Harvest claim on the backward compatibility was “Harvest server is backward compatible with harvest clients that are at same level ”, it means client components (workbench, agent, and client) that are at same release level can communicate through a higher release level broker. As an example 12.5 client and 12.5 agent should able to connect to 12.6 broker and able to perform basic operations. However the backward compatibility doesn’t work between clients (i.e. 12.5 client can’t connect 12.1 SP3 agent and vice versa).
Individually 12.5 workbench and 12.1 SP3 agent can connect to 12.6 broker and perform operations, however 12.5 workbench connected to 12.6 broker can’t work/connect with 12.1 SP3 agent. Higher level release clients can’t work with lower level agent as well (e.g. 12.6 hexecp command can’t be executed against 12.1 SP3/12.5 agent). In fact Harvest never claimed backward compatibility between different release level clients.
So if customer is going to use same level of clients (Workbench/client and agent) to communicate 12.6 broker then the backward compatibility is going work for them.
Please note that mixed client components at different release level would not able to communicate with each other.
Backward compatibility is supported for the transition of several clients in a phased manner for a very short duration. Any issues arising out of this would not be supported.