Is it safe to add properties to request/response types to activities? Will it break validation when the history is replayed?
It won’t if those types are backward compatibles and the workflow code itself is deterministic, meaning that the workflow won’t take a different path during replay based on the new properties added to the returned types.
I would use workflow replay to test the changes.
The information shared in this post can be useful too Changing WorkflowMethod param type
Antonio