Markdown Front-Matter Validator & Sanitizer

Markdown Front-Matter Validator & Sanitizer: Validate YAML, TOML & JSON Front Matter Free

Front matter looks harmless — a few lines of metadata sitting above your actual content, wrapped in three dashes. But it’s also one of the easiest places for a static site build to quietly break. A missing required field, an unquoted colon inside a title, a date that isn’t quite ISO-8601, and your build either fails outright or, worse, publishes something wrong without telling you. This tool exists to catch those problems before they reach your build pipeline, right in your browser, for free.

Why Front Matter Breaks More Often Than It Should

Front matter is technically just YAML, TOML, or JSON, but it’s usually hand-edited by people who are thinking about content, not syntax. That’s exactly the gap where mistakes creep in: a title with a colon in it that isn’t quoted, a date typed as “Aug 19 2026” instead of “2026-08-19”, a duplicate key that YAML silently overwrites without warning, or a required field a particular site generator expects that simply got left out.

None of these mistakes look wrong at a glance. They only show up when a build fails, or worse, when it doesn’t fail and just renders something incorrectly. Catching them at the point of writing — instead of at the point of deploying — is the entire premise of this tool.

What This Tool Actually Checks

This isn’t a generic YAML linter. It’s built specifically around how front matter gets used in real static site generators:

• Format auto-detection — paste your file and it identifies YAML (---), TOML (+++), or JSON front matter automatically, no need to specify it yourself.
• Generator-specific validation — select a preset for Jekyll, Hugo, Astro, Eleventy, Docusaurus, or Hexo, and the tool checks your fields against what that specific generator actually requires or recommends, not a one-size-fits-all rule set.
• Auto-fixes for common mistakes — titles with unsafe characters get automatically quoted, missing slugs get generated from your title, and stray trailing colons on keys get cleaned up.
• Date validation — flags any date that isn’t in proper ISO-8601 format, the format nearly every static site generator expects.
• SEO-aware checks — warns when a title or description runs outside the length search engines typically display well, and flags when a description is missing entirely.
• Duplicate key detection — YAML parsers silently keep only the last value when a key is repeated. This tool catches that before it becomes a confusing bug.
• Batch validation — paste multiple files separated by --- and validate all of them in one pass, with a downloadable pass/fail report.

Beyond Validation: Schema and CI Export

Most free front matter tools stop once they’ve told you something is valid or broken. This one goes a step further for teams that want consistency enforced automatically, not just checked manually:

• JSON Schema export — generates a schema from your current front matter fields, ready to use with schema-validation linters in your own build pipeline.
• GitHub Actions workflow export — copies a ready-to-use CI workflow that validates front matter automatically on every push or pull request touching your content files.

That combination — instant browser-based validation plus exportable schema and CI configuration — is what separates this from either a simple visual editor or a developer-only command-line linter. You get the immediate feedback of a browser tool and a direct path to enforcing the same rules in your actual pipeline, without writing the schema or workflow file by hand.

Who This Tool Is For

Static site authors using Hugo, Jekyll, Astro, Eleventy, Docusaurus, or Hexo can check a post’s front matter before committing it, catching missing fields or formatting issues immediately instead of after a failed build.

Technical writers and documentation teams can standardize metadata across dozens or hundreds of pages, using the JSON Schema export to keep everyone’s front matter consistent.

Developers setting up a new content pipeline can use the CI workflow export as a starting point for enforcing front matter rules automatically, without researching remark-lint configuration from scratch.

Anyone migrating content between static site generators can quickly see which fields are missing for the new generator’s conventions before doing a bulk migration.

How to Use It

1. Paste a Markdown file with front matter into the input panel, or click “Example” to see a sample first.
2. Leave the format on auto-detect, or choose YAML, TOML, or JSON manually.
3. Optionally select your static site generator to check against its specific required and recommended fields.
4. Review the sanitized output alongside any errors, warnings, and auto-fixes, each linked to its line number.
5. Copy or download the sanitized file, export a JSON Schema, or copy a ready GitHub Actions workflow for CI validation.

A Note on Privacy

Everything runs client-side in your browser — parsing, validation, sanitizing, and every export. Nothing is sent to a server. Your content is only kept locally through your browser’s autosave so you don’t lose your work on a refresh, and is never transmitted anywhere.

Try It Now

Paste your front matter above, or load the example to see it in action. No account, no install, no build step required — just validated, sanitized front matter in seconds.