Precisioncalc

A deterministic Model Context Protocol (MCP) server that gives LLM agents reliable, high-precision business, finance, and operational calculations.

LLMs routinely lose precision or hallucinate on multi-step financial formulas, currency conversions, business-day logic, and growth math. PrecisionCalc offloads that work to exact, transparent tools. Every monetary/financial value is computed with Python's decimal module (never floats), and every result is returned in a consistent, agent-parseable JSON envelope that includes the exact value, a human-readable value, the formula applied, the inputs used, the unit, and any assumptions/warnings.

v2 highlights: live + historical FX (ECB), 14 SaaS metrics, NPV/IRR, loan amortization, depreciation, a batch_calculate tool, per-country holidays, API-key auth + rate limiting + usage metering on the HTTP transport, structured JSON logging, optional OpenTelemetry tracing, and property-based tests.

🌐 Live hosted server (free, no install)

A public remote MCP server runs on Cloudflare's edge — point any Streamable-HTTP MCP client at it:

https://precisioncalc-mcp.pages.dev/mcp

Install via npm (stdio, no hosting)

Run the server locally over stdio with a single command — nothing to deploy:

npx -y precisioncalc-mcp

Claude Desktop / any stdio MCP client (claude_desktop_config.json):

{ "mcpServers": { "precisioncalc": { "command": "npx", "args": ["-y", "precisioncalc-mcp"] } } }

This is the same deterministic engine as the hosted server, running on your machine.

{ "mcpServers": { "precisioncalc": {
    "type": "http", "url": "https://precisioncalc-mcp.pages.dev/mcp" } } }

The edge build (worker-src/) is a Cloudflare Pages Function that mirrors the Python engine using decimal.js — verified 17/17 exact output parity. Landing page + docs: https://precisioncalc-mcp.pages.dev.

Plans (hosted endpoint)

Plan	Price	Daily calls	Live/historical FX	batch_calculate
Free (no key)	$0	15 / day (per IP)	❌ static only	❌
Starter	$12/mo	5,000 / day	✅	✅
Pro	$39/mo	50,000 / day	✅	✅

Checkout is Stripe (subscription). On success you get an API key instantly; send it as X-API-Key: <key> (or Authorization: Bearer <key>). Manage/cancel at /portal. When a limit is hit, tools return a structured status:"error" envelope with type, usage, and an upgrade block containing checkout URLs — so an agent can surface the paywall to the user and act on it. Self-host (below) for unlimited calls with your own keys.

Billing internals live in worker-src/billing.mjs (Stripe REST + Cloudflare KV for keys and daily counters). Server env: STRIPE_SECRET_KEY, STRIPE_WEBHOOK_SECRET, PRICE_STARTER, PRICE_PRO, FREE_DAILY, STARTER_DAILY, PRO_DAILY, and a PRECISIONCALC_KV namespace binding (see wrangler.toml).

Rebuild/redeploy the edge server:

npm install          # decimal.js + esbuild
npm run deploy       # bundles worker-src -> site/_worker.js and deploys to Pages

---

What it does

11 tools, all returning a uniform structured response:

Tool	Purpose
calculate_metric	14 SaaS/business metrics (LTV, CAC, churn, MRR growth, NRR, GRR, Rule of 40, magic number, break-even, ...)
currency_convert	Convert 9 major currencies; static (offline) or live/historical ECB rates
business_days	Add/count business days, next/previous; US/UK/EU + any ISO country + custom holidays
compound_growth	Future value, present value, CAGR; 7 compounding frequencies incl. continuous
net_present_value	NPV / discounted cash flow of a cashflow series
internal_rate_of_return	IRR (Newton + bisection fallback)
loan_amortization	Level-payment loan: payment, totals, full schedule, extra-payment payoff
depreciation	straight-line / declining-balance / sum-of-years-digits schedules
batch_calculate	Run many calculations in one request
list_metrics	Discovery: every metric with descriptions + required params
health_check	Server status, version, capabilities

Consistent response envelope

Success:

{
  "status": "success",
  "value": "1600",                       // exact, full-precision (string for money/rates)
  "formatted_value": "$1,600.00",        // human-readable
  "formula": "LTV = (ARPU * gross_margin) / churn_rate",
  "inputs_used": { "arpu": "100", "gross_margin": "0.8", "churn_rate": "0.05" },
  "unit": "USD",
  "notes": ["LTV = (ARPU x gross_margin) / churn_rate.", "..."]
}

Error (never raised across the tool boundary):

{
  "status": "error",
  "error": {
    "type": "missing_parameter",
    "message": "Missing required parameter 'churn_rate'.",
    "hint": "Include 'churn_rate' in params. See list_metrics for the full schema."
  }
}

