In the absence of a treaty protocol or verification regime, the Biological and Toxin Weapons Convention (BWC) instituted confidence-building measures (CBMs) as a mechanism to increase confidence in compliance by enhancing transparency and mitigating ambiguities regarding states parties' biological activities. While a promising tool to support treaty compliance, low participation, concerns regarding the completeness and accuracy of CBM submissions, a dearth of analysis, and restricted access to many submissions have limited CBMs' value. Through interviews with 53 international experts—38 from BWC delegations and 15 independent experts—we identified concrete opportunities to increase CBMs' value while mitigating the burden on states parties. This study supports states parties' efforts in the BWC Working Group on the Strengthening of the Convention, as part of a series of research on BWC assurance that aims to characterize challenges around BWC verification and increase certainty in BWC compliance.