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.

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:

Massive Language Coverage - Trained on over 1800 languages with progressive inclusion strategy
Modern Architecture - Built on ModernBERT foundation with Flash Attention 2 and unpadding techniques
Novel Training Recipe - Introduces inverse mask scheduling and temperature sampling
Open Training Data - Complete 3T+ token dataset publicly available
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:

Phase	Dataset	Tokens	Description
Pre-training P1	mmbert-pretrain-p1	2.3T	60 languages, foundational training
Pre-training P2	mmbert-pretrain-p2	-	Extension data for pre-training phase
Pre-training P3	mmbert-pretrain-p3	-	Final pre-training data
Mid-training	mmbert-midtraining	600B	110 languages, context extension to 8K
Decay Phase	mmbert-decay	100B	1833 languages, premium quality

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

Parameter	mmBERT-small	mmBERT-base
Layers	22	22
Hidden Size	384	768
Intermediate Size	1152	1152
Attention Heads	6	12
Total Parameters	140M	307M
Non-embedding Parameters	42M	110M
Max Sequence Length	8192	8192
Vocabulary Size	256,000	256,000
Tokenizer	Gemma 2	Gemma 2

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

Click to expand dense retrieval fine-tuning example

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

Click to expand multilingual classification fine-tuning example

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

Click to expand multilingual reranking fine-tuning example

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:

Phase	Dataset	Description
Pre-training P1	2.3T tokens	60 languages, diverse data mixture
Pre-training P2	-	Extension data for pre-training
Pre-training P3	-	Final pre-training data
Mid-training	600B tokens	110 languages, context extension
Decay Phase	100B tokens	1833 languages, premium quality

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}, 
}

"""

jhu-clsp/mmBERT-base

mmBERT: A Modern Multilingual Encoder

Table of Contents

Quick Start

Installation

Usage

Model Description

Novel Training Innovations

Model Family

Training Data

Model Architecture

Usage Examples

Masked Language Modeling

Cross-lingual Embeddings

Fine-tuning Examples

Dense Retrieval with Sentence Transformers

Cross-lingual Classification

Multilingual Reranking

Training Data

Citation

Deploy Model on Runcrate

Run mmBERT-base on Runcrate