I have ran into this issue and have been banging my head trying to troubleshoot it.
Since 5.1 came, I can’t get my GPT to use my MCP tool - it keeps telling me there is no tool / action available to it besides search, even when it is visible in the UI. Same thing happens when using the “Test” in the Custom GPT config UI - it shows the “Call the {URL} API with the {oprationID} operation” info box, but then tells me that it can’t use a non-existant tool or call web APIs directly.
I tried re-adding the action and creating a new GPT from scratch but neither yielded different result.
I also tried public GPTs and some of them with custom actions have a similar issue - they tell me they have tools avaialbe, but when trying to call them tell me that tool is disabled / unavilable.
(I am using the Go subscription for reference, so I can’t change the model to legacy ones)