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Introduction Rigid thoracoscopy is an established diagnostic instrument for pleural diseases; however, it is limited by challenges related to manoeuvrability, patient comfort and operator learning curve. A novel dual-function semi-rigid thoracoscope with associated semi-rigid forceps has been engineered to integrate the diagnostic efficacy of rigid biopsy forceps with the ergonomic advantages of a semi-rigid system. The Dual-functIon Semi-rigid thoraCoscOpy Versus rigid thoracoscopy for the diagnosis of plEuRal disease trial was designed to evaluate whether this device achieves a diagnostic yield that is non-inferior to that of traditional rigid thoracoscopy.Methods and analysis This study is designed as a prospective, randomised, open-label, controlled, non-inferiority trial. A cohort of 92 patients presenting with undiagnosed exudative pleural effusion across five centres in China will be randomly allocated to undergo pleural biopsy using either a dual-function semi-rigid thoracoscope or a conventional rigid thoracoscope. The primary endpoint of the study is the diagnostic yield. The key secondary endpoints include procedural analgesic requirements, specimen quality, operator-assessed device performance, postprocedural pain and safety. A non-inferiority margin of 10% will be applied, with analyses conducted on both the intention-to-treat and per-protocol populations.Ethics and dissemination The study protocol was approved by the Ethics Committee of China-Japan Friendship Hospital (2026-KY-033). Written informed consent will be obtained from all participants before enrolment. The trial will be conducted in compliance with the Declaration of Helsinki and Good Clinical Practice guidelines. The results will be submitted for publication in a peer-reviewed journal.Trial registration number NCT07114783.