Before my 1099-B was available from my broker, I went through the preparation process and answered "no" to the "have you received a 1099-B" question, resolving to update this when the 1099-B became available. When the 1099-B became available, I edited the existing investment information to provide my realized gains summary information. In hindsight, this was pretty stupid and set up this entire chain of events. What I should have done was delete my existing 1099-B record in turbotax & create a new one to answer "yes" to the 1099-B question. I was notified AFTER e-filing my federal taxes that I am now required to provide forms 8453 & 8949 by mail. Had this notification occurred before e-filing was submitted (it's not very good UX to be sidelined by this notification AFTER filing), it would definitely have been enough of a flag for me to review the 1099-B with more careful attention.

That aside, on form 8949 I should have checked box A for 1099-B showing basis was reported to the IRS (according to documents from my broker), which would have negated the necessity for 8453 & 8949. The confusion I have is: do I amend my filing to update the 8949 from box C to box A, or would mailing 8453 with the "recipient copy" of the 1099-B provided by my broker be adequate?

Thanks in advance.