Train Sentence Transformers

Hugging FaceOfficialPublished by Hugging Face

Skill
Workflow
Data and AI
Apache-2.0
Medium risk
Has scripts

From the author

Train or fine-tune sentence-transformers models across `SentenceTransformer` (bi-encoder, dense or static embedding model for retrieval, similarity, clustering, classification, paraphrase mining, dedup, multimodal), `CrossEncoder` (reranker, pair scoring for two-stage retrieval / pair classification), `SparseEncoder` (SPLADE, sparse embedding model for learned-sparse retrieval), and `MultiVectorEncoder` (ColBERT / late-interaction, per-token embeddings scored with MaxSim). Covers loss selection, hard-negative mining, evaluators, distillation, LoRA, Matryoshka, and Hugging Face Hub publishing. Use for any sentence-transformers training task.

— the description by Hugging Face

AutoGineer summary

Written by AutoGineer from the skill’s own files, not by the author

This skill provides a standardized approach to training various embedding and ranking models using the sentence-transformers framework. It acts as a routing system that directs users to specific production templates and technical references based on the chosen model architecture. The workflow covers the entire lifecycle from model selection and data preparation to loss function configuration, evaluation, and publishing to the Hugging Face Hub.

What it does

Routes users to appropriate training templates for bi-encoders, cross-encoders, sparse encoders, and multi-vector encoders.
Provides technical references for loss functions, evaluators, and model architectures.
Offers guidance on hard-negative mining and dataset formatting for training.
Includes instructions for hardware-specific configurations and Hugging Face Hub integration.
Defines mandatory script requirements such as smoke testing, logging, and verdict reporting.

When to use it

When you need to train or fine-tune a sentence-transformers model for retrieval, similarity, or ranking tasks.
When you are unsure which model architecture (e.g., bi-encoder vs. cross-encoder) is best suited for your specific use case.
When you require a structured, repeatable process for training and evaluating embedding models.
When you need to integrate advanced training techniques like distillation, LoRA, or Matryoshka embeddings.

Advantages

Enforces consistent training practices through standardized production templates.
Reduces common errors by providing specific references for loss-to-data-shape mapping.
Facilitates systematic experimentation with built-in logging and verdict reporting.
Supports a wide range of model types including modern late-interaction and sparse architectures.

Usage examples

“I need to train a bi-encoder for semantic search on my internal documentation.”: The agent will identify the [SentenceTransformer] type, provide the relevant loss and evaluator references, and guide you to the bi-encoder production template.
“How do I implement a two-stage retrieval system using a reranker?”: The agent will direct you to the [CrossEncoder] documentation and template, explaining how to use it to score pairs for two-stage retrieval.
“I want to train a SPLADE model for sparse retrieval.”: The agent will provide the [SparseEncoder] references, including the required SpladeLoss wrapper and specific evaluators for sparse retrieval.
“How can I improve my retrieval model using hard negatives?”: The agent will guide you to the hard-negative mining script and explain how to generate n-tuple datasets for training.

Install

Download the skill as a ZIP:
https://mp-staging.autogineer.com/agent-skills/huggingface/train-sentence-transformers/download

Unzip it into the cross-client skills folder of your project:

mkdir -p .agents/skills && unzip train-sentence-transformers.zip -d .agents/skills

Documentation by Hugging Face

Train a sentence-transformers Model

This SKILL.md is a router, not a manual. It tells you which references and example scripts to load for your task. The actual content (recommended losses, evaluators, training-script structure, model selection, training-arg knobs, troubleshooting) lives in references/ and scripts/.

Do not synthesize a training script from this file alone. Open the per-type production template (scripts/train_<type>_example.py) and copy it as your starting point. The templates contain load-bearing scaffolding (autocast helper, model-card class, logger silencing list, force=True, seed, TF32, version-compatible imports, named-evaluator metric handling) that prior agent runs have repeatedly missed when rolling their own from a synthesized snippet.

