ast-grep

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name: ast-grep description: Use ast-grep MCP tools for syntax-aware structural search, AST inspection, rule testing, and structural verification. Triggers: code-shape queries, structural invariants, pattern debugging, AST inspection, and multi-file syntax matching.

ast-grep MCP Workflow

When to use this skill

Use this skill when the question depends on code structure rather than plain text.

Good fits:

• Find all code matching a syntax shape.
• Verify a structural invariant across files.
• Debug why a pattern does or does not match.
• Inspect the AST or pattern structure before writing a rule.
• Plan or verify structural rewrite targets before editing.

Do not use ast-grep as the default repo navigation tool when cymbal can answer the question more directly.

Tool map

This local setup uses the official ast-grep/ast-grep-mcp server. Do not assume extra MCP tools exist beyond the four tools below.

• ast_grep_dump_syntax_tree

  • Use to inspect target code or understand how a pattern is parsed.
  • format: cst shows concrete syntax.
  • format: ast shows named syntax nodes.
  • format: pattern shows how ast-grep interprets your search pattern.

• ast_grep_test_match_code_rule

  • Use to test a YAML rule against example code before searching the repo.

• ast_grep_find_code

  • Use for simple single-pattern searches when one AST pattern is enough.

• ast_grep_find_code_by_rule

  • Use for complex searches that need relational rules like has, inside, precedes, follows, or composite rules like all, any, not.

Unavailable here:

• ast_grep_rewrite_code
• ast_grep_scan-code
• ast_grep_analyze-imports

Alternates:

• Use ast_grep_find_code or ast_grep_find_code_by_rule to verify structural candidates before editing.
• Use LSP diagnostics and project lint/typecheck/build/test commands for broad quality checks.
• Use cymbal, grep, and read for import discovery and narrow local inspection.

Default workflow

1. Clarify the target shape

Decide:

• which language is involved
• what should match
• what must be excluded
• whether a simple pattern is enough or a YAML rule is needed

2. Create a minimal example

Write a tiny code sample that should match. If exclusions matter, write one sample that should not match too.

3. Inspect syntax when uncertain

Use ast_grep_dump_syntax_tree on:

• the target code with format: cst or format: ast
• the candidate pattern with format: pattern

Do this when you are unsure about node kinds, nesting, or how metavariables are interpreted.

4. Start simple

• Use a plain pattern first when the shape is simple.
• Move to a YAML rule when you need context, ordering, exclusions, or relational logic.

Useful defaults:

• Use pattern for direct shape matches.
• Use kind plus relational rules for more complex structure.
• Use all, any, and not when combining conditions.
• For relational rules like has and inside, default to stopBy: end unless you need a tighter stop condition.

5. Test before repo-wide search

Use ast_grep_test_match_code_rule to validate a YAML rule against the minimal example.

If the rule fails:

• simplify it
• inspect the syntax tree again
• confirm the node kinds
• confirm metavariable placement

6. Search the repo

• Use ast_grep_find_code for straightforward pattern searches.
• Use ast_grep_find_code_by_rule for relational or composite logic.

Prefer the narrowest project path and language that answers the question.

Heuristics

• If text search would return many false positives, use ast-grep.
• If the match depends on syntax context, use a YAML rule.
• If the question is "how is this symbol connected?", use cymbal first.
• If you already know the exact file and just need lines for editing, use read.

Example progression

Simple pattern

Goal: find console.log(...) calls in JavaScript.

• Start with ast_grep_find_code
• Pattern: console.log($ARG)

Structural rule

Goal: find async functions that use await.

• Inspect a small example if needed.
• Test a YAML rule with ast_grep_test_match_code_rule.
• Then search with ast_grep_find_code_by_rule.

Example rule:

id: async-with-await
language: javascript
rule:
  kind: function_declaration
  has:
    pattern: await $EXPR
    stopBy: end

Exclusion rule

Goal: find async functions with await but without try/catch.

Use a composite rule with all and not.

References

• references/rule_reference.md contains the detailed ast-grep rule syntax.

Use the reference when you need exact YAML syntax for:

• atomic rules
• relational rules
• composite rules
• metavariables