You don't need to write code to make OpenTax better. The most useful contributions come from knowing tax. An AI assistant writes the code. You bring the judgment.
No coding requiredFreeStart in 5 minutes
Five ways in
From five minutes to an afternoon
Start at the top. Each step builds on the one before, and every one of them helps. Steps 3 and 4 need a one-time setup, covered below.
1
Run a return through it
5 minutes
The quickest way to understand the engine is to watch it work. Open a new chat in Claude and paste this:
Fetch and follow the OpenTax skill: https://raw.githubusercontent.com/filedcom/opentax/main/skills/opentax/SKILL.md
Claude sets everything up, then asks whether you want to prepare a return or review one. Give it a made-up W-2 and 1099 and watch it work through the 1040. Or give it a finished practice return and its source documents and it will check the return line by line.
Prefer a permanent setup? Add the OpenTax connector to Claude or ChatGPT once and it's there in every chat.
Client data
Practice with a made-up return. Before putting real client data into any hosted AI tool, check it against your firm's section 7216 policy. The command-line version runs entirely on your own machine.
2
Tell us where it's wrong
10 minutes
This is the single most valuable thing a tax professional can do. If OpenTax gets a number wrong, open an issue on GitHub (you'll need a free account). A good report has four parts:
Tax year and filing status:
What went in (made-up figures only):
What OpenTax produced (form, line, amount):
What it should be, and the IRS source (form instructions, publication, line):
The source matters most. "Form 8995 instructions, line 3" is what lets someone fix it quickly. You don't need to know why the engine got it wrong, only what the right answer is.
3
Turn a tricky scenario into a test case
30 minutes · needs setup
A test case is a made-up return: invented names and numbers that recreate a situation you know how to handle. It is never a client's actual return, and nothing about a real taxpayer goes into it.
OpenTax is checked against a public set of 133 of these fictional returns, each with values from IRS materials. A scenario only counts as passing when total tax, refund and amount owed all land within $5 of the right answer. Every case you add makes the engine harder to break.
The best candidates are the tricky situations you know cold: an education credit phase-out, a K-1 with a passive loss, a senior with Social Security and a pension. Describe the situation and Claude Code invents the taxpayer. Inside the project, type:
/tax-cases "MFJ, two W-2s, one child in college claiming the AOTC, 1098-T with scholarship"
You can also use a worked example from an IRS publication or VITA training exercise. Then do the part only you can do: check the correct values against your own calculation. When they match, ask Claude Code to open a pull request.
Made-up returns only
Never use a real client's return, even with the names removed. Test cases are public, and real return information is protected by section 7216. Build the scenario from scratch instead.
4
Fix a rule or add a form
An afternoon · needs setup
The project comes with tools that let Claude Code do the engineering. Start by asking where things stand:
/tax-status
Then either let it work through failing cases on its own with /tax-fix f1040:2025, or describe the rule in plain English. For example:
The engine isn't limiting the student loan interest deduction for MFJ filers above the phase-out range. Per the Schedule 1 line 21 worksheet in the 2025 Form 1040 instructions, it should phase out between $170,000 and $200,000 of modified AGI. Find where this is calculated, fix it, add a benchmark case that proves it, and run the tests.
You check the numbers. The maintainers check the code. Every change is tested and reviewed before it's merged, so you can't break the engine by trying.
5
Build your own tools on top
Whenever you're ready
The engine is a foundation for your own tools: a review checklist in your firm's style, a planning what-if tool, an intake step that feeds your existing software. The tax-reviewer skill is a good starting point:
Using the OpenTax tax-reviewer skill as a base, write a skill for our firm that also checks: estimated payments against last year's safe harbor, missing 1099-Bs when the prior year had them, and HSA contributions over the limit.
If you build something other firms could use, consider contributing it back. See the licence in plain English before you share or sell anything built on the engine.
One-time setup
Set up for steps 3 and 4
About fifteen minutes, once. After that, everything happens in plain-English conversation with Claude Code.
Create a free GitHub account at github.com/signup. This is where the code lives and where your changes get reviewed.
Install Claude Code. It's included with paid Claude plans and runs in the Claude desktop app or your terminal. claude.ai/code
Make your own copy of the project. Open github.com/filedcom/opentax and click Fork. That gives you a copy you can change freely.
Let Claude Code do the rest. Start it and paste this, with your GitHub username:
Clone github.com/YOUR-USERNAME/opentax into a folder called opentax, install Deno and anything else it needs, and run the tests to confirm it works.
Open Claude Code in that opentax folder and type /tax-status. If you see a table of forms and pass rates, you're ready.
Ground rules
Five things to keep in mind
Never use a real client's return
Not even with the names removed. Issues, test cases and pull requests are public. Every name, SSN and figure should be made up.
Always cite the source
Form instructions, publication and line. It's what makes a change reviewable.
One change at a time
A small, focused change gets reviewed and merged far faster than a big one.
You own the numbers, not the code
Your job is to confirm the answer is right. The maintainers review the code itself.
It calculates. It doesn't file.
OpenTax isn't IRS approved and doesn't submit returns. The preparer who signs a return is responsible for it, as with any software.
The licence in plain English
What you can do with it
Free to
Use it, study it, run it on your own machines and change it for your own work, at no cost, permanently.
If you share it
If you give a modified version to others, or let other people use it over a network, the AGPL v3 licence requires you to publish your source under the same licence.
Other options
A commercial licence is available if the AGPL doesn't suit you. Anyone planning to build an IRS e-file transmitter on the code needs written permission from Filed first.
For steps 1 and 2, no technical skills at all. For steps 3 and 4 you'll type instructions to Claude Code in plain English and it does the programming. Being comfortable installing an app is enough.
What if my change is wrong?
It won't go live. Every change is tested against the benchmark and reviewed by a maintainer before it's merged. A wrong change gets a comment, not a mistake in the engine.
Will I be credited?
Yes. Every contributor appears on the GitHub contributors page and by username in the release notes for the version their change ships in.
Can my firm use it with client data?
The command-line version runs on your own hardware and sends no return data anywhere. The only network call it makes is a daily check for new versions. Using a hosted AI assistant or the hosted connector with client data is a decision for your firm's section 7216 policy.
Which returns does it cover?
The federal Form 1040 for tax year 2025, with its schedules and common supporting forms. No state returns yet. Other return types are welcome contributions, and /tax-build is built for exactly that.
Who reviews what I send?
The OpenTax maintainers at Filed, as part of the Open Tax Technology Alliance with Crimson Tree Software. Questions that aren't bug reports can go in a GitHub issue too.
Start with a made-up return
Invent a return around a situation you know well and run it through the engine. If anything's off, tell us.