SpringMockK 4.0.0 is built against Spring Boot 3, with Java 17, and MockK 1.13.3.
Spying JDK proxies with Java 17 requires to pass a specific option to the JVM. See the README for details.
Important note: MockKs artifact ID has been modified from mockk to mockk-jvm. If you specify a specific version for MockK in your build rather than relying on SpringMockK's transitive dependency, make sure to use the new arctifact ID.