The Model Context Protocol in Python

Introduction

Every agent needs tools, and every tool needs a way to reach the model. Building Agentic Workflows in Python built that connection by hand — a hand-written JSON schema, a loop that dispatches on block.name. LLM Frameworks vs. the Raw SDK in Python showed LangChain’s @tool turning a plain function into that same schema via bind_tools. Both are still bespoke: the tool lives inside one process, wired to one agent, in one language.

The Model Context Protocol (MCP) solves a different problem: it standardizes the wire format between an AI application and a tool server, so the server doesn’t have to be rewritten per agent, per framework, or per language. This post covers what that buys you, builds a minimal MCP server and a client that consumes it — both on the official Python SDK — and gives an honest answer to when reaching for a protocol is worth it over a direct tool call.

The Problem MCP Solves

Without a shared protocol, every pairing of agent framework and tool needs its own glue code: a LangChain @tool wrapper, a hand-rolled schema for the raw SDK, a different wrapper again for whatever framework a teammate picks next — an integration per framework, per tool. That’s an M×N problem.

MCP flattens it to M+N. A server exposes tools, resources, and prompts once, over a standard JSON-RPC protocol. Any host application — Claude Code, Claude Desktop, VS Code, or your own agent — creates an MCP client that speaks that same protocol, regardless of which framework built the host. Write the server once; every MCP-aware host can use it without new integration code.

The protocol itself is intentionally boring: JSON-RPC 2.0 messages for lifecycle negotiation, tool discovery, and tool execution. Discovery (tools/list) and execution (tools/call) are the two calls that matter for this post:

// tools/list response (abbreviated)
{
  "jsonrpc": "2.0", "id": 2,
  "result": { "tools": [
    { "name": "get_account_balance",
      "description": "Look up the balance for an account by ID",
      "inputSchema": { "type": "object",
        "properties": { "account_id": { "type": "string" } },
        "required": ["account_id"] } }
  ]}
}

Two transports cover most cases: stdio (a local subprocess, one client per server — what this post uses) and Streamable HTTP (a remote server, many clients, standard bearer-token auth). Source: MCP architecture overview.

Building a Minimal MCP Server

The official Python SDK’s stable line is v1.x — v2 is alpha and explicitly not recommended for production (python-sdk README), so this post pins to it:

uv add "mcp[cli]"

FastMCP turns a type-hinted, docstring-described function into a tool — no hand-written JSON Schema, matching the “two type-hinted functions” pitch of the SDK’s own quickstart. The tool is a read-only account-balance lookup, deliberately keyed by ID rather than a free-text query, for reasons the next section makes concrete:

import re
from mcp.server.fastmcp import FastMCP

mcp = FastMCP("accounts-server")

# A fixed, in-memory dataset — stands in for a real accounts service
ACCOUNTS = {"ACC-100001": 542.10, "ACC-100002": 12.50}
ACCOUNT_ID = re.compile(r"^ACC-\d{6}$")

@mcp.tool()
def get_account_balance(account_id: str) -> str:
    """Look up the balance for an account by ID.

    Args:
        account_id: Account ID, e.g. ACC-100001.
    """
    # Validate/whitelist BEFORE using — account_id is model-provided, untrusted data
    if not ACCOUNT_ID.match(account_id):
        raise ValueError("Invalid account ID format")
    balance = ACCOUNTS.get(account_id)
    return f"Balance for {account_id}: ${balance}" if balance is not None \
        else f"No account found for {account_id}"

A read-only resource — MCP’s second primitive, for data the model can load into context without an action attached — is one more decorator:

@mcp.resource("docs://refund-policy")
def refund_policy() -> str:
    """Current refund policy text."""
    return "Refunds are issued within 5 business days."


if __name__ == "__main__":
    mcp.run(transport="stdio")  # never print() here — it corrupts the JSON-RPC stream on stdio

Source for FastMCP, @mcp.tool(), and @mcp.resource(): python-sdk v1.x README and the official Build an MCP Server tutorial. The stdio logging warning is language-agnostic: print() writes to stdout by default, which is the same stream the protocol uses — log to sys.stderr or a logging library instead.

The Trust Boundary Doesn’t Move

The account_id argument a tool function receives is the same untrusted, model-provided data as block.input in the raw SDK loop (post 15) or the arguments a @tool-decorated function receives from bind_tools/create_agent (post 27) — MCP standardizes the wire format, not the trust model. FastMCP does validate the shape of arguments against the schema it derives from your type hints before your function ever runs — but shape validation only confirms account_id is a string, not that it’s a safe string. The regex check above still has to happen inside the function body.

