Custom Action payload - what payload was sent to 3rd party?

​@Dex 666 Yes, you can run the journey in Test Mode with a test profile; the actionHistory log shows the exact request payload and response sent to the third party.

For more error detail, define a failure response payload on the custom action and branch on jo_status_code to surface the specific error fields.

For already-run production calls, this level of detail isn't retroactively visible, you'd need to raise a support case for backend logs.