MCP Gate

A transparent stdio gate that fixes the LLM parameter serialization bug in LLM-based agent CLI tools (Amp, Claude Code, Cursor, etc.) where tool parameters are sent as strings instead of their correct JSON types.

The Problem

LLMs non-deterministically serialize MCP parameters as strings:

• "true" instead of true (boolean)
• "5" instead of 5 (number)
• "[\"markdown\"]" instead of ["markdown"] (array)

How It Works

Amp → [gate] → MCP Server
        ↓
1. Caches tool schemas from tools/list
2. Intercepts tools/call requests
3. Coerces string params → correct types
4. Forwards to real server

Usage

Wrap any MCP server command with the gate. Works with Amp, Claude Code, Cursor, and other agent CLI tools:

Before (breaks)

{
  "amp.mcpServers": {
    "firecrawl": {
      "command": "npx",
      "args": ["-y", "firecrawl-mcp"]
    }
  }
}

After (fixed with mcp-gate)

{
  "amp.mcpServers": {
    "firecrawl": {
      "command": "mcp-gate",
      "args": ["npx", "-y", "firecrawl-mcp"]
    }
  }
}

Brave Search example

{
  "brave-search": {
    "command": "mcp-gate",
    "args": ["npx", "-y", "@nicepkg/brave-search-mcp"]
  }
}

Cursor config.json example

{
  "mcpServers": {
    "github": {
      "command": "mcp-gate",
      "args": ["npx", "-y", "github-mcp"]
    }
  }
}

What It Coerces

| From (string) | To (native) | Schema type | |---|---|---| | "true" / "false" | true / false | boolean | | "42" / "3.14" | 42 / 3.14 | number / integer | | "[\"a\",\"b\"]" | ["a","b"] | array | | "{\"k\":\"v\"}" | {"k":"v"} | object |

Also recurses into nested object properties and array items.

Zero Dependencies

Pure Node.js (>=18), no npm install needed.