Parcae

Parcae: Scaling Laws For Stable Looped Language Models
Hayden Prairie, Zachary Novack, Taylor Berg-Kirkpatrick, Daniel Y. Fu
Paper: https://arxiv.org/abs/2604.12946

About

Parcae is a new looped architecture that utilizes a handful of techniques to stabilize training. Parcae enables stable, hassle-free training of looped models, which we use to derive the first scaling laws for looping, finding that compute-optimal training scales looping and data in tandem.

Installation

Just wanna use off-the-shelf models? We make things easy with a PyPI package to access models. Install the package with the following:

pip install parcae-lm

If you are training models, then please clone the GitHub repository

git clone https://github.com/SandyResearch/parcae.git
cd parcae

and then follow the following:

Docker (recommended)

Our launch scripts handle everything automatically. Set PROJECT_DIR and DOCKER_IMAGE at the top of launch_job.slurm or launch_interactive.sh, then:

# Interactive development shell
bash launch_interactive.sh

# Submit a training job
CONFIG=launch_configs/parcae-small-140m.yaml sbatch launch_job.slurm

The Docker image is hosted publicly at ghcr.io/sandyresearch/parcae and will be pulled automatically.

Local

Requires Python 3.11+ and PyTorch 2.4+. Install PyTorch first, following pytorch.org, then:

pip install -e .

Usage

Models

We provide three ways to instantiate models: load pretrained weights with from_pretrained, build from a built-in config with create_model, or customize a config before building with create_config.

import parcae_lm

# Load a pretrained model from HuggingFace
model = parcae_lm.from_pretrained("SandyResearch/parcae-140m")

# Create a model from a built-in config
model = parcae_lm.create_model("parcae-small-140m")

# Or get the config, customize it, then build
config = parcae_lm.create_config("parcae-small-140m")
config.mean_recurrence = 16
model = config.construct_model()

Training

Downloading Data

python scripts/download_data.py fineweb-100bt   # FineWeb-Edu 100B tokens
python scripts/download_data.py fineweb-350bt   # FineWeb-Edu 350B tokens
python scripts/download_data.py huginn          # Huginn dataset

Training a Tokenizer

Train a GPT-4 style BPE tokenizer on your data:

python scripts/tok_train.py --data-dir fineweb --output-dir tokenizer/ --vocab-size 32768

Evaluate compression ratios against GPT-2 and GPT-4 tokenizers:

python scripts/tok_eval.py --tokenizer tokenizer/parcae_tokenizer --data-dir fineweb

Launching Training

Training is configured via YAML files in launch_configs/. Available configs:

Config	Architecture	Parameters
parcae-small-140m.yaml	Parcae	140M
parcae-medium-370m.yaml	Parcae	370M
parcae-large-770m.yaml	Parcae	770M
parcae-xlarge-1_3b.yaml	Parcae	1.3B
gpt-small-140m.yaml	GPT	140M
gpt-medium-370m.yaml	GPT	370M
gpt-large-770m.yaml	GPT	770M
gpt-xlarge-1_3b.yaml	GPT	1.3B

Single node:

bash runs/run_training.sh launch_configs/parcae-small-140m.yaml parcae-small 8

Multi-node (Slurm):

CONFIG=launch_configs/parcae-large-770m.yaml sbatch launch_job.slurm

Eval

Evaluate models using scripts/eval.py. Supports loading from HuggingFace or local checkpoints.

# Evaluate a pretrained model from HuggingFace
python scripts/eval.py --hf_repo SandyResearch/parcae-140m --eval_tasks core

# Evaluate a local checkpoint
bash runs/run_eval.sh outputs/parcae-small-140m eval_configs/eval-core.yaml 8

# Evaluate validation loss
python scripts/eval.py --hf_repo SandyResearch/parcae-140m --eval_tasks bpb \
    --tasks.bpb.val_data_dir /path/to/val/data

Available eval configs in eval_configs/:

• eval-core.yaml — Core benchmark suite
• eval-core-extended.yaml — Extended core benchmarks
• eval-val-loss.yaml — Validation loss / bits-per-byte
• eval-lambada.yaml — LAMBADA evaluation

Pretrained Models

Pretrained models are uploaded to Hugging Face: parcae-140m, parcae-370m, parcae-770m, parcae-1.3b, trained on the FineWeb-Edu dataset. Models will be auto-downloaded when using from_pretrained.

These models dimensions are:

Model	Parameters	Prelude	Core	Coda	Model dim.	Recurrence
Parcae-140M	140M	2	2	2	768	8
Parcae-370M	370M	4	4	4	1024	8
Parcae-770M	770M	6	6	6	1280	8
Parcae-1.3B	1.3B	8	8	8	1536	8

Note: these are base models without any form of downstream modification (instruction tuning, etc.).

Replicating Scaling Laws

The sweep scripts in runs/ reproduce the scaling law experiments from the paper. See runs/sweep_recurrence.sh for recurrence scaling and runs/sweep_flops.sh for compute-optimal scaling.

Citations

@misc{prairie2026parcaescalinglawsstable,
      title={Parcae: Scaling Laws For Stable Looped Language Models}, 
      author={Hayden Prairie and Zachary Novack and Taylor Berg-Kirkpatrick and Daniel Y. Fu},
      year={2026},
      eprint={2604.12946},
      archivePrefix={arXiv},
      primaryClass={cs.LG},
      url={https://arxiv.org/abs/2604.12946}, 
}

References

This code-base was built on karpathy/nanochat, seal-rg/recurrent-pretraining, and Lightning-AI/litgpt. While most code has been thoroughly adapted, we greatly appreciate the work that went into developing each of these training libraries.