Hello @255kb , sorry for the delay. I can finally return to the topic, and indeed, the solution you provided is almost correct. I say "almost" because:
If we take a closer look at the values of our "correlationId," we can see that:

Response:
"correlationId": "8c134edc-3b50-4360-a86e-b7cb20a1c906"
Callback:
"correlationId": "ef1723ac-638c-4cc5-95ef-a400e6d8ac51"

This suggests that referencing the variable correlationId causes it to be regenerated, leading to a mismatch.
I also tried creating an intermediate variable to avoid directly referencing the correlationId variable, but unfortunately, that didn’t help either. A potentially helpful detail is that I have a 1-second delay on the response and a 5-second delay on the callback.
Response definition:
{{setGlobalVar 'correlationId' (faker 'string.uuid')}} {{setGlobalVar 'anotherId' (getGlobalVar 'correlationId')}} { "expTime": "2020-05-05", "replayTo": "{{body 'replayTo' 'default'}}", "correlationId": "{{ getGlobalVar 'anotherId' }}" }
Callback definition:
{ "name": "It's callback", "correlationId": "{{ getGlobalVar 'anotherId' }}", "returnUrl": "{{body 'replayTo' 'default'}}", }