---
library_name: transformers
license: apache-2.0
language:
  - dna
tags:
  - dna
  - genomic
  - transformers
  - speculative-decoding
---

# Carbon-500M

A small generative DNA model from the **Carbon** family.

**Carbon-500M is intended primarily as a draft model for speculative decoding** — it shares the tokenizer and DNA template format of [Carbon-3B](https://huggingface.co/HuggingFaceBio/Carbon-3B) and [Carbon-8B](https://huggingface.co/HuggingFaceBio/Carbon-8B), so it can be paired with either as the target model to reduce wall-clock generation cost at no quality loss. It is not designed to be competitive with the 3B/8B Carbon models on downstream benchmarks.

For the full design rationale, tokenizer specification, evaluation protocol, and usage notes (DNA tag wrapping, 6-mer constraints, scoring helpers), please refer to the **[Carbon-3B model card](https://huggingface.co/HuggingFaceBio/Carbon-3B)** — this card focuses only on facts specific to Carbon-500M.

> TODO: update teh tokenizer code

## Facts

- **500M-parameter decoder-only autoregressive DNA model** (Llama-style architecture).
- **Hybrid tokenizer** shared with the rest of the Carbon family (6-mer for DNA + Qwen3 BPE for English text; each DNA token ≈ 6 bp).
- **Pre-training tokens:** 600B 6-mer tokens (≈ 3.6 T DNA base pairs).
- **Sequence length:** 8 192tokens (≈ 48 kbp).
- **Loss schedule:** cross-entropy 0 → 300 B tokens, then switch to the hybrid Factorised Nucleotide Supervision (FNS) loss from 300 B → 600 B tokens. The switch happens later than for Carbon-3B because Carbon-500M's training was very stable and tolerated the later transition.
- **Data mixture:** identical to the **decay-phase mixture used by Carbon-3B** — 50 % Generator-style eukaryotic genes / 25 % mature mRNA / 10 % splice-enriched mRNA / 15 % GTDB bacterial genomes. Same weights across the whole 600 B run.
- **Precision:** bfloat16. **Optimizer:** AdamW. **Positional embedding:** RoPE.
- **No long-context training stage** — the model stays at its 8 k-token native context so 48kbp.
- Released as a standard Hugging Face causal LM (`LlamaForCausalLM`).

## How to use

Wrap DNA in `<dna>...</dna>` exactly as for the larger models. See the [Carbon-3B card](https://huggingface.co/HuggingFaceBio/Carbon-3B#tokenizer-working-with-dna-inputs) for tokenizer details.

```python
from transformers import AutoModelForCausalLM, AutoTokenizer
import torch

repo = "HuggingFaceBio/Carbon-500M"
tok = AutoTokenizer.from_pretrained(repo, trust_remote_code=True)
model = AutoModelForCausalLM.from_pretrained(
    repo, torch_dtype=torch.bfloat16,
).cuda().eval()

prompt = "<dna>ATGCGCTAGCTACGATCGATCGTAGCTAGCTAGCTAGCTACG"
inputs = tok(prompt, return_tensors="pt", add_special_tokens=False).to("cuda")
out = model.generate(**inputs, max_new_tokens=64, do_sample=False)
print(tok.decode(out[0][inputs.input_ids.shape[1]:]))
```

### Recommended use: speculative decoding with Carbon-3B / Carbon-8B

Carbon-500M is most useful when paired with a larger Carbon model as the verifier. Hugging Face Transformers supports this natively through the `assistant_model` argument:

```python
from transformers import AutoModelForCausalLM, AutoTokenizer
import torch

tok    = AutoTokenizer.from_pretrained("HuggingFaceBio/Carbon-3B", trust_remote_code=True)
draft  = AutoModelForCausalLM.from_pretrained(
    "HuggingFaceBio/Carbon-500M", torch_dtype=torch.bfloat16
).cuda().eval()
target = AutoModelForCausalLM.from_pretrained(
    "HuggingFaceBio/Carbon-3B",   torch_dtype=torch.bfloat16
).cuda().eval()

prompt = "<dna>ATGCGCTAGCTACGATCGATCGTAGCTAGCTAGCTAGCTACG"
inputs = tok(prompt, return_tensors="pt", add_special_tokens=False).to("cuda")
out = target.generate(
    **inputs, max_new_tokens=256, do_sample=False,
    assistant_model=draft,
)
print(tok.decode(out[0][inputs.input_ids.shape[1]:]))
```

Output is guaranteed identical to greedy decoding with the target model alone; only wall-clock latency is reduced.

## Evaluation

Carbon-500M is benchmarked against  ≈ 1B-parameter DNA models on the standard Carbon evaluation suite. See the [Carbon-3B card](https://huggingface.co/HuggingFaceBio/Carbon-3B#evaluation) for the task definitions and methodology.

> TODO Loubna: add one downstream table comparing Carbon-500M to other 1B-class baselines. -->

## License

Apache 2.0.