The problem
How it works
Working in a local finances folder, he had Codex read his prior-year return, turn a long spoken braindump into fact-finding questions, and keep a README of facts and tax-treatment decisions. Codex collected documents from email and financial websites through Playwright while he handled every login himself, then wrote a Python tax engine because no existing tool covered his Washington capital gains situation. He gave an accountant the same documents in parallel.
Results and limitations
He says the accountant's first estimate was about $20k below Codex's; Codex suspected a missed escrow payment from a company sale, and after the accountant revised, the two figures nearly matched. He reports Codex's early estimate landed within 5% of the final number.
Before you try it
The return was not yet filed when he wrote (two K-1s were outstanding), and he did not review the tax engine's code himself. He later gave Codex bank access to pay the IRS despite his own stated caution, and he says most people should not build their own tax engine.
Tools and repositories
- Tools
- Codex · Playwright · Python
- Prerequisites
- Involves highly sensitive financial documents; keep work local and limit what accounts the agent can reach.
Try it yourself: Build a tax-year fact file and estimate for accountant review
Fill the fields to adapt the prompt. Empty fields stay as visible [placeholders].
Help me prepare for my [Tax year] [Tax jurisdiction] tax return using only the files in [Local finance folder]. First read last year's return and list every income source, deduction and life event you can find. Then ask me fact-finding questions, one topic at a time, about [Unusual items this year]. Keep a running [Fact file path] that records each fact, its source document, and the tax treatment you propose, with a link to official guidance. Produce a rough estimate with stated assumptions and a list of open questions for my accountant. Do not log in to any site, contact anyone, file anything or move money; tell me which documents to fetch and I will add them.Keep a qualified tax professional in the loop and never let the agent file a return or send a payment on its own.
Related videos
About openai/codex
Meet the New Codex CLI (opens in a new tab)
Published by the project’s official channel.
Watch on YouTube (opens in a new tab)Independent creators. Listing a video implies no endorsement, sponsorship or partnership.
Sources
- Codex, File My Taxes. Make No Mistakes. (opens in a new tab)
First-person account · corbt.com · Published 7 Mar 2026 · Source checked 9 Nov 2026, 22:16 UTC
What this source supports
- Codex researched tax rules and kept a facts README
- Playwright used to gather documents with manual logins
- Codex wrote a Python tax engine
- Accountant's estimate initially about $20k lower, later nearly matched
- Return not yet filed pending two K-1s