> ## Documentation Index
> Fetch the complete documentation index at: https://docs.usefused.com/llms.txt
> Use this file to discover all available pages before exploring further.

# MCP Client

> Connect agents to MCP servers with portable transports, identity, and policy.

`MCPClient` declares a connection to an external MCP server. Direct clients live under root `mcp/`; each public filename supplies the connection identity and exports a zero-argument `client()` factory returning `MCPClient`.

| Property            | Rule                                                     |
| ------------------- | -------------------------------------------------------- |
| Location            | `mcp/<name>.py`                                          |
| Export              | `client()`                                               |
| Factory arguments   | None                                                     |
| Preferred transport | Streamable HTTP                                          |
| Legacy transport    | SSE                                                      |
| Connection time     | Runtime, not compile time                                |
| Credentials         | Deployment environment or invocation credential provider |

```python theme={null}
import os

from harnest.mcp import MCPClient

def client():
    """Connect to the catalog MCP server."""

    return MCPClient.streamable_http(
        os.environ["CATALOG_MCP_URL"],
        headers={"Authorization": f"Bearer {os.environ['CATALOG_MCP_TOKEN']}"},
    )
```

Use `MCPClient.sse(...)` only for a legacy SSE server. Connections and remote tool discovery are lazy, so offline compilation and unit tests do not need the server.

## Framework adapters

| Framework | Adapter behavior                                           |
| --------- | ---------------------------------------------------------- |
| ADK       | Uses the native MCP toolset                                |
| LangGraph | Uses its MCP adapter and discovers remote tools at runtime |

Harnest keeps connection identity and approval policy consistent. Same-named tools on different servers remain distinct.

## Authentication and gateways

Read URLs and static credentials from the deployment environment. For user-scoped access, resolve a token through [Authentication and Credentials](/harnest/runtime/authentication-and-credentials).

Customize the HTTP client only for a gateway, mTLS, or transport policy. Put shared clients in lifecycle resources because adapters may call the factory more than once.

## Direct connections and plugins

Use root `mcp/` for a connection that is part of one agent. Use a [`plugins/<name>/`](/harnest/build/agent-plugins) bundle when the connection and its progressive usage skills should travel together as one reusable capability.