1. Identify the model type

Tag	Class	What it does	When to pick
[SentenceTransformer]	`SentenceTransformer` (bi-encoder)	Maps each input to a fixed-dim dense vector	Retrieval, similarity, clustering, classification, paraphrase mining, dedup
[CrossEncoder]	`CrossEncoder` (reranker)	Scores `(query, passage)` pairs jointly	Two-stage retrieval (rerank top-100 from bi-encoder), pair classification
[SparseEncoder]	`SparseEncoder` (SPLADE)	Sparse vectors over the vocabulary	Learned-sparse retrieval, inverted-index backends (Elasticsearch / OpenSearch / Lucene)
[MultiVectorEncoder]	`MultiVectorEncoder` (ColBERT)	One embedding per token, scored with MaxSim	Late-interaction retrieval, recall gains over bi-encoders at higher storage cost, multimodal (ColPali / ColQwen2)

Tiebreakers when the request is ambiguous: "embedding model" / "vector search" / "similarity" → [SentenceTransformer]. "rerank" / "ranker" / "two-stage" → [CrossEncoder]. "SPLADE" / "sparse" / "inverted index" → [SparseEncoder]. "ColBERT" / "late interaction" / "multi-vector" / "MaxSim" / "ColPali" / "ColQwen" → [MultiVectorEncoder]. If still unclear, ask.

2. Required reading

Read these in full before writing any code. Do not triage by perceived relevance.

Per-type: always required

[SentenceTransformer]

references/losses_sentence_transformer.md: loss-to-data-shape mapping, BatchSamplers.NO_DUPLICATES requirement for MNRL-family, Cached* ↔ gradient_checkpointing incompatibility.
references/evaluators_sentence_transformer.md: evaluator-to-task mapping, metric_for_best_model key construction (named vs unnamed), per-evaluator primary_metric values.
references/model_architectures.md: encoder vs decoder vs static vs Router pipelines, pooling rules (mean / cls / lasttoken), auto-mean-pooling behavior for fresh-start MLM bases.
scripts/train_sentence_transformer_example.py: production template. Copy this as your starting point.

[CrossEncoder]

references/losses_cross_encoder.md: pointwise / pairwise / listwise / distillation, pos_weight derivation, activation_fn=Identity() mandatory for non-BCE losses (silent eval-rank collapse otherwise).
references/evaluators_cross_encoder.md: CrossEncoderRerankingEvaluator recipe, named-evaluator key format eval_{name}_{primary_metric}.
scripts/train_cross_encoder_example.py: production template. Copy this as your starting point.

[SparseEncoder]

references/losses_sparse_encoder.md: SpladeLoss wrapper requirement, FLOPS regularizer weights, smoke-test active-dim ramp behavior.
references/evaluators_sparse_encoder.md: SparseNanoBEIREvaluator (English-only) and the in-domain alternative, eval_{name}_{primary_metric} key format.
scripts/train_sparse_encoder_example.py: production template. Copy this as your starting point.

[MultiVectorEncoder]

references/losses_multi_vector_encoder.md: MaxSim scoring, scale choice per scoring mode (scale=1.0 for MaxSim, roughly the average query length for MeanMaxSim), MNRL / CachedMNRL / MarginMSE / DistillKLDiv, XTR-vs-ColBERT scoring, CachedMNRL ↔ gradient_checkpointing incompatibility.
references/evaluators_multi_vector_encoder.md: MultiVectorNanoBEIREvaluator (English-only) and the in-domain alternative, eval_NanoBEIR_mean_maxsim_ndcg@10 key format, distillation-eval spearman variant.
scripts/train_multi_vector_encoder_example.py: production template. Copy this as your starting point.

Cross-cutting: always required (regardless of task)

references/training_args.md: TrainingArguments knobs, precision rules (load fp32 + autocast bf16/fp16, never torch_dtype=bfloat16), warmup_steps (float) vs deprecated warmup_ratio, save_steps must be a multiple of eval_steps for load_best_model_at_end, schedulers, HPO, tracker, resume, hub-push variants.
references/dataset_formats.md: column-matching rules (label name auto-detection, column-order-not-name), reshaping recipes, hard-negative mining options.
references/base_model_selection.md: discovery commands, per-type model namespaces, ModernBERT-family max_seq_length=8192 trap, datasets >= 4 script-loader rejection, non-English starting-point shortcuts.
references/troubleshooting.md: symptom-indexed failure recipes. Skim the section headings on every run, even a healthy one. The "Metrics don't improve" and "Hub push fails" entries cover bugs that bite frequently and are cheaper to recognize before they fire than to debug after.

