Every gap below comes straight from the map: an activity of a company's tax department or a tax authority that has no open code yet. Each runs on published rules or a standard schema. That makes them buildable: the rules are public, even where they are complex. Each gap says what to build on, what a first version could do, and which track on Learn helps. Below the gaps, the partly covered activities say what is open, what is still missing, and which projects to start from.
A model over provision and payment data. Depends on the provision gap being filled first.
A draft: not yet reviewed by a tax specialist.
See it on the mapHMRC's Senior Accounting Officer service is open: it files the certificate, not the controls behind it. Controls, owners, evidence and test results: fully specifiable, no open implementation yet.
A draft: not yet reviewed by a tax specialist.
See it on the mapThe rules-as-code frameworks on the list could power it. No open tool compares corporate structures yet.
A draft: not yet reviewed by a tax specialist.
See it on the map23 activities have open projects for part of the work. Each read says what is open and what is still missing; "Start with" names the projects to read first. A project that fills the rest belongs on the list too.
One French corporate model, a US tax engine, filing tools for Norway, the UK and German holding companies, an Italian incentive calculator and withholding tools for Bangladesh, Israel and Tunisia exist. The frameworks to encode any country’s rules are mature; the corporate models are not written.
Start withWenchect600-filingBoth file a real corporate return today, one country each; the frameworks (Catala, OpenFisca) are where a new country's rules would go.
A draft: not yet reviewed by a tax specialist.
See it on the mapA CC0 dataset of US dividend withholding rates, Taiwan's treaty list and rates as open data, an Irish withholding decision tool and an Indonesian treaty calculator are open; the treaty agents and a permanent establishment study tool are prototypes. Treaty logic, structure comparison and a treaty database with its texts: nothing open.
Start withus-dividend-whtTax Treaty Overview (Taiwan Ministry of Finance)Two open datasets of withholding rates, one of them a tax authority's. The treaty agents are prototypes to read.
A draft: not yet reviewed by a tax specialist.
See it on the mapFour open tools for companies, each a one-person project: a local GloBE calculator with tests from the OECD examples, a GIR XML preparer with French helpers, a GIR engine that applies the 2026 corrections, a projection for Hong Kong, Singapore and Japan groups; plus a generated Python client for HMRC's Pillar 2 submission API. Published rules and a standard GIR schema make the rest specifiable: the full rules, safe harbours in detail, local notifications.
Start withpillar-two-toolopen-globexmlThe calculator carries tests built from the OECD's own examples; the GIR preparer checks the XML against the schema. Both are one-person projects: read them before relying on them.
A draft: not yet reviewed by a tax specialist.
See it on the mapAgent skills and a deterministic harness compute a provision under IAS 12 and ASC 740, three single-author workpapers model one, and a dataset tracks how US filers apply ASC 740. A provision engine a group would close on, the ETR bridge, the tax note and uncertain tax positions: nothing open.
Start withfinanceskills: deferred tax skillaccounting-harnessA deferred tax skill with a calculator and evaluations, and a harness with tests and an open licence. The workpapers are one-person prototypes to read, not to close on.
A draft: not yet reviewed by a tax specialist.
See it on the mapTwo open tools check or build the CbC XML file: a validator with Luxembourg's rules and the safe harbour tests, and a builder for South Africa's SARS declaration. Producing the report, the GRI 207 tables and the EU public CbCR table from entity data: nothing open.
Start withCBCR-TPCountryByCountrySolution (SARS)Both are one-person tools for the XML file: the validator to check one, the SARS builder to make one. Nothing yet produces the report from entity data.
A draft: not yet reviewed by a tax specialist.
See it on the mapAgent skills hold the OECD methods and documentation checklists for about ten countries; small tools cover pieces: a Guidelines search, a Local File outline, segmented P&L scripts, a Rust library with the arm's length range, a CbC XML validator; a survey of German firms on transfer-pricing practice is open data. Most are one-person projects, many without a licence, none with a community. Still open: margin monitoring against the group's policy, year-end true-ups, benchmarking with data.
