jpa jars with ejb projects?

i just started using ejbs and when I added jpa annotations de my simple class there was a "Entity cannot be resolved to a type" error. I added simply jpa jars to my project in it worked ! my question is : do we need to add everytime the jpa jars to the project so it can work ?(cause I do remember that during my class we didnt add any jars to the project , and it worked ! )

Thanks