Refactor code structurally

This guide shows you how to get an agent to make a wide mechanical change — a codemod, a bulk API migration, a pattern you want gone — using structural search and rewrite, with the change staged for review before anything is written.

ast_grep matches code by shape. ast_edit rewrites it. resolve commits or discards.

Before you start

Structural tools cover the languages the code graph parses: Dart, JavaScript, TypeScript, TSX, PHP, Python, Rust, Zig, C, C++, Go, Java, Ruby, C#, Swift, Kotlin, R, Assembly, MATLAB and Ada. They need the tree-sitter grammars staged, which they always are in a shipped build — a missing grammar stops the server rather than degrading the search.

Find the shape first

Never rewrite something you have not looked at. Ask the agent to search:

Find every place we call dispose() on a nullable that we just null-checked.

ast_grep(pattern: "if ($X != null) $X.dispose()", language: "dart")

The output is file:line plus the matched source for each site.

What the pattern syntax buys you

Token	Matches
$NAME	Any single node, captured
$$$NAME	Any run of siblings — arguments, statements
$_	Anything, captured under no name

The one that matters: a repeated $NAME must capture the same text.

if ($X != null) $X.dispose()

finds a guard that disposes what it tested. It does not find if (a != null) b.dispose() — which is usually the bug you were looking for. No textual search can express that.

It cannot match inside a string or a comment

The pattern is parsed by the same grammar as the file, so dispose(x) written in a doc comment or a string literal is not a call and is not matched. That is the whole reason to reach for this over grep.

When the pattern finds nothing

Two things to try, in order:

1. Make it smaller. foo($X) before if (a) { foo($X); }. A pattern that is not valid code on its own often is not the construct you meant.
2. Name the language. language: is inferred only when path names a single file.

A pattern no parse context accepts is reported as unparseable — the tool says so rather than reporting “no matches” for something that was never being read the way it looks.

Rewrite, and look before it lands

ast_edit(
  pattern: "if ($X != null) $X.dispose()",
  rewrite: "$X?.dispose()",
  language: "dart",
)

The result is a proposal:

(proposed) ast_edit: if ($X != null) $X.dispose() → $X?.dispose()
12 replacements in 4 files:
  lib/features/a/x.dart (3)
  lib/features/b/y.dart (5)
  …
Nothing has been written. Call resolve with edit_id "edit_1" and action
"accept" to apply it, or "discard" to drop it.

Nothing is on disk yet. Read the list, check a couple of the sites the search reported, then:

resolve(edit_id: "edit_1", action: "accept")

or

resolve(edit_id: "edit_1", action: "discard")

The commit is all-or-nothing

Every file is compared against the content captured when the change was staged. If any one of them moved — the agent hand-edited it, a formatter ran, a diagnostics pass rewrote it — the whole change is refused and dropped. A partly-applied structural rewrite leaves a tree that compiles under neither the old shape nor the new one.

Re-run the tool against the current files; do not retry the stale id.

Check your work with the compiler

The point of doing this structurally is that it is wide. Wide changes are exactly where a plausible-looking rewrite breaks a type. Ask for diagnostics after resolving:

lsp(action: "diagnostics", file: "*")

file: "*" runs the project’s own checker (dart analyze, tsc --noEmit, cargo check, go build) rather than a per-file query — the right shape of question after a change that touched forty files.

Worked example: migrating a call

Say log(msg, level) is becoming log(level: level, message: msg).

ast_grep(pattern: "log($MSG, $LEVEL)", language: "dart")

Look at the hits. If some of them are a different two-argument log, narrow the pattern by scoping path to the directory that owns the one you mean, rather than trying to express the difference in the pattern.

ast_edit(
  pattern: "log($MSG, $LEVEL)",
  rewrite: "log(level: $LEVEL, message: $MSG)",
  path: "lib/features/telemetry",
  language: "dart",
)

Then resolve, then lsp(action: "diagnostics", file: "*").

When to use something else

Situation	Better tool
Renaming a symbol	lsp_rename — it follows references, not shapes
Finding a string, a comment or a filename	search (contents) / search_files (names)
“What breaks if I change this?”	The code graph
A language the grammars do not cover	search plus edit

Related

• Code intelligence — why structural matching is not a regex with extra steps
• Built-in agent tools — the full argument list
• Search code with the code graph — the symbol and impact side