Audit localhost, not just what you already shipped.

Paste a URL into any hosted audit tool, browser extension, or Slack bot and it does the same thing under the hood: a server somewhere else on the internet opens an outbound connection to your URL and reads what comes back. That works for https://your-app.com because it has a public address a stranger's server can route to.

http://localhost:3000 does not have one. Loopback addresses only resolve on the machine that opened them - that is what makes them loopback addresses. A hosted crawler asking the internet to route it to your laptop gets nowhere, the same way a letter addressed to "the kitchen" with no street address gets nowhere. This is not a rule anyone chose to enforce. It is how private networking has worked the whole time.

The LintPage CLI does not have that problem, because it never leaves your machine to look at your machine. It runs as a process on the same host as your dev server and requests the page directly, the same way your own browser tab does. That is the entire trick, and it is the same reason a fixed set of checks cannot run: anything that needs an outside vantage point - fetching your robots.txt as a bot would, checking whether an external service can reach you, crawling your site the way a search engine does - has no outside vantage point to use here. The disclosure below is that same fact, applied check by check.

Facebook's Sharing Debugger and Twitter's Card Validator have the same problem as any other hosted tool: they fetch your URL from their own servers, which cannot reach localhost. In practice that means most teams find out their Open Graph tags are wrong only after a redesign ships and the first Slack link unfurls with a broken image or last quarter's title.

LintPage's Open Graph and Twitter Card checks read the page the same way its other checks do - directly, from the document your dev server just rendered - which is why they are not on either list above. They run on localhost exactly as they run in production, so you can catch a missing og:image, a stale title, or a broken card type while you still have the component open in your editor, instead of after a teammate pastes the link somewhere.

next dev serves on localhost:3000 by default, so the command at the top of this page works unmodified against a fresh Next.js app. Run it while you work and you get the 44 checks that do not depend on a public address - meta tags, structured data, headings, Open Graph, accessibility, and more - without waiting for a build, a deploy, or a review.

The moment you open a pull request, Vercel builds a preview deployment with its own public URL. That URL is not a loopback address, so none of the mechanism above applies to it: point LintPage at the preview URL and all 60 checks run, including the 16 that localhost cannot reach and the 3 that localhost reports as informational. Run npx lintpage your-preview-url.vercel.app once the preview is up, the same way you would a production URL with the free page audit.