What is the MCP Debugger?

The MCP Debugger tests and diagnoses MCP (Model Context Protocol) server connections on Apify standby URLs. It sends a real protocol handshake — the same initialize request that Claude Desktop and other MCP clients send — and reports exactly what works, what fails, and how to fix it. MCP servers on Apify run as standby actors: always-on HTTP endpoints that accept JSON-RPC requests at a /mcp path. When they stop working, the error messages from MCP clients are frustratingly vague: 'connection failed', 'transport error', or just silence. The MCP Debugger replaces guesswork with diagnostics. Here is what the MCP Debugger checks. Connection success — can it reach the standby URL and get a response? Latency — how long does the initialize handshake take? Under 2 seconds is healthy, 2-5 seconds suggests a cold start, over 5 seconds is a warning. Protocol compliance — does the server respond with valid JSON-RPC? Tool discovery — how many tools does the server expose, and what are their names and descriptions? Authentication — does the server require a token, and does the provided token work? The debugger diagnoses common issues and maps them to actionable fixes. A 404 means your webServerMcpPath is wrong in actor.json. A timeout means standby mode is not enabled or the actor has not been warmed up. A 401 means the authentication token is missing or invalid. Zero tools returned means your server.tool() calls happen after the transport connects instead of before. The MCP Debugger costs $0.15 per debug run. To use it, go to the MCP Debugger page in the ApifyForge dashboard, enter your standby URL (e.g., https://username--actor-name.apify.actor/mcp), optionally provide an API token, and click Debug Connection. The report shows connection status, latency, available tools, detected issues, and fix suggestions. This tool is especially valuable when setting up a new MCP server, troubleshooting user-reported connection failures, and monitoring MCP server health over time. For related testing tools, see the questions about the Schema Validator, Test Runner, and Cloud Staging. Visit apifyforge.com/tools/mcp-debugger for documentation.