I believe the penalty is assessed if there is an amount due. If I am incorrect then someone will correct this.

I think IronMan is very agreeable, must be the post-extension Fireball, to agree to contradictory beliefs!

If no tax due, no late filing penalty.

Also no late filing penalty if 90% tax paid (not sure if it has to be paid by by Oct 15, I'll leave that one open)

With extension, and no tax due after Oct 15, no late filing penalty.

I agree with @PATAX No tax due no penalty. From IRS. "

• If you owe tax and don't file on time, there's also a penalty for not filing on time. The failure-to-file penalty is usually five percent of the tax owed for each month, or part of a month that your return is late, up to a maximum of 25%. If your return is over 60 days late, there's also a minimum penalty for late filing; it's the lesser of $485 (for tax returns required to be filed in 2024) or 100 percent of the tax owed. See Topic no. 304 for information about extensions of time to file if you can't file on time.