mdlint

An opinionated Markdown formatter and linter, written in Rust.

What ruff did for Python and gofmt did for Go, mdlint aims to do for Markdown: enforce a single, consistent canonical style so that style debates disappear and diffs stay meaningful. As AI coding agents increasingly read and write Markdown, well-structured files matter more than ever. Run mdlint format and stop thinking about it.

Project Status: Active development, in between my day job.

Features

• Formatter first: mdlint format rewrites files to a canonical style — no configuration required
• Linter second: mdlint check reports violations; most are auto-fixable by the formatter
• Fast: written in Rust for performance
• Portable: single, small, 0-dependency binary (Linux x86_64/ARM64, macOS Intel/Apple Silicon, Windows)
• Git-aware: respects .gitignore files by default

Installation

Pick between downloading binaries from GitHub releases, pulling from crates.io, or using a Docker container. A Homebrew formula is planned.

From GitHub Releases (Recommended)

Download the latest release for your platform from the releases page, or download the binary via the command line. For example, to download the latest Linux x86_64 build:

curl -LO https://github.com/swanysimon/mdlint/releases/latest/download/mdlint-linux-x86_64.tar.gz
tar xzf mdlint-linux-x86_64.tar.gz

# verify checksum
sha256sum -c mdlint-*.sha256

# add to PATH
sudo mv mdlint /usr/local/bin/

From crates.io

cargo install markdownlint-rs

From Docker

# check files in the current directory
docker run --rm -v "$PWD:/workspace" ghcr.io/swanysimon/mdlint:latest check

# format files in the current directory
docker run --rm -v "$PWD:/workspace" ghcr.io/swanysimon/mdlint:latest format

The Docker image supports both linux/amd64 and linux/arm64 platforms.

From pip

pip install markdownlint-rs

From npm

npm install --save-dev markdownlint-rs
npx mdlint check

Or globally:

npm install -g markdownlint-rs
mdlint check

From source

git clone https://github.com/swanysimon/mdlint.git
cd mdlint
cargo build --release
sudo cp target/release/mdlint /usr/local/bin/

Usage

Basic Usage

Check Markdown files for issues:

# check all Markdown files (auto-detected)
mdlint check

# check specific files or directories
mdlint check README.md docs/

# check and apply auto-fixes
mdlint check --fix

Format Markdown files (opinionated, fixes everything):

# format all Markdown files
mdlint format

# verify formatting without modifying files
mdlint format --check

# format specific files or directories
mdlint format README.md docs/

Command-Line Options

mdlint check

Lint Markdown files and report issues.

Usage: mdlint check [OPTIONS] [FILES]...

Arguments:
  [FILES]...              Files or directories to check (defaults to current directory)

Options:
      --fix               Apply auto-fixes where possible
      --format <FORMAT>   Output format: default or json [default: default]
      --exclude <PATH>    Exclude files or directories
      --config <CONFIG>   Path to configuration file
  -v, --verbose           Print each file name as it is checked
      --color <COLOR>     Color output: auto, always, never [default: auto]
  -h, --help              Print help

mdlint format

Format Markdown files with opinionated fixes.

Usage: mdlint format [OPTIONS] [FILES]...

Arguments:
  [FILES]...              Files or directories to format (defaults to current directory)

Options:
      --check             Only verify formatting, don't modify files
      --exclude <PATH>    Exclude files or directories
      --config <CONFIG>   Path to configuration file
      --color <COLOR>     Color output: auto, always, never [default: auto]
  -h, --help              Print help

Examples

Check with auto-fix:

mdlint check --fix

Check with custom config file:

mdlint check --config mdlint.toml

Check with JSON output:

mdlint check --format json

Check specific files:

mdlint check README.md CONTRIBUTING.md docs/

Format all files:

mdlint format

Verify formatting in CI:

mdlint format --check

Disable color output:

mdlint check --color never

Show each file as it is checked:

mdlint check --verbose

Configuration

mdlint uses TOML configuration files, similar to how ruff uses ruff.toml. The tool automatically discovers configuration files by searching up from the current directory.

Configuration File Locations

The tool searches for these files in order (first found wins per directory level):

1. mdlint.toml
2. .mdlint.toml

Configuration File Format

Create a mdlint.toml file in your project root:

# Enable all rules by default
default_enabled = true

# Respect .gitignore files when discovering files
gitignore = true

# Disable inline configuration comments
no_inline_config = false

# Rule-specific configuration
[rules.MD013]
line_length = 120
heading_line_length = 80
code_blocks = false

[rules.MD003]
style = "atx"

[rules.MD004]
style = "asterisk"

[rules.MD007]
indent = 2

# Disable specific rules
[rules.MD034]
enabled = false

Configuration Options

See mdlint.default.toml for every option with its default value and a description of what it does. The global options are summarised below.

