Pesantren Go Digital is an initiative by PT Telkom Indonesia to digitize the boarding school segment in Indonesia. One of the digital solutions developed is Kartu Santri, a cashless transaction service integrated with electronic money. However, the development of the Santri Card Monitoring website is still ongoing, and the API's performance is crucial for ensuring the website's features function properly. API performance sometimes encounters challenges, such as server failures, accuracy issues with retrieved data, or responses that do not align with the sent requests. This research aims to test the API's quality and functionality on the Santri Card Monitoring website using the Black Box Testing Equivalence Partitions method. This method allows detailed testing by determining valid and invalid data boundaries. The testing was conducted using the Postman tool, with results showing the API's effectiveness at 71,25% in the first iteration, which increased to 100% after improvement. Consequently, the RESTful API of the Santri Card Monitoring website was deemed "Very Good," with the improvements significantly enhancing its effectiveness. In conclusion, the Black Box Testing Equivalence Partitions method proves highly effective for testing API performance, contributing to the improved quality and functionality of the Santri Card Monitoring website in Pesantren Go Digital