Start withcorp-finance-coreAlgo-Trading-Skills: transfer pricing for trading groupsBoth compute something and ship tests under an open licence: the Rust library the arm's length range, the skill's Python engine intercompany fees. The country skills are text to read, not code to run.
A draft: not yet reviewed by a tax specialist.
See it on the mapThe UK's Online Trade Tariff and HMRC's declaration, transit, ruling and EORI services are open, ERP modules compute duties and Intrastat, MCP servers and data mirrors cover the US and EU schedules, and a dozen classification assistants exist, most of them prototypes. Customs valuation, origin determination and a classification engine a broker would rely on: nothing open yet.
Start withUK Online Trade TariffOCA intrastat-extrastatAn authority's own tariff service with an API, and an ERP module in use for HS codes and Intrastat: the two solid pieces. The classification assistants are prototypes to read, not to rely on.
A draft: not yet reviewed by a tax specialist.
See it on the mapDocument intelligence and four open tax law corpora (AU, BR, FR, IN) help with evidence and analysis. Case management and exposure tracking: nothing.
Start withBOFiP datasetCourtListenerFrench guidance as a dataset to put behind an agent, and US case law with a public API; the document tools sit on the Audit activity.
A draft: not yet reviewed by a tax specialist.
See it on the mapOne Chinese decision-support tool scores tax risk against Golden Tax IV rules; a payments governance toolkit and an uncertain tax position classifier are prototypes. Risk registers and a tax control framework as data: nothing open.
Start withtax-mindThe only maintained piece, for China. Read its rules to see what a risk model holds before writing your own.
A draft: not yet reviewed by a tax specialist.
See it on the mapOne engine computes US loss limitations after an ownership change (sections 382 and 383), without a README or a licence. Tax due diligence, SPA clause libraries and restructuring models: nothing open.
Start withsection382The only piece. Read its tests to see what it covers; its licence is unknown, so ask before reusing it.
A draft: not yet reviewed by a tax specialist.
See it on the mapSome payroll-tax logic inside US microsimulation, and the Norwegian Tax Administration's withholding-table routine. No payroll-tax engine for a multinational.
Start withTax-CalculatorThe US microsimulation covers payroll taxes as part of federal tax; nothing on the list runs a payroll.
A draft: not yet reviewed by a tax specialist.
See it on the mapScoring a reform is a solved problem in the open engines, and two nonprofit datasets size tax havens and unitary taxation. Monitoring legislation across jurisdictions is not.
Start withPolicyEngine USOpenFisca FranceModel a reform and see who gains: PolicyEngine for the US and UK, OpenFisca for France.
A draft: not yet reviewed by a tax specialist.
See it on the mapOpen tools prepare survey and tax microdata for the models, and research code and data estimate profit shifting and its revenue cost: the Atlas of the Offshore World series and its Stata replication, unitary taxation, trade misinvoicing, a licensing study and the Tax Justice Network's portal (non-commercial). Open tax-gap estimation and revenue forecasting on an administration's own data: none in the list.
Start withPSL taxdataCPS Tax Filing UnitsBoth build the microdata a tax model needs; the CBO's code shows how a public body does it.
A draft: not yet reviewed by a tax specialist.
See it on the mapAn open library validates tax identifiers for dozens of countries. The register itself has no open implementation in the list.
Start withpython-stdnumOne library validates identifiers for dozens of countries; the rest of the activity is a gap.
A draft: not yet reviewed by a tax specialist.
See it on the mapCommunity tools answer questions from official guidance in France, the UK and the US, an Australian guidance dataset is structured for agents, HMRC's take-home pay library and the IRS's withholding estimator, EITC assistant and the form builder behind them are open, and the UK Online Trade Tariff, a guidance service run by an authority, is public with its API and its Advance Tariff Rulings service. A guidance service for direct taxes run by an authority: none in the list.
