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Eleven text encoders hold sentiment at 91 percent probe accuracy while their similarity metric separates polarity by 0.003 to 0.064

AspireAI community · arXiv
The paper documents a dissociation in modern text embedders: sentiment is almost perfectly decodable from their embeddings, yet nearly absent from the metric that retrieval actually runs on. Across eleven frozen encoders from 2021 to 2026, including 2026-generation models and the 4B-parameter Qwen3-Embedding-4B, a linear probe recovers sentence polarity at up to 91.3 percent accuracy while cosine similarity separates polarity by a margin of only 0.003 to 0.064. Because retrieval, clustering, deduplication and retrieval-augmented generation all operate on that metric rather than on a trained probe, these systems are effectively blind to information their encoder demonstrably contains, and Qwen3-Embedding-4B reaches 91.3 percent probe accuracy with only 64.3 retrieval MAP. The fix is specialization, not a free win: a small supervised-contrastive head trained in seconds on CPU lifts probe accuracy by up to 5.8 points and retrieval MAP by up to 26.0, but the 128-d head loses 38 to 41 Spearman points on STS-B and a fused variant loses 2.4 to 12.0.
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