MCP Server Examples: A Tool, a Resource, and a Prompt

An MCP server example worth copying registers three things in one file: a tool the model calls, a resource the host reads, and a prompt the host fills in. The notes echo already lives in how to build an MCP server. This page is the other three, tested on the Python SDK.

By Tayyab Javed, AI agent developer

What does one MCP server example contain?

Tools are functions. Resources are data, read by URI. Prompts are reusable message templates. A server that only echoes a string teaches the tool and hides the other two. The file below is the set I actually imported. More of this cluster is on the MCP servers hub.

pip install "mcp[cli]==2.3.0"

from mcp.server import MCPServer

mcp = MCPServer("examples")

@mcp.tool()
def unit_price(cents: int, quantity: int) -> str:
    """Return the line total in cents."""
    return str(cents * quantity)

@mcp.resource("policy://returns")
def returns_policy() -> str:
    """The return rule the model should quote."""
    return "Returns are accepted within 30 days. Refunds go to the original payment method."

@mcp.prompt()
def explain_charge(sku: str) -> str:
    """Ask the model to explain one charge in plain language."""
    return f"Explain the charge for {sku} in two sentences. Do not offer a refund."

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

You do not write a JSON Schema. cents: int is the schema. The docstring is the description. On mcp 2.3.0, list_tools named the tool unit_price and required both integers. Calling it with {"cents": 250, "quantity": 3} returned the text 750. Reading policy://returns returned the 30-day sentence as text/plain. explain_charge with sku set to SKU-9 returned one user message: "Explain the charge for SKU-9 in two sentences. Do not offer a refund."

How do you run this MCP server example?

mcp.run() speaks stdio. A host such as Claude Desktop or Cursor starts the process and owns its lifetime. Log to stderr if you add logs. stdout is the protocol. I did not leave a process running. I connected mcp.Client to the server object in the same Python process, which is the SDK's in-process path, and called the three primitives from there.

A call with quantity missing did not reach unit_price. The client received is_error true and the text Error executing tool unit_price, naming quantity as a missing field. That is the check you want before a model is in the loop. Wiring a host, and the Inspector command, is the build guide linked above. When the tool should stay inside one program instead of a server, read MCP vs function calling.

What should you not copy from an MCP server example?

Copy the three registrations. Do not copy a toy tool onto a refund, a file write, or a database update and call it done. This example only multiplies two integers and returns text. A write needs a person, a timeout, and a credential the model cannot see. The reference servers that already do those jobs are on the MCP server list.

In practice

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