I found the answer in case anyone else comes across it. The problem is that the CosmosDB API for MongoDB does not support the connectionStatus command that Javier has shown: https://learn.microsoft.com/en-us/azure/cosmos-db/mongodb/feature-support-42
This same command is used by the Kafka connector sink code to validate the connection on startup. You can edit the connector to simply skip the user validation step if you want this to work. Change would go here → https://github.com/mongodb/mongo-kafka/blob/ee5edf317508da42a62917d32fb21c2d46660991/src/main/java/com/mongodb/kafka/connect/MongoSinkConnector.java#L105
I don’t know if the maintainers would be open to making the user validation step optional and exposing that in the configs?