Markdownlint

Validate markdown files follow formatting standards. Enables consistent documentation, readable diffs, and POSIX compliance.

What this skill does

1. Checks if markdownlint-cli is installed; installs if missing.
2. Detects project-specific markdownlint configuration or falls back to a minimal default config.
3. Validates markdown files for formatting, structure, and style.
4. Optionally auto-fixes common issues with --fix flag; reports errors with specific rules and fixes.

Installation check

# Check if markdownlint is available
markdownlint --version 2>/dev/null

If not installed, install globally:

npm install -g markdownlint-cli

If installation fails (permissions, no npm), stop validation and report the issue with remediation steps:

• Use sudo npm install -g ... or fix npm permissions
• Use a Node version manager (nvm, volta, fnm)
• Install Node.js if not present

Do not silently skip validation.

Config detection priority

1. .markdownlint.json in project root
2. .markdownlint.yaml in project root
3. .markdownlintrc in project root
4. markdownlint field in package.json
5. Default config (see references/markdownlint-config.json)

If no project config exists, create a temporary .markdownlint.json using the default config from the reference file before running validation.

Validation workflow

1. Check markdownlint installation
   ├─ Installed → continue
   └─ Missing → install, then continue (fail if install fails)

2. Detect project config
   ├─ Found → use it
   └─ Not found → use default config from references/

3. Run validation
   ├─ Check only:
   │    markdownlint <files-or-directories>
   └─ Auto-fix:
        markdownlint --fix <files-or-directories>

4. Report results
   ├─ Valid → confirm and proceed
   └─ Invalid → show errors, suggest fixes, re-validate after correction

Validation methods

Check files (default)

markdownlint README.md CHANGELOG.md docs/

Auto-fix common issues

markdownlint --fix README.md CHANGELOG.md docs/

Note: some issues (like MD001 heading-increment) cannot be auto-fixed and require manual correction. Re-run without --fix after fixing to confirm all issues resolved.

Error handling

Install failure

If markdownlint cannot be installed, report the error and provide remediation steps. Do not proceed with validation.

Validation failure

When validation fails, report each error with:

• The rule that failed (e.g., MD022/blanks-around-headings)
• What was wrong (e.g., heading not surrounded by blank lines)
• A concrete fix

Example:

Validation failed:

1. MD022/blanks-around-headings: heading not surrounded by blank lines
   File: README.md:15
   Fix: add blank lines before and after the heading

2. MD047/single-trailing-newline: file does not end with newline
   File: CHANGELOG.md:89
   Fix: add a single newline at end of file

After reporting errors, suggest running markdownlint --fix for auto-fixable issues and provide manual fixes for the rest. Re-validate after corrections.

Format reference

See references/markdownlint-rules.md for the full rule reference, common fixes, and auto-fixable rules.