Never skip that second check by trusting the type hint alone:

# UNSAFE — never do this: string-interpolating a tool argument into a query
sql = f"SELECT balance FROM accounts WHERE id = '{account_id}'"  # SQL injection

# SAFE — validate/whitelist first (as get_account_balance does above), then use a fixed
# lookup or a parameterized query — never a query string built from model input

The lesson from post 15’s execute_validated_tool carries over unchanged: whitelist the shape you expect, reject anything else, and never let a tool argument reach a shell command, file path, or SQL string by concatenation.

Building a Client That Consumes the Server

A client connects over the same stdio transport, discovers the tool, and calls it — no knowledge of how the server was implemented:

import asyncio
from mcp import ClientSession, StdioServerParameters
from mcp.client.stdio import stdio_client

server_params = StdioServerParameters(command="python", args=["accounts_server.py"])

async def main():
    async with stdio_client(server_params) as (read, write):
        async with ClientSession(read, write) as session:
            await session.initialize()

            tools = await session.list_tools()
            print("Available:", [t.name for t in tools.tools])

            result = await session.call_tool(
                "get_account_balance", arguments={"account_id": "ACC-100001"})
            print(result.content[0].text)

asyncio.run(main())

Source for ClientSession, StdioServerParameters, stdio_client, and session.call_tool: python-sdk v1.x docs/client.md. This is the same pattern modelcontextprotocol.io’s own Build an MCP Client tutorial uses — the client doesn’t care which SDK or language built the server on the other end of the transport.

Wiring MCP Into the Agent Loop from Post 15

The agent loop itself doesn’t change — only where the tool list and tool execution come from. Instead of a hand-written TOOLS list and a local execute_validated_tool function, list tools from the MCP session and route tool_use blocks through session.call_tool(...):

tools_response = await session.list_tools()
anthropic_tools = [{
    "name": t.name,
    "description": t.description,
    "input_schema": t.inputSchema,   # already the JSON-Schema shape the Messages API expects
} for t in tools_response.tools]

# Inside the loop from post 15, in place of execute_validated_tool(block.name, block.input):
mcp_result = await session.call_tool(block.name, block.input)
text_result = mcp_result.content[0].text

Everything else from post 15 — the MAX_ITERATIONS cap, claude-opus-4-8 with thinking={"type": "adaptive"}, the message bookkeeping — is unchanged. Only the tool’s implementation moved to a separate, reusable process, and t.inputSchema needs no manual conversion — MCP tool schemas are already the same JSON Schema shape tools= expects.

MCP vs. the Framework Tool Abstraction (Post 27)

LangChain’s @tool/bind_tools/create_agent (post 27) wrap a function living in your own Python process — convenient, but the tool only exists inside that one application. MCP wraps a separate process or service behind a standard protocol, so the same accounts server above can be launched by Claude Desktop, called from Claude Code, and consumed by this post’s plain client — three unrelated hosts, zero rewritten integration code (the “broad ecosystem support” modelcontextprotocol.io advertises).

The two aren’t mutually exclusive: LangChain ships langchain-mcp-adapters, which converts an MCP session’s tools directly into LangChain tools, so create_agent can drive them without a manual session.call_tool loop:

from langchain_mcp_adapters.tools import load_mcp_tools
from langchain.agents import create_agent

async with stdio_client(server_params) as (read, write):
    async with ClientSession(read, write) as session:
        await session.initialize()
        tools = await load_mcp_tools(session)          # MCP tools -> LangChain tools
        agent = create_agent(model, tools)              # same create_agent as post 27

Source: langchain-mcp-adapters README. Framework tool abstractions and MCP solve different layers of the same problem: one wires a function into a prompt; the other lets that function’s server outlive any one framework.

When MCP Earns Its Weight — and When It’s Overkill

Reach for MCP when the tool or data source needs to be shared across independently-built AI applications — your accounts lookup used by Claude Desktop for support staff and Claude Code for engineers and a custom internal agent — or when you’re consuming someone else’s server (Sentry, GitHub, a filesystem) without writing integration code per host. It’s the right call when the tool is a product in its own right, not a private implementation detail of one agent.

It’s ceremony when the tool has exactly one caller in one codebase: a separate process, a stdio or HTTP transport, and a protocol handshake are pure overhead next to a @tool-decorated function (post 27) or a hand-written JSON schema (post 15) that never leaves the interpreter. Build the direct version first; promote it to an MCP server only once a second, independent host actually needs to call it.

Practical Checklist

Final Thoughts

MCP doesn’t replace the agentic loop from post 15 or the framework tool abstractions from post 27 — it standardizes what sits behind either one: a tool server that any compliant client can discover and call, without per-host glue code. That standardization is worth real engineering cost when a tool needs to serve more than one application; for a tool with one caller, the direct approaches this series already covered remain the simpler, and correct, default.

• Python
• AI
• LLM
• MCP
• Anthropic