lang-check 0.6.0

A fast, extensible grammar and style checker for Markdown, HTML, LaTeX, Typst, and more. Ships as a VS Code / VSCodium extension and Neovim plugin, backed by a Rust core binary with LSP and Protobuf server modes.

[!NOTE] Language Check is in early development. Core checking works and is usable day-to-day, but expect rough edges — if you hit a bug, please open an issue and it will be addressed promptly. The configuration format may evolve between minor versions; backwards compatibility is a goal but not yet guaranteed.

Features

• Multi-engine checking — local Harper engine for instant feedback, optional LanguageTool for deep grammar analysis, optional Vale for style linting with its rich plugin ecosystem, optional Hunspell for spelling in the languages the others do not read, external binaries, and WASM plugins via Extism
• Tree-sitter parsing — extracts prose from Markdown, HTML, LaTeX, Typst, and more while skipping code blocks, math, and markup
• SpeedFix panel — keyboard-driven batch review: press 1–9 for suggestions, a to add to dictionary, i to ignore, Space to skip
• Inline suggestions — inlay hints, ghost text completions, and quickfix code actions
• Standalone CLI — language-check check, fix, list-rules, config, and packs subcommands
• Workspace indexing — background indexing with redb-backed caching
• Rule normalization — unified rule IDs across providers for consistent severity overrides
• Prose insights — word count, sentence count, and Automated Readability Index in the status bar
• Inspector — debug webview showing AST structure, prose extraction ranges, and check latency
• i18n — extension UI localized in English, German, French, and Spanish

Quick Start

VS Code Extension

1. Install the extension (publisher: KaiErikNiermann) — from the VS Code Marketplace, or from Open VSX on VSCodium and other non-Microsoft builds
2. Open a Markdown, HTML, LaTeX, or Typst file — checking starts automatically on save
3. Press Alt+F to open the SpeedFix panel

Neovim

-- lazy.nvim
{
  "KaiErikNiermann/lang-check.nvim",
  ft = { "markdown", "html", "latex", "typst", "restructuredtext",
         "org", "bibtex", "sweave" },
  opts = {},
}

-- Or with Neovim 0.11+ native LSP config:
vim.lsp.enable("lang_check")

See editors/nvim/README.md for full setup instructions.

CLI

# Build from source
cd rust-core
cargo build --release

# Check a file
./target/release/language-check check README.md

# Auto-fix high-confidence issues
./target/release/language-check fix docs/

# List available rules
./target/release/language-check list-rules

Project Structure

lang-check/
├── rust-core/          Rust core binary (server + CLI)
│   ├── src/
│   │   ├── engines/        Harper, LanguageTool, External, WASM engines
│   │   ├── orchestrator.rs  Multi-engine coordinator
│   │   ├── prose/           Tree-sitter prose extraction, one module per language
│   │   │   ├── gap.rs         What a language puts between two prose words
│   │   │   └── shared.rs      Range merging and scanning helpers
│   │   ├── grammars.rs      Bindings to the vendored tree-sitter grammars
│   │   ├── rules.rs         Rule normalization
│   │   ├── config.rs        YAML/JSON configuration loader
│   │   ├── workspace.rs     redb-backed workspace indexing
│   │   └── ...
│   └── src/bin/
│       ├── language-check-server.rs   LSP + Protobuf stdio server
│       └── language-check.rs          Standalone CLI
├── editors/nvim/       Neovim plugin (lang-check.nvim)
│   ├── lua/lang_check/   Plugin core (config, binary, health)
│   ├── lsp/              Neovim 0.11+ native LSP config
│   └── doc/              Vim help file
├── extension/          VS Code extension (TypeScript)
│   ├── src/
│   │   ├── extension.ts    Activation, commands, providers
│   │   ├── client.ts       Protobuf IPC client
│   │   ├── api.ts          Public API for other extensions
│   │   ├── downloader.ts   Core binary auto-download
│   │   └── trace.ts        Protobuf message tracing
│   ├── webview/            SpeedFix + Inspector UIs (Svelte 5 + Tailwind)
│   ├── l10n/               Runtime localization bundles
│   └── package.nls.*.json  Command/setting translations
├── examples/           A worked document per format, with its config
├── proto/              Protobuf schema (checker.proto)
├── docs/               Sphinx documentation
├── scripts/            Build utilities
└── docker-compose.yml  Local LanguageTool server

Configuration

examples/ has a working setup per format — a document, the .languagecheck.yaml it is written against, and a note on what a check reports and what it deliberately does not.

Create a .languagecheck.yaml in your workspace root:

engines:
  harper: true
  languagetool:
    enabled: true
    url: "http://localhost:8010"   # your LanguageTool server
  external:
    - name: vale
      command: /usr/bin/vale
      args: ["--output", "JSON"]
      extensions: [md, rst]
  wasm_plugins:
    - name: custom-checker
      path: .languagecheck/plugins/checker.wasm

rules:
  spelling.typo:
    severity: warning
  grammar.article:
    severity: off

performance:
  high_performance_mode: false
  debounce_ms: 500
  max_file_size: 1048576

auto_fix:
  - find: "teh"
    replace: "the"

See language-check config init to generate a default configuration.

Development

Prerequisites

• Rust 2024 edition (1.85+)
• Node.js 18+ and pnpm
• Protobuf compiler (protoc)

Building

# Rust core
cd rust-core
cargo build

# Extension
cd extension
pnpm install
pnpm run proto:gen
pnpm build

Testing

# Rust — unit tests + snapshot tests (insta)
cd rust-core
cargo test

# Extension — vitest
cd extension
pnpm test

Linting

# Rust
cargo clippy -- -D warnings
cargo fmt --check

# TypeScript
cd extension
pnpm lint

Local LanguageTool

docker compose up -d
# LanguageTool API available at http://localhost:8010

Git Hooks

The project uses Lefthook for pre-push checks (cargo check, clippy, fmt, test, and tsc).

Plugin System

Language Check supports three extension mechanisms:

See docs/advanced/plugins.md for the WASM plugin development guide.

Documentation

Full documentation is built with Sphinx:

cd docs
pip install -r requirements.txt
make html
# Open _build/html/index.html

License

MIT