Claude Desktop: substitute `${HOME}` in `user_config` defaults

[pl4nty]

Describe the bug

In Claude Desktop, ${HOME} is not substituted when it appears inside a user_config default value. When ${user_config.server_path} is resolved, the raw default string (e.g. ${HOME}/AppData/...) is passed through without further variable substitution.

Tried with multiple type values, with required: true, and using \\ instead of / - none resolve the issue.

To Reproduce

1. Create a manifest using ${HOME} inside a user_config default:

{
  "server": {
    "type": "binary",
    "entry_point": "${user_config.server_path}",
    "mcp_config": {
      "command": "${user_config.server_path}"
    }
  },
  "user_config": {
    "server_path": {
      "type": "file",
      "title": "WinGet MCP Server Executable",
      "description": "Path to WindowsPackageManagerMCPServer.exe. Override only if using a custom build.",
      "default": "${HOME}/AppData/Local/Microsoft/WindowsApps/Microsoft.DesktopAppInstaller_8wekyb3d8bbwe/WindowsPackageManagerMCPServer.exe"
    }
  }
}

2. Install the plugin without overriding the default value.
3. Observe that the server fails to start - ${HOME} is not expanded in the resolved path.

Expected behavior

${HOME} in a user_config default should be substituted, producing the same resolved path as if used directly:

{
  "server": {
    "type": "binary",
    "entry_point": "${HOME}/AppData/Local/Microsoft/WindowsApps/Microsoft.DesktopAppInstaller_8wekyb3d8bbwe/WindowsPackageManagerMCPServer.exe",
    "mcp_config": {
      "command": "${HOME}/AppData/Local/Microsoft/WindowsApps/Microsoft.DesktopAppInstaller_8wekyb3d8bbwe/WindowsPackageManagerMCPServer.exe"
    }
  }
}

(The above works correctly.)

Additional context

Workaround is to hardcode ${HOME} directly in entry_point/mcp_config, but this prevents users from overriding the path (e.g. for debug builds).