That's neat, what aspect(s) are you formalizing? That a given tax statement is correct?

Not quite. The formalization covers the IRS's published procedure, i.e. the questions and calculations that go into the published forms, worksheets, and instructions. It's not a formalization of the tax code itself, and it's not a tax calculator, though a client of the underlying code could function as a tax calculator. (The project includes such a client, a CLI tax interviewer, just to exercise the code.)

In more detail:

- every line of every form gets a YAML representation which includes the source text, a data type, and an expression built from a small set of lean atoms, plus a citation pointing to the exact location of the rule in the original official document.

- underneath the yaml layer there's a lean formalization of the atoms showing that they behave as specified, that all comuptations use only the allowed atoms and operations, etc. There's also a typescript evaluator that's used to cross check the lean for agreement, so we have the arithmetic represented two ways.

The critical failure point for an LLM-produced formalization like this is the interpretation of the form text. Since I'm not a tax lawyer and don't intend to become one, this is an explicitly owned weakness and one I've structured the project around. Publicly available tax calculators (PolicyEngine, UsTaxes, OpenTaxSolver, etc.) are used to cross check derived numbers from this system. Mutation testing ensures the test suite is robust against bugs and arithmetic errors. Interpretations are cross checked by different model families (Claude, GPT, Deepseek). Etc.

The idea is to derive confidence from the formal properties of the system itself, and to allow theoretical future users of such a system to derive confidence in the interpretation of specific forms by making it easy to cross check line-level formalizations against the source documents.

There's a lot more detail in the (alas, Claude-authored) README. When I have time I'm going to write my own version of the README expressing my vision of the project, but I'm not there yet.

Nice. Is the lean used to formalize formulas?

Sort of. The formulas for each line live in the form YAML I described above. The formulas are built out of a set of primitive operations that are defined in lean, with their properties proved there (non-negativity, monotonicity, etc.). There's also a lean evaluator that's run on compiled form data and cross checked with the typescript evaluator. (There's a lot more detail in the README.)

You are one sick puppy. This is a brilliant idea.

That's very cool but don't you think the Big AI labs will soon have collected enough data from us that they can easily do this?

I think that assuming work can be done more easily in the future should not stop people from doing work that needs to be done in the present.

(If a painter knew that the camera was coming out I would not want them to throw their canvas away)

> (If a painter knew that the camera was coming out I would not want them to throw their canvas away)

That's not a fair comparison because the painter produces different work, whereas tax computations will be the same.

[citation needed]

I'm not sure it matters. The IRS killed Direct File under lobbying pressure from the tax prep industry. In my opinion the public is owed a free version of this sort of tool. If the IRS doesn't produce it, then citizens ought to. If Google or Anthropic or whoever wants to offer a free version of this, more power to them, but I don't see why they'd bother. They could have done it decades ago.

And to be clear: my vision here isn't "have an agent do your taxes", it's to have agents build a hardened and thoroughly verified deterministic tool that can help you navigate the (somewhat bewildering) forms and instructions decision tree when filing your own taxes.

This is great, and I appreciate the approach. Yes, tax calculation should be a decision tree. Unfortunately it is truly pretty complicated (just this year I had to abandon FreeTaxUSA for TurboTax because the latter handled an important case I needed better). But I have a lot of faith that we can do this right. Good luck!

I have a probably stupid question: given that the IRS killed Direct File, even if your software can create ready-to-file (or even ones that need some work) tax returns, isn't that moot without a way to like... get the IRS to accept them?