These tools answer the questions that come up between a browser, a server and everything in between: what a response code means, which Content-Type a file should have, what a curl command actually sends, which browser is behind a user-agent string, and whether a crawler may fetch a URL.
The status code reference covers the codes you’ll meet, with what sends each one and the misuse to avoid, and it finds old reason phrases like Unprocessable Entity too. The MIME lookup gives the IANA-registered type for an extension and resolves the deprecated names it replaced. The curl converter turns a command, including Chrome’s Copy as cURL output, into fetch, axios, Python, Go or PHP, and lists every flag it left out. The robots.txt tester applies RFC 9309’s longest-match rule to a URL and a crawler, and the user-agent parser marks the parts of a string that browsers now freeze.
Two misunderstandings cause most of the trouble. Status codes are a contract: a 200 with an error in its body, or a 302 where a 301 belongs, changes what caches, clients and search engines do. And robots.txt is a request to well-behaved crawlers, not access control. A blocked page can still be indexed if other sites link to it; keeping it out of search takes noindex on a page crawlers are allowed to fetch.