mmBERT: A Modern Multilingual Encoder

TL;DR: A state-of-the-art multilingual encoder trained on 3T+ tokens across 1800+ languages, introducing novel techniques for learning low-resource languages during the decay phase.

mmBERT is a modern multilingual encoder that significantly outperforms previous generation models like XLM-R on classification, embedding, and retrieval tasks. Built on the ModernBERT architecture with novel multilingual training innovations, mmBERT demonstrates that low-resource languages can be effectively learned during the decay phase of training. It is also significantly faster than any previous multilingual encoder.

Table of Contents

• Highlights
• Quick Start
• Model Description
• Novel Training Innovations
• Model Family
• Training Data
• Usage Examples
• Fine-tuning Examples
• Model Architecture
• Citation

Quick Start

Installation

pip install torch>=1.9.0
pip install transformers>=4.21.0

Usage

from transformers import AutoTokenizer, AutoModel

tokenizer = AutoTokenizer.from_pretrained("jhu-clsp/mmBERT-base")
model = AutoModel.from_pretrained("jhu-clsp/mmBERT-base")

inputs = tokenizer("Hello world", return_tensors="pt")
outputs = model(**inputs)

Model Description

mmBERT represents the first significant advancement over XLM-R for massively multilingual encoder models. Key features include:

1. Massive Language Coverage - Trained on over 1800 languages with progressive inclusion strategy
2. Modern Architecture - Built on ModernBERT foundation with Flash Attention 2 and unpadding techniques
3. Novel Training Recipe - Introduces inverse mask scheduling and temperature sampling
4. Open Training Data - Complete 3T+ token dataset publicly available
5. Decay Phase Innovation - Demonstrates effective learning of low-resource languages in final training phase

The model uses bidirectional attention with masked language modeling objectives, optimized specifically for multilingual understanding and cross-lingual transfer.

Novel Training Innovations

Progressive Language Addition: Start with 60 high-resource languages, expand to 110 mid-resource languages, then include all 1833 languages in decay phase.

Inverse Mask Schedule: Reduce mask ratio from 30% → 15% → 5% across training phases for progressively refined learning.

Inverse Temperature Sampling: Adjust multilingual sampling from high-resource bias (τ=0.7) to uniform sampling (τ=0.3).

Model Merging: Combine English-focused, high-resource, and all-language decay variants using TIES merging.

Model Family

Model	Total Params	Non-embed Params	Languages	Download
mmBERT-small	140M	42M	1800+
mmBERT-base	307M	110M	1800+

Training Data

mmBERT training data is publicly available across different phases:

Data Sources: Filtered DCLM (English), FineWeb2 (multilingual), FineWeb2-HQ (20 high-resource languages), Wikipedia (MegaWika), code repositories (StarCoder, ProLong), academic papers (ArXiv, PeS2o), and community discussions (StackExchange).

Model Architecture

Usage Examples

Masked Language Modeling

from transformers import AutoTokenizer, AutoModelForMaskedLM
import torch

tokenizer = AutoTokenizer.from_pretrained("jhu-clsp/mmBERT-base")
model = AutoModelForMaskedLM.from_pretrained("jhu-clsp/mmBERT-base")

def predict_masked_token(text):
    inputs = tokenizer(text, return_tensors="pt")
    with torch.no_grad():
        outputs = model(**inputs)
    
    mask_indices = torch.where(inputs["input_ids"] == tokenizer.mask_token_id)
    predictions = outputs.logits[mask_indices]
    top_tokens = torch.topk(predictions, 5, dim=-1)
    
    return [tokenizer.decode(token) for token in top_tokens.indices[0]]

# Works across languages
texts = [
    "The capital of France is <mask>.",
    "La capital de España es <mask>.",
    "Die Hauptstadt von Deutschland ist <mask>."
]

for text in texts:
    predictions = predict_masked_token(text)
    print(f"Text: {text}")
    print(f"Predictions: {predictions}")

Cross-lingual Embeddings

from transformers import AutoTokenizer, AutoModel
import torch
from sklearn.metrics.pairwise import cosine_similarity

tokenizer = AutoTokenizer.from_pretrained("jhu-clsp/mmBERT-base")
model = AutoModel.from_pretrained("jhu-clsp/mmBERT-base")

def get_embeddings(texts):
    inputs = tokenizer(texts, padding=True, truncation=True, return_tensors="pt")
    
    with torch.no_grad():
        outputs = model(**inputs)
        embeddings = outputs.last_hidden_state.mean(dim=1)
    
    return embeddings.numpy()

multilingual_texts = [
    "Artificial intelligence is transforming technology",
    "La inteligencia artificial está transformando la tecnología",
    "L'intelligence artificielle transforme la technologie", 
    "人工智能正在改变技术"
]

embeddings = get_embeddings(multilingual_texts)
similarities = cosine_similarity(embeddings)
print("Cross-lingual similarity matrix:")
print(similarities)

