AST editing

ast_edit is the first AI coding tool to edit TypeScript and JavaScript through the AST instead of text matching. It addresses symbols by kind and name, mutates them structurally, and writes the result back. Micro-edits cost a handful of tokens and never fail on whitespace.

You don't invoke ast_edit directly. The agent picks it automatically for TypeScript and JavaScript files. This page is a reference for what it can do.

Origin

The AST-first approach comes from my own Master's thesis in Computer Science, *Typed vs Untyped Programming Languages* (co-authored with Artur Matusiak, 2022), and its reference implementation JS Typer. The thesis demonstrated that programmatic JavaScript-to-TypeScript conversion could be done entirely through ts-morph AST mutations rather than string manipulation. ast_edit is the direct descendant of that work, applied to AI coding.

Examples

Change a function's return type:

ast_edit({
  path: "src/api.ts",
  action: "set_return_type",
  target: "function",
  name: "fetchUser",
  value: "Promise",
})

Make a function async and add a parameter - one atomic batch:

ast_edit({
  path: "src/api.ts",
  operations: [
    { action: "set_async", target: "function", name: "fetchUser", value: "true" },
    { action: "add_parameter", target: "function", name: "fetchUser", value: "cache: boolean" },
    { action: "add_named_import", value: "./types", newCode: "User" },
  ],
})

Rename a class method:

ast_edit({
  path: "src/store.ts",
  action: "rename",
  target: "method",
  name: "UserStore.fetch",
  value: "load",
})

Replace a whole function body:

ast_edit({
  path: "src/util.ts",
  action: "set_body",
  target: "function",
  name: "parseInput",
  newCode: "return JSON.parse(raw);",
})

Batch edits are all-or-nothing - if any fails, none are applied.

Targets

Action reference

Supported files

.ts, .tsx, .js, .jsx, .mts, .cts, .mjs, .cjs. For other files the agent falls back to edit_file / multi_edit.

Safety

• File snapshotted before the edit - rejects if changed on disk mid-flight.
• Atomic batches: all or none.
• undo_edit reverses ast_edit the same as edit_file.
• Pre/post diagnostics - new type errors flow back to the agent immediately.