Tax engines
Rules-as-code that compute liabilities, returns and reforms.
Open source tax technologyProjects · Map · Learn · Build
Tax is public law. Its software should be too.
Find the open tax projects that exist, see where they fit in a company's tax department and a tax authority, learn how they work with AI, and build what is missing. No licence fees. No black boxes. No lock-in.
01 — Projects
Everything a tax function runs on. Mapped in the open.
From the engine that computes a liability to the agent skill that explains it — every layer of modern tax technology, with the open alternatives listed, compared and maintained by the people who use them.
01 · 39 projects
02 · 3 projects
Agent skills
Packaged tax expertise an AI agent can follow, step by cited step.
03 · 13 projects
MCP
Open connectors and protocols between agents and legislation, case law and ledgers.
04 · 1 project
Plugins
Bundles of skills, tools and agents, installable in one go.
05 · 54 projects
APIs
Validation, rates, filings and registries you can call directly.
06 · 1 project
Tax calendars
Filing and payment deadlines as open, machine-readable feeds.
07 · 21 projects
Automations
Reusable workflows for provisions, reconciliations and compliance runs.
08 · 15 projects
Tax datasets
Rates, treaties and statistics, open and versioned.
09 · 5 projects
Legal datasets
Statute, case law and guidance your agents can cite.
10 · 3 projects
Benchmarks
Evals that show whether a model reasons about tax, or guesses.
11 · 3 projects
All kindsModels
Open AI models trained or tuned for tax text.
All 158 projects
Search them by name, tax, country or purpose.
01 — Projects · Featured
10 to start with.
A first cut of the projects already out there. Some are run by governments, some by researchers, some by a single stubborn maintainer. The open slot is an invitation.
ProjectTypeJurisdictionWhat it does
OpenFiscaEngineFR · GlobalRules-as-code framework: legislation written in Python, served as a web API.PolicyEngineEngineUS · UKOpen tax-benefit models with a public app for scoring policy reforms.IRS Direct FileEngineUSShut down by the IRS for 2026, but its code, Fact Graph included, is public. Forks live on.EUROMODEngineEU-27The EU tax-benefit microsimulation model, maintained by the Joint Research Centre.Tax-CalculatorEngineUSFederal income and payroll tax microsimulation from the Policy Simulation Library.python-stdnumAPIGlobalParse and validate VAT, TIN and company numbers for dozens of countries.Model Context ProtocolMCPGlobalThe open standard for connecting AI agents to tools, data and sources.EUR-LexLegal dataEUEU legislation and case law, with machine-readable open data.CourtListenerLegal dataUSFree Law Project archive of opinions and dockets, with a public API.SARADatasetUSStatutory reasoning benchmark built on the US Internal Revenue Code.[Open slot]SkillAnyReverse-charge checker, treaty WHT lookup, Pillar Two data validator — publish the first.02 — The map
Where open source already does tax work. And where it doesn't yet.
A company's tax department and a tax authority, drawn team by team: 19 teams and 35 activities. Each team shows what is open for its activities, and where the gaps are.
Company tax department
- Direct & International Tax5 projects
- Tax Accounting & Reporting0 projects
- Transfer Pricing0 projects
- Indirect Tax / VAT / GST68 projects
- Controversy & Tax Risk11 projects
- M&A / Transactions / Planning0 projects
- Employment & Payroll Tax4 projects
- Tax Technology, Data & Transformation32 projects
- Tax Operations / PMO0 projects
- Tax Policy & Stakeholder Engagement11 projects
Tax authority
- Policy, Legislation & Revenue Analysis47 projects
- Registration1 project
- Taxpayer Services11 projects
- Returns, Payments & Debt0 projects
- E-invoicing & Real-time Reporting4 projects
- Compliance Risk & Audit6 projects
- Disputes & Appeals2 projects
- International Cooperation0 projects
- Data, IT & Analytics12 projects
03 — Learn
Written for fiscalists, not engineers.
Your judgement is the scarce part. Learn how open engines, agents and protocols work, so you can make that judgement run at scale — without handing it to a vendor.
- 01
Rules as code
Read how OpenFisca and Direct File turn statute into logic you can test, diff and review.
- 02
Agents and skills
Package your expertise as an agent skill: the steps, the sources, the checks — in plain language.
- 03
MCP and APIs
Connect agents to legislation, case law and your own ledgers through open protocols.
- 04
Evidence and review
Benchmarks, source citation, and the four-eyes review no model replaces.
--- name: eu-vat-reverse-charge description: Checks whether a cross-border B2B service falls under the EU reverse charge, and cites the provision it relies on. --- # Steps 1. Confirm supplier and customer are both taxable persons. 2. Validate the customer's VAT number via VIES. 3. Apply the general B2B place-of-supply rule — Art. 44, Directive 2006/112/EC 4. If it applies, the customer accounts for VAT — Art. 196 5. Cite every article. Flag exceptions for human review.
04 — Build
15 activities with no open project. Pick one and build it.
Three to start with. Each runs on published rules or a standard schema. On the Build page, every gap says what to build on, what a first version could do, and which track on Learn helps.
Company tax departmentDirect & International TaxPillar Two / global minimum taxA first versionCompute the effective tax rate and top-up tax for one jurisdiction from a handful of inputs, with tests built from the examples in the Commentary.Company tax departmentIndirect Tax / VAT / GSTCustoms & trade taxA first versionAn agent skill that proposes a tariff code for a product description, cites the heading and notes it used, and hands off to a person.Tax authorityInternational CooperationExchange of information & MAPA first versionA validator that checks a CRS or CbC file against the OECD schema and user guide, and lists every error in plain words.
Build for a gapA tax system runs on trust, and trust needs checking.
Read the manifesto