mcp-90-mre

A minimal Model Context Protocol server that replicates a bug with the Claude Desktop app's MCP implementation. Specifically, using allOf or anyOf with an object doesn't appear to work correctly.

Also see discussion #90 on the model context protocol.

https://github.com/user-attachments/assets/e22b0d06-f463-4c74-a081-cb0a631cb5e9

Usage

To use this server with the Claude Desktop app, first clone this repository, install deps (with npm install) and build the repo (with npm run build).

Then, add the following configuration to the "mcpServers" section of your claude_desktop_config.json:

{
  "mcpServers": {
    "postgres": {
      "command": "node",
      "args": [
        "/path/to/mcp-90-mre/dist/index.js"
      ]
    }
  }
}

Components

Tools

• format_hello (Currently not working)

  • Format a hello message
  • Input parameters:
    • obj: allOf:
      • Base object with timeOfDay (string)
      • Either:
        • Group type: { type: "group", people: string[] }
        • Individual type: { type: "individual", person: string }

• format_goodbye (Currently not working)

  • Format a goodbye message
  • Input parameters:
    • obj: anyOf:
      • Group type: { timeOfDay: string, type: "group", people: string[] }
      • Individual type: { timeOfDay: string, type: "individual", person: string }

• format_sorry (Working)

  • Format a sorry message
  • Uses a nested object containing a union, which appears to work correctly
  • Input parameters:
    • obj: Object containing:
      • nested: anyOf:
        • Group type: { timeOfDay: string, type: "group", people: string[] }
        • Individual type: { timeOfDay: string, type: "individual", person: string }

Bug Description

The server implements three different approaches to combining object types:

1. format_hello uses allOf to combine a base object with a discriminated union
2. format_goodbye uses a direct union of complete objects
3. format_sorry uses a nested object containing a union

Currently, only the third approach (format_sorry) works correctly in the Claude Desktop app. Both format_hello and format_goodbye fail to validate input properly, suggesting an issue with how the Claude Desktop app handles certain JSON Schema constructs.

Contributing

Pull requests are welcomed on GitHub! To get started:

1. Install Git and Node.js
2. Clone the repository
3. Install dependencies with npm install
4. Run npm run test to run tests
5. Build with npm run build

• You can use npm run build:watch to automatically build after editing src/index.ts. This means you can hit save, reload Claude Desktop (with Ctrl/Cmd+R), and the changes apply.