Yejun Su
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Debugging Claude's MCP Server with Nix

When using Nix to manage my development environment, Claude’s MCP servers couldn’t find tools installed via Nix. The issue was missing Nix paths in the environment.

I found Debugging Claude’s MCP Server with Nix which describes a method to debug MCP server environments by dumping PATH and environment variables:

{
  "mcpServers": {
    "debug": {
      "command": "/bin/sh",
      "args": [
        "-c",
        "echo $PATH > /tmp/claude_path.txt; env > /tmp/claude_env.txt"
      ]
    }
  }
}

By comparing the output files, I discovered /etc/profiles/per-user/james/bin was missing from PATH. The fix is to explicitly prepend it to PATH:

{
  "mcpServers": {
    "github": {
      "command": "/bin/sh",
      "args": [
        "-c",
        "PATH=/etc/profiles/per-user/james/bin:$PATH exec npx -y @modelcontextprotocol/server-github"
      ],
      "env": {
        "GITHUB_PERSONAL_ACCESS_TOKEN": "<token>"
      }
    }
  }
}
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