The @ command is broken in the Custom GPTs right now.
This seems to be the case since either /agent or /study mode appeared, I suppose.
Workaround:
So you cannot call another GPT in your current one by using the @key.
BUT, at least for now you can still use this in your non-GPT chats.
In case you created quite some Custom GPTs, be a little patient.
The list will pop up. It’s definitely NOT broken in the normal chats.
I hope it wouldn’t stay like this until OpenAI doesn’t intend to add more features to the Custom GPTs. Like for example maybe advanced voice mode, finally? etc.
Noticed the same. It’s a shame because calling other GPTs with @ from a custom GPT adds so much power… I hope Open.ai does not abandon GPTs there have been no updates to the Custom GPT concept for at least 2 years. Maybe this will change when GPT 5 arrives.