I have written a custom connector to update an object using a vendor's REST API. When I test the custom connector using the test facility Microsoft provides within the custom connector itself, it works perfectly. However, when I try to use the exact same data within a flow, the flow fails and the vendor's API returns a 500 unknown error.
I have been trying to debug this for several weeks. I have already tried creating a duplicate of the connector and sending the output to Postman. It looks perfect. The headers are perfect. I've tried sending the data to the vendor's API via the HTTP connector. That works fine as well. The only thing that does not work is when you try to use the custom connector in a flow.
The vendor does not log requests and refuses to look into this on their side. But clearly, since the request works when using the HTTP connector and in the test facility within the connector itself, the problem is not on the vendor's side.
I'm wondering if anyone has any ideas. Because it works perfectly in Test and not in a flow using the same data, I really don't know where to go from here.
I personally believe now (after weeks of looking at this bug), that this is a bug in Power Automate.
Sure, here's a picture of the flow:
The reasons for 500 could be a extra headers/ip blocking/auth issues/how your flow is triggering in run/ however since its a very specific case i would suggest you to debug using Support.
Can you try posting your complete flow? Have you tried using only the custom connector action?
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Thanks,
Rahber