Should I amend immediately after a CP2000 missing-basis notice?

Usually no. A CP2000 is typically a proposed change based on data the IRS matched against the return, often without full basis information. If the problem is missing basis on securities sold, the stronger first move is to respond to the notice with the basis records and the corrected gain computation.

Here is the key distinction:

• the IRS may know proceeds
• the IRS may not know adjusted basis
• the notice may therefore overstate the taxable gain

If you send an amended return without following the notice instructions, you can create a messy duplicate trail. A cleaner workflow is:

1. confirm the notice number and response deadline
2. gather broker statements or transaction detail
3. compute the actual gain or loss
4. send the notice response package in the format requested

If the notice explicitly instructs a different path, follow that notice. But for a standard missing-basis mismatch, the taxpayer should usually answer the proposed change first rather than reflexively amending.