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The Selection Effect: Why Compact LLM Verifiers for Emotion Classification Fail Behind Encoder Confidence Gates
Is a compact LLM verifier cost-effective for closed-label emotion classification when a supervised encoder is already available? We formulate inference-time escalation as cost-aware selective routing: an encoder predicts first, and only uncertain examples (via confidence, disagreement, or novelty gates) may be forwarded to a verifier. We run a Pareto search over thresholds and seven verifier slots, including zero-shot Qwen3.5-4B, LoRA-tuned Qwen, voting, self-consistency, MultiAgentESC-style routing, heterogeneous voting, and a second encoder. Across three-seed RoBERTa experiments, DeBERTa robustness checks, and an additional Russian XLM-R diagnostic, no compact-LLM verifier improves the high-quality low-cost frontier. The mechanism is a selection effect: encoder gating forwards a conditional slice on which compact LLMs perform no better than the encoder itself, or substantially worse, erasing any standalone LLM advantage. Encoder-only gating, split-conformal acceptance, and a small RoBERTa ensemble are strictly preferable alternatives.