💰 Multi-User Expense Tracker MCP Server

A production-ready Async Multi-User Expense Tracker MCP Server built using FastMCP, PostgreSQL (Neon), and Python.

This server is designed to support multiple users, isolating budgets, expenses, and monthly finance per user via their email address (auto-creating users dynamically on first use). It validates categories and subcategories dynamically using a localized configuration file, and isolates credit card self-usage from borrowed/lent expenses.

🚀 Key Features

👤 1. Zero-Config Multi-Tenant Isolation

• Dynamic Onboarding: No complex sign-up flows, passwords, or authentication overhead. The first time a user's email address is supplied in a tool call, a new user account is generated automatically.

• Strict Data Isolation: Every database query is restricted using WHERE user_id = %s. It is impossible for a user to view, edit, or delete another user's financial details.

• Race-Condition Free: Uses atomic INSERT ... ON CONFLICT DO NOTHING SQL followed by a SELECT query inside transactions to prevent duplicate user creation under high concurrent loads.

🏷️ 2. Dynamic Category & Subcategory Validation

• Local Rules Engine: Uses category.json as the source of truth for allowed expense classifications.

• Smart Normalization: Automatically normalizes input casing, strips trailing/leading whitespaces, and converts spaces to underscores (e.g., inputting "Food" and "Dining Out" normalizes cleanly to "food" and "dining_out").

• Tool Discovery: Exposes an MCP resource expense://categories containing the JSON config. This lets AI clients query and inspect valid classifications before calling tools.

💳 3. Borrowed Expense Isolation (Credit Card & Cash)

• Separate Summaries: Expenses flagged as borrowed are excluded from the user's monthly budget, personal spend, and average transaction stats.

• Auto-Detection Rules: The system automatically detects a borrowed expense if:

  1. The category is "credit_card_usage" and the subcategory is "borrowed_to_friend".

  2. The note contains keywords like "borrowed to friend", "borrowed to a friend", or "lent to" (case-insensitive).

• Virtual Categorization: During monthly summary generations, all borrowed expenses are grouped and returned under a separate virtual category called "borrowed", allowing clean data visualization.

Related MCP server: MCP Agent - AI Expense Tracker

🛠️ Tech Stack

• Python 3.11+

• FastMCP: Modern tool-first MCP server framework.

• PostgreSQL (Neon-compatible): High-performance relational database.

• psycopg3: Modern, type-safe async driver for PostgreSQL.

• psycopg-pool: Connection pooler designed for high concurrent throughput.

• Pydantic: Robust runtime model typing and constraint validation.

• uv: Next-generation Python package management.

📁 Project Structure

expense-tracker/
│
├── app/
│   ├── config.py         # Loads database settings & pool sizing from environment
│   ├── databases.py      # Configures & lifecycle-manages psycopg connection pools
│   ├── schema.py         # Defines table queries, keys, indexes, and cascades
│   ├── models.py         # Contains Pydantic models for validation and type-hints
│   ├── resources.py      # Implements category loading and validation/normalization
│   │
│   ├── repository/       # Database query operations (zero business logic)
│   │      expense_repository.py
│   │      finance_repository.py
│   │      user_repository.py
│   │
│   └── services/         # Orchestrates business logic, validations, & orchestrates transactions
│          expense_service.py
│          finance_service.py
│          summary_service.py
│
├── category.json         # Source of truth for categories & subcategories
├── main.py               # Instantiates FastMCP, sets tool endpoints, & binds lifespan
├── pyproject.toml        # Declares project dependencies & tool mappings
├── uv.lock               # Deterministic dependency resolution lockfile
└── README.md             # Project documentation

📊 Database Schema Details

1. users

Represents the primary tenant identification.

CREATE TABLE users (
    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
    email TEXT UNIQUE NOT NULL,
    full_name TEXT DEFAULT '',
    created_at TIMESTAMPTZ DEFAULT NOW()
);

2. expenses

Stores transaction items linked to users.

CREATE TABLE expenses (
    id SERIAL PRIMARY KEY,
    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
    date DATE NOT NULL,
    amount NUMERIC(12,2) NOT NULL,
    category TEXT NOT NULL,
    subcategory TEXT DEFAULT '',
    note TEXT DEFAULT '',
    is_borrowed BOOLEAN DEFAULT FALSE
);

-- Optimize queries by user, dates, categories, and borrow status
CREATE INDEX idx_expense_user ON expenses(user_id);
CREATE INDEX idx_expense_user_date ON expenses(user_id, date);
CREATE INDEX idx_expense_user_category ON expenses(user_id, category);
CREATE INDEX idx_expense_user_borrowed ON expenses(user_id, is_borrowed);

3. monthly_finance

Stores budget limits and credit allocations.

