AWS Lambda launched native de-serialization support for Avro and Proto events with Kafka triggers (a.k.a Event Source Mappings) - https://aws.amazon.com/about-aws/whats-new/2025/06/aws-lambda-native-support-avro-protobuf-kafka-events/

There are options to perform de-serialization on Key and/or Value fields and receive the de-serialized payload in JSON format in your C# Lambda without having to deal with the de-serialization nuances.

https://docs.aws.amazon.com/lambda/latest/dg/services-consume-kafka-events.html