JSON-LD Validator — Free Schema Markup Checker

Introduction

Structured data tells search engines what your page content means — FAQ answers, article headlines, breadcrumb trails — but a single missing comma or absent required field can invalidate the entire JSON-LD block. Catching errors locally saves deployment cycles, prevents silent failures in production, and reduces back-and-forth with Google's Rich Results Test when you already know the markup is malformed.

Certoflow's JSON-LD Validator parses structured data pasted into the editor and reports syntax errors, missing @type values, and type-specific requirement gaps for FAQPage, Article, BlogPosting, and BreadcrumbList schemas. Validation runs entirely in your browser — staging URLs, unreleased product copy, and client markup never upload to a server.

Generate schema first with tools like the FAQ Schema Generator or Breadcrumb Schema Generator, then validate here. For general JSON syntax checks unrelated to schema, use the JSON Validator.

What this tool does

The validator accepts JSON-LD markup and produces a structured report:

Feature	Behavior
Input	Paste or edit JSON-LD in a monospace textarea
JSON parsing	Catches invalid syntax before schema checks run
@type detection	Identifies schema type from the root object or @graph array
Type-specific rules	Validates required fields per detected schema type
Issue list	Errors and warnings with descriptive messages

The default sample shows a minimal FAQPage block so you can see valid output before pasting your own markup. Results update automatically as you edit the input.

How it works

Certoflow's validateJsonLd function runs a multi-step pipeline:

1. Parse JSON — Invalid syntax returns an immediate error without further checks.
2. Extract nodes — Handles single objects, arrays, and @graph collections.
3. Read @type — Identifies the primary schema type for targeted validation.
4. Check common fields — Warns if @context is missing.
5. Apply type rules — Runs schema-specific requirement checks.

Type-specific validation includes:

Schema type	Key checks
FAQPage	mainEntity array with at least one Question
Article / BlogPosting	headline and datePublished warnings if missing
BreadcrumbList	itemListElement array required

A result is marked valid only when no error-level issues remain. Warnings do not block validity but flag recommended fields.

All processing runs client-side in JavaScript with no network requests.

Worked example

Paste this incomplete FAQPage:

{
  "@context": "https://schema.org",
  "@type": "FAQPage"
}

The validator reports:

• Error: FAQPage requires mainEntity array with at least one Question.

Add a proper mainEntity block and the error clears:

{
  "@context": "https://schema.org",
  "@type": "FAQPage",
  "mainEntity": [{
    "@type": "Question",
    "name": "Is this tool free?",
    "acceptedAnswer": {
      "@type": "Answer",
      "text": "Yes, it runs in your browser."
    }
  }]
}

Real-world examples

Pre-deploy CI check

A developer copies JSON-LD from a CMS preview into Certoflow before merging a pull request. A missing datePublished on BlogPosting is caught as a warning, prompting a fix before production.

Agency client handoff

An SEO consultant validates breadcrumb schema generated for an e-commerce category page. The itemListElement error flags an empty array caused by a template bug on paginated URLs.

Certoflow tool pages

After generating FAQ schema with the FAQ Schema Generator, the team validates output here, then beautifies with the JSON Formatter before embedding in page templates.

Migration from Microdata

A site converting legacy Microdata to JSON-LD pastes the new script block. A JSON syntax error from an unescaped quote in an answer string is caught immediately.

Staging environment without public URL

Google's Rich Results Test requires a live URL. Certoflow validates markup locally when staging is behind authentication.

Common mistakes

Treating local validation as final approval. This tool catches common syntax and required-field issues. Google's Rich Results Test still determines rich-result eligibility for live URLs.

Validating minified JSON with hidden errors. Run through the JSON Formatter first if the block is one long line — easier to spot structural problems.

Mixing schema types without @graph. Multiple root types in one script block may need @graph structure. The validator checks the first detected node.

Ignoring warnings. Missing datePublished on Article will not fail validation but may reduce rich-result eligibility.

Pasting HTML instead of JSON. The validator expects raw JSON-LD content, not the full <script> wrapper. Paste only the JSON object.

Schema that does not match visible content. Valid syntax does not mean Google will show rich results. FAQ answers must match on-page text.

Use cases

Pre-publish QA — Catch errors before deployment when CMS preview is available.

Developer workflow — Quick feedback loop without opening external testing tools.

Client deliverables — Document validation status alongside generated schema.

Learning structured data — Error messages teach required fields for common types.

Template debugging — Isolate whether a CMS template produces valid JSON-LD across page variants.

FAQ

Does this replace Google's Rich Results Test?

No. This catches common syntax and required-field issues locally. Use Google's test for final rich-result eligibility on live URLs.

Which schema types are supported?

FAQPage, Article, BlogPosting, and BreadcrumbList checks are included. Other types receive basic @context and @type validation only.

Are errors and warnings different?

Errors block validity — the markup has a structural problem. Warnings flag recommended fields that may affect rich-result eligibility but do not invalidate the JSON.

Can I validate @graph arrays?

Yes. The validator extracts nodes from @graph and validates the first detected type.

Is my markup sent to a server?

No. Validation runs locally in JavaScript. Your JSON-LD never leaves your device.

Can I use this offline?

After the page loads once, validation works offline because logic runs entirely client-side.

What should I do after validation passes?

Deploy the markup, then verify with Google's Rich Results Test and monitor Search Console for structured data reports.