Choosing Embedding Models for RAG

Embeddings are the foundation of semantic search. The right model balances retrieval accuracy (MTEB score) with vector database cost and latency.

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Dimensionality Trade-offs

Higher dimensions (e.g., 1536) capture more nuance but require significantly more RAM/storage in a vector database like Pinecone or Qdrant. A 384-dimensional open-source model (like BGE-small) can often provide 95% of the retrieval performance at 25% of the storage cost.

Context Window Limits

Standard embedding models chunk text at 512 tokens. If your chunks are larger (e.g., full pages), you need models specifically trained for long contexts (e.g., Nomic Embed), or you must implement rigorous document chunking strategies before embedding.

Local vs API Embeddings

Because embedding generation is highly parallelizable and doesn't require auto-regressive generation, it is exceptionally fast to run locally. Self-hosting embedding models via ONNX or sentence-transformers is almost always preferable to paying API costs at scale.

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