While there have been some claims and conspiracy theories about ghostwriters or collaborators, there's no credible proof to support them. Agatha Christie was a prolific writer, and she carefully guarded her own work.
It's important to note that:
* She was a very private person. This makes it unlikely she would have shared her writing process or let anyone else write under her name.
* Her writing style is highly distinct. It's very difficult to mimic her unique voice and plot structures.
* There's no known documentation or testimony from anyone who worked with her to suggest collaboration.
While it's always fun to speculate, the evidence overwhelmingly points to Agatha Christie being the sole author of her own work.