Nostr Devtools: https://chromewebstore.google.com/detail/nostr-devtools/ogoaodeklpfbboeopebgdggammkcndcp
Get clear insight of what relays are serving what subscriptions and what events came from where.
Everything is filterable by arbitrary strings, groupable by relay, event, subscription or by any arbitrary Nostr filter elements (click on them, or Ctrl+click to add to existing).
It only captures stuff while the devtools is open, so open it first then refresh the page to use, otherwise you'll get incomplete data.