Cross-cutting: load when applicable

references/hardware_guide.md: VRAM sizing, multi-GPU, FSDP / DeepSpeed, HF Jobs flavors. Required for >24GB models, multi-GPU, or HF Jobs runs.
references/hf_jobs_execution.md: required when running on HF Jobs.
references/prompts_and_instructions.md: required when using prompt-tuned bases (E5, BGE, GTE, Qwen3-Embedding, Instructor, Nomic, etc.) or adding query: / passage: style prefixes.

Variant scripts (open when the task matches)

[SentenceTransformer] scripts/train_sentence_transformer_<matryoshka|multi_dataset|with_lora|distillation|make_multilingual|static_embedding>_example.py.
[CrossEncoder] scripts/train_cross_encoder_<distillation|listwise>_example.py.
[SparseEncoder] scripts/train_sparse_encoder_distillation_example.py.
Hard-negative mining CLI: scripts/mine_hard_negatives.py.

3. Defaults

Override only if the user specifies otherwise:

Local execution. Pitch HF Jobs only if local hardware can't fit the job.
Single run. After it completes, propose experimentation if the user would benefit (weak/marginal verdict, "see how high you can push it" framing, etc.). Iteration rules in references/training_args.md (Experimentation section).
Public Hub push at end-of-run, wrapped in try-except. On HF Jobs (ephemeral env) ALSO enable in-trainer push (push_to_hub=True + hub_strategy="every_save"). Details in references/hf_jobs_execution.md.

4. Constraints the produced script must satisfy

These are non-negotiable contracts. Implementation lives in the production templates and references. Do not reinvent.

Capture the pre-training evaluator score as baseline_eval before trainer.train().
Emit a single end-of-run line: VERDICT: WIN|MARGINAL|REGRESSION | score=... | baseline=... | delta=.... A monitor scrapes for this.
Silence httpx, httpcore, huggingface_hub, urllib3, filelock, fsspec to WARNING (otherwise HF download URLs flood the agent's context).
Tee logs to logs/{RUN_NAME}.log.
End with model.push_to_hub(...) wrapped in try/except.
Smoke-test before any long run (max_steps=1 + tiny dataset slice). The production templates show one common pattern (SMOKE_TEST env var).
[CrossEncoder] Include EarlyStoppingCallback(patience>=3). CE rerankers often peak mid-training and regress.
[SparseEncoder] Log query_active_dims / corpus_active_dims on the verdict line. High nDCG with collapsed sparsity is not a win. The keys come back name-prefixed (e.g. ..._query_active_dims). Use suffix matching to pluck them. See the SPARSE production template for the exact pattern.
[MultiVectorEncoder] Match scale to the scoring mode on any MNRL-family loss: near 1.0 for unnormalized MaxSim (do not copy scale=20.0 from bi-encoder MNRL), roughly the average query length with length-normalized MeanMaxSim, since each score is divided by its query's token count. XTRScores is a train-only similarity_fct: the evaluators reject it, so evaluation always scores with MaxSim, including for XTR-trained models.

5. Workflow

Identify the model type (§1). Ask if ambiguous.
Load the §2 required-reading files for that type.
Open scripts/train_<type>_example.py and copy it as your starting point.
Replace MODEL_NAME, DATASET_NAME, RUN_NAME, the loss, and the evaluator with the user's task. Cross-check loss/data-shape match against references/losses_<type>.md. Cross-check the metric_for_best_model key against references/evaluators_<type>.md (named evaluators format the key as eval_{name}_{primary_metric}).
Smoke-test (max_steps=1).
Run.
After the run, append to logs/experiments.md and propose iteration if the verdict is weak/marginal.

Prerequisites

pip install "sentence-transformers[train]>=5.0"        # add [train,image] / [audio] / [video] for [SentenceTransformer] multimodal
                                                       # [MultiVectorEncoder] requires >=6.0
pip install trackio                                    # optional tracker (or wandb / tensorboard / mlflow)
hf auth login                                          # or set HF_TOKEN with write scope (for Hub push)

GPU strongly recommended. CPU works only for demos and [SentenceTransformer] StaticEmbedding.

