What gets hashed
RFC 7638 hashes only the required public members for the key type: crv, kty, x, and y for EC; e, kty, and n for RSA; and crv, kty, and x for OKP. Extra public metadata is not part of the thumbprint. The calculator checks the public-key shape, validates supported curve points locally, hashes the canonical UTF-8 JSON with SHA-256, then Base64URL-encodes the digest without padding.
{"crv":"P-256","kty":"EC","x":"…","y":"…"}The displayed thumbprint is an identifier for the public key, not a replacement for the key pair or a credential. A non-Nightflyer key can still be good for its own protocol; it simply cannot create Nightflyer request proofs.
When an online example fails
A JWK is one key; a JWK Set is an object with a keys array. For a public EC key, kty must be EC; for RSA, it must be RSA and the public numbers are n and e. alg is optional algorithm metadata, not a substitute for kty. Long Base64URL values in specifications are sometimes wrapped only for display, so remove line breaks and spaces before pasting.