IRS E-File API?
My company, http://www.aplossoftware.com is an Authorized IRS e-File Provider. It is quite the process to get approved to become an e-file provider for the IRS. Several layers of application, fingerprinting, FBI background checks, to extremely buggy sdk... with few log explanations.
If you are serious about getting into the IRS e-file program, Publication 3112 was a helpful overview of process: https://www.irs.gov/pub/irs-pdf/p3112.pdf
IRS recently updated their site and changed all the urls with no forwarding...
Here is the Modernized e-File main page: https://www.irs.gov/e-file-providers/modernized-e-file-program-information
Another helpful document is Publication 4164, Modernized e-File Guide for Software Developers and Transmitters.
E-file application questions are handled by the e-Help Desk. Here is the e-Help desk email inbox: [email protected] You can email that inbox for any specific inquiries.
IRS also does have an SDK. You just have to email the help desk to request it and they'll send you a form to fill out to officially request it.
Good luck..
It looks like this page has the list of documentation for the efile process: http://www.irs.gov/taxpros/providers/article/0,,id=97657,00.html