Anthropic reports large gains from contextual retrieval. What exactly did they measure, and what does it cost?

1 − recall@20: the share of relevant chunks missing from the top 20. Contextual embeddings cut it 35% (5.7% → 3.7%); adding contextual BM25, 49%; adding a reranker, 67% (to 1.9%), on their evaluation sets.

The cost is a model call per chunk at indexing time, repeated whenever documents change, plus longer chunks to embed and store.