Do you want the Tax Collector to also be the Tax Preparer?

As part of the Inflation Reduction Act just passed, the IRS is working on a program to file your tax return for you.

Here is what has been proposed:
The IRS will send you a pre-filled return that will show either what you owe or what you will be refunded. If you are accepting of this, simply sign and mail back.

Here are my questions:
What data is being used to calculate the return?
What would happen to me if I do not accept their return?
What if I also do my own return and reach a different result, can I re-file or am I stuck?

Most of my clients only pay around $200/year for their returns to be done by an accountant, I would feel more comfortable with that vs. the IRS doing a return for me.