The open source tax atlas.
Tax software doesn't travel — so this is a map, not a list. Where the open ecosystems run deep, where mandates called them into existence, and where the map is still blank.
Governments now ship open source
The IRS published the Tax Year 2024 Direct File code, now archived as a historical reference. Poland's Ministry of Finance maintains official KSeF tooling on GitHub. France's OpenFisca powers state-run simulators, and New Zealand piloted rules-as-code inside government. Authority publication is strong provenance evidence, but maintenance, security, support, and production fitness still require separate review.
The closed-API ceiling
The pattern that repeats in almost every jurisdiction: open source thrives at calculation, formats, and preparation — then stops at submission, because the filing channel is gated. Germany's ERiC library is closed, the Dutch Belastingdienst exposes no public filing API, Canada's NETFILE certification is closed, MTD requires recognised software. Plan for open source to carry you to the last mile, and treat the filing step as its own decision.