Frontmatter Validator

Validate Jekyll / Astro / Next MDX YAML + SEO length checks.

Validate YAML frontmatter for Jekyll, Astro & Next MDX — check syntax, structure, and SEO title/description length. Free, runs in your browser, nothing…

About Frontmatter Validator

Frontmatter Validator is a free, browser-based tool that checks the YAML frontmatter block of your Markdown/MDX files against the schema your static-site generator expects — Jekyll, Astro, or Next (MDX) — and flags missing required keys, type mismatches, and SEO title/description length problems. Paste a full file or just the `--- … ---` block and it validates live as you type using the js-yaml parser, reporting YAML syntax errors with the offending line number. It also outputs a normalized, copy-ready version of your frontmatter. Everything runs locally in your browser — nothing is uploaded and there is no signup.

How to use Frontmatter Validator

  1. Pick your static-site target with the Jekyll / Astro / Next (MDX) segmented control (Astro is the default).
  2. Paste your Markdown or MDX into the textarea — either the whole file or just the --- … --- frontmatter block — or click 'Load sample' to drop in an example for the selected target.
  3. Watch the live results: validation runs automatically as you type, so no submit button is needed.
  4. Review the keys panel to see required keys present (green), missing (red), type mismatches (amber), recognized optional keys, and unknown keys for that target.
  5. Check the SEO panel for title length (recommended 30–60 chars) and description length (recommended 70–160 chars), each with a colored progress bar, plus a date-parseability check for the date/pubDate field.
  6. Copy the 'Normalized frontmatter (YAML)' output with the copy button to paste a clean, re-serialized version back into your file.

Frequently asked questions

Is my file or frontmatter uploaded to a server?
No. The YAML is parsed entirely in your browser with js-yaml, and all validation happens locally on your device — nothing is ever uploaded.
Is it free and do I need an account?
Yes, it's completely free with no signup, account, or watermark.
Which frontmatter formats and frameworks does it support?
It validates YAML frontmatter for three targets — Jekyll (required: layout, title), Astro (required: title, description, pubDate), and Next/MDX (required: title, description, date, slug) — each with its own known optional keys.
What does it actually check beyond syntax?
Besides parsing YAML and reporting syntax errors with the line number, it flags missing required keys, type mismatches (e.g. a field that should be an array but isn't), unknown keys for the chosen target, SEO title and description lengths, and whether the date/pubDate value parses as a real date.
What are the recommended title and description lengths it checks for?
It marks titles as 'good' between 30 and 60 characters and descriptions as 'good' between 70 and 160 characters, showing 'too short' or 'too long' otherwise. Lengths are counted by visual grapheme clusters via Intl.Segmenter, so emoji and combining marks count as one character.
Is there a size limit on what I can paste?
Yes — input is capped at about 1,000,000 characters so the live parse never blocks the page. Real frontmatter blocks are tiny, so this only guards against accidental multi-megabyte pastes.

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