Should I submit the legal name or the DBA for TIN verification?

The IRS matches against the name in its own records, so a DBA will usually return no match even when the EIN is correct - unless the IRS has knowledge of that alternative name.

The payload for Baselayer's business search accepts both name and alternative_names. We would recommend sending the legal name of the business in the first one, and only submit DBAs in the second field.

If you get tin_matched=FALSE and suspect the fields were swapped, a second check with the other name through the TIN verification endpoint would be the quickest way to confirm.