Test MCP Server Online

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What This Tool Does

The Model Context Protocol (MCP) is an open protocol created by Anthropic that defines how AI applications communicate with external tools and data sources. If you're building an MCP server — or evaluating someone else's — this tool lets you connect, list available tools, and execute them with custom JSON arguments, all from a browser GUI.

It sends JSON-RPC 2.0 requests either directly via HTTP POST or through a proxy for SSE-based servers. No client-side code to write, no CLI to install.

How MCP Works (Briefly)

An MCP server exposes three categories of capabilities:

• Tools — functions the AI can call (e.g., search_database, send_email). Each tool has a name, description, and a JSON Schema for its input parameters.
• Resources — read-only data the AI can access (e.g., file contents, database rows).
• Prompts — reusable prompt templates with variables.

The wire protocol is JSON-RPC 2.0. A client sends a request like:

{
  "jsonrpc": "2.0",
  "method": "tools/list",
  "id": 1
}

And the server responds with a list of available tools, including their input schemas. The client can then call a specific tool:

{
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "name": "search_database",
    "arguments": { "query": "active users" }
  },
  "id": 2
}

This tool automates that flow. You connect, it calls tools/list, and you pick a tool to execute.

Connection Methods

What You Can Do

Discover tools: After connecting, the sidebar lists every tool the server exposes, along with its description and input schema. This is useful when you're integrating with a third-party MCP server and want to see what's available without reading source code.

Execute tools: Select a tool, fill in the JSON arguments (the schema is shown for reference), and hit Execute. The response panel shows the server's return value or error.

Monitor logs: The log panel records every JSON-RPC request and response with timestamps. This helps with debugging issues like malformed input schemas, authentication failures, or unexpected error codes.

Building Your Own MCP Server

If you're implementing a server, here's what this tester expects:

1. Accept POST requests with Content-Type: application/json
2. Implement at minimum tools/list (returns your tool catalog) and tools/call (executes a tool)
3. Return valid JSON-RPC 2.0 responses (including jsonrpc, result or error, and the matching id)

The MCP specification covers the full protocol. Anthropic maintains reference implementations in TypeScript and Python.

Common issues when testing:

• CORS — if your server doesn't set Access-Control-Allow-Origin, browser requests will fail. The proxy mode bypasses this.
• Missing id in response — JSON-RPC 2.0 requires the response id to match the request id.
• Schema validation — if your tool's input schema declares a field as required but the tester sends arguments without it, you'll get a validation error. The tool shows the expected schema so you can match it.

Frequently Asked Questions

What is MCP? The Model Context Protocol, created by Anthropic, is a standard for AI applications to interact with external tools and data. The full specification is at spec.modelcontextprotocol.io.

Can I test a server running on localhost? Not directly. Since this tool runs in the browser, it can't reach your local network. Use ngrok or a similar tunnel to get a public URL.

Does this store my API keys or auth headers? No. If your server requires authentication, the headers are passed through the proxy in memory. Nothing is logged or persisted.

Why does tools/list return an empty array? Your server is reachable but hasn't registered any tools. Check your server's tool registration code.

Does it support streaming responses? Not yet. Tool calls use standard request-response. Streaming resource access is a planned addition.

Is JSON-RPC 2.0 required? Yes. MCP uses JSON-RPC 2.0 exclusively for message exchange. The request must include "jsonrpc": "2.0" and the response must match the specification.

Related Tools

• API Response Mocker - Mock API responses while building MCP integrations
• Schema Converter - Generate TypeScript types for MCP tool schemas
• JSON Validator - Validate JSON-RPC payloads
• HTTP Header Analyzer - Audit security headers on MCP endpoints