An Open Tax Technology Alliance project
Open source 1040 tax calculation engine for AI agents and developers. Runs on your own hardware, with no account and no cloud dependency. For filing your taxes, visit irs.gov.
Built and maintained by Filed
Open Tax Technology Alliance
Filed and Crimson Tree Software founded the Alliance and co-develop both projects. Neither will be monetised.
OpenTax
A complete federal 1040 calculation engine, free under AGPL v3. One binary, 186 registered nodes and 131 input types, covering W-2s, the 1099 series, K-1s and Schedules A, C and E. It checks returns against the IRS MeF business rule set and exports Modernized e-File XML.
Open Tax Document
A free YAML standard for creating and reading Schedule K-1 data, published as a public draft. Readable and implementable by anyone. There is no licence to acquire and no vendor to go through.
OpenTax has an application in with the IRS Modernized e-File system. On acceptance it would be the first open-source route into federal e-filing, and the Alliance intends to run it as a free one, open to individuals, firms, developers and AI agents.
Your AI does the thinking. OpenTax does the calculations.
Install as a plugin
Keeps the skills always available, including /opentax:tax-preparer and /opentax:tax-reviewer.
Example 1
Calculate my 2025 taxes — drop your W-2s and 1099s, Claude handles the rest.
Skill loaded. Following it now.
Step 1: Checking for the CLI
Not installed. Installing now:
opentax 1.0.2 installed and ready.
Please upload some source docs and let's talk tax.
Calculate my taxes for 2025. Here are my W2 and 1099MISC
The documents came through as blank templates — the actual values (wages, withholding, SSN, employer info, etc.) aren't filled in. Let me read the raw files directly.
These are form PDFs — let me extract the form field data directly.
Got all the data. Now let me fetch the Tax Preparer skill and run the CLI.
Good. I have everything I need. Let me parse the docs and proceed through the phases.
Extracted from documents:
Before I proceed to compute, I need a few more things per the skill:
1. Single, Age - 30, Dependents - No, no other docs
Got it. Here's the full picture before I compute:
Dante Hicks — 2025 Federal Tax Return
| Filing status | Single |
| Age | 30 |
| Dependents | None |
Income:
No additional deductions or credits noted.
Does this look complete and correct?
yes
Clean result. Here's Dante's full return:
Form 1040 — Dante Hicks — 2025
| Line | Description | Amount |
|---|---|---|
| 1a | Wages (Quick Stop W-2) | $72,000 |
| 8 | Additional income (rental, Sch. E) | $24,000 |
| 9 | Total income | $96,000 |
| 11 | Adjusted gross income | $96,000 |
| 12a | Standard deduction (single) | $15,750 |
| 15 | Taxable income | $80,250 |
| 16 | Income tax | $12,569 |
| 24 | Total tax | $12,569 |
| 25a | W-2 federal withholding | $7,454 |
| 33 | Total payments | $7,454 |
| 37 | Amount owed | $5,114.96 |
Plain summary:
Dante owes $5,114.96 by April 15, 2026. His W-2 withholding covered the wage income reasonably well, but the $24,000 in rental income had zero withholding applied — that's the main driver of the balance due.
Worth noting: Since Dante has rental income with no withholding, he may be subject to an underpayment penalty (Form 2210). To avoid this next year, he should consider making quarterly estimated tax payments (Form 1040-ES) on the rental income.
Would you like me to:
Claude is AI and can make mistakes. Please double-check responses.
Example 2
Already have a draft return? Drop your source docs and the finished 1040 — Claude audits it line by line and flags what's wrong.
Step 3: What would you like to do?
OpenTax is ready. What would you like to do?
1. Prepare a return — I'll walk you through it step by step. Drop in your W-2s, 1099s, and other tax documents (photos, PDFs, or just type the values). I'll ask questions until I have everything, confirm the details with you, then compute your full 1040 with all schedules.
2. Review a completed return — Already have a return prepared by someone else? Upload it along with your source documents and I'll independently compute everything, compare line-by-line, and flag any discrepancies — missed income, incorrect deductions, unclaimed credits, or compliance risks.
Now fetching the Tax Reviewer skill before diving in.
