What this tool checks
The tester downloads /robots.txtfrom the site's origin, parses it into user-agent groups with their Allow and Disallow rules, and lists any Sitemap: declarations. If you supply a path, it evaluates that path the way Google does: it picks the group whose user-agent token best matches the crawler (longest match, with * as the fallback), then applies the most specific rule — longest pattern wins, allow beats disallow on ties. Wildcards (*) and end-anchors ($) are supported.
Why robots.txt matters
robots.txt is the first file every well-behaved crawler requests, and it is one of the few places where a one-character mistake can deindex-in-effect an entire business. A stray Disallow: /stops all crawling; a malformed group header silently detaches the rules beneath it; blocking your CSS and JavaScript directories prevents Google from rendering pages properly, which hurts rankings even though the HTML itself is crawlable. The file has also become the control point for AI: OpenAI's GPTBot, Anthropic's ClaudeBot, PerplexityBot and Google-Extended all read it to decide whether your content can be fetched for training or answering. Whether you allow or block them should be a decision, not an accident.
How to fix common problems
Everything blocked: look for Disallow: / under User-agent: * — usually a leftover staging config. Delete it and resubmit your sitemap in Search Console. Rules not applying: rules only attach to the group above them; a blank User-agent line or a typo like User Agent: orphans everything that follows. Blocked assets: never disallow /wp-includes/, /assets/ or similar folders that hold CSS/JS — Google needs them to render the page. Case sensitivity: paths are case-sensitive; /Admin/ and /admin/ are different rules. Missing sitemap line: adding Sitemap: https://example.com/sitemap.xml is free crawl-discovery — do it even if you have submitted the sitemap manually.
Remember that robots.txt is public — anyone can read it, so never use it to "hide" sensitive URLs. Auth and noindex are for privacy; robots.txt is only for crawl budget.