Tax is public law. Its software should be too.

The statute is public. The directive is public. The rulings, the guidance, the treaty texts: public. The only closed part of the chain is the software that turns them into a number on a return. That is an accident of history, not a requirement of the work.

When the code that applies a rule can be read, it can be checked against the article it claims to implement. When it cannot, you are trusting a vendor's reading of the law, and paying for the trust.

A rule written as code has a history. Every change is dated, attributed, and sits next to the reason for it. A threshold moves in the budget; the diff shows the line that moved, the test that proves it, and the person who signed it.

This is how software has been reviewed for decades. It is also, not by coincidence, how a Tax Control Framework wants a tax position to be reviewed. Open rules give a tax function the audit trail it is already required to keep, for free, at the source.

A tax adviser's expertise is the product. The tool that packages it, whether a spreadsheet, a rule engine or an agent skill, should compound in the adviser's hands, not accrue to a licence holder. If you wrote the reverse-charge check, you should be able to read it, change it, run it anywhere, and give it to a colleague without asking permission.

Open source is the only licensing model under which that is simply true. Everything else is a lease.

Agents will do more of the work: research, extraction, reconciliation, monitoring, drafting. We welcome that, on one condition. Every answer cites the statute, the ruling or the dataset it rests on, and every action is logged so a person can retrace it. No citation, no answer.

And the person stays. No agent selects a material tax position, settles an audit or signs a return. Autonomy is for the bounded, reversible work. Accountability is for humans, with names.