Train Sentence Transformers

Training Arguments

SentenceTransformerTrainingArguments, CrossEncoderTrainingArguments, and SparseEncoderTrainingArguments all inherit from Hugging Face's TrainingArguments, so 95% of the arguments are the same.

This reference covers the arguments that actually matter for embedding-model training.

The recommended default set

Start with this and adjust only what you have a reason to change:

from sentence_transformers import SentenceTransformerTrainingArguments
from sentence_transformers.base.sampler import BatchSamplers

args = SentenceTransformerTrainingArguments(
    output_dir="models/my-model",

    # Duration
    num_train_epochs=1,
    # max_steps=10_000,                    # alternative to epochs

    # Batch size
    per_device_train_batch_size=64,
    per_device_eval_batch_size=64,
    gradient_accumulation_steps=1,

    # Optimizer
    learning_rate=2e-5,
    warmup_steps=0.1,                      # transformers v5.2 deprecated `warmup_ratio`; pass the ratio as a float directly to `warmup_steps`
    lr_scheduler_type="linear",
    weight_decay=0.0,

    # Precision
    bf16=True,                             # fp16=True on older GPUs (T4, V100)

    # Sampler (bi-encoder + sparse-encoder)
    batch_sampler=BatchSamplers.NO_DUPLICATES,

    # Eval + checkpointing
    eval_strategy="steps",
    eval_steps=0.1,                        # fraction: 10 evals/epoch, scales with dataset size
    save_strategy="steps",
    save_steps=0.1,                        # keep aligned with eval_steps for load_best_model_at_end
    save_total_limit=2,
    load_best_model_at_end=True,
    metric_for_best_model="eval_NanoBEIR_mean_cosine_ndcg@10",
    greater_is_better=True,

    # Logging
    logging_steps=0.01,                    # fraction: ~100 log lines/epoch
    logging_first_step=True,
    run_name="my-model",
    report_to="trackio",                   # or "wandb", "tensorboard", "mlflow", "none"
)

Duration

• num_train_epochs: most common. 1 for large datasets (>500k), 3-10 for small.

• max_steps: use instead of epochs when you want a fixed compute budget. Overrides num_train_epochs.

• For huge datasets where 1 epoch is overkill, pick max_steps matching your compute plan.

Batch size

Effective batch size = per_device_train_batch_size × num_gpus × gradient_accumulation_steps.

Rules of thumb:

• Contrastive losses (MNRL, GIST, SMNRL): push per_device_train_batch_size as high as VRAM allows. Larger in-batch negatives = better gradients. 64-256 typical.

• Regression losses (CoSENTLoss, CosineSimilarityLoss, etc.): batch size matters less. 16-64 works.

• Cross-encoders: batch size is less critical for quality. 32-128 typical.

• If you can't fit the desired per-device batch, use gradient_accumulation_steps to simulate it, but for MNRL-family losses this does not provide the same benefit as a real batch (in-batch negatives are still only per-device). Use CachedMultipleNegativesRankingLoss instead.

Learning rate and schedule

• 2e-5 is the safe default for full fine-tuning of BERT-family encoders.

• 1e-4 to 5e-4 for LoRA / PEFT adapters.

• 2e-1 for training a StaticEmbedding model from scratch (much higher than transformers because each token is a free-floating vector with no upstream gradients).

• lr_scheduler_type="linear" with warmup_steps=0.1 is standard (a float < 1 is interpreted as a fraction of total steps). "cosine" works equally well. "constant_with_warmup" is fine for very short runs. The legacy warmup_ratio was deprecated in transformers v5.2 in favor of warmup_steps accepting floats. Passing warmup_ratio=... still works but emits a DeprecationWarning.

• If loss goes NaN, drop LR first before anything else.

Precision

The non-negotiable rule: load the model in fp32 (the default). Don't pass torch_dtype=torch.bfloat16 to the model constructor or model_kwargs. Use the bf16=True / fp16=True flags below to enable autocast, not a weight cast. The trainer keeps the model and optimizer state in fp32 and autocasts activations to bf16/fp16 at forward/backward time. This preserves Adam's full-precision moments while giving you most of the bf16 throughput.

Casting weights to bf16 before the optimizer is created puts the Adam state (exp_avg, exp_avg_sq) in bf16 too: bf16's 7-bit mantissa is too coarse for small gradient moments and you get silent quality regressions across runs.

Do not set both bf16=True and fp16=True.