Fine-tuning Examples

Dense Retrieval with Sentence Transformers

import argparse
from datasets import load_dataset
from sentence_transformers import (
    SentenceTransformer,
    SentenceTransformerTrainer,
    SentenceTransformerTrainingArguments,
)
from sentence_transformers.evaluation import TripletEvaluator
from sentence_transformers.losses import CachedMultipleNegativesRankingLoss
from sentence_transformers.training_args import BatchSamplers

def main():
    parser = argparse.ArgumentParser()
    parser.add_argument("--lr", type=float, default=8e-5)
    parser.add_argument("--model_name", type=str, default="jhu-clsp/mmBERT-base")
    args = parser.parse_args()
    
    lr = args.lr
    model_name = args.model_name
    model_shortname = model_name.split("/")[-1]

    model = SentenceTransformer(model_name)

    dataset = load_dataset(
        "sentence-transformers/msmarco-co-condenser-margin-mse-sym-mnrl-mean-v1",
        "triplet-hard",
        split="train",
    )
    dataset_dict = dataset.train_test_split(test_size=1_000, seed=12)
    train_dataset = dataset_dict["train"].select(range(1_250_000))
    eval_dataset = dataset_dict["test"]

    loss = CachedMultipleNegativesRankingLoss(model, mini_batch_size=16)
    run_name = f"{model_shortname}-DPR-{lr}"
    
    training_args = SentenceTransformerTrainingArguments(
        output_dir=f"output/{model_shortname}/{run_name}",
        num_train_epochs=1,
        per_device_train_batch_size=512,
        per_device_eval_batch_size=512,
        warmup_ratio=0.05,
        fp16=False,
        bf16=True,
        batch_sampler=BatchSamplers.NO_DUPLICATES,
        learning_rate=lr,
        save_strategy="steps",
        save_steps=500,
        save_total_limit=2,
        logging_steps=500,
        run_name=run_name,
    )

    dev_evaluator = TripletEvaluator(
        anchors=eval_dataset["query"],
        positives=eval_dataset["positive"],
        negatives=eval_dataset["negative"],
        name="msmarco-co-condenser-dev",
    )
    dev_evaluator(model)

    trainer = SentenceTransformerTrainer(
        model=model,
        args=training_args,
        train_dataset=train_dataset,
        eval_dataset=eval_dataset,
        loss=loss,
        evaluator=dev_evaluator,
    )
    trainer.train()

    model.save_pretrained(f"output/{model_shortname}/{run_name}/final")
    model.push_to_hub(run_name, private=False)

if __name__ == "__main__":
    main()

Cross-lingual Classification

from transformers import (
    AutoTokenizer,
    AutoModelForSequenceClassification, 
    TrainingArguments,
    Trainer
)
from datasets import load_dataset
import numpy as np
from sklearn.metrics import accuracy_score, f1_score

def compute_metrics(eval_pred):
    predictions, labels = eval_pred
    predictions = np.argmax(predictions, axis=1)
    return {
        'accuracy': accuracy_score(labels, predictions),
        'f1': f1_score(labels, predictions, average='weighted')
    }

def main():
    model_name = "jhu-clsp/mmBERT-base"
    
    tokenizer = AutoTokenizer.from_pretrained(model_name)
    model = AutoModelForSequenceClassification.from_pretrained(
        model_name,
        num_labels=3
    )
    
    dataset = load_dataset("xnli", "all_languages")
    
    def tokenize_function(examples):
        texts = [f"{p} {tokenizer.sep_token} {h}" 
                for p, h in zip(examples["premise"], examples["hypothesis"])]
        
        return tokenizer(
            texts,
            truncation=True,
            padding=True,
            max_length=512
        )
    
    train_dataset = dataset["train"].map(tokenize_function, batched=True)
    eval_dataset = dataset["validation"].map(tokenize_function, batched=True)
    
    training_args = TrainingArguments(
        output_dir="./mmbert-xnli",
        learning_rate=3e-5,
        per_device_train_batch_size=32,
        per_device_eval_batch_size=32,
        num_train_epochs=3,
        weight_decay=0.01,
        evaluation_strategy="epoch",
        save_strategy="epoch",
        load_best_model_at_end=True,
        metric_for_best_model="f1",
        greater_is_better=True,
    )
    
    trainer = Trainer(
        model=model,
        args=training_args,
        train_dataset=train_dataset,
        eval_dataset=eval_dataset,
        compute_metrics=compute_metrics,
    )
    
    trainer.train()

if __name__ == "__main__":
    main()

