budget-mcp

MCP server for budget tooling, built on the official MCP Python SDK.

Project layout

budget-mcp/
├── pyproject.toml
├── budget.db              # SQLite db, created by init_db.py (gitignored)
├── src/
│   └── budget_mcp/
│       ├── __init__.py
│       ├── server.py      # MCPServer instance + tools
│       ├── db.py          # schema DDL + connection helper
│       ├── init_db.py     # resets budget.db and seeds example rows
│       ├── money.py       # dollars <-> integer-cents conversion
│       ├── dates.py       # date validation + period resolution
│       ├── categorizer.py # LLM-based transaction categorizer
│       └── eval_categorizer.py  # accuracy check against a labeled set
├── tests/
│   ├── conftest.py         # puts src/ on sys.path for test collection
│   └── test_tools.py       # end-to-end tool tests, see "Tests" below
└── .venv/                 # local virtualenv (gitignored)

Related MCP server: Hello World MCP Server

Requirements

• Python 3.10+ (this project was set up with Python 3.12 via Homebrew: brew install python@3.12)

Setup

cd budget-mcp
/opt/homebrew/bin/python3.12 -m venv .venv
.venv/bin/pip install -e ".[dev]"
PYTHONPATH=src .venv/bin/python -m budget_mcp.init_db   # creates + seeds budget.db

macOS + iCloud Drive gotcha: if ~/Desktop is synced via iCloud Drive, the editable install's _editable_impl_budget_mcp.pth file in .venv/lib/python3.12/site-packages/ can end up with the macOS "hidden" file flag set, which makes Python 3.12 skip it — a bare import budget_mcp (e.g. in a REPL or test) will fail with ModuleNotFoundError even though the install "succeeded". If that happens, run:

chflags nohidden .venv/lib/python3.12/site-packages/_editable_impl_budget_mcp.pth

This doesn't affect running the server or the init script below — both are invoked with PYTHONPATH=src, which sidesteps the editable-install mechanism entirely.

categorize_transaction calls the Anthropic API, so it needs a key:

export ANTHROPIC_API_KEY=sk-ant-...

Add the same variable to the env block in the Claude Desktop config below so it's available when Claude Desktop launches the server.

Database

Three tables (see src/budget_mcp/db.py for the full DDL):

• budgets(category PK, monthly_limit)

• transactions(id, date, amount, category -> budgets.category, source, note)

• savings_goals(id, name, target_amount, current_amount, account_type)

All money columns are integer cents (e.g. 50000 == $500.00) to avoid float rounding — SQLite has no real DECIMAL type. transactions.amount is signed: negative = expense, positive = income.

Re-run PYTHONPATH=src .venv/bin/python -m budget_mcp.init_db any time to wipe budget.db and reset it to the seed data.

Tools

• ping() — placeholder, returns "pong".

• add_transaction(date, amount, category, source, note=None) — inserts a transaction and returns it plus the category's running total for that transaction's calendar month. amount is dollars (e.g. -45.67); date must be YYYY-MM-DD; category must already exist in budgets. Rejects bad dates, zero/non-finite/sub-cent amounts, unknown categories, and empty source with a clear error message (no stack traces).

• get_spending_summary(period, group_by, start_date=None, end_date=None) — sums expense transactions (amount < 0) over period ("this_month", "last_month", or "custom" with start_date/end_date), grouped by "category" or "source", and returns each group's total plus % of total spend. Rejects unknown period/group_by values and missing/invalid custom-range dates.

• categorize_transaction(description) — classifies a raw statement line (e.g. "TESCO STORES 3421 LONDON") into one of rent, food, transport, savings, business_expense, entertainment, other, via a Claude Haiku 4.5 call with structured JSON output (category, confidence, reasoning). The returned category is checked against the fixed list before being returned — an invalid category from the model surfaces as an error rather than being trusted. Requires ANTHROPIC_API_KEY (see Setup above).

Tests

.venv/bin/pytest tests/ -v

Each test spawns a real server subprocess and drives it through the actual MCP protocol (same path Claude Desktop uses), against a fresh throwaway SQLite db (via BUDGET_MCP_DB_PATH) — your real budget.db is never touched. Covers both tools' happy paths (including that running totals and spend percentages come out exact, not float-drifted) and every validation rejection (bad dates, zero/non-finite/sub-cent amounts, unknown categories, empty source, bad period/group_by, missing custom-range dates).

categorize_transaction isn't in this suite — LLM output isn't deterministic, so it doesn't belong in a pass/fail unit test. Instead, check its accuracy against 15 hand-labeled realistic statement lines:

PYTHONPATH=src .venv/bin/python -m budget_mcp.eval_categorizer

This calls the live API (needs ANTHROPIC_API_KEY) and prints a pass/fail per example plus overall accuracy.

Running locally

Run the server directly (it speaks MCP over stdio):

PYTHONPATH=src .venv/bin/python -m budget_mcp.server

It will sit waiting for an MCP client to talk to it over stdin/stdout — that's expected, it's not meant to be run interactively.

To poke at it with the official inspector UI instead:

PYTHONPATH=src .venv/bin/mcp dev src/budget_mcp/server.py

Connecting to Claude Desktop

Edit ~/Library/Application Support/Claude/claude_desktop_config.json and add an mcpServers entry:

{
  "mcpServers": {
    "budget-mcp": {
      "command": "/Users/hamza/Desktop/projects/budget-mcp/.venv/bin/python",
      "args": ["-m", "budget_mcp.server"],
      "env": {
        "PYTHONPATH": "/Users/hamza/Desktop/projects/budget-mcp/src",
        "ANTHROPIC_API_KEY": "sk-ant-..."
      }
    }
  }
}

Then fully quit and reopen Claude Desktop. In a new conversation, the tools should be available (look for the tools/hammer icon) — try asking Claude to call ping, then to add a transaction or get a spending summary.

Notes

• Uses MCP Python SDK v2 (mcp.server.MCPServer, formerly FastMCP in v1.x). Requires mcp>=1.2.0 per pyproject.toml, but what's actually installed here is the current 2.x line.

• Tool validation errors are raised as mcp.server.mcpserver.exceptions.ToolError, which the framework returns to the client as a plain error message — never a Python traceback.