It was all going so well for Meta stock. But after the rally comes the uncertainty.

The early success of its Muse personal AI agent catapulted the shares higher for most of September but they've fallen 8% over the past two trading days. The stock pointed 0.5% higher ahead of the open Tuesday and it's still up 25% this month, through Monday's close.

But it feels like it's at a bit of a crossroads, especially ahead of OpenAI's developer conference on Tuesday. The AI company could unveil an always-on assistant, expected to be called "O", according to a report from Testing Catalog. OpenAI didn't to respond to a previous request for comment on the report.

Something like that could be a direct rival to Muse and the stock market will be eager to quickly assess just how much of a challenge it might pose.

While speculation is rife over what CEO Sam Altman may reveal during his keynote speech at 1 p.m. Eastern time, the company confirmed it's scrapping its latest AI model over safety concerns.

Meta's Muse was always going to face tough competition at some point, and it won't just be from OpenAI.

The real battle for the best personal AI agents is only just beginning.