The JSON parser uses the "appendNumber" function to append a NumberLong value to the resulting BSON object. However, this function chooses a different type depending on the value of the number: https://github.com/mongodb/mongo/blob/r2.6.0-rc1/src/mongo/bson/bsonobjbuilder.h#L234
This means that when using a NumberLong type in a JSON object that is being imported by mongoimport, the imported type can be an int, a double, or a long, depending on the value.
The solution would be to use "BSONObjBuilder::append" and force a specific type (as is done inside the "appendNumber" function).
Steps To Reproduce: