When updating an Azure APIM API using Pulumi (pulumi up) we started seeing this obscure error.

First thing to do is get more info via debug:
pulumi up --debug
(if you're running this in a shell, make sure you've increased the buffer for the window (in options) because there's alot of extra logs!)

Searching through that output we can see the PUT that tries to update the Open API (swagger), followed by a few GET polls to the same API. The value of note is the provisioningState, the first couple of calls were:

"provisioningState": "InProgress"

followed by a final:

"provisioningState": "Failed"

But no real info on what the issue actually is. To get that info we need to go into the APIM and manually try to update the Open API.

In the Azure portal, navigate to APIs in the APIM, each API name has an ellipse next to it, click the ellipse for the API that is having the issue, then select "Import", then OpenAPI and select the json file that has your OpenAPI from your machine:

Click Import, then after a short amount of time you should see an error, ours came up like this:

From there should have enough info to resolve whatever the error is!