Ast Grep

---
name: ast-grep
description: 'Use when asked to run AST-based structural search, lint, or rewrite of code when regex is too fragile. Not for remote, credential, publish, deploy, or irreversible changes.'
---

# ast-grep

## Contract

| Field | Bound contract |
|---|---|
| Trigger | AST-based modification, structural search, lint, or replacement too fragile for regex. |
| Authority | Reversible local: writes only VCS-tracked source files (search is read-only; rewrites apply only through the helper after dry-run review); rollback is version control. No remote mutation. |
| Side effect | Local file writes through the helper two-pass validate/dry-run/apply flow; no remote, credential, or published mutation. |
| Done | Pattern validated and blast radius reviewed; any rewrite landed at the correct scope (search/lint-only runs are valid). |

## Inputs

- An ast-grep pattern, single-quoted in the shell so `$VAR` reaches ast-grep unexpanded.
- A language (`--lang`) or a single target path whose extension auto-detects it; required for stdin patterns.
- For rewrites: a rewrite template and one or more target paths (defaults to the current directory).
- Optional: include/exclude globs (repeatable, prefix `!` to exclude), context lines, JSON output mode.

## Procedure

1. Confirm the task is structural (call, function, class, or import shaped like a pattern), not text/regex/filename matching (use grep) or semantic type/reference lookup (use LSP or the compiler). ast-grep matches syntax, not bytes. Done when: the task is confirmed structural.
2. Validate the pattern before searching: `python3 scripts/ast_grep_helper.py validate '<pattern>' --lang <L>`. Exit 0 means ast-grep parses it cleanly; exit 2 means malformed (the helper prints the parsed pattern tree showing the ERROR node). Fix and re-validate. Done when: validate exits 0.
3. For a rewrite, run the dry-run: `python3 scripts/ast_grep_helper.py replace '<pattern>' '<rewrite>' --lang <L> <paths>`. Read the diff and the `N matches across M files` count. If the blast radius is wrong, stop and refine the pattern (tighten meta-variables, add `--lang`, add context); re-run the dry-run. Done when: the dry-run diff and match count are correct.
4. Apply only after the dry-run diff is correct: `python3 scripts/ast_grep_helper.py replace '<pattern>' '<rewrite>' --lang <L> <paths> --apply`. The helper writes via a separate `--update-all` pass. Done when: files are updated via `--update-all`.
5. Invoke `ast-grep`, never `sg`: `sg` collides with the `setgroups` binary on many systems. Done when: `ast-grep` is invoked, not `sg`.

Pattern syntax: `$VAR` matches any single node; `$$$ARGS` matches zero or more nodes; `$_` matches any node (non-capturing).

Invariants: validate before searching; dry-run before applying; single-quote patterns; `--lang` is required for stdin; a pattern is code, not regex; switch to grep the moment `|`, `.*`, `\w`, or `[...]` would be needed. The helper keeps `--json` and `--update-all` as separate passes because combining them makes `--json` silently win and the write is dropped with no error.

For complex tasks, ast-grep supports YAML rule files (`sgconfig.yml`, `ast-grep new project`) with `rule`, `fix`, `inside`, `any`, and `matches` fields; invoke `ast-grep scan`/`run`/`test` directly for these.

## Failure and recovery
- Malformed pattern: `validate` exits 2 and prints the pattern debug tree with the ERROR node. No files touched. Fix the pattern and re-validate.
- Wrong blast radius: the dry-run diff or match count is not as expected. Do not `--apply`. Refine the pattern and re-run the dry-run. No files touched.
- Zero matches unexpectedly: run the 0-matches ladder in order: (1) validate the pattern; (2) check `--lang` (`tsx` is not `ts`; the wrong dialect silently matches nothing); (3) `ast-grep run -p '<pattern>' -l <L> --debug-query=pattern` and look for `ERROR`; (4) inspect the target's actual tree with `--debug-query=ast` on a known-matching snippet; (5) reproduce in the online playground. No files touched until a correct match is confirmed.
- ast-grep binary absent: `validate` skips the parse check (regex-smell only) and warns; `replace` exits 2 without running. Install ast-grep before proceeding.
- Apply pass failure: the helper reports the ast-grep error and returns non-zero. Revert the partially-written VCS-tracked targets via version control and re-run the full dry-run/apply sequence.
- Partial-result rule: a failed apply pass leaves whatever ast-grep wrote; never report done. Recover via version control and re-run from the dry-run.

## Output
- `validate`: exit 0 (valid) or 2 (malformed, with the pattern debug tree); advisory regex-smell hints on stderr.
- `replace` dry-run: a compact unified diff and an `N matches across M files` summary; no files modified.
- `replace --apply`: VCS-tracked files updated via `--update-all` plus a confirmation line.
- Direct `ast-grep` search: matched code locations.