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