ast-grep

Use this skill when the task benefits from AST-based search, linting, or rewrites instead of regex- or line-based editing.

Use this skill when

• You need structural search over code, lint-rule authoring, or rewrite changes with AST-aware patterns.
• You want repeatable, reviewable edits across many files.
• You want to preview candidate matches before applying a codemod.
• A request explicitly mentions ast-grep, sg, codemod, rule files, or syntax-aware edit work.

Do not use this skill when

• The task is simple literal string search; route to normal rg/glob usage.
• The request is under-specified and needs sharpening first; route to reverse-prompt.
• The main work is root-cause analysis of a failure; route to systematic-debugging.
• The task is writing behavior tests first for a feature or bugfix; route to test-driven-development.

Routing boundary

Situation	Use this skill?	Route instead
"Author a lint rule for a structural pattern"	Yes	-
"Replace a JS API call pattern across all TS files safely"	Yes	-
"Find where this exact string appears"	No	direct rg search
"I have a failing test and do not know why"	No	systematic-debugging
"Before coding, rewrite my vague request into an executable brief"	No	reverse-prompt
"Implement feature X with failing test first"	No	test-driven-development

Inputs to gather

Required before editing

• Goal: search, lint, or rewrite.
• Target language(s) and search scope (paths/directories).
• Structural pattern and, for rewrites, replacement pattern or rule file.

Helpful if present

• Existing ast-grep rule files in the repository.
• Known exclusions (dist, generated code, vendored directories).
• Expected output format (human-readable vs JSON for tooling).

Only investigate if encountered

• Parser/language mismatch warnings.
• Rewrite collisions where one pass affects later matches.
• File-encoding or formatting side effects.

First move

1. Choose mode: search, lint, or rewrite.
2. Build a narrow command from the reference templates and run a preview first.
3. Expand scope only after the preview matches the intended shape.

Workflow

1. Build a search or rewrite command from references/command-patterns.md and constrain it by language/path.
2. For search or lint work, inspect representative results before broadening scope.
3. If rewrite is needed, choose replacement (-r) or rule-file mode (--rule) and re-run preview.
4. Apply rewrite only after preview is clean and exclusions are explicit.
5. Review resulting diffs and check for unintended changes using the safety checklist in references/safety-and-review.md.
6. If output is noisy, roll back and tighten pattern/scope before retrying.

Outputs

• A documented ast-grep command (or rule invocation) used for the task.
• For rewrites, a clean diff showing intended structural edits only.
• For linting, a tested rule file or rule invocation.
• A short summary of matched scope, applied exclusions, and any rollback/retry decisions.

Guardrails

• Default to non-destructive flow: preview before apply.
• Never run broad rewrites without explicit language and scope constraints.
• Prefer the full ast-grep binary name in docs and generated commands; use sg only if local shell compatibility requires it.
• Keep command examples portable (repo-relative paths only, no user-specific absolute paths).
• Treat helper automation as optional convenience; do not require it to use this skill.
• If pattern intent is ambiguous, narrow to search mode first instead of guessing rewrite behavior.

Validation

• Run metadata/structure validation:
  • node skills/skill-authoring/scripts/validate-skill-library.mjs skills/ast-grep/SKILL.md
  • node skills/skill-authoring/scripts/validate-skill-library.mjs
• Confirm every support file listed in ## Reference files exists and is linked.
• Smoke-test description-level routing with one trigger and one near-miss:
  • should trigger: Use ast-grep to rewrite obj.val && obj.val() to obj.val?.() in TypeScript, preview first
  • should not trigger: Find the exact string "obj.val && obj.val()" in this repository.

Examples

• Use ast-grep to search for \console.log($X)` in `src/` with `--lang ts` and return JSON output.`
• Use ast-grep to rewrite \var code = $PAT` to `let code = $PAT` in JavaScript after previewing matches.`
• Use ast-grep to rewrite \obj.val && obj.val()` to optional chaining in TypeScript after a narrow preview.`

Reference files

• references/command-patterns.md - search/rewrite command templates and scoped usage patterns
• references/safety-and-review.md - review checklist, rollback flow, and failure-mode handling
• assets/examples.md - trigger and near-miss prompt matrix for activation tuning
• scripts/scaffold-ast-grep-command.mjs - optional helper to scaffold portable ast-grep commands