Evaluator calls outside the trainer (typically the pre-training baseline + a final post-training pass) don't get the trainer's autocast. Wrap them manually for the speedup, and note that the wrap is only strictly required when the model uses attn_implementation="flash_attention_2", since FA2 kernels need bf16/fp16 inputs to function. Without FA2 the wrap is a throughput optimization, not a correctness requirement:

import torch
from contextlib import nullcontext

def autocast_ctx():
    if not torch.cuda.is_available():
        return nullcontext()
    dtype = torch.bfloat16 if torch.cuda.is_bf16_supported() else torch.float16
    return torch.autocast("cuda", dtype=dtype)

with autocast_ctx():
    evaluator(model)            # baseline
trainer.train()
with autocast_ctx():
    evaluator(model)            # post-training

FlashAttention 2 wants bf16/fp16 inputs but doesn't require bf16 weights. Pass model_kwargs={"attn_implementation": "flash_attention_2"} without torch_dtype, and let bf16=True autocast feed bf16 activations to FA2. Weights stay fp32, optimizer state stays fp32.

Batch sampler (bi-encoder + sparse-encoder)

batch_sampler=BatchSamplers.NO_DUPLICATES is critical for contrastive losses. Without it, the same (anchor, positive) can appear multiple times in one batch, turning legitimate positives into false negatives.

Use BatchSamplers.NO_DUPLICATES for MNRL / SMNRL / CachedMNRL / GIST (the default recommendation). Use BatchSamplers.GROUP_BY_LABEL for batch-triplet losses (BatchAllTripletLoss, BatchHardTripletLoss). Use BatchSamplers.NO_DUPLICATES_HASHED only for very large datasets where per-batch string comparison gets slow.

For multi-dataset training, the analogous MultiDatasetBatchSamplers class controls how to draw from each dataset (ROUND_ROBIN, PROPORTIONAL). Under DDP, each dataset is sharded automatically per process, no extra config needed. Set multi_dataset_batch_sampler=... once and it works for 1-GPU and N-GPU runs identically.

Evaluation and checkpointing

eval_strategy="steps",
eval_steps=0.1,                        # evaluate every 10% of training
save_strategy="steps",
save_steps=0.1,                        # save at the same cadence (required for load_best_model_at_end)
save_total_limit=2,
load_best_model_at_end=True,
metric_for_best_model="eval_<EvaluatorName>_<metric>",
greater_is_better=True,

Prefer fractional values over absolute step counts. eval_steps=0.1 / save_steps=0.1 / logging_steps=0.01 are interpreted as fractions of total training steps (10 evals per epoch, 100 log lines per epoch) and auto-scale when the dataset size or epoch count changes. The HF Trainer converts a float < 1 to int(total_steps * fraction) at init time, so the same config works whether you're training on 10k or 10M rows. No need to recompute absolute step counts each time.

Use an absolute integer (e.g. eval_steps=500) only when you have a specific reason: comparing runs at known step counts, or when max_steps is set to an unusual value that makes fractions awkward.

Non-negotiable rules:

1. save_steps must be a multiple of eval_steps (or equal) when load_best_model_at_end=True, so the best-eval checkpoint is actually on disk. Matching them is the simplest path (e.g. both at 0.1).

2. If eval_strategy="steps" and you don't pass eval_dataset, training hangs. Either provide an eval dataset or set eval_strategy="no".

3. metric_for_best_model must match the exact key the evaluator writes. The pattern is usually f"eval_{evaluator.primary_metric}". Common values:

   • NanoBEIREvaluator (bi-encoder, cosine): eval_NanoBEIR_mean_cosine_ndcg@10

   • SparseNanoBEIREvaluator (sparse, dot): eval_NanoBEIR_mean_dot_ndcg@10

   • CrossEncoderNanoBEIREvaluator (rerank from BM25 top-100): eval_NanoBEIR_R100_mean_ndcg@10

   • EmbeddingSimilarityEvaluator(name="sts-dev"): eval_sts-dev_spearman_cosine

Early stopping

Add EarlyStoppingCallback via callbacks=[...]:

from transformers import EarlyStoppingCallback

trainer = SentenceTransformerTrainer(
    ..., callbacks=[EarlyStoppingCallback(early_stopping_patience=3)],
)

This requires load_best_model_at_end=True and metric_for_best_model=... to be set.

early_stopping_patience=3 means "stop if the best metric hasn't improved in 3 consecutive eval rounds." Use early_stopping_threshold=0.001 to require a minimum improvement.

When it actually matters:

• Cross-encoders: strongly recommended. CE rerankers typically peak mid-training and then degrade. The best checkpoint is rarely the last. Early stopping saves compute and guards against quality regression.

• Bi-encoders and sparse encoders: usually plateau rather than regress, so early stopping fires much less often. load_best_model_at_end=True alone gives you the right final model. Adding the callback is a belt-and-suspenders safety net.

Resuming training

trainer.train(resume_from_checkpoint=True) resumes from the latest checkpoint in output_dir. Pass a specific path to resume from a specific step: resume_from_checkpoint="models/my-model/checkpoint-500".

State that persists across resumption: optimizer, scheduler, random seeds, trainer step counter. State that does not: dataset iteration order for IterableDataset. If you use streaming datasets, you must handle resumption yourself.

