@rsvelte/fmt

A Rust-powered formatter for Svelte projects. rsvelte-fmt formats .svelte files in-process with the rsvelte formatter and delegates every other file — and the <script> / <style> bodies of each Svelte component — to oxfmt. On a directory it covers the full oxfmt-supported set (.ts / .js / .css / .json, plus .md / .yaml / .toml / .html, …), so it formats the same files oxfmt . would. Both pipelines run in parallel, so a mixed project formats in a single pass with no Node startup cost on the Svelte side and no Prettier doc-IR round-trip.

⚠️ Early stage (v0). Output and flags are stabilising.

Install

npm install -D @rsvelte/fmt
# pnpm add -D @rsvelte/fmt
# yarn add -D @rsvelte/fmt

The package resolves the right prebuilt native binary for your platform via optionalDependencies. Supported targets:

| OS | Architecture | |---|---| | macOS | arm64, x64 | | Linux | x64 (glibc ≥ 2.35), arm64 (glibc ≥ 2.35) | | Windows | x64 (MSVC) |

If your platform isn't listed, please open an issue.

How the CLI is launched

rsvelte-fmt's bin entry is a small Node launcher that resolves the platform-native binary from optionalDependencies (plus your oxfmt, if installed) and execs it, forwarding argv/stdio and the exit code — one Node cold start per invocation. There is no install-time step and nothing to allow in a package manager's build-script gate (e.g. pnpm's onlyBuiltDependencies).

oxfmt daemon (warm CSS formatting)

On POSIX, inline <style> blocks are formatted through a short-lived oxfmt daemon kept warm across invocations, so re-formatting a changed <style> costs a ~ms socket round-trip instead of a fresh oxfmt Node start (~370ms → ~5ms). It idle-exits after 60s and is keyed to your oxfmt version. Output is identical to the non-daemon path; set RSVELTE_FMT_NO_DAEMON=1 to disable it.

oxfmt (optional, bring-your-own version)

Non-.svelte files and the CSS inside <style> blocks are formatted by oxfmt. It's declared as an optional peer dependency — install whatever version you like and rsvelte-fmt will use it automatically:

npm install -D oxfmt

rsvelte-fmt resolves your installed oxfmt (or, failing that, an oxfmt on $PATH). Pass --oxfmt-bin <path> to point at a specific binary. Without any oxfmt, .svelte markup still formats; only the non-Svelte files are skipped.

Usage

# Format the current directory in place (no path = cwd, like `oxfmt`)
npx rsvelte-fmt

# Format everything under src/ in place
npx rsvelte-fmt src/

# Check mode — exits non-zero if anything would change (CI gate)
npx rsvelte-fmt --check

# Editor / stdin mode
cat App.svelte | npx rsvelte-fmt --stdin --stdin-filepath App.svelte

When no path is given, rsvelte-fmt formats the current directory — matching oxfmt, where the path argument is optional and defaults to the cwd.

Add it to your package.json:

{
  "scripts": {
    "format": "rsvelte-fmt src/",
    "format:check": "rsvelte-fmt --check src/"
  }
}

Directory inputs hand every non-.svelte file to oxfmt (which respects .gitignore / .prettierignore and skips node_modules), while .svelte files are walked in-process, skipping node_modules, target, dist, build, and hidden directories.

Configuration (.oxfmtrc)

rsvelte-fmt reads the project's oxfmt config (.oxfmtrc.json / .oxfmtrc.jsonc, discovered upward from the working directory, or --config <path>). Standalone files already pick it up via oxfmt; the inline <script> and <style> blocks inside .svelte files now honor it too, so settings like singleQuote, semi, printWidth, trailingComma, quoteProps, arrowParens, bracketSpacing, and endOfLine apply consistently across standalone files and embedded blocks. Explicit --print-width / --tab-width / --use-tabs flags override the config.

Svelte markup & sort options

The following .oxfmtrc keys also drive .svelte formatting, matching oxfmt + prettier-plugin-svelte:

| Key | Default | Effect | |---|---|---| | singleAttributePerLine | false | Break every attribute onto its own line when an element has more than one | | bracketSameLine | false | Keep a wrapped open tag's > / /> on the last attribute's line | | sortImports | off | Sort imports inside embedded <script> (and standalone JS/TS) — accepts true or the full oxfmt object form | | svelte.allowShorthand | true | Collapse name={name} / class:x={x} / style:x={x} / bind:x={x} to shorthand; set false to always emit the full form | | svelte.indentScriptAndStyle | true | Indent the body of <script> / <style> one level under its tag; false keeps it flush | | svelte.sortOrder | options-scripts-markup-styles | Print order of the top-level sections (any permutation of options/scripts/markup/styles, or none to keep source order) |

sortTailwindcss sorts the classes in static class attributes (the value must be a plain string — values with {expr} interpolation are left untouched).

