Introduction
You are trying to configure a custom MCP Connector, but you see the following error message:
MCP connection requires additional information or is invalid.
🔑 In short, this means that the initial connection check to your server could not be completed. The issue is usually related to the URL, authentication, or server response.
👨🔧 This article (intended for a technical audience) explains what happens under the hood and how to resolve the issue.
Prerequisites
Before proceeding, make sure that:
☑️ You are a developer or technical user connecting a custom MCP server to Mistral AI.
☑️ You have reviewed the MCP documentation on building a server.
☑️ You have access to the server code or logs and can run basic
curlcommands.
Common causes
This error indicates that the connection to your MCP server could not be established.
The most common causes include:
The URL points to the wrong endpoint (SSE or MCP).
The server is unreachable (invalid domain, firewall rule, or certificate issue).
The authentication flow fails (OAuth metadata not found or API key rejected).
The MCP server doesn’t respond correctly to the
initializeJSON-RPC call.
Let’s review each point one by one.
Quick checklist
Go through the following checks in order.
1. Confirm the correct endpoint
Ensure your server correctly exposes the transport endpoint:
/mcpormcpin the URL for MCP (Streamable HTTP)./ssefor Server-Sent Events.
If neither endpoint is found, the connection will fail.
🔑 SSE is deprecated, and we strongly recommend using Streamable HTTP instead.
2. Verify that the server is reachable
Use a valid HTTPS domain with a trusted TLS certificate.
3. Check your authentication method
OAuth 2
The server must return 401 with a valid WWW-Authenticate header that includes the resource_metadata endpoint.
API key
The endpoint must return 200 when the correct key is provided.
No authentication
The endpoint must return 200 on the first request.
4. Verify the response to the “initialize” call
The connection must remain open and stream events or heartbeats. Ensure that it responds correctly to the initialize JSON-RPC call.
🔎 Refer to the official MCP documentation for further details on the Connector’s lifecycle.
How to test your server
You can manually test your setup using curl.
📌 Alternatively, you may use MCP Inspector to test your MCP server through its user interface.
1. Check reachability
curl -i https://your-domain.example.com/mcp
What to expect:
Authentication required? | Response Status | Notes |
❌ No authentication required | 200 | The server responds successfully. |
✅ Yes (via OAuth) | 401 | The request fails and the server’s response include a |
🙋 Quick question: What if the path isn’t found in the WWW-Authenticate header?
🙋 Quick question: What if the path isn’t found in the WWW-Authenticate header?
Verify OAuth discovery manually using the following commands:
bash curl -i https://mcp.your-domain.example.com/.well-known/oauth-protected-resource
bash curl -i https://your-domain.example.com/.well-known/oauth-protected-resource
2. Test API key
If your server uses a API key based connection, test:
curl -i -H "Authorization: Bearer <API_KEY>" https://your-domain.example.com/mcp
The server should return a 200.
3. Test MCP initialize
curl -sS -X POST https://your-domain.example.com/mcp \
-H 'Content-Type: application/json' \
-d '{
"jsonrpc": "2.0",
"id": 1,
"method": "initialize",
"params": {
"protocolVersion": "<your-supported-version>",
"capabilities": {}
}
}'
Conclusion
You should now be able to resolve the issue and connect your MCP server. 🚀
📩 Still blocked? If the message persists after performing the curl tests and validating with the MCP Inspector, please contact our support team and include your server logs.