Mongos batches writes together and even if individual writes fail, the command response can return {ok: 1}. However, if any of the individual writes failed with an error label, that label will be lost.

The batch write path should be modified to preserve any error labels returned by writes in a batch.

This ticket includes determining how error labels encountered during batch writes should be handled, e.g. if two writes in an unordered batch fail but only one has the TransientTransactionError label, should the batch response include the label or not. Mongod addresses this by throwing if any statement in a batch fails within a transaction, so the solution may be adding this behavior to mongos.

This work was spun off from SERVER-35708.