Train Sentence Transformers

Hugging FaceOfficialPublished by Hugging Face

Skill
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Data and AI
Apache-2.0
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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/dataset_formats.md

Dataset Formats

This reference covers: how datasets map to losses, how to reshape data when it doesn't fit, and how to mine hard negatives.

The two rules

From the sentence-transformers training overview:

If the loss requires a label, the dataset must have a column named label, labels, score, or scores. Any column with one of these names is the label.
All other columns are inputs. The loss defines how many input columns it expects. Column names don't matter. Order does.

Example: CoSENTLoss expects 2 inputs + a float label. A dataset with columns ["premise", "hypothesis", "score"] works. A dataset with ["score", "premise", "hypothesis"] does not. Reorder first.

Per-loss data shapes

The per-type loss references (losses_sentence_transformer.md, losses_cross_encoder.md, losses_sparse_encoder.md) are the canonical mappings from data shape to loss. Cross-cutting recipe bits those tables don't show:

CosineSimilarityLoss wants score normalized to [0, 1]. CoSENTLoss / AnglELoss are pairwise-ranking and ignore absolute scale, so on stsb (raw 0-5) divide by 5 only when using cosine-similarity.
BatchAllTripletLoss / BatchHardTripletLoss / BatchSemiHardTripletLoss need batch_sampler=BatchSamplers.GROUP_BY_LABEL so multiple samples per label appear in the same batch.
MSELoss (distillation) label is the teacher's full embedding vector (a list of floats), not a scalar score.
MarginMSELoss (distillation) label is teacher_score(q, pos) - teacher_score(q, neg), precomputed per row.
N-tuple shape for MNRL (anchor, positive, negative_1, negative_2, ..., negative_N) (1-indexed) is produced by mine_hard_negatives(..., output_format="n-tuple"). "labeled-list" output_format produces the CrossEncoder listwise shape.

Reshaping operations

If your data doesn't fit the loss's expected shape:

Reorder columns

# Columns are ["hypothesis", "premise", "score"] but CoSENTLoss expects premise first.
dataset = dataset.select_columns(["premise", "hypothesis", "score"])

Rename label column

# Your label is called "relevance" but ST wants "label".
dataset = dataset.rename_column("relevance", "label")

Drop extra columns

# ST will treat every non-label column as an input. Drop metadata.
dataset = dataset.remove_columns(["source_id", "created_at", "language"])

Convert dtypes

# Label is str, need float for CoSENTLoss.
dataset = dataset.map(lambda x: {"label": float(x["label"])})

Hard-negative mining

mine_hard_negatives (in sentence_transformers.util) produces a training dataset with mined negatives using a retriever. Hard negatives are the single highest-leverage lever for retrieval-model quality.

Basic usage

from sentence_transformers import SentenceTransformer
from sentence_transformers.util import mine_hard_negatives

retriever = SentenceTransformer("sentence-transformers/all-MiniLM-L6-v2")

mined = mine_hard_negatives(
    dataset=train_pairs,                # has (anchor, positive) or (q, a) columns
    model=retriever,
    num_negatives=5,
    range_min=0, range_max=100,         # rank window to sample hard negatives from
    sampling_strategy="top",            # "top" = rank-1 hardest; "random" = random in window
    output_format="n-tuple",            # "triplet" | "n-tuple" | "labeled-pair" | "labeled-list"
    use_faiss=True,
)

Output formats

"triplet": (anchor, positive, negative) triplets. One row per (query, negative) pair.
"n-tuple": (anchor, positive, negative_1, negative_2, ..., negative_N) (1-indexed). One row per query.
"labeled-pair": (anchor, text, label) with label=1 for positives and label=0 for negatives. Good for BinaryCrossEntropyLoss.
"labeled-list": (anchor, texts, labels). One row per query with a list of candidates. Good for listwise losses.

Filtering false negatives

If the retriever returns "negatives" that are actually relevant, they become false negatives and hurt training. Filter them:

mined = mine_hard_negatives(
    dataset=train_pairs,
    model=retriever,
    cross_encoder=CrossEncoder("cross-encoder/ms-marco-MiniLM-L-6-v2"),   # score candidates
    num_negatives=5,
    max_score=0.9,                      # drop candidates scoring above 0.9
    relative_margin=0.05,               # require neg_score <= pos_score - abs(pos_score) * 0.05
    absolute_margin=0.2,                # require neg_score <= pos_score - 0.2
    output_format="n-tuple",
    use_faiss=True,
)

Use either relative_margin or absolute_margin, usually not both. max_score is independently useful as a hard ceiling.

CLI

scripts/mine_hard_negatives.py is a CLI wrapper. See it for a ready-to-run command.

Choosing the right `range_min` / `range_max`

range_max=None is the default. Pass an integer to cap how far down the ranked list to sample from.

range_min=0, range_max=100: sample from the top-100 retrieved. Good default.
range_min=10, range_max=100: skip the top-10 (often contains true positives). Safer if you lack a cross-encoder.
range_min=0, range_max=1000: wider net, more diverse negatives, slower.
sampling_strategy="top": always pick the rank-1 hardest. Maximum training signal per row.
sampling_strategy="random": pick randomly within the range. More robust if your retriever is itself noisy.

Quick Hub-side dataset checks

hf datasets sql "SELECT * FROM 'hf://datasets/<id>/<split>' LIMIT 5" streams rows via DuckDB without load_dataset(...). This is the fastest way to confirm column names match your loss before a full validation run. hf datasets info <id> shows config / splits / size. hf datasets card <id> --text renders the README.

Gotchas

remove_unused_columns=True (default): the trainer drops columns that aren't passed to the model's forward. Usually fine, but if you rely on a custom collator that uses metadata columns, set remove_unused_columns=False.
Floats stored as strings after CSV load: load_dataset("csv", ...) keeps columns as strings by default. Cast with .map(lambda x: {"label": float(x["label"])}).
Mined hard negatives with include_positives=True include the positive as a negative in the output list, only useful when you're building an evaluator or want to measure rank of the positive. For training, leave it False.

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

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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

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