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.
