FAQ

FAQ and troubleshooting

Which upstream transports does vMCP support?

Only HTTP endpoints: Streamable HTTP and legacy HTTP+SSE. vMCP does not launch upstream server processes.

Resources and prompts?

Curated resources, templates and prompts are supported with explicit selections, aliases, namespace validation, completion and subscription grants. See catalog contract.

Do I need Redis?

No. Redis only makes rate limits global across nodes (Rate limits). Everything else uses PostgreSQL.

A tool I selected does not show up

Check, in order: the upstream is enabled and healthy; the tool still exists after a refresh (orphaned selections are not served); the policy does not block it (read_only needs readOnlyHint from the upstream); the exposed name is unique.

401/403 from the data plane

• For agent keys: the virtual server requires auth (authRequired), the key does not list that virtual server, it expired or was revoked, or your IP is outside ipAllowlist (behind a proxy, set VMCP_TRUSTED_PROXIES).
• For OAuth JWTs: check WWW-Authenticate header. 401 indicates a missing or expired/invalid token (must be an asymmetric JWT with aud matching VMCP_AGENT_OIDC_AUDIENCE). 403 indicates no identity binding matched the token's claims, or the matching binding does not authorize this virtual server or client IP.

429 Too Many Requests

A rate limit was hit; honour Retry-After. Without Redis each node counts separately.

Upstream calls fail with "circuit open"

The circuit breaker saw repeated transport failures. It probes again after the cooldown; check the upstream's health in the UI or GET /api/v1/upstreams. Note that OAuth token acquisition failures (IdP down or invalid client credentials) do not count toward circuit breaker failure thresholds.

TLS errors

See the table in Upstream TLS.

The data plane did not pick up my change

It reloads via PostgreSQL LISTEN/NOTIFY. Check data-plane logs and that it uses the same database as the control plane.

Stored secrets stopped working

VMCP_MASTER_KEY differs from the key that encrypted them (both services must share it).

Browser verification fails, but a native client works

Check the configured public gateway URL and use HTTPS for both the UI and gateway when the UI is served over HTTPS. For separate origins, the data-plane origin allowlist and the reverse proxy's CORS responses must both permit the exact UI origin. The gateway does not add CORS headers itself. See browser verification setup.

Internal upstream connections are denied

Grant the upstream's actual private IP/subnet with VMCP_EGRESS_ALLOW_CIDRS on both planes and restart them. A hostname allowlist alone does not authorize a private address. Container localhost points to that container, not your host. Use the final endpoint URL; redirects are rejected. See Network security.

I created an agent key, but the server still allows anonymous access

Edit the virtual server and enable authentication required. Issuing a key does not change authRequired:false. Also check that the key grants access to the intended virtual server. See Authentication.

Known limitations

• Protocol baseline is 2025-06-18; newer revisions and March batching are not advertised.
• Resource update hints can be missed during physical watcher reconnection; durable gateway replay does not make upstream events exactly once.
• Unsafe calls with ambiguous outcomes are not automatically repeated.
• Circuit-breaker state and local fallback rate limits are per node; shared strict rate limits require Redis.
• Regex masking does not guarantee privacy or prompt-injection prevention. Binary content is denied when inspection is required.
• JWT early revocation uses exact-token digests; arbitrary opaque tokens/introspection are unsupported.
• Tenant isolation uses dedicated plane/database deployments; shared-schema tenant switching is excluded.
• Production HA, restore, workload scaling and compliance gates require operator evidence. See deployment.