The Certificate of Completed Work under a contract for services holds significant importance in the legal context, serving as a document that confirms the fact of work performed or services rendered, as well as the fulfillment of the parties’ contractual obligations. This article examines the legal status of such certificates in the judicial practice of the Republic of Kazakhstan, where they are analyzed as legal instruments confirming the actual performance of work (services). The regulatory framework governing certificates of completed work is closely linked to the fundamental principles of civil law, such as the fulfillment of contractual obligations and the protection of infringed rights. However, the legal nature of such certificates remains a matter of debate, as judicial practice in Kazakhstan demonstrates a variety of approaches to recognizing unilateral certificates of completed work as valid. On the one hand, a certificate may be regarded as confirmation of contractual performance; on the other, it may be treated as a transaction that entails legal consequences for the parties. Particular attention is given to issues related to proving the fact of work completion and the proper preparation of the corresponding documents. The article analyzes the impact of certificates of completed work on the legal relationships between the parties, their use as evidence in court proceedings, and their role in law enforcement practice. The legal status of certificates of completed work requires clearer legal regulation, driven by the need to eliminate legal uncertainty in order to prevent disputes over such documents.