I’m not sure whether these messages from @deanrie is being triaged by a bot or by a person, but I think the core issue is being missed.
The problem is not the API key itself, nor the custom backend configuration in general. The issue is specifically that when GPT-5.5 is selected, Cursor does not send the request to the configured overridden OpenAI base URL at all.
The same custom endpoint and API key work correctly with GPT-5.4. The failure only happens with GPT-5.5, which shows that GPT-5.5 requests are bypassing the custom base URL path and are either being validated internally or routed against api.openai.com instead.
This may also be related to the existing BYOK/custom endpoint issue where image attachments appear to fall back to the official OpenAI endpoint instead of the overridden base URL.
So the expected fix seems straightforward:
When GPT-5.5 is selected and the user has configured an overridden OpenAI-compatible base URL, Cursor should send the GPT-5.5 request to that overridden base URL, exactly as it already does for GPT-5.4.
Repeatedly asking users to confirm their API key or endpoint does not address the bug. The request is not reaching the custom backend in the first place.