Debugging Passwordless Authentication Failures Over gRPC

The request hit production and then failed. Logs showed it wasn’t the passwordless authentication itself—it was a gRPC error.

Passwordless authentication over gRPC can fail in subtle ways. The error might appear as UNAVAILABLE, UNAUTHENTICATED, or INTERNAL. Each code has a specific cause. UNAVAILABLE often points to a network transport issue, a bad TLS handshake, or a misconfigured service endpoint. UNAUTHENTICATED means the server rejected the request’s credentials, even if your local token cache says they’re valid. INTERNAL is a catch‑all; if you see it, inspect server logs immediately.

Most passwordless flows over gRPC use short‑lived access tokens or signed requests. If token refresh fails—due to clock drift, expired claims, or missing metadata—the server will treat the request as unauthenticated. Check system clocks, token lifetimes, and signing keys. Hellos to the service that fail silently can still return errors deep in your call stack.

Another frequent cause is metadata propagation. With gRPC, every call’s context must carry the authentication headers. If you forget to propagate them in streaming calls or retries, the server sees an anonymous client. Verify middleware and interceptors are adding the expected authorization header, and confirm it persists across retries.

TLS settings also matter. A passwordless authentication gRPC error can stem from a mismatch in supported cipher suites, incorrect root CA bundles, or ALPN negotiation failures. Enable debug logs for TLS on both client and server to see which stage fails.

When debugging, isolate the failing call. Use grpcurl or a minimal test client to reproduce the request with the exact metadata and payload. This ensures you’re testing the full handshake, not just a mocked path. Then adjust connection settings, token generation logic, and header injection until the gRPC error disappears.

Every millisecond counts when your authentication layer fails. A single gRPC error can cascade into outages for every dependent service. Get proactive. Instrument token generation, log metadata handling, and surface handshake failures before they hit your users.

Want to see a working passwordless authentication flow over gRPC with zero guesswork? Spin it up at hoop.dev and watch it run live in minutes.