TaxableTurtle1,
If Fidelity has cost basis numbers you trust, then by all means use them whether or not they were reported to the IRS. There are some odd situations related to spinoffs, mergers and the like where new shares are issued but their basis depends on a prior long-term holding for which the broker is not required to report the basis even though they do have those figures. I've also seen some similar chat about crypto transactions.
