Week 7 — MCP (giving an agent its own tools, the standard way)

Part of my journey leveling up automation skills as the field shifts toward AI. This week is about Model Context Protocol (MCP) — instead of pasting data into a prompt, the LLM is handed a small set of tools and decides for itself which ones to call, in which order, to answer a question.

Why this week matters: hardcoding data into a prompt doesn't scale past a demo. MCP is the emerging standard for "here are your tools and data sources, go" — it's what lets an agent plug into a live system instead of a snapshot.

The architecture

[client.py asks a question]
            |
            v
[Claude sees 3 tool definitions — no invoice data in the prompt]
            |  decides which tools to call, with what arguments
            v
[MCP server (server.py): list_invoices / get_invoice / get_purchase_order]
            |  returns live data over stdio
            v
[Claude reasons over the results and answers]

Continuity: Week 2 extracted an invoice from a document. Week 6 decided what to do with an invoice exception, given data the bot already had in hand. This week, the data itself lives behind an MCP server — an agent fetches its own facts instead of having them handed to it. Combine this with Week 6's policy-guarded decision service and you get Portfolio Project #4 on the roadmap: an agent that both looks things up and hands the judgement call to an auditable rule layer.

Related MCP server: Invoice MCP Server

What's here

1. Run it

python -m venv .venv
.venv\Scripts\Activate.ps1
pip install -r requirements.txt
# copy .env.example to .env and paste your key

python src/client.py

client.py spawns server.py itself over stdio (that's how MCP typically works for local tools — no HTTP port to manage, unlike Week 6). Watch the [MCP tool call: ...] lines print as Claude decides what it needs to look up.

Run the unit tests for the tool logic (no API key needed):

pytest

2. The key design idea: tools + data over a standard protocol

Before MCP, "give the model a tool" meant hand-rolling a JSON schema and a dispatch if/elif for every project. MCP standardizes that: server.py declares tools once with @mcp.tool(), and any MCP-speaking client — this repo's client.py, Claude Desktop, another team's agent — can discover and call them the same way. That's the "connecting agents to real tools" pitch: the tool-provider and the agent don't need to be written by the same person.

3. What I learned / production-ready vs. not

• MCP separates who owns the data from who reasons about it — the server doesn't know or care which LLM is calling it.

• Model choice matters here too: this is orchestration (deciding which tool to call), not hard judgement, so a cheap/fast model (claude-haiku-4-5) is the right call — save Opus-tier reasoning for the kind of decision Week 6 makes.

• Not production-ready: the "ERP" is an in-memory dict, the server has no auth, and there's no real MCP resource (only tools) — a production version would likely expose invoice records as MCP resources too. (Weeks 8+.)

Stack

Python · MCP (official Python SDK) · Claude (claude-haiku-4-5 for tool-driving) · Anthropic SDK · pytest