Troubleshoot MCP tunnels

A tunnel that won't accept traffic can fail at three layers; diagnose them in order: the outbound connection to the tunnel edge, the inner TLS handshake from Anthropic to your proxy, then upstream routing and IP validation.

Quick reference

OAuth fails behind a source-IP allowlist

OAuth flows fail when your authorization server's source-IP allowlist blocks Anthropic's backend from reaching /token, /register, and the discovery endpoints. If you'd rather not allowlist Anthropic's egress ranges, you can route the backend-to-backend OAuth calls through the tunnel while keeping the browser-facing /authorize endpoint on your existing public hostname.

With this configuration, the user's browser hits /authorize on your existing hostname (which your allowlist already permits), while Anthropic's backend reaches /token, /register, and the discovery documents through the tunnel.

Setup Job authentication failures

The Helm setup Job and the Compose setup service authenticate to the Tunnels API by exchanging an OIDC JWT through your federation rule. When the exchange fails, see Troubleshoot a failed exchange in the Workload Identity Federation reference; the failure modes (subject, audience, issuer, JWKS, lifetime) are the same.

Tunnels-specific causes:

• The chart's default audience is api.anthropic.com (no scheme). If your rule's audience is https://api.anthropic.com, set api.wif.audience to match.
• A 403 from the Tunnels API after a successful exchange means the rule's scope doesn't include org:manage_tunnels, or the rule's service account isn't a member of the tunnel's workspace. Set the scope and add the service account to the workspace.

The Helm setup Job runs as a pre-install hook. On failure, the Job is left behind for inspection (kubectl logs job/mcp-tunnel-setup -n mcp-tunnel). Helm doesn't manage hook resources, so delete it before retrying:

helm uninstall mcp-tunnel -n mcp-tunnel
kubectl -n mcp-tunnel delete job mcp-tunnel-setup

Tunnel won't connect

Check the cloudflared logs first. Common causes:

• The TUNNEL_TOKEN is missing, expired, or copied incorrectly.
• A firewall is blocking outbound TCP/UDP on port 7844 to the tunnel edge.

cloudflared may also log warnings about UDP receive buffer sizes; this is a QUIC tuning hint, not an error.

Certificate errors

When Anthropic rejects the proxy's certificate during the inner TLS handshake, the proxy logs tls handshake failed. Verify that:

• The server certificate has not expired.
• The certificate's Subject Alternative Name matches *.<tunnel-domain>.
• The signing CA is registered with Anthropic for this tunnel.

See the certificate requirements for the full validation rules.

Upstream IP validation

For SSRF protection, the proxy only dials addresses in the RFC1918 private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) by default. Only IPv4 is supported.

If the proxy logs IP validation failed: <ip> is not a private address, the upstream hostname resolved outside that set. On Kubernetes, some managed distributions allocate the Service CIDR outside RFC1918; if kubectl get svc kubernetes -n default -o jsonpath='{.spec.clusterIP}' returns an address outside the private ranges, look up your cluster's Service CIDR and add it.

If the address is legitimate, add the narrowest covering CIDR to upstream.allowed_ips. Setting allowed_ips replaces the RFC1918 default rather than extending it, so include the private ranges your other upstreams use:

upstream:
  allowed_ips:
    - 10.0.0.0/8
    - 172.16.0.0/12
    - 192.168.0.0/16
    - 127.0.0.0/8       # loopback, for local testing only