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 (using old tools which use the c++ parser):

$ cat input.json
{ "_id" : { "$oid" : "532205cba066bc569f1e8626" }, "x" : { "$numberLong" : "1" } }
{ "_id" : { "$oid" : "53220603dccaa782b7fdc2d9" }, "x" : { "$numberLong" : "4398046511104" } }
$ ./mongoimport --db test --collection test input.json
connected to: 127.0.0.1
2014-03-13T15:28:42.646-0400 imported 2 objects
$ ./mongo
MongoDB shell version: 2.6.0-rc2-pre-
connecting to: test
> db.test.find()
{ "_id" : ObjectId("532205cba066bc569f1e8626"), "x" : 1 }
{ "_id" : ObjectId("53220603dccaa782b7fdc2d9"), "x" : NumberLong("4398046511104") }
>

Or in c++ server code:

fromjson("{a:{$numberLong:1}}"}