Function algs_for_key 

fn algs_for_key(
    allowed: &[Algorithm],
    kty: &str,
) -> Result<Vec<Algorithm>, String>

Narrows the configured algorithms to those a kty key can verify.

This is not an optimization — it is required for correctness. jsonwebtoken validates the whole algorithm list against the key family before it even looks at the token:

for alg in &validation.algorithms {
    if key.family != alg.family() { return Err(InvalidAlgorithm); }
}

So a list spanning two families can never verify anything. The default token_signing_alg_values_expected spans RSA and EC, which meant every JWKS-verified token — bearer tokens and interactive id_tokens alike — was rejected with InvalidAlgorithm unless the operator happened to pin a single family. Filtering per key keeps the permissive default working with whichever key the IdP actually published.