_grep_helper.py

A single-file Python 3 stdlib wrapper. Same on every OS. The agent's default entry point.

search — find all matches of a pattern

python3 scripts/ast_grep_helper.py search 'console.log($MSG)' --lang ts src/

Validates the pattern offline first. If the pattern looks like regex (\w, .*, |, etc.) the helper exits with a hint and never calls sg — saves a round-trip. Pass --force to skip validation.

Flags:

• --lang ts (or any of the 25 languages; aliases like js, py, rs, kt accepted)
• --globs '!**/*.test.ts' (repeatable; prefix ! to exclude)
• -C 3 (context lines)
• --json-out (raw JSON instead of human format)

replace — rewrite by pattern, dry-run by default

# Dry-run preview (default — no files mutated)
python3 scripts/ast_grep_helper.py replace 'console.log($MSG)' 'logger.info($MSG)' --lang ts src/

# Actually apply
python3 scripts/ast_grep_helper.py replace 'console.log($MSG)' 'logger.info($MSG)' --lang ts src/ --apply

The helper:

1. Validates both pattern and rewrite for hint-detectable mistakes.
2. Runs pass 1 with --json=compact to collect matches and show a preview.
3. If --apply is set, runs pass 2 with --update-all to mutate files.

scan — run YAML rules

# Discover sgconfig.yml from cwd and run all rules
python3 scripts/ast_grep_helper.py scan src/

# Run a single rule file
python3 scripts/ast_grep_helper.py scan -r rules/no-console.yml src/

# Apply auto-fixes
python3 scripts/ast_grep_helper.py scan -U src/

# CI-friendly GitHub annotations
python3 scripts/ast_grep_helper.py scan --report-style short src/

validate — offline pattern check (no sg call)

Useful for CI lints, pre-commit hooks, and quick sanity checks:

python3 scripts/ast_grep_helper.py validate '\w+' --lang ts
# → exit 2: regex \w not supported. Use $VAR for identifiers.

python3 scripts/ast_grep_helper.py validate 'console.log($MSG)' --lang ts
# → exit 0: pattern looks plausible for ast-grep.

langs / doctor / install

python3 scripts/ast_grep_helper.py langs       # list 25 supported languages and aliases
python3 scripts/ast_grep_helper.py doctor      # check ast-grep binary availability
python3 scripts/ast_grep_helper.py install     # delegate to install.sh / install.ps1

new and test subcommands proxy directly to sg new and sg test.

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Direct sg use (when the helper isn't enough)

The helper is opinionated. For full control, drop to sg. The skill ships a CLI cheat sheet in references/cli.md. The minimal idioms:

# Search
sg run -p 'console.log($MSG)' --lang ts src/

# Search with JSON for scripting
sg run -p 'console.log($MSG)' --lang ts --json=compact src/ | jq '.[] | .file'

# Rewrite, dry-run
sg run -p 'console.log($MSG)' -r 'logger.info($MSG)' --lang ts --json=compact src/

# Rewrite, apply
sg run -p 'console.log($MSG)' -r 'logger.info($MSG)' --lang ts --update-all src/

# Pattern from stdin (great for ad-hoc experiments)
echo 'console.log("hi")' | sg run -p 'console.log($MSG)' --lang js --stdin

# Debug a pattern that returns 0 matches
sg run -p '<your pattern>' --lang <lang> --debug-query=ast --stdin <<< '<sample-code>'

# Run YAML rules
sg scan src/

# Inline YAML rule (one-off)
sg scan --inline-rules '
id: no-todo
language: TypeScript
severity: warning
rule: { pattern: TODO }' src/

When using sg directly in a shell, always single-quote patterns so $VAR is not expanded by the shell.

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Decision tree — what to use, when

USER asks for "find/rewrite/codemod"
│
├─ structural pattern (function shape, call, class, import, control flow)
│  └→ ast-grep (this skill)
│
├─ text pattern (regex, alternation, character classes, file names)
│  └→ rg / grep
│
├─ semantic question (what variable does this refer to? does this throw?)
│  └→ LSP tools, TypeScript compiler, Pyright, Semgrep with type inference
│
└─ multiple repos / federated search
   └→ a search engine + then ast-grep / rg / LSP per-repo

If the user says "find all" or "every", default to ast-grep when the target is shaped (function, class, call, import, statement). Default to rg when the target is text (string content, comment, license header, file name, identifier substring).