Good. Now let me work through the review systematically. I have all the source documents and the draft return. Let me extract and compute everything.
Based on my independent read of all source documents vs. the draft return, here's a phase-by-phase audit:
Phase 1 — Source Document Extraction ✅
Oliver K. Filed | SSN: 576-54-3211 | Single | DOB 1/1/1986 | Denton, MD 21629
| Source | Key Values |
|---|---|
| W-2 (False Landscape Supply) | $102,000 wages, $8,786 fed withheld, $5,280 state withheld |
| K-1 FOJ Ventures (50%) | $434,800 ordinary, $80,000 rental, $14,200 interest, $24,330 dividends, $44,000 §179, $26,000 charitable |
| K-1 MHJB Investments (50%) | $191,584 ordinary, $24,000 rental, $2,637 interest, $8,413 dividends, $70,000 distributions |
| 1099-INT Fidelity | $2,400 interest |
| 1099-DIV Fidelity | $22,475 ordinary, $21,874 qualified, $8,800 §199A |
| 1099-B | ST noncovered RSUs: $4,747/+$276 adj gain; LT covered: $8,449/$111; LT noncovered: $158 |
| 1098 XYZ Bank | $13,045 mortgage interest, $7,292 property taxes |
| 1098-SA / §498-SA | HSA contributions $3,200, distributions $3,871 (all qualified) |
Phase 2 — Line-by-Line Comparison & Findings
Here's what my independent calculation shows vs. the draft return:
✅ Items that check out
| Line | Description | Draft | My Calc | |
|---|---|---|---|---|
| 1a | W-2 wages | $102,000 | $102,000 | ✅ |
| 2b | Taxable interest | $19,237 | $19,237 | ✅ |
| 3b | Ordinary dividends | $55,218 | $55,218 | ✅ |
| 3a | Qualified dividends | $51,089 | $51,089 | ✅ |
| 7a | Capital gain | $772 | $772 | ✅ |
| Sch A L8a | Mortgage interest | $13,045 | $13,045 | ✅ |
| Sch A L8b | Real estate taxes | $7,292 | $7,292 | ✅ |
| Sch SE | Self-employment tax | ~$24,785 | ~$24,785 | ✅ |
| Form 8960 | NIIT | $2,850 | $2,850 | ✅ |
🔴 FINDING 1 — HSA Deduction Not Taken (Missing $4,300)
Draft return Schedule 1, line 13 (HSA deduction) = $0. Your §498-SA shows $3,200 in contributions. Form 8889 correctly computes a $4,300 deduction limit — but line 13 shows $0. The deduction was computed but not linked to Schedule 1.
Impact: ~$1,600–$1,700 in overpaid tax at ~37% marginal rate on $4,300.
🔴 FINDING 2 — §199A REIT Dividends Not Captured ($800)
The Fidelity 1099-DIV shows $800 in Section 199A dividends (box 5). The return's Form 8995-A shows the MHJB/FOJ QBI but the $800 REIT dividend is not appearing on Form 8995-A Part IV (line 28). Potential $160 additional deduction.
🟡 FINDING 3 — Qualified Dividends Classification (FOJ K-1)
The FOJ K-1 shows $24,330 in ordinary dividends (box 6a). The return picks these up on Schedule B. However, there's no qualified dividends breakdown from FOJ — the Statement A doesn't indicate any are qualified. The return treats all K-1 dividends as ordinary (non-qualified), which is the conservative correct approach absent additional information. No error, but worth confirming with the partnership.
Claude is AI and can make mistakes. Please double-check responses.
Install the CLI for macOS, Linux, or Windows. Runs locally. No cloud, no signup required.
186 registered nodes. Every IRS form piped as JSON. Inspect schemas, create returns, compute taxes — all from your terminal.
Create a full return using the CLI
FAQ
If something here is wrong or missing, open an issue on GitHub.
OpenTax is a free, open source federal tax calculation engine. It computes Form 1040 and its schedules from source-document inputs, checks the result against the IRS Modernized e-File business rule set, and exports MeF XML or a filled IRS PDF. It ships as a single binary for macOS, Linux and Windows, and it is built to be driven by AI agents as much as by people.
