llm-semantic-router / mmbert-32k-yarn

mmBERT-32K-YaRN

Modern Multilingual BERT with 32K context length - Extended from 8K to 32K tokens using YaRN RoPE scaling.

This model extends jhu-clsp/mmBERT-base (Modern Multilingual BERT supporting 1800+ languages) from 8,192 to 32,768 maximum context length using YaRN (Yet another RoPE extensioN) scaling method.

Model Description

Intended Use

This model is designed for:

• Long-document understanding in any of 1800+ languages
• Semantic routing for LLM request classification
• Document classification with extended context
• Information retrieval from long texts
• Multilingual NLP tasks requiring long context

Part of the vLLM Semantic Router Mixture-of-Models (MoM) family.

Evaluation Results

Distance-Based Retrieval (Key Metric for Long-Context)

Summary : Perfect retrieval up to 2048 tokens. Long-range capability improved from baseline ~33% to 50% (averaged across all distances ≥1024).

Multilingual MLM Accuracy

Overall : 100% (10/10 languages tested)

Perplexity by Context Length

Position-wise Accuracy (16K context)

Training Details

Training Configuration

base_model: jhu-clsp/mmBERT-base
rope_scaling_type: yarn
original_max_position_embeddings: 8192
target_max_position_embeddings: 32768
scaling_factor: 4.0
yarn_beta_fast: 32.0
yarn_beta_slow: 1.0

# Training hyperparameters
learning_rate: 1e-5
batch_size: 1 (effective: 16 with gradient accumulation)
gradient_accumulation_steps: 16
num_epochs: 1
warmup_steps: 100
lr_scheduler: constant_with_warmup
mlm_probability: 0.3
bf16: true

Training Data

• Dataset : CC-100 (Common Crawl) multilingual corpus
• Samples : 30,774 sequences
• Sequence Length : 32,768 tokens each
• Total Tokens : ~1B tokens

Hardware

• GPU : AMD Instinct MI300X (192GB VRAM)
• Training Time : ~6.5 hours
• Framework : PyTorch 2.3 + ROCm 6.2

Usage

Basic Usage

from transformers import AutoModelForMaskedLM, AutoTokenizer

model = AutoModelForMaskedLM.from_pretrained("llm-semantic-router/mmbert-32k-yarn")
tokenizer = AutoTokenizer.from_pretrained("llm-semantic-router/mmbert-32k-yarn")

# Multilingual MLM example
text = "The capital of France is <mask>."
inputs = tokenizer(text, return_tensors="pt")
outputs = model(**inputs)

mask_idx = (inputs["input_ids"] == tokenizer.mask_token_id).nonzero()[0, 1]
logits = outputs.logits[0, mask_idx]
top5 = tokenizer.decode(logits.topk(5).indices)
print(top5)  # ['Paris', 'Strasbourg', 'Nice', 'Brussels', 'Lyon']

Long Context Usage (32K tokens)

# Process long documents (up to 32K tokens)
long_document = "..." * 30000  # Your long text in any of 1800+ languages
inputs = tokenizer(
    long_document, 
    return_tensors="pt", 
    max_length=32768, 
    truncation=True
)
outputs = model(**inputs)

Feature Extraction

import torch

# Get embeddings for downstream tasks
with torch.no_grad():
    outputs = model(**inputs, output_hidden_states=True)
    # Use last hidden state or pooled output
    embeddings = outputs.hidden_states[-1].mean(dim=1)  # Mean pooling

Limitations

• Long-range retrieval : While the model handles 32K context, retrieval accuracy drops significantly beyond 2048 tokens distance
• Training data : Trained on CC-100 which may have biases from web crawl data
• Compute requirements : Full 32K context requires significant GPU memory (~180GB for batch size 1)

Citation

@misc{mmbert-32k-yarn,
  title={mmBERT-32K-YaRN: Extended Context Modern Multilingual BERT},
  author={vLLM Semantic Router Team},
  year={2026},
  publisher={Hugging Face},
  url={https://huggingface.co/llm-semantic-router/mmbert-32k-yarn}
}

References

• mmBERT - Modern Multilingual BERT (1800+ languages)
• ModernBERT - Base architecture
• YaRN - Yet another RoPE extensioN method
• vLLM Semantic Router - Mixture-of-Models routing

License

MIT License (same as mmBERT base model)