Start withtax-kalculatorBOFiP datasetAn authority's own calculation library, and official French guidance as a dataset an agent can cite.
A draft: not yet reviewed by a tax specialist.
See it on the mapThe code of the IRS's Direct File and of HMRC's filing services (VAT, Pillar Two, Digital Services Tax, Income Tax, country-by-country reporting, CRS/FATCA, customs declarations and transit) is public, Norway's and Hungary's tax administrations publish their filing APIs, validators and samples, Denmark's its VAT sample clients, and community SDKs wrap the German and Norwegian filing APIs. Pre-filling engines: none in the list.
Start withHMRC VAT APIeric-rsAn authority's own filing API, and a community SDK for Germany's ELSTER library: the two ways filing code becomes public.
A draft: not yet reviewed by a tax specialist.
See it on the mapThe IRS's Fact Graph, the DGFiP's M-language rules with the Mlang compiler, the Belastingdienst's ALEF rules tool and HMRC's Self Assessment calculation APIs are open assessment engines; Norway's and Hungary's return validation code and the IDB's CACAO receive and check filings. A processing pipeline another authority could adopt whole: nothing open.
Start withIRS Fact GraphDGFiP calculette-irTwo authorities' own calculation cores: a rules engine for the US code, and the rules of the French engine, with a compiler to run them.
A draft: not yet reviewed by a tax specialist.
See it on the mapHMRC's penalties, Time to Pay, payments, repayment-tracker and refund services and its Self Assessment accounts and financial-transactions APIs are open: one authority's whole payments stack, tied to its platform. A taxpayer ledger or an interest and penalty engine another authority could reuse: nothing open.
Start withHMRC Self Assessment accounts APIHMRC penalties serviceThe account API shows what a taxpayer ledger holds; the penalties service shows the rules applied to it.
A draft: not yet reviewed by a tax specialist.
See it on the mapNo tax authority publishes its clearance server. Open Peppol access points (phase4, phoss AP, Oxalis-NG), the phoss SMP and the Commission's Domibus are the open receiving infrastructure, and Hungary's NAV publishes the interfaces of its reporting platforms. Two community clearance engines are pilots.
Start withphase4NAV Online InvoiceThe AS4 library most access points build on, and a live reporting platform's published interface.
A draft: not yet reviewed by a tax specialist.
See it on the mapOne open research framework ranks US non-profit filings for human review, HMRC Assist applies the authority's risk messages to a return before it is filed, and a small pipeline flags price outliers in trade statistics. No general case-selection tool is in the list.
Start withtaxguard-xaiHMRC Assist (Self Assessment)A research framework with explainable signals, and an authority's own pre-filing risk messages: the two open pieces of a case-selection tool.
A draft: not yet reviewed by a tax specialist.
See it on the mapOpen document AI helps read evidence, Norway's SAF-T schemas are published and the IDB's CACAO receives and analyses accounting data. Audit case management: none in the list.
Start withpaperless-ngxdocTRStore and search the evidence with the document system; read it with the OCR library.
A draft: not yet reviewed by a tax specialist.
See it on the mapOpen case-law and legislation archives help with research, and HMRC's penalty appeals frontends are open. Dispute case management: none in the list.
Start withCourtListenerEUR-LexCase law with APIs for the US and the EU; nothing yet manages an appeal.
A draft: not yet reviewed by a tax specialist.
See it on the mapOpen tools build and lint CRS, FATCA and DAC7 files, HMRC's CRS/FATCA, mandatory disclosure and digital platform reporting services are public, and the IRS's IDES samples are archived but readable. The exchange platforms themselves, request handling and MAP case tooling: nothing open.
Start withcrs-lintHMRC CRS/FATCA reporting serviceA linter that applies the OECD status-message rules offline, and an authority's own reporting service: read these before the older converters.
A draft: not yet reviewed by a tax specialist.
See it on the mapEvery project on the list is public and about tax, and anything to check first is marked. Tell us about yours, or one you know. We review it before it goes on the list and the map.
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