Yes. The engine is licensed under AGPL v3 at no cost. A separate commercial licence is available for organisations whose policies prohibit AGPL. Both founding members of the Open Tax Technology Alliance have committed that neither OpenTax nor the Open Tax Document standard will be monetised.
No. OpenTax calculates and validates returns, and it can export e-file-ready MeF XML, but it is not a filing service and it does not transmit anything to the IRS. To file your taxes, go to irs.gov. An application to the IRS Modernized e-File system is pending; if it is accepted, the Alliance intends to operate a free e-filing route.
Form 1040 for tax year 2025, across 186 registered nodes and 131 input types. That covers W-2s, the 1099 series, K-1s, and Schedules A, C and E, along with the major credits and capital transactions. Additional forms and state returns are planned but not implemented. Run opentax node list to see everything currently registered.
No. The binary runs locally and stores returns as files on your own disk. There is no account to create, no cloud service in the path, and no telemetry on your return data. A firm can run OpenTax on its own hardware and re-check returns it has already filed without any client data leaving the building.
The repository includes 133 benchmark scenarios drawn from official IRS material, including VITA Publication 4491 training exercises, Publication 17 worked examples, and MeF test packages. Each case pairs inputs with expected output so regressions surface immediately. Returns are also validated against the IRS MeF business rules, with hard failures separated from softer warnings. The benchmark suite and the tests are public, so you can run them yourself with deno task bench.
Three ways. Add the MCP connector, which works across all sessions in either assistant. Paste the skill URL into a Claude chat, which fetches and installs everything for you with no prior setup. Or, in Claude Code, install it as a plugin with /plugin marketplace add filedcom/opentax followed by /plugin install opentax@opentax.
Open Tax Document is a free YAML standard for creating and reading Schedule K-1 data, published as a public draft at opentaxdocument/otd-spec. It is the second Alliance initiative, separate from the engine. Anyone can read and implement it, with no licence to acquire and no vendor to go through.
Filed and Crimson Tree Software founded the Open Tax Technology Alliance and co-develop both initiatives. The engine's day-to-day maintenance runs largely through AI agents that read IRS instructions, write node implementations, generate test cases from official IRS exercises, and fix regressions, with IRS publications as the source of truth and a human reviewing changes before merge.
Yes, and the Alliance is open to any firm, developer or individual. You can extend the K-1 standard or build additional IRS forms into the engine. One restriction: the right to apply for IRS MeF Software Developer or Transmitter authorisation using this codebase is reserved to Filed and its authorised partners, so contact Filed in writing first if that is your plan.
One command installs it: curl -fsSL https://raw.githubusercontent.com/filedcom/opentax/main/install.sh | sh. It detects your OS and architecture and puts the right binary on your PATH. Signed release binaries for Apple Silicon, Intel Mac, Windows x64, Linux x64 and Linux ARM are also on the GitHub releases page. The CLI tells you when a new version exists, and opentax update replaces the binary in place.
OpenTax is the calculation layer on its own: deterministic arithmetic, no interface, no document reading. Filed is the commercial product that reads source documents the way a preparer does and then prepares and reviews returns inside a firm's existing tax software. OpenTax gives an AI assistant a calculator it can trust; Filed does the professional work around it.
The Alliance
Federal tax calculation has been closed software since the 1990s, sold on annual licences by a small number of vendors. The arithmetic is no longer the hard part of tax work, and keeping it proprietary holds back everyone building on top of it.
Founding members
Filed
AI that works inside a tax firm's existing software. Reads the same source documents a preparer reads, then prepares and reviews returns. Supports 1040, 1041, 1065, 1120, 1120-S and 990, and works with CCH Axcess, UltraTax, Lacerte, Drake and ProConnect.
Crimson Tree Software
Partnership tax automation for large public accounting firms, covering federal, state and international compliance for complex allocations and tiered structures. Joins as co-founding member and co-develops both initiatives.
Contributors can extend the K-1 standard or build additional IRS forms into the engine. Both founders have committed that neither initiative will be monetised.
Firms intending to build an MeF transmitter on the codebase should contact Filed for written authorisation first.