MCP Client - Fused

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

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. 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>/ bundle when the connection and its progressive usage skills should travel together as one reusable capability.