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svelte-check-native

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Blazing fast svelte-check (<1.0s) built with Rust

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<h1>svelte-check-native</h1>

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Blazing fast CLI type-checker for **Svelte** projects. Drop-in replacement for [`svelte-check`](https://www.npmjs.com/package/svelte-check) — compatible flags, byte-identical diagnostics, same exit codes. Single Rust binary, powered by TypeScript 7's native `tsc`, incremental via `tsbuildinfo`. Built for AI agents, CI/CD, and pre-commit hooks that actually stay enabled.

Not an LSP, CSS linter, or formatter.

## Speed

Measured on a SvelteKit + TypeScript monorepo with
1359 `.svelte` files (Svelte 5 runes), M1 Pro 8C, mean of 2 runs each:

`svelte-check-native --tsconfig tsconfig.json --diagnostic-sources 'ts,svelte'`

```
  tool                  cold     warm     dirty   speedup   errors/warnings/problems
──────────────────────────────────────────────────────────────────────────────────────
svelte-check-native      2.5s     0.2s     0.9s      40x           0/49/17
svelte-check            40.0s    41.0s    41.6s     1.0x           0/49/17
svelte-check --tsgo     18.3s    18.6s    17.9s     2.9x           1/49/18
svelte-check-rs         12.2s     5.5      4.4s     7.5x         732/44/261
```

Diagnostic counts match `svelte-check` with same flags.

## Install

```sh
npm i -D svelte-check-native typescript@^7.0.0
```

TypeScript 7 provides the native `tsc` binary required at check time;
it is never imported at runtime. `@typescript/native-preview` (tsgo,
the nightly channel) works too, as the fallback when `typescript` 7+
isn't installed — so you can keep `typescript@6` for your own
toolchain and check with tsgo. TypeScript 6 and below is never used
as the check engine.

## Use

```sh
npx svelte-check-native --workspace .
```

Or add it to `package.json`:

```json
{
  "scripts": {
    "check": "svelte-check-native --workspace ."
  }
}
```

Same flags as `svelte-check`. See `npx svelte-check-native --help`.

## How it works

Single Rust binary. Each `.svelte` file flows through a handful of
crates in one process:

| Crate             | What it does                                                                                    |
| ----------------- | ----------------------------------------------------------------------------------------------- |
| `parser`          | Parses `.svelte` source into a Svelte-5 AST (script + template).                                |
| `analyze`         | Semantic passes — runes, prop shapes, events, bindings, scope — feeding emit and lint.          |
| `emit`            | Generates the `.svelte.ts` overlay TypeScript will type-check. Imports rewritten so `tsc` lands on it. |
| `svn-lint`        | Native Rust port of `svelte/compiler`'s warning pass. Covers all known codes; no subprocess.    |
| `svelte-compiler` | Fallback bridge to the user's `svelte/compiler` over a persistent `bun`/`node` worker pool.     |
| `typecheck`       | Owns the TypeScript overlay tsconfig + the `tsbuildinfo` cache; invokes `tsc`; maps diagnostics back. |
| `core`            | Shared types (spans, diagnostics, position maps) + the canonical `TsConfig` struct.             |
| `cli`             | Entrypoint. Flag parsing, file discovery, output formatting, exit codes.                        |

## Flags

Every flag not listed below behaves the same as `svelte-check`.

### New flags

```
--svelte-warnings <mode>      How to source Svelte compiler warnings:
                              native | bridge
                              native: rust port faster
                              bridge: js bridge same as svelte-check, slower by 1.5-2s
--timings                     Phase-by-phase wall-clock breakdown
--debug-paths                 Print resolved binaries, exit
--tsgo-version                Print tsgo version, exit
--tsgo-diagnostics            Print tsgo's perf/memory stats after the run
--emit-ts                     Print generated TypeScript per file, exit
```

### Not supported

- `--watch` / `--preserveWatchOutput` — use [`watchexec`](https://github.com/watchexec/watchexec)
  or your editor's file watcher externally.
- `--no-tsconfig` — errors out. A tsconfig is required.
- `--incremental` — always on. TypeScript's `tsbuildinfo` handles it.
- `--tsgo` — always on. TypeScript 7's native `tsc` is used.
- `--diagnostic-sources css` — accepted but no-op (a CSS language
  service isn't bundled). Roadmap below.

Run `svelte-check-native --help` for the full list.

Output defaults to `machine` when run from a coding-agent CLI:
`CLAUDECODE=1` (Claude Code), `GEMINI_CLI=1` (Gemini CLI), or `CODEX_CI=1` (OpenAI Codex CLI).

## Environment variables

- `TSGO_BIN` — override discovery; accepts an absolute path to a
  platform-native `tsgo`/`tsc` binary or JS wrapper. Useful when
  neither engine package is in `node_modules` (e.g. a monorepo where
  TypeScript lives elsewhere).
- `SVN_BRIDGE_WORKERS` — number of `svelte/compiler` worker
  subprocesses. Default `cores/2`, capped at 8; tracks the perf-core
  count on Apple Silicon. Override if you hit IPC contention on very
  large core counts.
- `CLAUDECODE` / `GEMINI_CLI` / `CODEX_CI` — any set forces `machine`
  output for agent-friendly parsing.

## Exit codes

- `0` — no errors (and no warnings if `--fail-on-warnings`)
- `1` — errors detected (or warnings with `--fail-on-warnings`)
- `2` — invocation error (bad flag, missing tsconfig, TypeScript not found)

## Roadmap

- [ ] **CSS lint diagnostics**

## Troubleshooting

**Stale errors after editing `tsconfig.json` or path aliases** — wipe
the cache: `rm -rf node_modules/.cache/svelte-check-native` and re-run.
The overlay config is regenerated from your live tsconfig on every
run, but tsgo's `tsbuildinfo` can hold onto stale resolution state.

**TS2321 "Excessive stack depth"** on your own types — usually a
`UnionToRecord<T>` that round-trips through `UnionToTuple<T>[number]`.
Iterate `T` directly in the mapped-type key instead.

## Prior art

- [`svelte2tsx`](https://github.com/sveltejs/language-tools/tree/master/packages/svelte2tsx)
  / [`svelte-check`](https://github.com/sveltejs/language-tools/tree/master/packages/svelte-check)
  — transpiler + CLI whose output shape and flags we
  match. The `.v5` fixture corpus from `svelte2tsx` is our parity gate.
- [TypeScript 7](https://devblogs.microsoft.com/typescript/announcing-typescript-7-0/)
  — the native compiler that does the actual type-checking.

## License

MIT

svelte-check-native

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