Ahh, I see. If I open the Coding Assistant tab and start a new conversation from there, I indeed get a list of agents to select from. I was initially clicking the 'New Conversation' button in the top toolbar, which evidently just implicitly set Codex as the agent without giving me a choice. Now having started a conversation from the Coding Assistant tab using an agent I've configured (Gemini), the button in the toolbar is now defaulting to Gemini.

Ahh, I see. If I open the Coding Assistant tab and start a new conversation from there, I indeed get a list of agents to select from. I was initially clicking the 'New Conversation' button in the top toolbar, which evidently just implicitly set Codex as the agent without giving me a choice. Now having started a conversation from the Coding Assistant tab using an agent I've configured (Gemini), the button in the toolbar is now defaulting to Gemini.