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Always run dry-run first when rewriting

A bad pattern silently rewrites the wrong thing. The helper's replace defaults to dry-run for this reason. The flow is:

1. Search to confirm matches: helper search '<pattern>' --lang X .
2. Dry-run rewrite: helper replace '<pattern>' '<rewrite>' --lang X . (no --apply)
3. Inspect the dry-run summary: number of matches, files affected, the per-location preview.
4. If wrong: refine pattern, go back to step 1.
5. If right: helper replace '<pattern>' '<rewrite>' --lang X . --apply.

Never apply a rewrite that you have not first dry-run.

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When sg returns 0 matches but you know the code is there

In priority order:

1. Run helper validate '<pattern>' --lang <lang> — catches regex misuse, missing function bodies, Python trailing colons.
2. Check --lang — sg infers from extension; if you pass a .tsx file with --lang ts (not tsx), JSX won't parse.
3. Inspect the parsed pattern: sg run -p '<pattern>' --lang <lang> --debug-query=ast --stdin <<< '<sample>'. If it shows ERROR nodes, the pattern is malformed.
4. Check the AST of the target file: sg run -p '$_' --lang <lang> --debug-query=cst path/to/file | head -40 — find the kind you're trying to match.
5. Try the playground: <https://ast-grep.github.io/playground.html> — paste code + pattern, see what's happening.

Do not blindly retry with variations. Each failure has a reason; surface it.

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When to use YAML rules vs inline -p patterns

Use inline -p when:

• One-off ad-hoc query.
• The pattern is simple (no constraints, no fix template).
• You're exploring.

Use YAML rules (file under rules/, run via sg scan) when:

• The pattern is reused (lint rule, codemod that runs in CI).
• You need constraints, transform, complex inside/has, or composite logic.
• You want auto-fix (fix: field).
• You want to test the rule (snapshot tests via sg test).

The full YAML rule schema is in references/yaml-rules.md. Project setup (sgconfig.yml, ruleDirs, utilDirs) is in references/sgconfig.md.

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Output discipline

• sg run --json=compact produces an array of match objects: { file, range: {start, end}, text, replacement?, lines, language, ... }. Pipe through jq for further processing.
• Without --json, sg produces human-readable colored output suitable for terminals.
• The helper's default output is human-readable (file:line:column + match preview). Pass --json-out for raw JSON.
• The helper's replace always summarizes: number of matches, number of files, per-location preview.

When summarizing for the user, always include the count of files affected, not just the count of matches. Users care about blast radius.

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Required reading (in order of priority)

1. references/patterns.md — meta-variables, naming rules, strictness levels. Read when you're unsure why a pattern doesn't match.
2. references/pitfalls.md — the failure-mode field guide. Read when 0 matches surprises you.
3. references/recipes.md — copy-paste patterns by language. Read first when you start a new task.
4. references/cli.md — sg run, sg scan, sg test, sg new, sg lsp. Read when the helper isn't enough.
5. references/yaml-rules.md — YAML rule schema. Read when you outgrow inline patterns.
6. references/sgconfig.md — project-level configuration. Read when you set up sg scan for a real project.
7. references/install.md — per-OS install methods. Read only if install.sh / install.ps1 fail.

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Invariants (do not break)

• Validate before searching. When emitting a pattern programmatically, call helper validate first. It catches the regex-misuse class of mistakes that account for ~70% of "0 matches" debug sessions.
• Dry-run before applying. Never run sg run -r ... --update-all without first inspecting the matches. The helper's replace enforces this by default.
• Two-pass writes. When using sg directly to both preview and apply, run two invocations — --json ignores --update-all.
• Single-quote patterns in shell. '$VAR' not "$VAR". The shell expands $VAR to the empty string in double quotes, breaking the pattern.
• Pattern is code, not regex. When the pattern would need |, .*, \w, or [a-z], switch to rg instead. Don't try to force ast-grep into a regex shape.
• --lang is required for stdin. When piping with --stdin, set --lang explicitly; sg cannot infer from extension.
• Linux: prefer ast-grep over sg because sg collides with setgroups. The helper handles this; if you call sg directly, alias it: alias sg=ast-grep.