Sadly, Kagi uses a Cloudflare turnstile that's set to paranodial mode, so my Firefox on Linux is rejected. I suspect it's because the turnstile wants to use some WebGPU APIs to fingerprint my browser, and those APIs don't exist.
[Cloudflare Turnstile] Error: 600010.
600* Generic challenge failure Bot behavior detected.
Basically, you'd have to submit a Device ID that reliably identifies you.