/tools/headers does a single GET on the URL with all redirects followed manually (capped at 10 hops). The result page shows:

• The final HTTP status, colour-coded by class (2xx teal, 3xx purple, 4xx amber, 5xx red).
• Total round-trip time in milliseconds.
• The redirect chain — every hop with its status and Location target — so you can spot a chain that's too long or that goes from HTTPS back to HTTP somewhere.
• Every response header, alpha-sorted.

Useful for debugging an OG card that won't render (need to check what content-type Twitter saw), a missing cookie (which Set-Cookie actually came back), or a CDN that's adding a stale Cache-Control somewhere.