Several suites use txn_override.js to bundle operations in jscore tests into multi-statement transactions, which involves attaching a read concern to the first command in each bundle so it can start a transaction. The override checks if an operationTime exists on the default session for a database and attaches it as afterClusterTime in the command's readConcern if it does. All sessions track operationTime, so this value will almost always be set and almost all transactions started by the override will be using causal consistency.

Causal consistency is not required to read your own writes if all reads and writes are sent to the same node without failovers, which is guaranteed in several of these passthroughs, so modifying the override to allow disabling causal consistency and doing so in the passthroughs where it is not required would increase coverage of transactions.