Skip to main content

MCPX_STDIO_SPAWN_ENOENT

Severity: error Domain: STDIO

What happened​

mcpproxy tried to spawn a stdio MCP server, but the operating system reported ENOENT — the configured command does not exist on the user's PATH. The subprocess never started, so the MCP initialize handshake timed out.

A typical error chain looks like:

failed to connect: stdio transport (command="uvx", docker_isolation=false):
  server did not respond to MCP initialize within 30s
  recent stderr: zsh:1: command not found: uvx

Common causes​

  • Tool not installed — uvx, npx, python3, bunx, etc. are missing from the host.
  • Wrong PATH — the GUI launcher (Finder, systemd, launchd) doesn't see the shell-only PATH that includes ~/.local/bin, /opt/homebrew/bin, NodeSource paths, etc.
  • Wrong working directory — the command exists at a relative path that's only valid inside the server's working_dir.
  • Typo in the command field of the upstream entry.

How to fix​

1. Check what mcpproxy's environment sees​

which npx && which uvx && which python3
echo "$PATH"

If the command is missing, install the runtime:

# uvx (Python tools) — recommended
curl -LsSf https://astral.sh/uv/install.sh | sh

# npx (Node tools)
brew install node            # macOS
sudo apt install nodejs npm  # Debian/Ubuntu

2. Make the GUI inherit the shell PATH (macOS)​

When launched from a macOS GUI/launchd context (Launchpad, the login item, or the tray spawning the core), mcpproxy inherits a launchd-minimal environment rather than your interactive shell PATH. Since the daemon now hydrates the login-shell environment once at startup — sourcing your login shell to merge in PATH (and curated DOCKER_*/proxy/tool-home vars) — uvx/npx installed in /opt/homebrew/bin, ~/.local/bin, or a version-manager shim directory are normally found automatically. (Hydration is a no-op when mcpproxy is started from a terminal whose PATH is already comprehensive.)

If the tool still isn't found — e.g. it lives in a directory your login shell doesn't export, or your rc files don't run non-interactively — either:

  • Move the binary to a system directory: sudo ln -s "$(which uvx)" /usr/local/bin/uvx, or
  • Set an absolute path in the upstream config: "command": "/Users/you/.local/bin/uvx".

3. Use Docker isolation​

If you don't want to install language runtimes on the host, enable Docker isolation for the server. mcpproxy will run the command inside a container that already has uvx / npx available.

{
  "name": "my-server",
  "command": "uvx",
  "args": ["some-mcp-server"],
  "isolation": { "enabled": true }
}

See Docker Isolation.

4. Inspect recent server logs​

mcpproxy upstream logs <server-name> --tail 50

The last lines of stderr usually identify exactly which executable was missing.