Comment by bd_at_rivenhill

You have to assume that the brokerage must retain and report the information, otherwise you can lie all you want about your cost basis without any way for the IRS to cross check at scale. If caught in an audit, you can just claim a bookkeeping error; probably not what they want given the lengths they've gone to in order to fill common holes (c.f. the whole "provide a 1099 to pretty much everyone you have a financial relationship with" pile of bullshit).