Sentence Transformers - State-of-the-Art Embeddings

Python framework for sentence and text embeddings using transformers.

When to use Sentence Transformers

Use when:

• Need high-quality embeddings for RAG

• Semantic similarity and search

• Text clustering and classification

• Multilingual embeddings (100+ languages)

• Running embeddings locally (no API)

• Cost-effective alternative to OpenAI embeddings

Metrics:

• 15,700+ GitHub stars

• 5000+ pre-trained models

• 100+ languages supported

• Based on PyTorch/Transformers

Use alternatives instead:

• OpenAI Embeddings: Need API-based, highest quality

• Instructor: Task-specific instructions

• Cohere Embed: Managed service

Quick start

Installation

pip install sentence-transformers

Basic usage

from sentence_transformers import SentenceTransformer

Load model

model = SentenceTransformer('all-MiniLM-L6-v2')

Generate embeddings

sentences = [ "This is an example sentence", "Each sentence is converted to a vector" ]

embeddings = model.encode(sentences) print(embeddings.shape) # (2, 384)

Cosine similarity

from sentence_transformers.util import cos_sim similarity = cos_sim(embeddings[0], embeddings[1]) print(f"Similarity: {similarity.item():.4f}")

Popular models

General purpose

Fast, good quality (384 dim)

model = SentenceTransformer('all-MiniLM-L6-v2')

Better quality (768 dim)

model = SentenceTransformer('all-mpnet-base-v2')

Best quality (1024 dim, slower)

model = SentenceTransformer('all-roberta-large-v1')

Multilingual

50+ languages

model = SentenceTransformer('paraphrase-multilingual-MiniLM-L12-v2')

100+ languages

model = SentenceTransformer('paraphrase-multilingual-mpnet-base-v2')

Domain-specific

Legal domain

model = SentenceTransformer('nlpaueb/legal-bert-base-uncased')

Scientific papers

model = SentenceTransformer('allenai/specter')

Code

model = SentenceTransformer('microsoft/codebert-base')

Semantic search

from sentence_transformers import SentenceTransformer, util

model = SentenceTransformer('all-MiniLM-L6-v2')

Corpus

corpus = [ "Python is a programming language", "Machine learning uses algorithms", "Neural networks are powerful" ]

Encode corpus

corpus_embeddings = model.encode(corpus, convert_to_tensor=True)

Query

query = "What is Python?" query_embedding = model.encode(query, convert_to_tensor=True)

Find most similar

hits = util.semantic_search(query_embedding, corpus_embeddings, top_k=3) print(hits)

Similarity computation

Cosine similarity

similarity = util.cos_sim(embedding1, embedding2)

Dot product

similarity = util.dot_score(embedding1, embedding2)

Pairwise cosine similarity

similarities = util.cos_sim(embeddings, embeddings)

Batch encoding

Efficient batch processing

sentences = ["sentence 1", "sentence 2", ...] * 1000

embeddings = model.encode( sentences, batch_size=32, show_progress_bar=True, convert_to_tensor=False # or True for PyTorch tensors )

Fine-tuning

from sentence_transformers import InputExample, losses from torch.utils.data import DataLoader

Training data

train_examples = [ InputExample(texts=['sentence 1', 'sentence 2'], label=0.8), InputExample(texts=['sentence 3', 'sentence 4'], label=0.3), ]

train_dataloader = DataLoader(train_examples, batch_size=16)

Loss function

train_loss = losses.CosineSimilarityLoss(model)

Train

model.fit( train_objectives=[(train_dataloader, train_loss)], epochs=10, warmup_steps=100 )

Save

model.save('my-finetuned-model')

LangChain integration

from langchain_community.embeddings import HuggingFaceEmbeddings

embeddings = HuggingFaceEmbeddings( model_name="sentence-transformers/all-mpnet-base-v2" )

Use with vector stores

from langchain_chroma import Chroma

vectorstore = Chroma.from_documents( documents=docs, embedding=embeddings )

LlamaIndex integration

from llama_index.embeddings.huggingface import HuggingFaceEmbedding

embed_model = HuggingFaceEmbedding( model_name="sentence-transformers/all-mpnet-base-v2" )

from llama_index.core import Settings Settings.embed_model = embed_model

Use in index

index = VectorStoreIndex.from_documents(documents)

Model selection guide

Model Dimensions Speed Quality Use Case

all-MiniLM-L6-v2 384 Fast Good General, prototyping

all-mpnet-base-v2 768 Medium Better Production RAG

all-roberta-large-v1 1024 Slow Best High accuracy needed

paraphrase-multilingual 768 Medium Good Multilingual

Best practices

• Start with all-MiniLM-L6-v2 - Good baseline

• Normalize embeddings - Better for cosine similarity

• Use GPU if available - 10× faster encoding

• Batch encoding - More efficient

• Cache embeddings - Expensive to recompute

• Fine-tune for domain - Improves quality

• Test different models - Quality varies by task

• Monitor memory - Large models need more RAM

Performance

Model Speed (sentences/sec) Memory Dimension

MiniLM ~2000 120MB 384

MPNet ~600 420MB 768

RoBERTa ~300 1.3GB 1024

Resources

• GitHub: https://github.com/UKPLab/sentence-transformers ⭐ 15,700+

• Models: https://huggingface.co/sentence-transformers

• Docs: https://www.sbert.net

• License: Apache 2.0