Definition
MCP connection debugging is the process of diagnosing why an MCP (Model Context Protocol) server fails to connect to a client, typically manifesting as 404 errors, 'Failed to connect' messages, or silent tool loading failures in Claude Code, Cursor, or VS Code.
In Depth
MCP connection failures fall into four categories: (1) Transport mismatch -- the server expects stdio but the client sends HTTP, or the server uses SSE but the client expects streamable HTTP. (2) URL format errors -- missing /mcp suffix, wrong protocol (http vs https), or trailing slashes that break routing. (3) Authentication failures -- missing or malformed API key headers, wrong header name (Authorization vs Authorization), or expired tokens. (4) Server-side issues -- the MCP server process crashed, the port is occupied, or the server has not started. Debugging checklist: verify the transport type matches between server config and client config, test the server URL directly with curl, confirm the API key is set in environment variables, check server logs for startup errors. For hosted MCP servers like Scavio (https://mcp.scavio.dev/mcp), the transport is HTTP and the only requirement is a valid Authorization header. For local MCP servers, check that the process is running and the stdio/SSE transport is correctly configured in the client's mcp.json.
Example Usage
A developer gets '404 Not Found' when connecting BetterStack MCP to Claude Code. Debugging reveals the URL in .mcp.json was missing the /mcp path suffix. After fixing the URL and moving the key into the header that server actually expects, the connection succeeds. Auth headers differ per vendor, so check the docs rather than assuming: Scavio's remote MCP server at mcp.scavio.dev takes the key in x-api-key, while its REST API takes Authorization: Bearer. Total debug time: 15 minutes following a systematic checklist.
Platforms
MCP Connection Debugging is relevant across the following platforms, all accessible through Scavio's unified API: