Copy command, args and env from the config into a terminal. See whether it starts at once or waits for interactive input. A stdio server should not print a banner or a progress bar to stdout; those bytes break the protocol.
Once it is up, make a read-only call: list tools, read a harmless resource. The result should be structured, not a whole README. Then try one write you are willing to approve, and check that failure is a clear error.
For remote HTTP, handshake the same URL first. Expired OAuth, a company proxy or a wrong path all show up as a hang. See debugging MCP connections and transports on this site.
Only then put it in the project config. Pin the version. Keep secrets in the user environment. Do not commit the first working experiment with a token still in it.