BACKGROUND The digitalization of healthcare has increased the demand for efficient data exchange, emphasizing semantic interoperability. SNOMED CT, a comprehensive terminology with over 360,000 medical concepts, supports this need. But it cannot cover all medical scenarios, particularly in complex cases. To address this, SNOMED CT allows postcoordination, where users combine precoordinated concepts to new expressions. Despite SNOMED CT’s potential, the creation and validation of postcoordinated expressions (PCE) remain challenging due to complex syntactic and semantic rules. OBJECTIVE This work aims to develop a tool that validates postcoordinated SNOMED CT expressions, focusing on providing users with detailed, automated correction instructions for syntactic and semantic errors. The goal is not just validation, but also offering user-friendly, actionable suggestions for improving PCEs. METHODS A tool was created using the FHIR service $validate-code and the terminology server Ontoserver to check the correctness of PCEs. When errors are detected, the tool processes the SNOMED CT Concept Model in JSON format and applies predefined error categories. For each error type, specific correction suggestions are generated and displayed to users. The tool was integrated into a web application, where users can validate individual PCEs or bulk-upload files. The tool was tested with real existing PCEs, which were used as input and validated. In the event of errors, appropriate error messages were generated as output. RESULTS In the validation of 136 PCEs from 304 FHIR Questionnaires, 18 PCEs (13.23%) were invalid, with the most common errors being invalid attribute values. Additionally, 868 OncoTree codes were evaluated, resulting in 161 PCEs (20.85%) containing inactive concepts, which were successfully replaced with valid alternatives. A user survey reflects a favourable evaluation of the tool's functionality. Participants found the error categorization and correction suggestions to be precise, offering clear guidance for addressing issues. However, there is potential for enhancement, particularly regarding the level of detail in the error messages. CONCLUSIONS The validation tool significantly improves the accuracy of postcoordinated SNOMED CT expressions by not only identifying errors but also offering detailed correction instructions. This approach supports healthcare professionals in ensuring that their PCEs are syntactically and semantically valid, enhancing data quality and interoperability across systems.