In looking into why a 4.0.3RC2 PooledInvokerProxy could not be read by the 4.0.2 codebase, the problem is that the override of the writeClassDescriptor/readClassDescriptor in the Optimized*Stream classes is throwing away information that would allow binary compatible changes such as the addition of new fields to the ServerAddress class to be dealt with. Instead, such a change is resulting in an EOFException. The same proxy when read over a standard ObjectInputStream during the bootstrap lookup from jndi is read without any problem.