A stock, zero-config Tailwind v4 setup — a stylesheet that is essentially @import "tailwindcss"; with no @plugin, @utility, @custom-variant, @theme, or @config, and no v3 tailwind.config.js — is sorted natively in Rust, byte-for-byte, with no subprocess.

A custom config (any of the above) changes the class order in ways that depend on the project's compiled CSS, so rsvelte-fmt shells out to a one-shot Node sidecar running the real prettier-plugin-tailwindcss — the same plugin (and API) oxfmt uses — resolving your project's tailwindcss and producing the same order oxfmt would. This needs a Node interpreter (the one the JS launcher passes via RSVELTE_FMT_NODE) and the bundled plugin; if either is unavailable, rsvelte-fmt prints a warning and leaves class names unchanged rather than reorder them wrongly. The attributes option is honored (default ["class"]); functions (e.g. cn(...)) are not yet supported on either path.

sortTailwindcss.strategy (an rsvelte-only extension, not an oxfmt key) selects the sorter:

| strategy | Behavior | |---|---| | auto (default) | Stock config → native Rust; custom config → Node/JS oracle | | native | Always pure-Rust; a custom config warns and is left unsorted | | js | Always the Node/JS oracle, even for a stock config (bypasses the native sorter's rare edge cases) |

With the default auto, an existing oxfmt sortTailwindcss config works unchanged.

Notes:

• Under auto, the choice between native and JS relies on a heuristic that detects a stock stylesheet. If it ever misclassifies your setup, set strategy: "js" to force the exact oxfmt oracle for every file.
• The Node sidecar path bundles prettier-plugin-tailwindcss as a dependency of @rsvelte/fmt (a few MB), installed for all users even if they never enable sortTailwindcss. Because it is a Prettier plugin, some package managers may print a Prettier peer-dependency warning at install time; it is harmless — rsvelte-fmt uses only the plugin's sorter API, not Prettier itself.

CLI flags

| Flag | Default | Effect | |---|---|---| | --write | (implied for paths) | Write formatted output back to source files | | --check | off | Exit 1 if any file would change; no writes | | --stdin | off | Read source on stdin, write result to stdout | | --stdin-filepath PATH | — | Filename used to pick the engine (required with --stdin) | | --print-width N | .oxfmtrc / 80 | Maximum line width before breaking | | --tab-width N | .oxfmtrc / 2 | Spaces per indent level | | --use-tabs | .oxfmtrc / off | Indent with tabs | | --config PATH, -c | discovered | .oxfmtrc to apply to inline <script> / <style> blocks | | --oxfmt-bin PATH | resolved / oxfmt | Override the oxfmt binary used for non-.svelte files | | --no-style-cache | off | Disable the on-disk cache of formatted inline <style> blocks |

Run rsvelte-fmt --help for the authoritative list.

Inline <style> cache

Inline <style> CSS is delegated to oxfmt for output parity with standalone .css files. To avoid re-running that round-trip on every invocation, formatted results are cached on disk (keyed by the oxfmt version, the resolved .oxfmtrc, and the exact <style> body), so an unchanged block is served from cache and skips oxfmt entirely on subsequent runs. Cache hits are byte-identical to a fresh format. Disable with --no-style-cache or the RSVELTE_FMT_NO_CACHE environment variable; relocate it with RSVELTE_FMT_CACHE_DIR (defaults to the platform cache dir, e.g. ~/.cache/rsvelte-fmt).

Exit codes

| Code | Meaning | |---|---| | 0 | Success (write applied or check passed) | | 1 | --check found at least one file that would change | | 2 | Internal error (parse failure, oxfmt missing, IO error, …) |

Performance

rsvelte-fmt is part of the rsvelte project. The Svelte parser is rsvelte (Rust, in-process) and the JS rewrite is straight oxc_formatter — no Node calls and no Prettier round-trip for the Svelte markup itself.

Formatting 3,854 real .svelte files (Apple M1 Pro, 10 iterations / 3 warmup), the Svelte engine runs 18× faster single-threaded and 114× faster multi-threaded than prettier-plugin-svelte. Live numbers and reproduction steps are on the benchmark page.

License

MIT