CREATE TABLE monthly_finance (
    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
    month DATE NOT NULL,
    budget NUMERIC(12,2) DEFAULT 0,
    credit NUMERIC(12,2) DEFAULT 0,
    PRIMARY KEY (user_id, month)
);

CREATE INDEX idx_finance_user_month ON monthly_finance(user_id, month);

⚙️ Setup & Installation

1. Re-initialize Database (Breaking Schema Change)

If you had an old version of the database running, you must drop the old tables first:

DROP TABLE IF EXISTS expenses, monthly_finance, users CASCADE;

2. Setup Environment Variables

Create a .env file in the root directory:

DATABASE_URL=postgresql://neondb_owner:********@ep-example.us-east-1.aws.neon.tech/neondb?sslmode=require
POOL_MIN_SIZE=2
POOL_MAX_SIZE=10

3. Synchronize Dependencies

Install python packages using uv:

uv sync

🚀 Running the Server

Start the server using uv run:

uv run fastmcp run main.py:mcp --transport http --host 0.0.0.0 --port 8000

This launches a SSE/HTTP server listening at http://localhost:8000/mcp.

🛠️ MCP Tools Reference

Every tool automatically takes email: str as the first parameter to identify the caller and scope operations.

1. add_expense

Adds a new expense.

• Arguments:

  • email (str): Email address of the user.

  • date (str): Date formatted as "YYYY-MM-DD".

  • amount (float): Positive numeric value.

  • category (str): Key inside category.json.

  • subcategory (str): Sub-item matching category.

  • note (str): Optional description.

  • is_borrowed (bool, optional): Force flags the transaction as borrowed.

2. list_expenses

Fetch transactions for a given date range.

• Arguments:

  • email (str)

  • start_date (date)

  • end_date (date)

3. update_expense

Update a transaction (scopes updates to user owning the expense_id).

• Arguments:

  • email (str)

  • expense_id (int)

  • date (str)

  • amount (float)

  • category (str)

  • subcategory (str)

  • note (str)

  • is_borrowed (bool)

4. delete_expense

Permanently delete an expense.

• Arguments:

  • email (str)

  • expense_id (int)

5. recent_expenses

Get latest transactions.

• Arguments:

  • email (str)

  • limit (int, default=10)

6. set_monthly_budget / set_monthly_credit

Update financial targets for a specific month (composite PK handles conflicts).

• Arguments:

  • email (str)

  • month (str, format "YYYY-MM")

  • budget/credit (float)

7. summarize

Generates a comprehensive financial overview of the month.

• Arguments:

  • email (str)

  • start_date (date)

  • end_date (date)

  • category (str, optional)

• Example Output:

  {
    "status": "ok",
    "month": "2026-07",
    "budget": 50000.0,
    "credit": 70000.0,
    "total_expense": 23500.0,      // Excludes borrowed amounts
    "total_borrowed": 1500.0,      // Tracks borrowed separately
    "remaining_budget": 26500.0,
    "remaining_credit": 46500.0,
    "budget_used_percent": 47.0,
    "is_over_budget": false,
    "over_budget_amount": 0.0,
    "transactions": 38,
    "average_transaction": 618.42,
    "average_daily_spend": 758.06,
    "largest_category": "food",
    "largest_category_amount": 12000.0,
    "category_summary": [
      { "category": "food", "total_amount": 12000.0 },
      { "category": "transport", "total_amount": 8000.0 },
      { "category": "borrowed", "total_amount": 1500.0 }
    ]
  }

🤝 Integrating with Clients

Claude Desktop Configuration

Add the following block to your claude_desktop_config.json:

{
  "mcpServers": {
    "expense-tracker": {
      "command": "uv",
      "args": [
        "--directory",
        "/absolute/path/to/expense-tracker",
        "run",
        "fastmcp",
        "run",
        "main.py:mcp"
      ]
    }
  }
}

Note: Make sure to replace /absolute/path/to/expense-tracker with the actual path to your directory.