Application Programming Interface (API) testing automation enhances software reliability through service communication support that minimizes both human error and manual testing work. Postman, SoapUI, JMeter and Rest Assured serve as existing API testing tools that perform functional as well as performance tests yet struggle with high maintenance needs, false results and integration problems. The proposed AutoTest API method integrates self-healing test scripts together with intelligent test execution, built-in security testing, accuracy, efficiency and scalability features. The system maintains test scripts through automatic updates which decreases the need for maintenance work. The system combines automated security vulnerability detection together with performance testing to achieve strong API validation. The Auto Test API method stands out from other tools through its ability to decrease test failures and boost security while speeding up the process of identifying and resolving bugs according to comparative analysis. The research brings improved API testing frameworks into modern software design by deploying smart automation and optimized testing adaptation techniques which boosts developmental reliability through efficient development environments.