Files

ATTRIBUTION.mdAdded by AutoGineer898 B
LICENSELicence file added by AutoGineer11.1 KiB
SKILL.md10.3 KiB
references/base_model_selection.md7.5 KiB
references/dataset_formats.md6.3 KiB
references/evaluators_cross_encoder.md5.5 KiB
references/evaluators_multi_vector_encoder.md6.6 KiB
references/evaluators_sentence_transformer.md5.8 KiB
references/evaluators_sparse_encoder.md4.9 KiB
references/hardware_guide.md5.9 KiB
references/hf_jobs_execution.md7.1 KiB
references/losses_cross_encoder.md9.5 KiB
references/losses_multi_vector_encoder.md10.5 KiB
references/losses_sentence_transformer.md10.7 KiB
references/losses_sparse_encoder.md5.1 KiB
references/model_architectures.md9.0 KiB
references/prompts_and_instructions.md6.4 KiB
references/training_args.md15.0 KiB
references/troubleshooting.md12.9 KiB
scripts/mine_hard_negatives.pyScript8.3 KiB
scripts/train_cross_encoder_distillation_example.pyScript9.2 KiB
scripts/train_cross_encoder_example.pyScript9.0 KiB
scripts/train_cross_encoder_listwise_example.pyScript10.2 KiB
scripts/train_multi_vector_encoder_example.pyScript8.8 KiB
scripts/train_sentence_transformer_distillation_example.pyScript11.9 KiB
scripts/train_sentence_transformer_example.pyScript7.5 KiB
scripts/train_sentence_transformer_make_multilingual_example.pyScript11.5 KiB
scripts/train_sentence_transformer_matryoshka_example.pyScript7.1 KiB
scripts/train_sentence_transformer_multi_dataset_example.pyScript9.9 KiB
scripts/train_sentence_transformer_static_embedding_example.pyScript10.3 KiB
scripts/train_sentence_transformer_with_lora_example.pyScript10.3 KiB
scripts/train_sparse_encoder_distillation_example.pyScript9.4 KiB
scripts/train_sparse_encoder_example.pyScript8.1 KiB

references/prompts_and_instructions.md

Prompts and Instructions

Modern embedding models (E5, BGE, GTE, Qwen3-Embedding, Nomic, Instructor, etc.) use prompts / instructions at encode time (short prefixes like "query: ", "passage: ", or "Represent this sentence for retrieval:") to signal task intent.

In sentence-transformers, prompts work during both training and inference, and the library keeps them aligned automatically.

The prompt is literally prepended to the input text before tokenization.

Model-level prompts

Set prompts on the model so encode(), encode_query(), encode_document() use them automatically:

model = SentenceTransformer("your-base-model")
model.prompts = {
    "query":    "query: ",
    "document": "passage: ",
}
model.default_prompt_name = "document"        # used when none is specified

At inference:

q_emb = model.encode_query("What is the capital of France?")
# Internally: encodes "query: What is the capital of France?"

d_emb = model.encode_document(["Paris is the capital of France.", "Berlin is the capital of Germany."])
# Internally: encodes "passage: Paris..." etc.

# Explicit prompt_name:
emb = model.encode(["some text"], prompt_name="query")

Training with prompts

Set prompts on the training args so they're applied automatically to the right columns during training. Four shapes, increasingly specific:

1. Single prompt (applied everywhere)

args = SentenceTransformerTrainingArguments(
    ...,
    prompts="Represent this sentence for similarity: ",
)

Every input column gets the prefix. Simplest and often sufficient for single-task training.

2. Per-column prompts

prompts = {
    "anchor":   "query: ",
    "positive": "passage: ",
    "negative": "passage: ",
}

Keys are column names. Different prefixes for different roles.

3. Per-dataset prompts (multi-dataset)

prompts = {
    "all-nli": "Classify the entailment relationship: ",
    "stsb":    "Score semantic similarity: ",
}

Keys are dataset names (matching the train_dataset dict keys).

4. Per-dataset + per-column

prompts = {
    "all-nli": "",                                          # all-nli: no prompt
    "msmarco": {                                            # msmarco: per-column
        "query":    "query: ",
        "positive": "passage: ",
        "negative": "passage: ",
    },
}

Cross-encoder specifics

Cross-encoders restrict prompts to single-value or per-dataset only (no per-column). This is because cross-encoders take text pairs, not separate columns.

args = CrossEncoderTrainingArguments(
    ...,
    prompts="Rank this passage for the query: ",   # single prompt
)
# or
args = CrossEncoderTrainingArguments(
    ...,
    prompts={"msmarco": "...", "gooaq": "..."},    # per-dataset
)

Sparse encoders (SPLADE)

Sparse encoders support prompts with the same four shapes as bi-encoders (added in v5.x). Same model.prompts = {...} API.

