Worked on enhancing Kotlin compiler infrastructure, focusing on contract programming and type system improvements in the google/kotlin and JetBrains/kotlin repositories. Addressed a contract issue in ConeKotlinType.classId by updating its contract to returnsNotNull, improving type-safety and aligning nullability semantics for downstream code. Expanded test coverage for contract validation, introducing robust error reporting for invalid contract bodies to increase reliability and developer feedback. Updated ContractBuilder documentation to relax constraints on 'callsInPlace' effects, enabling broader usage patterns. Utilized Kotlin and Java, applying skills in compiler development, diagnostic reporting, and language design to deliver targeted fixes and improved API flexibility.