Hub push

push_to_hub=True + hub_model_id="your-username/my-model" + hub_strategy="every_save" is the standard pattern. On HF Jobs, also pass secrets={"HF_TOKEN": "$HF_TOKEN"} on the job submission. The four hub_strategy values: "every_save" (each checkpoint, mandatory for HF Jobs), "end" (final only), "checkpoint" (latest, overwrite), "all_checkpoints" (each as a separate commit).

Logging

logging_steps=0.01,             # fraction: ~100 log lines per epoch (use an int for a fixed cadence)
logging_first_step=True,        # log before any training; useful sanity check
logging_dir=None,               # defaults to output_dir/runs
report_to="trackio",            # or ["trackio", "tensorboard"] for multiple; "none" disables all
run_name="meaningful-name",     # shown in the tracker UI

Tracker recommendation:

• Trackio (default) for solo / small-team work: zero friction beyond HF_TOKEN. Auto-creates a Space at https://huggingface.co/spaces/<your-username>/trackio on the first run. Subsequent runs append and group by run_name.

• W&B for larger teams or sweep / report features. pip install wandb && wandb login (or set WANDB_API_KEY).

• TensorBoard for air-gapped environments. No remote dashboard.

• MLflow when it's already the org standard.

For trackio sweeps / ablations, use trackio.init(project="...", name="...", group="v1", config={...}) before training to group related runs side-by-side. Without trackio.init(), defaults are derived from run_name and HF username.

Tracker gotchas:

• report_to="all" enables every installed integration (usually more than you want). "none" disables everything (the current transformers default). Always set explicitly.

• Trackio on HF Jobs without secrets={"HF_TOKEN": "$HF_TOKEN"} fails silently. W&B on HF Jobs needs WANDB_API_KEY in secrets.

• The HF Trainer logs only on rank 0 under DDP. Custom logging in your script may need explicit rank checks to avoid duplicate writes.

Memory-saving arguments

gradient_checkpointing=True,    # trades compute for memory. ~30% slowdown, ~40% less memory.
gradient_checkpointing_kwargs={"use_reentrant": False},
torch_empty_cache_steps=1000,   # periodically clear PyTorch allocator cache
dataloader_num_workers=2,       # parallel data loading, past 2 adds startup cost but no throughput
dataloader_persistent_workers=True,  # recommended on Windows/macOS, can be False on Linux
dataloader_pin_memory=True,

Do not combine gradient_checkpointing=True with any Cached* loss. They conflict.

Hyperparameter search

trainer.hyperparameter_search(...) is supported for all three trainers via Hugging Face's Trainer API (uses Optuna, Ray Tune, Sigopt, or W&B as backends).

Minimal example:

def model_init(trial):
    return SentenceTransformer("microsoft/mpnet-base")

def hp_space(trial):
    return {
        "learning_rate": trial.suggest_float("learning_rate", 1e-6, 1e-4, log=True),
        "num_train_epochs": trial.suggest_int("num_train_epochs", 1, 3),
        "per_device_train_batch_size": trial.suggest_categorical(
            "per_device_train_batch_size", [32, 64, 128]
        ),
    }

trainer = SentenceTransformerTrainer(
    model=None, model_init=model_init,
    args=args, train_dataset=train_dataset, eval_dataset=eval_dataset,
    loss=lambda model: MultipleNegativesRankingLoss(model),   # function that takes model -> loss
    evaluator=evaluator,
)

best_run = trainer.hyperparameter_search(
    hp_space=hp_space,
    direction="maximize",
    n_trials=10,
    backend="optuna",
)
print(best_run)

Install a backend: pip install optuna (or ray[tune]).

HPO is expensive. Don't reach for it until a single manually-tuned run is working end-to-end. For most production models, picking a reasonable LR from the range above and tuning batch size is enough.

Multi-task training args (brief)

When training on a dict of datasets with a dict of losses, add:

multi_dataset_batch_sampler=MultiDatasetBatchSamplers.PROPORTIONAL,  # or ROUND_ROBIN

See ../scripts/train_sentence_transformer_multi_dataset_example.py (docstring covers per-dataset losses, single-loss + DatasetDict variant, samplers, gotchas).

Don't

• Don't set eval_strategy="epoch" without setting save_strategy="epoch". Checkpoint/eval alignment matters for load_best_model_at_end.

• Don't set remove_unused_columns=False unless you have a custom collator that consumes metadata columns the loss doesn't see. The default (True) is safer. It drops unused columns automatically.

• Don't set seed to verify reproducibility and then expect bit-for-bit identical runs on different GPUs or across PyTorch versions. Exact reproducibility across hardware is not guaranteed.

• Don't tune adam_beta1 / adam_beta2 / adam_epsilon unless you have a specific reason. The defaults are fine for 99% of cases.