Multilingual Reranking

import logging
from datasets import load_dataset
from sentence_transformers.cross_encoder import (
    CrossEncoder,
    CrossEncoderModelCardData,
    CrossEncoderTrainer,
    CrossEncoderTrainingArguments,
)
from sentence_transformers.cross_encoder.evaluation import CrossEncoderNanoBEIREvaluator
from sentence_transformers.cross_encoder.losses import BinaryCrossEntropyLoss
from sentence_transformers.util import mine_hard_negatives
from sentence_transformers import SentenceTransformer
import torch

def main():
    model_name = "jhu-clsp/mmBERT-base"
    train_batch_size = 32
    num_epochs = 2
    num_hard_negatives = 7

    model = CrossEncoder(
        model_name,
        model_card_data=CrossEncoderModelCardData(
            language="multilingual",
            license="mit",
        ),
    )
    
    full_dataset = load_dataset("sentence-transformers/gooaq", split="train").select(range(50_000))
    dataset_dict = full_dataset.train_test_split(test_size=1_000, seed=42)
    train_dataset = dataset_dict["train"]
    eval_dataset = dataset_dict["test"]

    embedding_model = SentenceTransformer("sentence-transformers/paraphrase-multilingual-MiniLM-L12-v2", device="cpu")
    hard_train_dataset = mine_hard_negatives(
        train_dataset,
        embedding_model,
        num_negatives=num_hard_negatives,
        margin=0,
        range_min=0,
        range_max=100,
        sampling_strategy="top",
        batch_size=2048,
        output_format="labeled-pair",
        use_faiss=True,
    )

    loss = BinaryCrossEntropyLoss(model=model, pos_weight=torch.tensor(num_hard_negatives))

    nano_beir_evaluator = CrossEncoderNanoBEIREvaluator(
        dataset_names=["msmarco", "nfcorpus", "nq"],
        batch_size=train_batch_size,
    )

    args = CrossEncoderTrainingArguments(
        output_dir="./mmbert-reranker",
        num_train_epochs=num_epochs,
        per_device_train_batch_size=train_batch_size,
        per_device_eval_batch_size=train_batch_size,
        learning_rate=2e-5,
        warmup_ratio=0.1,
        fp16=False,
        bf16=True,
        dataloader_num_workers=4,
        load_best_model_at_end=True,
        metric_for_best_model="eval_msmarco_ndcg@10",
        eval_strategy="steps",
        eval_steps=1000,
        save_strategy="steps",
        save_steps=1000,
        save_total_limit=2,
        logging_steps=200,
        seed=42,
    )

    trainer = CrossEncoderTrainer(
        model=model,
        args=args,
        train_dataset=hard_train_dataset,
        loss=loss,
        evaluator=nano_beir_evaluator,
    )
    trainer.train()

    model.save_pretrained("./mmbert-reranker/final")

if __name__ == "__main__":
    main()

Training Data

mmBERT was trained on a carefully curated 3T+ token multilingual dataset:

Primary Sources:

• Filtered DCLM: High-quality English content
• FineWeb2: Broad multilingual web coverage (1800+ languages)
• FineWeb2-HQ: Filtered subset of 20 high-resource languages
• Code: StarCoder and ProLong repositories
• Academic: ArXiv papers and PeS2o scientific content
• Reference: Wikipedia (MegaWika) and textbooks
• Community: StackExchange discussions

Citation

If you use mmBERT in your research, please cite our work:

@misc{marone2025mmbertmodernmultilingualencoder,
      title={mmBERT: A Modern Multilingual Encoder with Annealed Language Learning}, 
      author={Marc Marone and Orion Weller and William Fleshman and Eugene Yang and Dawn Lawrie and Benjamin Van Durme},
      year={2025},
      eprint={2509.06888},
      archivePrefix={arXiv},
      primaryClass={cs.CL},
      url={https://arxiv.org/abs/2509.06888}, 
}

"""