---
title: MCP Tools
description: Integrate external tool ecosystems via the Model Context Protocol
---

CowAgent supports the [Model Context Protocol (MCP)](https://modelcontextprotocol.io), allowing the Agent to directly invoke tens of thousands of community MCP tools. Configure `mcp.json` once and the tools are exposed to the LLM in exactly the same way as built-in tools — automatically selected and invoked.

## Configuration File

CowAgent reads `~/cow/mcp.json`. If the file does not exist, no MCP tools are loaded — and no error is raised.

For Docker deployments, the official `docker-compose.yml` already mounts the host's `./cow` directory to `/home/agent/cow` inside the container (i.e. the container user's `~/cow`). Just drop `mcp.json` into the host's `./cow/` directory and it will take effect.

### Standard Format

Fully compatible with the MCP community standard, identical to Claude Desktop / Cursor:

```json
{
  "mcpServers": {
    "<server-name>": {
      "command": "npx",
      "args": ["-y", "some-mcp-package"],
      "env": {
        "API_KEY": "your-key-here"
      }
    }
  }
}
```

| Field | Required | Description |
| --- | --- | --- |
| `command` | stdio | Executable to launch the server (e.g. `npx`, `python`, `uvx`) |
| `args` | No | Arguments passed to `command` |
| `env` | No | Environment variables for the subprocess, commonly used for API keys |
| `url` | SSE | SSE endpoint URL (alternative to `command`) |
| `disabled` | No | When `true`, this server is skipped — handy for temporary disabling |

### Full Example

```json
{
  "mcpServers": {
    "fetch": {
      "command": "uvx",
      "args": ["mcp-server-fetch"]
    },
    "github": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-github"],
      "env": {
        "GITHUB_PERSONAL_ACCESS_TOKEN": "<YOUR_TOKEN>"
      }
    }
  }
}
```

- **fetch**: Generic web page fetcher that returns page text content. No API key required.
- **github**: Access GitHub repos, issues, PRs, etc. Requires a Personal Access Token.

## Let the Agent Configure It for You

CowAgent ships with `read` / `write` / `edit` tools, so **you can simply send the MCP config to the Agent and ask it to write the file**:

For example:

```markdown
Add this MCP to ~/cow/mcp.json:

{"mcpServers":{"fetch":{"command":"uvx","args":["mcp-server-fetch"]}}}
```

The Agent will:

1. Read the existing MCP config and merge the new server entry, preserving existing ones
2. Hot-reload the new MCP server, so the corresponding tools become available on the next message

## How It Works

- **Async loading at startup**: All servers configured in `mcp.json` are loaded asynchronously in the background, never blocking the main loop — chat is usable immediately.
- **Hot reload**: When you or the Agent modifies `mcp.json`, changed servers are automatically reloaded after the current message — no need to restart cow.
- **Flat exposure**: Each method exposed by an MCP server appears as an individual tool. The LLM picks one directly without a second-stage decision.

## Supported Transports

| Transport | Description | Config Field |
| --- | --- | --- |
| **stdio** | Subprocess communication. The most common option, with the richest community ecosystem. | `command` + `args` |
| **SSE** | HTTP Server-Sent Events, suitable for remotely hosted MCP services. | `url` |

## Troubleshooting

| Symptom | What to Check |
| --- | --- |
| Agent has no MCP tools after startup | Verify that `~/cow/mcp.json` exists and contains valid JSON |
| A specific server fails to load | Look for `[MCP] Server 'xxx' load failed` in startup logs — usually missing dependencies or API keys |
| Changes to `mcp.json` aren't applied | Changes take effect on **the next message**. If the server config didn't actually change (e.g. only comments edited), no restart is triggered |
| Docker deployment | Make sure host's `./cow` is mounted to `/home/agent/cow` in the container, then just drop `mcp.json` into host's `./cow/`. Or just ask the Agent to do it |

## Recommended MCP Marketplaces

You can browse third-party MCP marketplaces and copy a JSON config to use directly, for example:

- [mcp.so](https://mcp.so) — Global MCP service index
- [ModelScope MCP Hub](https://modelscope.cn/mcp) — ModelScope's MCP hub, more reliable from mainland China

Any MCP server that follows the standard protocol (stdio / SSE) integrates with CowAgent out of the box.
