Why social intelligence matters for interactive AI. Social intelligence (SI)—the capacity to perceive, interpret, and respond effectively to social situations—is the meta-capability that governs all sustained human-AI interaction. Unlike task-specific competence (coding, math, summarization), SI cannot be evaluated through isolated test items because it is inherently relational, temporal, and context-dependent: the same utterance may be socially brilliant in one context and disastrous in another, and its quality depends on the accumulated relational history, not just the current turn. For interactive AI agents—conversational companions, game NPCs, therapeutic bots, tutoring systems—SI is not one capability among many; it is the capability that determines whether the interaction feels authentic, productive, and worth sustaining. An agent with perfect factual accuracy but no SI (ignoring emotional cues, violating social norms, failing to adapt to relational dynamics) will produce interactions that humans abandon. This positions SI evaluation as a necessary prerequisite for any meaningful evaluation of interactive AI quality—and explains why MarioEval's architecture is designed around the cognitive structures of social cognition (Theory of Mind, appraisal theory, narrative engagement) rather than around linguistic features.