Add embeddings & reranking via Sentence Transformers

[tomaarsen]

Description

• Add Sentence Transformers embeddings & reranking

Related Issues

None

Changes Made

1. Add sentence_transformers_embed with auto embedding_dim and max_token_size, allowing you to use any model from https://huggingface.co/models?library=sentence-transformers
2. Add sentence_transformers_rerank, allowing you to use any model from https://huggingface.co/models?pipeline_tag=text-ranking&library=sentence-transformers
3. Update READMEs
4. Add a demo script based on lightrag_hf_demo.py

Checklist

• Changes tested locally
• Code reviewed
• Documentation updated (if necessary)
• Unit tests added (if applicable)

Additional Notes

I wasn't able to get lightrag_hf_demo.py to run nicely, so I couldn't extensively test the new demo lightrag_sentence_transformers_demo.py either, but the embedding and reranking components do work on their own.

• Tom Aarsen

[danielaskdd]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

LightRAG/lightrag/api/config.py

Lines 242 to 246 in 23e7ffb

	choices=[
	"lollms",
	"ollama",
	"openai",
	"azure_openai",

Allow CLI to select sentence_transformers embedding binding

Even though create_app adds logic for a sentence_transformers embedding binding, the CLI still restricts --embedding-binding to the list shown here, so passing --embedding-binding sentence_transformers results in an argparse "invalid choice" error. Because of this omission the new embedding provider cannot be enabled from the server’s documented configuration interface.

---

LightRAG/lightrag/api/config.py

Lines 254 to 257 in 23e7ffb

	"--rerank-binding",
	type=str,
	default=get_env_value("RERANK_BINDING", DEFAULT_RERANK_BINDING),
	choices=["null", "cohere", "jina", "aliyun"],

Add sentence_transformers to rerank binding choices

Similarly, the argparse definition for --rerank-binding still lists only null|cohere|jina|aliyun even though the server now maps a sentence_transformers reranker (lightrag/api/lightrag_server.py lines 943‑998). Attempting to run the API with --rerank-binding sentence_transformers is rejected before the app starts, making the newly added reranker unreachable via the supported CLI/environment configuration.

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[chatgpt-codex-connector]

Wire sentence_transformers binding with required model

Selecting the new sentence_transformers embedding binding crashes immediately. The server block calls actual_func(texts, embedding_dim=embedding_dim) without ever constructing or passing a SentenceTransformer instance, yet sentence_transformers_embed (lightrag/llm/sentence_transformers.py lines 13‑32) requires a model positional argument and does not accept embedding_dim. As soon as the binding is chosen the API raises TypeError (missing model / unexpected embedding_dim), so the embedding provider cannot be used at all.

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