Debugging MCP Connection Timeouts and Tool Calling Failures in Claude, Cursor, and SadaSend

A developer at SadaSend published a diagnostic guide for debugging MCP server connection timeouts and tool calling failures in Claude Desktop, Cursor, and SadaSend. The guide identifies four common causes—stdio pollution, missing environment variables, schema type mismatches, and path resolution failures—and provides verification methods using raw JSON-RPC handshakes and the official MCP Inspector.

When your MCP server fails with “Connection closed” or tools silently fail to execute in Claude Desktop, use this systematic diagnostic guide to fix the underlying issue. The top 4 reasons MCP servers fail in Claude Desktop Stdio Pollution Console Logging : In stdio transport, stdout is reserved strictly for JSON-RPC messages. If your code calls console.log "Starting server..." , the text corrupts the JSON-RPC stream, causing Claude to terminate the connection instantly. Always use console.error for debug logs. Missing Environment Variables: Claude Desktop does not inherit your full terminal shell profile .zshrc / .bashrc . Explicitly define environment variables inside claude desktop config.json. Schema Type Mismatches: If a tool parameter is marked as number in Zod but the LLM supplies "50" as a string without coercion, the server will reject the call with code -32602. Path Resolution Failures: Using relative paths e.g. ./server.js fails because Claude spawns processes from the application root. Always use absolute paths or npx. Test raw MCP handshake over stdio echo '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2026-07-28","capabilities":{},"clientInfo":{"name":"test-client","version":"1.0.0"}}}' | npx tsx server.ts Verifying with the Official MCP Inspector Run npx @modelcontextprotocol/inspector tsx server.ts to open a local web UI where you can inspect every JSON-RPC frame, inspect error codes, and benchmark tool latencies. Read more about MCP Server Debugging: https://sadasend.com/blog/debugging-mcp-connection-timeouts-tool-failures https://sadasend.com/blog/debugging-mcp-connection-timeouts-tool-failures