Per the IRS Partner's Instructions for Schedule K-1
(Form 1065), page 2:
The partnership is not responsible for keeping the information needed to figure the basis of your partnership interest. Although the partnership does provide an analysis of the changes to your capital account in item L of Schedule K-1, that information is based on the partnership's books and records and cannot be used to figure your basis.
You can figure the adjusted basis of your partnership interest by adding items that increase your basis and then subtracting items that decrease your basis.
Use the worksheet on page 2 of these instructions to figure the basis of your interest in the partnership.
For more details on the basis rules, see IRS Publication 541 Partnerships,