Every UCP validator I could find, and what each one actually checks

UCP is six months old and there are already at least five tools that will tell you whether your manifest is valid. That sounds like a solved problem. It is not, because they do not agree on what "valid" means, and a green tick from one does not imply a green tick from another.

I went through every UCP validation tool I could find and read what each one documents about its own checks. This is that list, with the caveat up front: this is a survey of what these tools say they do, not a lab test. Where I have run something myself I say so.

ucpchecker.com

The most complete of the set, and the one people search for by name. It has a domain checker that fetches your live manifest, a standalone validator that runs against three published spec versions (2026-01-11, 2026-01-23 and 2026-04-08), a Chrome extension, bulk checking, and monitoring with alerts. Their validator runs client-side, which means your manifest never leaves your browser. That is a genuinely good property for anything you have not shipped yet.

Two things worth knowing. First, it is UCP only. If your site also exposes WebMCP tools, that is a separate problem with separate tools. Second, their headline numbers come from crawling stores that never signed up, which is the right way to build a market view, but it means "monitored" does not mean "opted in".

ucptools.dev

A free validator, simpler than ucpchecker's, aimed at checking manifest structure. Fine for a quick shape check while you are writing the file.

aggentic.ai

Their validator page covers UCP alongside ACP, which matters if you have not yet picked which protocol you are implementing. If you are unsure how the protocols relate, the short version is in UCP vs ACP vs AP2.

Nextwaves UCP tester

A tester in a general tools collection. It exists, it fetches a manifest, and it is not trying to be more than that.

Chrome DevTools and the extensions

Not UCP, but worth naming because people conflate them: the WebMCP side has its own tooling now, including a panel in Chrome DevTools itself. If your question is about page tools rather than commerce manifests, that is the place to start, and we wrote up how to expose WebMCP tools separately.

What none of them tell you

A manifest that parses is not a store that works. The file can be valid and stale. It can be valid and blocked by your bot rules, so no agent ever fetches it. It can declare capabilities the endpoints behind it no longer serve.

That gap is why our own check runs both protocols in one pass and includes the access layer, the robots rules and rate limits in front of everything else. Validation is one third of the answer. Whether an agent can reach the file, and whether what it declares still matches reality a month later, are the other two thirds, and they are the parts that break silently.

Run more than one tool. They are all free, they disagree in useful ways, and the disagreements are usually where your actual problem is.