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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