Formatters after edits

After a file-edit tool (write, edit, apply_patch) succeeds, the formatter for the file’s language runs on the edited file and one line is appended to the tool result, the way language-server diagnostics are appended:

formatted (prettier): /work/src/app.ts

The agent sees that the file changed under it and reads it again before the next edit. Language-server diagnostics, when enabled, are computed after formatting, so they describe the formatted file.

Nothing is bundled or downloaded. A formatter is a command; when the command is not on PATH, nothing runs and nothing is appended.

Defaults

Language	File extensions	Commands tried in order
typescript	ts, tsx, mts, cts	biome format --write (when biome.json exists), prettier --write
javascript	js, jsx, mjs, cjs	same as typescript
json	json, jsonc	same as typescript
css	css, scss, less	same as typescript
markdown	md, mdx	prettier --write
yaml	yml, yaml	prettier --write
html	html, vue, svelte	prettier --write
python	py, pyi	ruff format, black -q
go	go	gofmt -w
rust	rs	rustfmt --edition 2021

biome is tried only when a biome.json or biome.jsonc exists in a directory above the file, so a project that uses prettier is not touched by a globally installed biome. The first command found on PATH wins; detection is cached per tool instance.

Configuration

{
  "format": {
    "enabled": true,
    "onEdit": {
      "python": "yapf -i {file}",
      "go": false,
      "sql": "sqlfmt \"{file}\""
    },
    "timeoutMs": 10000,
    "maxOutputChars": 2000
  }
}

• onEdit.<language> replaces the default command for that language; {file} is the edited path and is appended when absent. Quotes group arguments; there is no shell.
• onEdit.<language>: false turns a default off.
• A language without defaults (sql above) matches files whose extension equals the language id.
• enabled: false turns the feature off.
• timeoutMs stops a formatter that runs too long (default 10 s); maxOutputChars bounds the formatter output kept for the result line (default 2000).

Result lines

Line	Meaning
formatted (<command>): <path>	the command exited 0 and the file’s bytes changed
already formatted (<command>): <path>	the command exited 0 and the file did not change
formatter failed (<command> exit <n>): <path> + first output line	the command exited non-zero; the edit itself stands
formatter timed out (<command>) after <ms> ms: <path>	the command was stopped at timeoutMs; the file may be half-written only if the tool itself does that; the edit stands

A formatter failure never fails the edit: the edit result is the tool’s own result plus the line.

Limits

• Formatters run in the file’s directory with the gateway’s environment; they do not run in the sandbox (agents.defaults.sandbox), where edits are not formatted.
• One formatter per file per edit. apply_patch formats every added and modified file.
• The formatter must accept a file path argument and write in place.