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checks YAML / TOML front matter for syntax + duplicates

Validate Markdown front matter

Catch malformed front matter before it breaks a static-site build — missing closing delimiter, duplicate keys, malformed key: value pairs are reported with line numbers.

Front-Matter Validator
Input
120 B
11 lines
Output
73 B
4 lines
Status
ready
live
Ratio
0.61×
out/in
input · markdown + front matter
output · validation report
Format: YAML
Keys (5): title, date, author, tags, draft

No issues found.
Quick Start

How to validate front matter

Detects --- (YAML) or +++ (TOML) on line 1, verifies the closing delimiter, then parses key: value pairs reporting duplicate keys and malformed lines with line numbers.

  1. 01

    Paste a post

    With YAML or TOML front matter on top.

    Parsing runs through marked with GFM extensions enabled — tables, task lists, strikethrough, and fenced code blocks all match GitHub’s rendering.

    inputGFMmarkedCommonMarklive
    input · markdownmd · GFM
    ---
title: P
---
    loaded · GFMGFM · ready
  2. 02

    Detect block

    Opening / closing delimiter validated; format identified.

    Soft-break behaviour is configurable: by default a single newline becomes a space (CommonMark), toggle on for chat-style <br> insertion.

    blocksoft-breaksoptionsconfiglive
    fmt · optionsyaml · live
    Format: YAML
    options applied · livelive
  3. 03

    Parse fields

    Key: value pairs are extracted and checked for duplicates.

    The preview is scoped under a .mde class so its CSS doesn’t bleed into the rest of the page. Output HTML is whitespace-controlled for clean diffs.

    parsepreviewscoped CSSrenderstable
    keys · renderedn · ready
    title, date
    rendered · liverendered
  4. 04

    Fix issues

    Report lists each error with its line number.

    Export covers both raw Markdown and rendered HTML; downloads stream as text/markdown or text/html via Blob + object URL.

    fixclipboard.md.htmlblob
    ✓ · exportedok · done
    ✓ clean
    shipped · ready to shipsuccess
Pro tips

Author Markdown that renders everywhere

GFM is the safe default

GitHub-Flavored Markdown is the de-facto baseline (tables, task lists, strikethrough). CommonMark is stricter but ubiquitous as a fallback.

Soft-break behaviour varies

A single newline is a space in CommonMark, a <br> in GFM. Toggle soft breaks when source matters.

Escape inside code fences

Backticks inside fenced blocks need a longer fence — three becomes four. Use ~~~ if your content has many backticks.

Anchors are slugified

GitHub lowercases, strips punctuation, and joins with hyphens. Duplicate headings get -1, -2 suffixes.

Tables need a header row

The dash row sets alignment (:---, :-:, ---:). Use Table Aligner for diff-friendly columns.

Front matter is YAML

Triple-dash blocks at the top of files are parsed by static-site generators. Use the Front-Matter Validator to catch syntax errors.

Related toolsFM StripperFM EditorMD to YAMLLinterStyleEditor
Use Cases

When to validate front matter

Save yourself from a red CI build.

Hugo / Jekyll

Pre-build sanity check before deploy. Render path uses marked with GFM extensions enabled (tables, task lists, strikethrough, autolinks), so what you preview matches what GitHub publishes bit-for-bit. Soft-break behaviour is configurable for chat-style vs prose Markdown.

GFM-compliant

Astro content

Catch malformed metadata locally. Pairs cleanly with the Markdown Cheatsheet for syntax you can't quite remember — callout blocks, footnotes, definition lists. Output stays diff-friendly so style-only changes don't drown out real edits in PRs.

GitHub-perfect

Bulk imports

Validate after migrating from another tool. Cross-platform safe: anchors, code fences, and tables behave identically on GitHub, GitLab, Bitbucket, and Gitea. Heading-anchor slugs follow GitHub's lowercase-and-hyphenate rules so deep links survive a docs migration.

diff-friendly

PR hygiene

Block bad front matter at review time. No server roundtrip — perfect for proprietary content you can't paste into a public tool. The full marked + GFM bundle ships with the page, so it works offline once loaded.

render-stable

CI guards

Inspect FM before invoking your build. Front matter (YAML / TOML / JSON) passes through unchanged so static-site pipelines (Jekyll, Hugo, Astro, Eleventy) read it correctly. Pair with the Front-Matter Validator to catch syntax errors before deploy.

cross-platform

Multi-author blogs

Help contributors get YAML right. Output is reproducible: identical input always produces identical Markdown, so doc-generation scripts can rely on it for snapshot tests and embedded into release workflows.

SEO-friendly
FAQ

Frequently asked questions

YAML (---) and TOML (+++) blocks at line 1. JSON front matter is not currently parsed.
Not yet — only structural checks (delimiter pairing, duplicate keys, malformed lines). Pair with a schema validator in CI.
Sub-blocks under a key are accepted but not deep-parsed; they show as the literal scalar.
No — it only reports. Use the Front-Matter Editor or Stripper to make changes.
No. Validation runs locally in your browser.
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