Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025
Cash flows from operating activities:
Net loss	$ (80,293)	$ (67,236)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	99	85
Stock-based compensation expense	9,001	5,232
Net amortization of premiums and accretion of discounts on short-term investments	(793)	(2,720)
Non-cash lease expense	307	272
Changes in operating assets and liabilities:
Accounts receivable	(104)	(324)
Prepaid expenses and other assets	(7,903)	(15,658)
Accounts payable	(412)	(1,456)
Accrued expenses and other current liabilities	(3,060)	1,674
Operating lease liabilities	(315)	(273)
Net cash used in operating activities	(83,473)	(80,404)
Cash flows from investing activities:
Purchases of short-term investments	(65,363)	(323,138)
Maturities of short-term investments	123,524	122,577
Purchases of equipment	0	(47)
Net cash provided by (used in) investing activities	58,161	(200,608)
Cash flows from financing activities:
Proceeds from issuance of common stock in connection with at-the-market offering, net of commissions	2,083	0
Proceeds from issuance of common stock under stock plans	1,048	612
Payments of initial public offering costs	0	(34)
Net cash provided by financing activities	3,131	578
Net decrease in cash, cash equivalents and restricted cash	(22,181)	(280,434)
Cash, cash equivalents and restricted cash at beginning of period	101,772	326,086
Cash, cash equivalents and restricted cash at end of period	79,591	45,652
Cash, cash equivalents and restricted cash at end of period:
Cash and cash equivalents	79,397	45,458
Restricted cash	194	194
Total cash, cash equivalents and restricted cash at end of period	$ 79,591	$ 45,652

Non-cash Lease Expense

No definition available.

Name:	upb_NonCashLeaseExpense
Namespace Prefix:	upb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The sum of the periodic adjustments of the differences between securities' face values and purchase prices that are charged against earnings. This is called accretion if the security was purchased at a discount and amortization if it was purchased at premium. As a noncash item, this element is an adjustment to net income when calculating cash provided by or used in operations using the indirect method.