[jeremyjh]

The client is already participating in a transaction in a distributed system. There is no way to change the reality of that. Suggestions about masking this only make the composite system unsound and will not improve net service reliability improvement.

That doesn't mean that idempotency keys have to be used. You can certainly hash message content if that is documented behavior. That probably only makes sense when there is already some logical session or transaction identifier that makes dedupe semantics clear.

The system you propose might be sound and might be necessary in some systems, but I can't think of what they might be that wouldn't be better served by the simpler solution that is already widely used for this purpose.