Power your own tax calculator

This is the lightest way to build on OpenAccountants: take only the data. If you're a neobank or invoicing tool building your own in-house tax calculator, you want accurate, cited inputs — local rates and allowances — and you'll do the computation yourself.

1. Fetch the rules for a jurisdiction

One anonymous request returns every published rule. Ask for JSON — it's the structured shape a calculator wants (each fact with its value, unit, quality tier, and citation), so there's no markdown to parse:

curl -s "https://www.openaccountants.com/api/bundle/US-CA?format=json"

No auth, no key. The {jurisdiction} segment takes an ISO code (US-CA, GB, MT) or a country name (malta). Drop ?format=json for the human markdown version. See the HTTP API reference for the full pack shape, headers, and caching.

Pin a version for reproducibility. So your calculator can't silently change mid-release, pin an immutable snapshot with ?version=2026.2 — a byte-frozen pack that never shifts. Discover the current version per jurisdiction from /api/catalog, and watch /api/changelog so you re-pin deliberately when a figure is corrected.

2. Parse what you need

Each rule is cited to its source, and its quality tier travels with it — Q1 (accountant-verified) or Q2 (source-cited draft). Pull the figures you care about (brackets, thresholds, allowances, deadlines) into your own model. A common pattern:

fetch pack  →  read each fact's value + tier + source link (+ review_status)
            →  store them with the pack's version as the "as of"
            →  re-pin when /api/changelog reports a bump (or refresh the ~1h rolling cache)

Surface the tier and the source link in your own UI where it matters — a user (or your compliance team) should be able to see whether a number is accountant-verified and where it came from.

3. Do the maths

The bundle gives you the inputs, not a computed answer. You apply them to the user's numbers in your own engine. That's the trade-off of the data-only path: maximum control, and you own the calculation.

When to reach for more

• Want the reasoning done for you — estimates, set-asides, deduction detection? That's the MCP intelligence tools.
• Want a credentialed human to review and sign before a user files? That's accountant handoff.

This guide is block one of the three composable building blocks on /for-platforms — take just the data, or combine it with the others.