---

Project structure

precisioncalc-mcp/
├── server.py                 # MCP server: tool definitions + transports
├── security.py               # API-key auth + token-bucket rate limit + metering (ASGI)
├── observability.py          # Structured JSON logging + optional OpenTelemetry
├── requirements.txt / pyproject.toml
├── Dockerfile / .dockerignore
├── fly.toml / render.yaml    # One-click hosting configs
├── .env.example
├── calculations/
│   ├── _util.py              # Decimal coercion, validation, formatting
│   ├── metrics.py            # 14 business/SaaS metrics + catalog
│   ├── currency.py           # FX: static + Frankfurter (live/historical) providers
│   ├── business_days.py      # Region-aware holidays (built-in + `holidays` lib)
│   ├── growth.py             # FV / PV / CAGR
│   └── finance.py            # NPV / IRR / loan amortization / depreciation
├── schemas/responses.py      # Response envelope helpers
├── examples/agent_example.py # End-to-end MCP client demo
├── site/                     # Static landing/docs page (Cloudflare Pages)
└── tests/                    # 49 unit tests + Hypothesis property tests

---

Requirements

• Python 3.11+ (developed/tested on 3.12)
• Core: mcp, python-dateutil
• Recommended: uvicorn + starlette (HTTP transport), holidays (per-country calendars)
• Optional: opentelemetry-sdk (tracing), pytest + hypothesis (tests)

The server auto-detects the SDK layout and works with mcp >= 2.0 (MCPServer), mcp 1.x (FastMCP), or the standalone fastmcp package.

---

Run it locally

cd precisioncalc-mcp
python -m venv .venv && source .venv/bin/activate
pip install -r requirements.txt          # or: pip install -e ".[all]"

# stdio transport (default; how MCP clients launch it)
python server.py            # or: precisioncalc-mcp   (console entrypoint)

# Streamable HTTP transport (endpoint: /mcp)
python server.py http
PRECISIONCALC_API_KEYS=key1,key2 PRECISIONCALC_FX_PROVIDER=frankfurter python server.py http

Demo + tests:

python examples/agent_example.py         # live end-to-end over stdio
python tests/test_calculations.py        # 28 core tests (no pytest needed)
python tests/test_v2.py                  # 17 v2 tests
python tests/test_properties.py          # Hypothesis property tests
# or simply:  pytest -q

Register with an MCP client (stdio)

{ "mcpServers": { "precisioncalc": {
    "command": "python", "args": ["/absolute/path/to/precisioncalc-mcp/server.py"] } } }

---

Deploy

Docker

docker build -t precisioncalc-mcp .
docker run --rm -p 8000:8000 -e PRECISIONCALC_API_KEYS=your-key precisioncalc-mcp
docker run --rm -i precisioncalc-mcp python server.py stdio

Fly.io

fly launch --no-deploy
fly secrets set PRECISIONCALC_API_KEYS=key1,key2
fly deploy

Render.com

Push to GitHub, then New + → Blueprint and point at the repo (render.yaml). Set PRECISIONCALC_API_KEYS as a secret in the dashboard.

---

Configuration (env vars)

Var	Default	Purpose
PRECISIONCALC_HOST / PRECISIONCALC_PORT	127.0.0.1 / 8000	HTTP bind
PRECISIONCALC_API_KEYS	(empty)	Comma-separated keys. Empty = open mode (still metered/limited by IP)
PRECISIONCALC_RATE_LIMIT_PER_MIN / _BURST	120 / 40	Token-bucket limits
PRECISIONCALC_METRICS_PATH	/metrics	Usage-metrics endpoint
PRECISIONCALC_FX_PROVIDER	static	static or frankfurter (live/historical ECB)
PRECISIONCALC_FX_TTL / _TIMEOUT	3600 / 4	FX cache TTL / HTTP timeout (s)
PRECISIONCALC_LOG_LEVEL / _LOG_JSON	INFO / 1	Logging
PRECISIONCALC_OTEL	0	1 enables OpenTelemetry tracing if SDK present

---

Tools & parameters

calculate_metric(metric, params, currency="USD")

Rates/margins are decimals (0.05 = 5%).

metric	params	unit
ltv	arpu, churn_rate, gross_margin(=1)	currency
cac	total_spend, new_customers	currency
ltv_cac_ratio	ltv, cac	ratio
payback_period_months	cac, monthly_revenue_per_customer, gross_margin(=1)	months
contribution_margin	revenue, variable_costs	currency
gross_margin	revenue, cogs	percent
churn_rate	customers_lost, customers_at_start	percent
mrr_growth_rate	beginning_mrr, ending_mrr	percent
arr	mrr	currency
break_even_units	fixed_costs, price_per_unit, variable_cost_per_unit	units
nrr	starting_mrr, expansion_mrr, contraction_mrr, churned_mrr	percent
grr	starting_mrr, contraction_mrr, churned_mrr	percent
rule_of_40	growth_rate, profit_margin	percent
magic_number	current_quarter_revenue, prior_quarter_revenue, prior_quarter_sm_spend	ratio

currency_convert(amount, from_currency, to_currency, date=None, live=None)

