kotlin-stdlib on the classpath (correct, Kotlin BOM imported first):
  kotlin-stdlib-2.3.10.jar

Same classpath with kotlin-stdlib 1.9.25, which is what the Spring Boot 3.5.16 BOM picks when it comes first:
  kotlin-stdlib-1.9.25.jar

$ java -cp <that classpath> com.ankurm.koog.TrapMain
  Exception in thread "main" java.lang.NoClassDefFoundError: kotlin/time/Instant
  	at ai.koog.utils.time.KoogClock.<clinit>(KoogClock.kt:27)
  	at ai.koog.prompt.executor.model.PromptExecutorBuilder.openAI$default(PromptExecutorBuilder.kt:85)
  	at ai.koog.prompt.executor.model.PromptExecutorBuilder.openAI(PromptExecutorBuilder.kt)