Ok anyone got any ideas and before anyone asks, I’ve done a butt tonne of API work in Create so I know my way round 
I’m sending a valid JSON request to a 3rd party service (Companies House if anyone is interested or has an opinion) and I’m getting a 500 response with no return payload.
The error message seems to be … Invalid response JSON structure. … suggesting the service is sending a garbage response payload.
Problem is the same request / response from postman works fine (and the response JSON validates as expected) - as do other API calls I’m making from Create to the same service.
I’m at a loss - even if I leave the response payload empty (in the API call object) I get the same result. I just cannot see any further debug or logging so I’m fishing in the dark !!!
Thanks.
Could you try creating a response payload pre-processor and dumping the response payload into the Detective log? Might be able to catch it before it errors?
A 500 response code would imply an error on their side and therefore probably not getting any response payload back.
Are you saving the API requests as record so you can access the full headers etc?
If so can you see the Content-Length header, to confirm if there was a response payload at all?
Thats a good call - will certainly look at that shortly.
hmm suspicious - 0 length 
yes am doing all the usual API logging
Almost like its being blocked or stopped but then other APIs are working fine to the same service and even a call from Postman (to the same service endpoint) works exactly as I would expect.
There is no request payload in this case - its all URL based with suitable parameters. Only the Authorization header is sent additionally.
Odd that they return a 500 if it was just a network/permission issue for example, that would imply a crash on their side instead.
Why postman doesn’t give same result is interesting. Have you checked the hidden headers in postman, it often adds several default ones? Maybe it turns out one of those is essential, like Content-type for example I know has tripped me up in the past if you don’t specify it with some APIs.
Right - Found it !!!
This particular call is very dependant on the value in the Accept request header.
All the others don’t seem to care 
Thanks again Bob - you are on the ball here too !!!
