MCP server not connecting: what our checks find
We check 37,683 servers over the network, the way a client connects to them. This is how their newest checks came out, and what to look at for each.
| Newest check | What it means, and what to check | Servers |
|---|---|---|
| Started | It started and listed its tools. Nothing to fix on the server side. | 22,341 (59%) |
| Needs a key | It starts only with a key or setting we do not give it. Set the environment variables or headers its documentation names. | 683 (2%) |
| Asks to sign in | It answered and asked for a sign-in or a key first. Sign in from the client, or give the key in a header if the client can send one. | 10,786 (29%) |
| No answer in time | Nothing came back within our time limit. Open the address or run the command once by hand and see how long the first answer takes. | 227 (1%) |
| Not MCP | It answered with something that is not valid MCP. Check that the address is the MCP endpoint itself, not a web page or an API next to it. | 1,654 (4%) |
| Unclear | It failed three times in a row without a cause we could name. Try again later; a server that keeps failing shows it on its page. | 1,992 (5%) |
Which clients speak which transport
A client connects only over the transports it supports: Streamable HTTP (HTTP below) or the older HTTP+SSE (SSE). It sends a key only if it can set a header.
| Client | HTTP | SSE | Header |
|---|---|---|---|
| Claude Desktop | Yes | Yes | No |
| Claude Code | Yes | Yes | Yes |
| Cursor | Yes | Yes | Yes |
| VS Code | Yes | Yes | Yes |
| Devin Desktop | Yes | Yes | Yes |
| Zed | Yes | No | Yes |
| Cline | Yes | Yes | Yes |
| Continue | Yes | Yes | No |
| Goose | Yes | No | Yes |
| LM Studio | Yes | Yes | Yes |
| Gemini CLI | Yes | Yes | Yes |
| Codex | Yes | No | Yes |
| mcp-client-cli | No | No | No |
| oterm | Yes | Yes | Yes |
| TUUI | No | No | No |
| BoltAI | No | No | No |
| Agenvoy | Yes | No | Yes |
| Amp | Yes | Yes | Yes |
| AnythingLLM | Yes | Yes | Yes |
| Augment Code | Yes | Yes | No |
| Bob Shell | Yes | No | Yes |
| ECA | Yes | Yes | Yes |
| Kiro | Yes | No | Yes |
| Kiro CLI | Yes | No | Yes |
How our check connects
- Over HTTPS only, to ports 443 and 8443, following at most three redirects.
- It reads at most 1 MB of an answer and gives up after 45 seconds.
- It never sends a key, so a server that asks for one counts as answering.