MCP from the Tool Author's View

Write the tool once

Without a standard, every agent or app needs its own adapter for each tool. The Model Context Protocol (MCP) defines how a client discovers (tools/list) and calls (tools/call) tools on a server, so one server can serve many hosts. Everything from this course still applies: write clear schemas and descriptions, return helpful errors, keep write tools idempotent, and limit what the server's credentials can do.

Our tools as an MCP server

With the official Python SDK, a decorated function becomes a tool and its schema comes from the type hints. Install with pip install "mcp[cli]". Details are in the dedicated MCP course.

from mcp.server.fastmcp import FastMCP

mcp = FastMCP("accounts")

@mcp.tool()
def get_balance(account_id: int) -> str:
    """Return the current balance of one account. Read-only."""
    return f"account {account_id}: 1500"

if __name__ == "__main__":
    mcp.run()

Version your tool servers

When many agents depend on one tool server, changing a tool name or schema can break all of them. Add new tools instead of renaming, and announce removals ahead of time.

Quick check: What is the main benefit of exposing tools through MCP?

• One tool server can work with many compatible agents and apps
• Tools no longer need descriptions
• It removes the need for security
• It makes tools free