MCP Server Tester

Why test your MCP server here?

If you are building an MCP server, the worst time to find a schema bug is when a Claude Desktop user files an issue. This MCP server tester lets you exercise every JSON-RPC method your server implements — initialize, tools/list, tools/call, prompts/get, resources/read, completion/complete, logging/setLevel — and see the raw request and response.

It is what you would write yourself with curl and a JSON pretty-printer, but already built. Point it at http://localhost:3000 during development, or your deployed staging URL before you publish.

A pre-publish testing checklist

1. initialize responds with protocolVersion, serverInfo, and the capabilities you actually implement
2. tools/list returns every tool with a complete inputSchema (no any, no missing required fields)
3. tools/call returns content arrays with the correct MIME types — not bare strings
4. Tool errors use isError: true with a useful message; do not throw raw exceptions
5. Schema enums and patterns are honored — try invalid input and confirm a clean rejection
6. Long-running tools stream progress notifications or finish under your client's timeout
7. Pagination works on tools/list / resources/list if your server has >50 entries
8. Auth rejects unauthenticated requests on every protected method (not just initialize)

Common MCP server bugs this catches

• Schema drift — your TypeScript type says userId: number but the schema you advertise says string
• Missing required — a field your code assumes is present is actually optional in the schema
• Bare-string responses — returning "ok" when the spec requires a {type:"text", text:"ok"} content item
• Stack traces in production — uncaught exceptions leaking file paths to the client
• Initialize advertising capabilities you didn't implement — clients then call methods that 500
• Inconsistent IDs — sending a notification response when the client sent a request (or vice versa)

Testing local servers

For HTTP / SSE / Streamable HTTP local servers, paste http://localhost:PORT and connect — this works for most modern frameworks (FastMCP, the TypeScript SDK's StreamableHTTPServerTransport, Python's mcp.server.streamable_http).

For STDIO servers, use the official npx @modelcontextprotocol/inspector tool — browsers cannot spawn child processes. Many developers start STDIO during prototyping and switch to Streamable HTTP for distribution; this server tester then becomes the primary tool.

Frequently Asked Questions