In most cases, you don’t need to file a full 1040-X amendment just to fix your IRA basis, but you should correct the record to avoid being double-taxed later.
Assuming you entered the wrong basis but correcting it wouldn't change the actual amount of tax you paid (e.g., you didn't take any distributions or do a Roth conversion in 2024), you have two options:
1. Correct 2024: File a standalone Form 8606 for 2024. The IRS allows you to file this form by itself if it doesn't change your tax liability. Just download the 2024 Form 8606, fill it out with the correct numbers, sign page 2, and mail it to the IRS.
2. Correct for 2025: Simply enter the correct basis when TurboTax asks for your "prior year basis" while you are filing your 2025 return. While this doesn't technically "fix" the 2024 filing, it ensures your 2025 (and all future) math is accurate.
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