Global Options

• default_enabled (boolean): When true, all rules are enabled unless explicitly disabled. Default: false
• gitignore (boolean): Respect .gitignore files when discovering markdown files. Default: true
• no_inline_config (boolean): Disable inline configuration via HTML comments. Default: false
• exclude (array): Paths to exclude from file discovery; merged with any --exclude CLI flags. Default: []
• custom_rules (array): Paths to custom rule modules (future feature). Default: []
• front_matter (string): Pattern for front matter detection. Default: auto-detects YAML (---) and TOML (+++)
• fix (boolean): When true, mdlint check automatically applies all auto-fixable violations, equivalent to passing --fix on the command line. Default: true

Rule Configuration

Rules can be configured in three ways:

1. Enable/Disable a rule:

   [rules.MD013]
   enabled = false
   

2. Configure rule parameters:

   [rules.MD013]
   line_length = 100
   code_blocks = false
   

3. Use both (parameters implicitly enable the rule):

   [rules.MD003]
   style = "atx"
   

Configuration Hierarchy

Configurations are discovered by walking up the directory tree. When multiple configs are found, they are merged with the following precedence (highest to lowest):

1. Command-line options (--config)
2. Local directory config (mdlint.toml in current dir)
3. Parent directory configs (walking up to root)
4. Default configuration

Later configs override earlier ones for scalar values. When a rule is configured in multiple places, the most specific configuration wins.

See the markdownlint rules documentation for details on each rule and its configuration options.

Inline Configuration

Rules can be suppressed for specific lines using HTML comments, without modifying mdlint.toml:

<!-- mdlint-disable-next-line MD013 -->
This line may be longer than the configured limit.

<!-- mdlint-disable MD033 -->
<div>Raw HTML block that needs to stay as-is</div>
<!-- mdlint-enable MD033 -->

Multiple rules: <!-- mdlint-disable MD001 MD013 --> — space-separate rule codes. Set no_inline_config = true in mdlint.toml to ignore all inline comments.

Exit Codes

• 0: Success - no linting errors found (or files successfully formatted with format)
• 1: Linting errors found (or formatting issues found with format --check)
• 2: Runtime error (invalid config, file not found, etc.)

Use exit codes in CI/CD pipelines:

# Fail build on linting errors
mdlint check || exit 1

# Fail build if files need formatting
mdlint format --check || exit 1

Supported Rules

mdlint implements the markdownlint rule set. Rules marked ✅ are enforced automatically by mdlint format; rules marked ❌ are reported by mdlint check only.

Pre-commit Hooks

Native git hook

Create .git/hooks/pre-commit (and make it executable with chmod +x):

#!/bin/sh
mdlint format --check

This causes git commit to fail if any staged Markdown file needs formatting.

pre-commit framework

Add to .pre-commit-config.yaml:

repos:
  - repo: https://github.com/swanysimon/mdlint
    rev: v0.3.3  # use the latest release tag
    hooks:
      - id: mdlint-format-check
        name: mdlint format --check
        language: system
        entry: mdlint format --check
        types: [markdown]
      - id: mdlint-check
        name: mdlint check
        language: system
        entry: mdlint check
        types: [markdown]

Or use mdlint check --fix to auto-fix and stage the result:

      - id: mdlint-fix
        name: mdlint check --fix
        language: system
        entry: mdlint check --fix
        types: [markdown]
        pass_filenames: false

GitHub Actions

- name: Check Markdown formatting
  run: mdlint format --check

- name: Lint Markdown
  run: mdlint check

Contributing

Contributions are welcome!

Development setup

Prerequisites: mise and Rust. Optionally, Docker is needed for Dockerfile linting. uv is required only if working on the Python package.

git clone https://github.com/swanysimon/mdlint.git
cd mdlint
mise install   # installs prek, tombi, hadolint
cargo build

Code quality

All quality checks run via prek run -a. This must pass before submitting a pull request.

Pull request process

1. Create a feature branch from main
2. Make focused commits with clear messages
3. Add tests for new functionality
4. Run prek run -a and fix any failures
5. Submit a PR with a description of what changed and why

Release process

Releases use cargo-release, which bumps all package manifests in sync and pushes the tag that triggers CI to build, package, and publish everything automatically:

cargo release patch --execute   # or minor / major

Once the tag is pushed, CI verifies manifest versions, builds binaries for all 7 platforms, and publishes to crates.io, PyPI, and npm via trusted publishing (no tokens required).

License

The Unlicense - see LICENSE for details.

Acknowledgments

• markdownlint by David Anson — original rule definitions
• mdformat — inspiration for the formatter-first approach
• pulldown-cmark — Markdown parsing

Resources

• Documentation
• Issue Tracker
• Changelog
• markdownlint Rules Reference