Výpoveď spolupracujúcej osoby v trestnom konaní: pravdivosť a hranice jej dôkazného významu

The testimony of a cooperating person represents a specific evidentiary source in criminal proceedings, as its epistemic function is directly linked to the author's personal interest in obtaining a criminal law benefit. While the legitimacy of this institute within the Slovak legal order is not disputed, its evidentiary assessment raises questions concerning the limits of its independent probative weight. The article proceeds from the thesis that incentivised testimony entails a structurally increased risk of distortion of factual findings and therefore requires qualitative limits derived from the principle nemo debet esse testis in propria mercede, the case law of the European Court of Human Rights, and the constitutional framework of criminal procedure. Particular attention is devoted to the distinction between credibility and truthfulness and to the material understanding of a fair trial. The article was finalised in the context of the decision of the Constitutional Court of the Slovak Republic (Order No. PL. ÚS 2/2026 of 11 February 2026), which suspended the effectiveness of Section 119(6) of the Criminal Procedure Code as introduced by Act No. 416/2025 Coll. This intervention reopened the question of how the evidentiary significance of cooperating testimony should be normatively framed within the system of free evaluation of evidence. In response to the resulting normative uncertainty, the article provides doctrinal clarification and outlines de lege ferenda proposals aimed at enhancing legal certainty, transparency of judicial reasoning, and material fairness while preserving the legitimacy of the institute itself.