What this tool checks
Every <script type="application/ld+json"> block on the page: whether it parses as valid JSON at all, which schema.org @type(s) it declares (including types nested inside an @graph array, a common pattern for pages describing multiple entities at once), and whether the required fields for 6 commonly-used types — Article, Organization, Product, FAQPage, BreadcrumbList, and WebSite — are present.
Why syntax errors here are invisible everywhere except here
A broken JSON-LD block causes no visible symptom: the browser doesn't render it (it's not meant to be visible), and most JSON-LD is added once during development and never looked at again. The only way to know it's broken is to either check Google Search Console's rich results report (which can take days to reflect a change) or parse the block directly — which is exactly what this tool does, instantly.
Why required fields matter beyond just valid syntax
Valid JSON that happens to declare "@type": "Article" without a headline or datePublished is still valid JSON — but it doesn't qualify for the rich-result treatment Article markup is meant to unlock. This tool checks both layers: does it parse, and does it actually satisfy what that specific type needs to be useful.