Boy, do I have bad news for you.

Edit: To be clear, as a sibling post said, the basic arithmetic is easy enough. It's the tax opinion stuff that is absolutely not deterministic. If your situation is even moderately complex, there's a vast number of ways to describe your deductions, each with different tax implications and multi-year requirements. I'm not talking about being Jeff Bezos, either. Is your spouse an independent contractor? Do you own a home? Do you have stock options? Do you have a home office? These alone are enough to make some pretty creative reporting situations.

I think it's a very common misconception among programmers that the law is a sort of natural language 'program' where you can consistently deduce that x input generate y output.

And there are basic things that shouldn’t be subjective at all but that the IRS refuses to give a clear answer to, like if/how the SALT cap affects deduction for NIIT. There are at least 3 possible interpretations and no consensus.

I know my own taxes pretty well. I don’t follow the tax code changes but could fill out a 1040 form on my own. Even did for a short time.

I use tax prep software because I do NOT want to worry whether I copied the amount from line C to line K correctly. The IRS forms are a nightmare!

The postscript in PDF should allow something more sane than what we have today in IRS forms, but that’s just wishful thinking.

That’s a true side fact, but that has nothing to do with how the software behaves once you input your answers.

Is your spouse an independent contractor? Do you own a home? Do you have stock options? Do you have a home office? These alone are enough to make some pretty creative reporting situations.

Those questions all have discrete answers, including the much-misunderstood home office. The correct tax outcomes are very much deterministic, in the sense that the same inputs always result in the same outputs. It's simply that there are a lot of options to change the inputs (for example, choosing FIFO vs LIFO for stock sales, using an S-corp vs a sole proprietorship for a personal business).

For example, the home office deduction is only available to individuals who have a home office that is used exclusively for their individual business, not as an employee for someone else's business (the only exception being for flow-through entities where the taxpayer is a shareholder/partner). The "exclusive use" is meant in the all-or-nothing sense. Use your office computer after hours for games? The home office deduction no longer applies. Uses the office space to store personal documents, or really any other activity except for the business activity? The HOD no longer applies. Don't have any income from that business activity you claim are doing in the home office? The HOD no longer applies.

It's simply that enforcement is not deterministic, so people think they get away with a lot of positions that do not survive actual audits. Talk to an IRS agent that handles audits and you'll learn that a failed home office deduction claim is the #2 adjustment to the tax returns of white collar professionals.[1] At a relatively recent tax mixer, IRS agents from the Los Angeles branch office could only recall about a dozen cases in which the home office deduction actually survived an audit, out of thousands, and those taxpayers were extremely rigorous about following the rules to the letter (to the extent that all of them locked their home offices when they were not being used for work).

[1] The #1 reason for adjustments to the tax returns of white collar professionals is attempting to claim business expense deductions without matching business income to deduct against. Technically, the home office deduction is one of these deductions, which is why it is #2 and not #1.