MCP On-Demand

A lightweight HTTP session manager for Model Context Protocol (MCP) servers. Load MCP servers on-demand instead of keeping them in context permanently. Zero token overhead when not in use.

For Humans

What this does: Use any MCP server without permanent context pollution.

Get started:

• Create Command - Build /commands that use MCP On Demand
• Create Skill - Build auto-activating skills that use MCP On Demand

For LLMs

Prerequisites

1. Node.js 22+ is required
2. MCP servers must be pre-installed on the user's system
3. MCP-on-demand must be installed and configured

Installation

IMPORTANT: This document assumes MCP servers have already been installed on the system.

1. Install mcp-on-demand globally

   npm install -g mcp-on-demand

   The installation automatically runs setup, which creates ~/.mcp-on-demand/installation.json to make the CLI self-locating. This allows you to use mcp-on-demand commands from anywhere.

2. Create configuration file

   Create ~/.mcp-on-demand/mcp-configs.json with user's MCP installation paths:

   {
     "chrome-devtools-mcp": {
       "command": "node",
       "args": ["/absolute/path/to/chrome-devtools-mcp/build/src/index.js"]
     }
   }

   Ask the user for their MCP installation paths. Do not assume locations.

3. That's it! The session manager auto-starts when you run any command.

   To manually check status:

   mcp-on-demand status

   To manually start (only needed for troubleshooting):

   mcp-on-demand manager &

Configuration Format

The ~/.mcp-on-demand/mcp-configs.json file maps MCP names to their launch commands:

{
  "mcp-name": {
    "command": "executable",
    "args": ["/absolute/path/to/mcp/entrypoint.js", "additional", "args"]
  }
}

Examples:

Node.js MCP:

{
  "chrome-devtools-mcp": {
    "command": "node",
    "args": ["C:/Dev/chrome-devtools-mcp/build/src/index.js"]
  }
}

Python MCP:

{
  "example-python-mcp": {
    "command": "python3",
    "args": ["/home/user/mcp-servers/example/main.py"]
  }
}

Binary MCP:

{
  "example-binary-mcp": {
    "command": "/usr/local/bin/example-mcp",
    "args": ["--flag", "value"]
  }
}

Usage Pattern

Use the mcp-on-demand CLI to interact with MCP sessions. The CLI communicates with the session manager via HTTP API on http://127.0.0.1:9876.

Check status:

mcp-on-demand status

Start MCP session:

mcp-on-demand start chrome-devtools-mcp

Discovering Available Tools

When you start an MCP session, the available tools are automatically displayed with their full schemas:

mcp-on-demand start chrome-devtools-mcp

Output example:

{
  "success": true,
  "mcpName": "chrome-devtools-mcp",
  "toolCount": 15,
  "message": "Session started with 15 tools",
  "tools": [
    {
      "name": "navigate_page",
      "description": "Navigate to a URL",
      "inputSchema": { ... }
    },
    ...
  ]
}

Best practice: Review the tools output to understand what capabilities the MCP provides, then use the appropriate tools for your task. This ensures you're always working with the current set of tools and their actual schemas.

Hide tools list: Use --no-show-tools to suppress the tools output:

mcp-on-demand start chrome-devtools-mcp --no-show-tools

Call tool:

mcp-on-demand call chrome-devtools-mcp navigate_page '{"url": "https://example.com"}'

Batch call:

mcp-on-demand batch chrome-devtools-mcp '[
  {"tool": "navigate_page", "args": {"url": "https://example.com"}},
  {"tool": "take_screenshot", "args": {"format": "png"}}
]'

Stop session:

mcp-on-demand stop chrome-devtools-mcp

List active sessions:

mcp-on-demand list

Shutdown session manager:

mcp-on-demand shutdown

File References with file://

The session manager automatically resolves file:// references in tool arguments:

mcp-on-demand call chrome-devtools-mcp evaluate_script '{
  "function": "file://./scripts/check-buttons.js"
}'

What happens:

1. Session manager detects file:// prefix
2. Reads file contents from disk
3. Replaces string with file contents
4. Executes tool call with resolved content

Supported paths:

• Relative: file://./script.js (relative to session manager working directory)
• Absolute: file:///absolute/path/to/script.js

Works everywhere: File resolution recursively traverses all objects and arrays in args.

API Reference

All requests are POST to http://127.0.0.1:9876 with JSON payloads.

