[opinion-is-bad]

I find it very unlikely the IRS is not already writing tax software. They would need fairly sophisticated in-house software to evaluate returns, so it would just be the creation a new UI to be tax-payer facing. I would imagine all the required logic is already coded.

[jxramos]

I know at the least they seem to entirely outsource their Free File Fillable Forms to Intuit:

Search this page for Intuit https://www.freefilefillableforms.com/static/fed_terms_of_se...

And here's where you search for xml error codes when your return gets rejected.

https://www.irs.gov/filing/individuals/free-file-fillable-fo...

This thread a replier expresses first hand knowledge of doing government software development contracted out to their company: https://news.ycombinator.com/item?id=28003793