As generative AI chatbots become more personalized and emotionally responsive, they increasingly serve as companions, friends, and romantic partners. Yet these relationships are accompanied by significant uncertainty: users question the AI's identity and agency, the authenticity of its emotional responses, and the stability of the relationship amid system updates, policy changes, or platform shutdowns. Drawing on in-depth interviews with 25 users of AI companions, this study identifies three forms of uncertainty: ontological uncertainty concerning the AI's nature and agency, structural uncertainty arising from platform control and system instability, and normative uncertainty regarding the legitimacy and boundaries of human-AI intimacy. These uncertainties are shaped by technical and social factors, such as algorithmic opacity, platform changes, and social stigma, often inducing frustration, self-doubt, and distress. Participants managed these uncertainties through information seeking, topic avoidance, expectation adjustment, and disengagement. This study extends interpersonal uncertainty theories to human-AI communication and contributes to HCI research by conceptualizing uncertainty in AI companionship as a socio-technical phenomenon with potential socio-emotional harms. We discuss implications for designing safer AI companionship through contextual transparency, user control, update notice, and relational safeguards.

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