writeAllFields
config is not working because that is only meaningful for non-phantom records; any fields of a phantom (new) record will be viewed as "changed" and therefore included in the request packet.
To exclude specific fields from being included in the request, add persist:false to the fields you don't want to include.
That being said, I don't quite understand why you'd only want to write the ids of a newly added record. Unless you are explicitly having Ext JS create those ids for you, what are you actually going to be sending to server? I don't know your use case, but typically it is desirable to have your persistence layer (e.g., a database) assign identifiers to your records.