Small things built while working on something else, then cleaned up because they turned out to be useful. Free, no accounts, no tracking on the inputs. Where a tool quotes a number, that number was measured rather than estimated, and the method is written at the bottom of the page so you can check it.
Runs in your browser
Paste an X.509 certificate and see its structure: every object with its byte offset, object identifiers decoded from raw bytes, key usage bits translated, and a mode that diffs a broken certificate against one that works. Nothing is uploaded, because the parser is in the page.
Built on measured data
Work out whether a post-quantum certificate chain fits inside TCP's initial congestion window, and which client stacks can actually complete the handshake. The usual "keep it under 10KB" advice states the limit on the wrong quantity.
Measured corpus, projected verdicts
Check any public domain: top-10k domains answer from a measured corpus, everything else is measured live, and either way you see the chain the site actually serves and what it weighs once ML-DSA signs it, checked against the server's first flight. Existing checkers report what a site negotiates today; this one asks what the signature migration does to its chain.
Runs on your machine
Ask your own TLS server which certificate chain it sends, and to whom. Five connections, each a different client story, including one whose constraints excluded the chain that arrived anyway. It runs locally because it has to: the server's answer is encrypted, so no passive scan can see it. The measurement behind it is the write-up "Three of five sent it anyway".