Pooling and prompt tokens (bi-encoder)

When using mean/last-token pooling with prompt prefixes, the prompt tokens themselves can either:

Be included in the pooled embedding (default behavior). Simpler, what most models do.
Be excluded: only the actual content tokens are pooled.

To exclude, flip include_prompt on the Pooling module, found via isinstance rather than a fixed index, since multimodal / Router pipelines don't always put Pooling at position 1:

from sentence_transformers.sentence_transformer.modules import Pooling

for module in model:
    if isinstance(module, Pooling):
        module.include_prompt = False

Or use the helper if your model exposes one (e.g. SparseEncoder.set_pooling_include_prompt(False)).

When to exclude: when the prompt is purely a task signal and you don't want it to dilute the semantic representation. Most E5 / BGE / GTE models leave prompts included.

During training with prompts=, the include_prompt setting is respected. The trainer also tracks include_prompt_lengths internally so pooling skips the prompt tokens correctly when include_prompt=False.

Inference alignment

If you trained with prompts={"query": "query: ", ...}, you must:

Save the prompts on the model: model.prompts = args.prompts before save_pretrained, OR let the library do this automatically via the model card data.
Use the same prompts at inference: call encode_query() / encode_document() and the saved prompts are applied.

If the saved model's config_sentence_transformers.json has prompts + default_prompt_name, anyone who loads it gets the correct prompts for free.

Instruction tuning (different from prompts)

Some models (Instructor, Qwen3-Embedding) use instructions, longer descriptions like "Represent the biomedical query for retrieving relevant passages:". In sentence-transformers these are modeled the same way: just set them as prompts.

The model-card convention is to list available instructions/prompts in a dict:

model.prompts = {
    "msmarco-query":    "Represent the query for MS MARCO retrieval: ",
    "msmarco-doc":      "Represent the passage for MS MARCO retrieval: ",
    "sts":              "Represent the sentence for semantic similarity: ",
    "classification":   "Represent the sentence for topic classification: ",
}

Users call model.encode(["..."], prompt_name="msmarco-query").

Gotchas

Training with prompts but forgetting to set them on the model before save_pretrained: the saved model won't know about the prompts. Users will encode without the prefix and get bad results. Fix: model.prompts = args.prompts before saving, or use SentenceTransformerModelCardData(...) with the prompt info.
Using encode_query() at inference but didn't train with a "query" prompt: the method will just call regular encode (no prefix applied), which is fine. But the documentation implies you use it, and users may be confused.
Trailing spaces matter: "query: " vs "query:". The former has a trailing space before the real text. Inspect your training data to know which one your model was trained with.
Mixing prompted and non-prompted data in multi-dataset training is fine as long as your prompts dict covers it per-dataset (e.g. {"dataset_a": "query: ", "dataset_b": ""}).
include_prompt=False with small datasets: the model may under-fit because you've effectively shortened every input. Usually leave as True.

Requirements and compatibility

Works with: Claude Code, Claude apps, OpenAI Codex, GitHub Copilot, Cursor, Gemini CLI, Kiro, OpenCode, Goose, Amp, Roo Code, JetBrains Junie, Factory Droid, Devin (ex-Windsurf), Mistral Vibe, OpenHands, Letta

“Works with” means the tool reads the open Agent Skills format; a skill written for one tool may use features others lack.

Needs: Needs Python

Security

What our scanner noticed· Medium risk

Instructions that use the network
Downloads piped into a shell
Scripts that your agent may run

This skill includes scripts that your agent may run. Read them before you use it.

A static scan is not a guarantee. Skills are third-party content: review a skill before you let an agent use it.

Attribution and licence

Published by

Hugging Face

Publisher

Hugging Face (Official)

Source

huggingface/skills · skills/train-sentence-transformers · commit 25910b3

Licence

Apache License 2.0 (redistribution allowed)

LICENSE (added by AutoGineer from LICENSE)
ATTRIBUTION.md (written by AutoGineer)

Synced

2026-11-08

AutoGineer is not affiliated with or endorsed by Hugging Face.

Download ZIP

The ZIP holds the skill’s files, its licence files and an ATTRIBUTION.md. Download train-sentence-transformers.zip