USD, EUR, GBP, JPY, CAD, AUD, CHF, CNY, INR. date (YYYY-MM-DD) or live=true uses live/historical ECB rates (frankfurter.app), with automatic static fallback on any network failure. Returns rate, provider, is_live, and timestamps.

business_days(operation, start_date, days=None, end_date=None, region="US", custom_holidays=None)

operation: add_business_days | count_business_days (inclusive) | next_business_day | previous_business_day. region: US | UK | EU | NONE, or any ISO country code when the holidays package is installed (DE, FR, CA, AU, JP, IN, ...).

compound_growth(operation, rate, years, present_value, future_value, begin_value, end_value, compounding="annually", currency="USD")

operation: future_value | present_value | cagr. compounding: daily | weekly | monthly | quarterly | semiannually | annually | continuous.

net_present_value(rate, cashflows, currency="USD")

NPV = Σ CFₜ/(1+rate)ᵗ. cashflows[0] = period 0 (usually the negative outlay).

internal_rate_of_return(cashflows, guess=0.1)

Per-period rate where NPV = 0. Requires a sign change in the cashflows.

loan_amortization(principal, annual_rate, term_months, extra_payment=0, currency="USD", include_schedule=false)

Returns monthly payment, months-to-payoff, total interest, total paid, and (optionally) the full month-by-month schedule.

depreciation(method, cost, salvage_value, useful_life_years, currency="USD")

method: straight_line | declining_balance | sum_of_years_digits. Returns the full yearly schedule; book value converges to salvage_value.

batch_calculate(calls)

calls: list of {"tool": <name>, "arguments": {...}} (max 100). One item failing never aborts the batch.

list_metrics() / health_check()

Discovery + status. No parameters.

---

Example MCP tool-call payloads

{ "name": "calculate_metric",
  "arguments": { "metric": "rule_of_40", "params": { "growth_rate": 0.30, "profit_margin": 0.15 } } }

{ "name": "currency_convert",
  "arguments": { "amount": 5000, "from_currency": "EUR", "to_currency": "GBP", "date": "2024-01-15" } }

{ "name": "net_present_value",
  "arguments": { "rate": 0.10, "cashflows": [-10000, 3000, 4200, 6800] } }

{ "name": "loan_amortization",
  "arguments": { "principal": 250000, "annual_rate": 0.065, "term_months": 360, "include_schedule": false } }

{ "name": "batch_calculate",
  "arguments": { "calls": [
    { "tool": "internal_rate_of_return", "arguments": { "cashflows": [-10000, 3000, 4200, 6800] } },
    { "tool": "depreciation", "arguments": { "method": "declining_balance", "cost": 50000, "salvage_value": 5000, "useful_life_years": 5 } }
  ] } }

---

Design decisions & assumptions

• Decimal everywhere money/rates matter; value is serialized as a string to prevent float loss in JSON, with a separate pretty formatted_value. Precision = 50 sig figs.
• Rates/margins are decimals (0.05 = 5%), documented in every tool.
• FX: static USD-based table (as_of 2024-06-01) is the offline default; frankfurter provider adds live + historical ECB rates with in-memory TTL cache and graceful static fallback.
• Business days: holidays computed per-year (floating US, Easter-based UK/EU); count is inclusive; add accepts negatives; custom holidays unioned; any ISO country via holidays lib.
• IRR uses Newton's method with a bracketed bisection fallback; requires a sign change.
• Errors never cross the tool boundary as exceptions — always status:"error" with a machine type + actionable hint.
• HTTP hardening is opt-in via env: API keys, token-bucket rate limiting, /metrics usage.
• SDK compatibility shim runs on mcp>=2.0, mcp 1.x, or standalone fastmcp unchanged.

---

Monetization hooks

• Auth — PRECISIONCALC_API_KEYS; requests need X-API-Key or Authorization: Bearer.
• Rate limiting — per-key token bucket (per-IP in open mode); swap for Redis to scale.
• Usage metering — in-memory counters exposed at /metrics; the seam for per-key billing.
• FX provider — calculations/currency.py::RateProvider is the drop-in point for a licensed feed.

---

Roadmap (post-v2)

1. Redis-backed rate limiting + billing-grade usage metering.
2. Persisted historical FX + more providers; multi-currency carry through metrics.
3. Bond pricing/yield, WACC, options (Black-Scholes), tax/VAT, unit conversions.
4. Prometheus exporter + Grafana dashboard alongside OTel traces.
5. Published PyPI package + Docker image on GHCR; hosted multi-tenant SaaS.