GPTs Builder action call Azure API function app error

I would like to call Azure API python function app from a custom GPTs. I have working Azure API, tested externally from Developer portal. When testing action from my GPT, GPT corrrectly formulate request, but later I get the following error: UnrecognizedFunctionError. Anybody got such an error? Where can I find end-to-end working somple of GPTs → Azure API function app integration?
I provide Open API json schema to be used by GPTs. When testing action in builder it makes first call with parameters callected from test case, and second call without parameters (is it GPT hallucination?), and next reports above error. My Azure API is supposed to respond once, with the number from <0, 1>, as calculation of the function app that takes parameters of the call as input.