Ast Grep

ast-grep Structural Search

Use ast-grep when the user needs code structure, not just text. The useful outcome is a validated rule plus a clear statement of what it matches and what it may miss.

Triage

Start with the smallest tool that can answer the question.

• Use rg for exact names, strings, comments, or simple text.
• Use language tooling for semantic facts such as type resolution, references, imports, or rename safety.
• Use ast-grep for syntax shape: descendants, ancestors, call forms, decorators, missing constructs, nested contexts, or multi-language structural searches.

If ast-grep is not installed, say so and offer an rg fallback only when the fallback will not pretend to be structural.

Workflow

1. Identify the language and target files.

   • Infer from paths and extensions when possible.
   • Ask only if the language or include/exclude scope cannot be inferred.

2. Write a tiny fixture before searching the real repository.

   • Include at least one positive example.
   • Add a negative example when false positives would be costly.

3. Start with the simplest pattern.

   • Use ast-grep run --pattern ... --lang ... for a single-node search.
   • Move to a YAML rule when the search needs has, inside, not, any, all, regex, nthChild, or reusable metadata.

4. Debug the parsed structure when the first rule misses.

   • Use ast-grep run --pattern '<sample code>' --lang <lang> --debug-query=ast.
   • --debug-query prints diagnostic output and may exit non-zero; treat the printed tree as useful debug output, not as a failed repository search.

5. Validate on the fixture before running on the repository.

   • Keep the rule as simple as possible until it matches the fixture.
   • Then scan the real path and report expected false-negative boundaries.

6. Report the final rule, command, and caveats.

   • Name node kinds or language constructs intentionally excluded.
   • Prefer JSON output when a downstream script will consume matches.

Rule File First

Rule files avoid most shell quoting problems and are the preferred form for complex searches.

On Windows, save rule files as UTF-8 without BOM. If ast-grep scan --rule reports missing field language even though the YAML has language, check for a BOM at the start of the file.

# async-await.yml
id: async-await
language: javascript
rule:
  any:
    - kind: function_declaration
    - kind: arrow_function
    - kind: method_definition
  has:
    pattern: await $EXPR
    stopBy: end

$rule = @'
id: async-await
language: javascript
rule:
  any:
    - kind: function_declaration
    - kind: arrow_function
    - kind: method_definition
  has:
    pattern: await $EXPR
    stopBy: end
'@
[System.IO.File]::WriteAllText(
  "async-await.yml",
  $rule,
  [System.Text.UTF8Encoding]::new($false)
)

ast-grep scan --rule async-await.yml .\src
ast-grep scan --rule async-await.yml .\src --json

ast-grep scan --rule async-await.yml ./src
ast-grep scan --rule async-await.yml ./src --json

Inline Smoke Tests

Use inline rules only for quick checks. In PowerShell, single-quoted here-strings preserve $META variables without escaping.

$rule = @'
id: console-call
language: javascript
rule:
  pattern: console.log($$$ARGS)
'@

"console.log('debug', value);" | ast-grep scan --inline-rules $rule --stdin

In POSIX shells, quote the YAML with single quotes or escape $.

printf '%s\n' "console.log('debug', value);" |
  ast-grep scan --inline-rules 'id: console-call
language: javascript
rule:
  pattern: console.log($$$ARGS)' --stdin

Every scan --inline-rules example must include id, language, and rule.

Debugging Checklist

When a rule produces no matches:

1. Confirm the language: --lang javascript is different from --lang tsx.
2. Print the query tree with --debug-query=ast or --debug-query=cst.
3. Check that rule files are UTF-8 without BOM.
4. Replace the complex rule with one pattern or one kind.
5. Add relational rules back one at a time.
6. Add stopBy: end to deep has / inside searches.
7. Check whether metavariables occupy a whole syntax node.
8. Broaden node kinds if the language has several forms of the same concept.

Avoid Silent False Negatives

Do not assume one node kind covers a user concept.

For JavaScript and TypeScript, "function" may include:

• function_declaration
• function
• arrow_function
• method_definition
• class fields or object properties containing functions

For React, decide whether the user means .js, .jsx, .ts, or .tsx.

For Python, inspect the parsed shape before relying on indentation-sensitive multi-line patterns.

Output Contract

When answering an ast-grep task, include:

1. Rule: the YAML rule or inline pattern.
2. Validation: the fixture command you used or recommend.
3. Repository command: the command for the user's target path.
4. Caveats: likely false positives, false negatives, and language forms not covered.

Reference

Load references/rule_reference.md when the task needs detailed syntax for atomic rules, relational rules, composite rules, metavariables, or troubleshooting.