Json with SMT on SourceConnector

I’m trying to apply different configurations on a MongoSourceConnector to have data on a kafka topic in json format, but with some transormations (replaceField, time conversion): seems that the only way to have messages as json is with output.format.value = json and value.converter = org.apache.kafka.connect.storage.StringConverter, but in this way I’m not able to apply transformations (SMT), with error: ‘transformation supports only Struct objects’.
Is there a way to not use schema, having json format as output, and use SMT ?

Kafka SMTs require a schema to be defined on the source. Thus, you’ll need to define one

Thanks for your prompt support! Do you mean that I need to specify output.format.value: schema to use SMT or I’m losting somewhere? Is necessary that any message on topic include schema and payload subobjects?
My goal is to have a message on json format with standard changeStream fields (_id, operationType, fullDocument,…) but with SMTs: can you help to understand if is this possible or not?

if its just replacing a field is all you need SMTs for, why not just use the pipeline ? https://www.mongodb.com/docs/kafka-connector/current/source-connector/usage-examples/custom-pipeline/#customize-a-pipeline-to-filter-change-events.

then you dont need to define the schema or use SMTs at all and it will be the best performance. (SMTs can be slow)

Sorry to coming back here after a time: for field replacing pipeline is enough, but what if I need something more? For example it could be helpful to move some values on header, what is the suggested approach?