Actions:

| Action | Parameters | Description | |--------|------------|-------------| | start | mcpName (required), showTools (optional, default: true) | Start MCP session | | call | mcpName, toolName, args | Call single tool | | batch | mcpName, toolCalls (array) | Call multiple tools sequentially | | stop | mcpName | Stop MCP session | | list | (none) | List active sessions | | shutdown | (none) | Shutdown session manager |

Response format:

Success:

{"success": true, ...}

Error:

{"error": "error message"}

Example: Web Debugging Workflow

# Start chrome-devtools-mcp session (daemon auto-starts if needed)
mcp-on-demand start chrome-devtools-mcp

# Execute debugging workflow
mcp-on-demand batch chrome-devtools-mcp '[
  {"tool": "navigate_page", "args": {"url": "https://example.com"}},
  {"tool": "evaluate_script", "args": {"function": "file://./check-buttons.js"}},
  {"tool": "take_screenshot", "args": {"filePath": "./screenshot.png"}}
]'

# Stop session when done
mcp-on-demand stop chrome-devtools-mcp

Token Economics

• Native MCP: 5000+ tokens per MCP permanently in context
• MCP On Demand: 0 tokens when not in use, ~5000 tokens only when session active
• Per-call overhead: Identical (~30 tokens)
• Value: Use 10+ MCPs without permanent context cost

Error Handling

Common errors:

| Error | Cause | Solution | |-------|-------|----------| | Unknown MCP: xyz | MCP not in config | Add to ~/.mcp-on-demand/mcp-configs.json | | Session xyz already running | Duplicate start | Use existing session or stop first | | No active session for xyz | Session not started | Call start action first | | Failed to read file: ENOENT | File not found | Check file:// path | | Connection refused | Session manager not running | Start session manager |

Session resilience:

• Malformed requests do not corrupt sessions
• Failed tool calls do not invalidate sessions
• Sessions are independent and can be restarted

Architecture

┌─────────────────┐
│   Client/LLM    │ (Claude Code, mcp-on-demand CLI)
└────────┬────────┘
         │ HTTP POST :9876
         │
┌────────▼────────────────────┐
│  Session Manager            │
│  - Loads ~/.mcp-on-demand/  │
│    mcp-configs.json         │
│  - Manages MCP sessions     │
│  - Routes tool calls        │
│  - Resolves file:// refs    │
└────────┬────────────────────┘
         │ stdio transport
         │
┌────────▼────────────────────┐
│  MCP Server(s)              │
│  (chrome-devtools-mcp, etc) │
└─────────────────────────────┘

File Structure

mcp-on-demand/
├── src/
│   └── session-manager.js         # Main HTTP server & MCP client
├── bin/
│   └── mcp-on-demand.js           # CLI executable
├── scripts/
│   ├── mcp-call.js                # Helper script for tool calls
│   └── setup.js                   # Setup script for installation.json
├── mcp-configs.example.json       # Example configuration
├── package.json
└── README.md

User configuration:

~/.mcp-on-demand/
├── installation.json              # CLI self-location (auto-generated by npm run setup)
├── mcp-configs.json               # User's MCP paths (required)
└── session.json                   # Runtime state (auto-generated)

Platform Notes

Windows (MSYS/Git Bash):

• Use forward slashes: C:/Dev/chrome-devtools-mcp/...
• Works correctly in MSYS/Git Bash environments

macOS/Linux:

• Standard Unix paths
• Consider nohup for background session manager

Troubleshooting

Check daemon status:

mcp-on-demand status

Session manager not responding?

# The daemon auto-starts, but if you have issues:
mcp-on-demand shutdown  # Stop any existing daemon
rm ~/.mcp-on-demand/session.json  # Clean up stale session file
mcp-on-demand start <mcp-name>  # Will auto-start fresh daemon

MCP won't start?

1. Check config exists: cat ~/.mcp-on-demand/mcp-configs.json
2. Verify MCP path: ls /path/to/mcp/index.js
3. Check Node.js version: node --version (need v22+)
4. Test MCP directly: node /path/to/mcp/index.js

Port 9876 in use?

• Change PORT constant in src/session-manager.js:12

Tool call timeout?

• Session remains functional - retry the call
• Consider restarting the session if persistent

Design Philosophy

Separation of Concerns:

• MCP-on-demand does not install or manage MCP servers
• Users maintain their own MCP installations
• Session manager provides runtime orchestration only

Universal Adapter Pattern:

• Standardized HTTP API for any MCP server
• Swap MCP servers without changing client code
• Works with any stdio-based MCP implementation

Zero Overhead:

• No token cost when sessions inactive
• Sessions started/stopped as needed
• Multiple concurrent sessions supported

License

MIT

Related Projects

• Model Context Protocol
• Chrome DevTools MCP
• MCP SDK