opentax-engine

A US tax calculator that shows its work — and can prove it.

Three ideas, that's the whole project:

1. Ask it a tax question with plain facts (--wages 50000).
2. It answers with a proof: every rule it applied, with the law it came from (26 U.S.C. § 63(c) …), every assumption it made.
3. Anyone can verify that proof offline. If it can't derive an answer from its encoded rules, it refuses — it never guesses.

Open source (AGPL-3.0, with commercial licenses available), exact to the cent (no floating point), built to be extended.

Try it

pnpm install && pnpm build
pnpm opentax eval --status mfj --wages 120000 --kids 2

us.federal.income_tax_after_credits  $5,640.00
├─ us.federal.income_tax_before_credits  $10,040.00   [26 U.S.C. § 1(j); Rev. Proc. 2025-32]
│  └─ us.federal.taxable_income  $87,800.00           [26 U.S.C. § 63(b)]
│     ├─ us.federal.agi  $120,000.00
│     │     ├─ fact wages = $120,000.00
│     │     └─ assumed taxableInterest = $0.00 (default)
│     └─ us.federal.standard_deduction  $32,200.00    [Rev. Proc. 2025-32 § 4.14]
└─ us.federal.ctc  $4,400.00                          [§ 24, as amended by OBBBA § 70104]

Assumptions (6): taxableInterest = $0.00, isAge65OrOlder = false, …
corpus sha256:6ff44719…

It answers under the law in force on the date you ask about — --as-of defaults to today; pass --as-of 2025-12-31 and the same facts derive under TY2025 rules instead (different standard deduction, different brackets, cited to the other Rev. Proc.).

Money is written in dollars (50000, "$50,000", "1234.56"). Not sure what inputs exist? pnpm opentax facts lists them all. More facts than fit in flags? Use a JSON file: --facts examples/mfj_120k_2kids.json.

Or skip the terminal entirely — the engine is pure TypeScript with zero platform dependencies, so the whole thing runs in a browser:

pnpm -F @invaro/opentax-playground build && open packages/playground/dist/index.html

One 130 KiB self-contained HTML file: engine + full rule corpus + verifier, no server, no network, works from file://. Compute, read the proof tree, download the proof — then paste it into the Verify tab and watch your own browser re-derive every step (alter one byte and it says exactly what broke). Same corpus Merkle root the CLI prints.

Save a proof, verify a proof: