How enable subagent mode in ChatGPT Pro6 again

A user reported that their ChatGPT Pro 6 (Astra) account unexpectedly entered a "subagent mode" while in regular Chat mode last week. This behavior persisted for nearly the entire week until the user exhausted their 50-message Pro subscription limit. The user is now seeking an explanation for how this subagent mode works and how to re-enable it.

This is the first user report of ChatGPT Pro 6 (Astra) spontaneously entering a "subagent mode" in regular chat, unlike previous discussions that focused on explicit agentic features.