npm vs npx: When to Use Each in MCP Server Configuration

npm manages packages. npx executes them. This distinction is critical when configuring MCP servers.

Core Difference

npm (Node Package Manager) installs and manages dependencies:

npm install @modelcontextprotocol/server-memory

npx (Node Package Execute) runs packages without permanent installation:

npx @modelcontextprotocol/server-memory

npx checks in order: $PATH → local node_modules/.bin → downloads temporarily if not found.

Essential npm Commands

# Install to local node_modules/
npm install <package>
npm i <package>

# Install globally (available system-wide)
npm install -g <package>

# Install as dev dependency
npm install --save-dev <package>
npm i -D <package>

# Create package.json with defaults (skip prompts)
npm init -y

npx in MCP Configuration

MCP servers use npx to execute packages on-demand:

{
  "mcpServers": {
    "memory": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-memory"]
    }
  }
}

The -y flag auto-confirms installation prompts. Without it, MCP clients hang waiting for user input that never comes.

Server with Arguments

{
  "mcpServers": {
    "filesystem": {
      "command": "npx",
      "args": [
        "-y",
        "@modelcontextprotocol/server-filesystem",
        "/Users/me/allowed-directory"
      ]
    }
  }
}

Arguments after the package name pass directly to the server.

When to Use Each

Use npx for MCP when:

• Testing or developing locally
• Running latest version automatically
• Avoiding global package pollution
• Quick debugging sessions

Consider npm install when:

• Production deployments (predictable versions)
• Offline environments
• Containerized setups with locked dependencies

Key Differences

Aspect	npm	npx
Purpose	Install packages	Execute packages
Persistence	Permanent	Temporary
Version control	Uses installed	Can specify any
MCP use case	Production lock	Development/testing

Version Pinning with npx

Specify exact versions for reproducibility:

{
  "mcpServers": {
    "memory": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-memory@1.0.0"]
    }
  }
}

Troubleshooting

Server not starting?

# Test manually first
npx -y @modelcontextprotocol/server-memory

# Check if package exists
npm view @modelcontextprotocol/server-memory

Permission errors?

# Clear npx cache
rm -rf ~/.npm/_npx

# Verify node/npm installation
which node && which npx

The -y flag is non-negotiable for MCP configs. Automated systems cannot respond to interactive prompts.