To follow-up on the comments from @AliciaP1, your 1099-B will reflect whether the cost basis was reported to the IRS. Transactions where cost basis is reported to the IRS are designated as "covered" on your 1099-B. If you have transactions that are listed on your 1099-B as "uncovered," for those transactions your brokerage firm is not required to report cost basis to the IRS and probably has not. For uncovered transactions, the taxpayer needs to report basis to the IRS.
When you see on your 1099-B that short term transactions should be reported on Form 8949 with Box A checked, those are transactions that are covered where basis has been reported to the IRS. Long-term transactions that need to be reported on Form 8949 with Box D checked are covered and thus, cost basis has been reported to the IRS.
@Mattman
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