Paste your robots.txt, enter a path and a crawler, and see whether it is allowed or blocked, and which rule decided it. Runs in your browser.
A Disallow rule blocks this path.
Matched: Disallow: /admin
Paste your robots.txt, enter a path, and pick a crawler. The tester tells you whether that path is allowed or blocked and, just as useful, which line in the file made the call.
It all runs in your browser, so you can test changes to a draft robots.txt without pushing anything live.
Real robots.txt files use wildcards (*) and end-of-path anchors ($), and rules often overlap. When more than one rule could apply, the longest matching path wins, not the first one listed. Getting that wrong is how pages end up blocked by accident.
The tester applies these rules the way crawlers do, so the result matches what would happen in production instead of a rough guess.
A single stray Disallow can wipe a section out of search results, and the damage is easy to miss because nothing visibly breaks. Testing the exact paths that matter is the fastest way to confirm your file does what you meant.
Check different crawlers too. A rule under one user-agent may not apply to another, and your robots.txt can treat them differently on purpose.