Why PROBE Says “Unknown”

A username checker is expected to give a simple yes or no, but websites rarely make that easy. HTTP 403 might mean the site blocked our request, the profile is private, access varies by region, or the whole platform is having trouble. Even HTTP 200 can lead to a real profile, an empty site template, or a polished “user not found” page.

“Unknown” is still useful

PROBE only says found or not found when a site gives enough evidence through its response code, redirects, title, or page content. Challenges, rate limits, and unclear responses remain blocked or unknown.

This means fewer definite answers, but the answers you do get are more useful. In our latest test using 648 usernames expected not to exist, PROBE did not report any false matches.

Why generic rules fail

One site may use 404 for a missing account, another may return 200 with a “user not found” title, and another may send existing accounts through a login page. A universal “200 means found” rule creates false matches. A universal “login means missing” rule also fails on sites that hide real profiles behind authentication.

Where we can improve

The most common unclear response in our test was HTTP 403: 149 cases across 53 sources. Four sources returned HTTP 429, and six showed challenge-page titles. Some sources appear in more than one group, so those figures cannot simply be added together.

We will only add proxy or browser-based checks when testing shows that they answer more cases without creating extra false matches. The normal request remains the starting point, and every fallback should be visible in the result.