Parable-Nanbeige4.2-3B-Claude-Fable-5

Part of the Parable series: small local LLMs fine-tuned on genuine agent traces. This is the reasoning-style chat variant of Nanbeige4.2-3B (to our knowledge the first published fine-tune of this base), tuned on real Claude Fable 5 agent transcripts so its step-by-step reasoning voice carries into local use.

GGUF quants for llama.cpp / Ollama / LM Studio: Parable-Nanbeige4.2-3B-Claude-Fable-5-GGUF

Usage

trust_remote_code=True is required: the architecture ships its own modeling code (modeling_nanbeige.py, included in this repo).

from transformers import AutoModelForCausalLM, AutoTokenizer

tok = AutoTokenizer.from_pretrained(
    "AnkitAI/Parable-Nanbeige4.2-3B-Claude-Fable-5", trust_remote_code=True)
model = AutoModelForCausalLM.from_pretrained(
    "AnkitAI/Parable-Nanbeige4.2-3B-Claude-Fable-5",
    torch_dtype="bfloat16", device_map="auto", trust_remote_code=True)

msgs = [{"role": "user", "content": "Write a python function that reverses a string."}]
ids = tok.apply_chat_template(msgs, add_generation_prompt=True, return_tensors="pt").to(model.device)
print(tok.decode(model.generate(ids, max_new_tokens=512, use_cache=False)[0]))

Note use_cache=False: the base architecture's modeling code was written against transformers 4.42 and calls a cache API that transformers removed in later versions, so cached generation crashes with 'DynamicCache' object has no attribute 'get_max_length' on transformers 4.47+. Either pass use_cache=False (shown above, slower) or pin transformers>=4.42,<4.47. Training and loss computation are unaffected.

Output begins with a <think>...</think> reasoning block, then the answer. If you are building on top of this model, parse and strip the think block before showing text to end users.

The architecture runs its 22-layer stack twice (num_loops: 2, effective depth 44). This is encoded in the GGUF and handled automatically; expect roughly 2x the per-token compute of a typical 22-layer model of this width.

Model details

Base: Nanbeige/Nanbeige4.2-3B (4.17B params, 3B non-embedding, Apache-2.0)
Method: QLoRA (nf4, r=16, alpha=32) on the standard 7-module target set, all 22 layers (154 adapted modules)
Data: 845 genuine Claude Fable 5 agent-session traces (chat variant: prose answers, <think> reasoning preserved) + 362 replay rows (OpenCoder educational_instruct + tulu-3 mix) to limit forgetting; 98% of rows retain their full assistant span at the 3,072-token training window
Schedule: 380 optimizer steps, effective batch 8, completion-only loss masking, cosine LR
Held-out trace loss: 2.333 (base) → 2.133 (tuned), same 225-trace held-out set never used in training

Evaluation

The number above is a held-out language-modeling loss on real agent traces: it measures how much better the model fits the Fable-5 reasoning distribution, and it is the honest headline for what this fine-tune does.

We do not claim benchmark gains over the base model. Nanbeige4.2-3B's model card reports strong agentic results (SWE-bench Verified 63.6, Terminal-Bench 2.0 44.1, vendor self-reported); this variant trains the reasoning voice, not tool-calling, and prose-only SFT should be expected to trade a little benchmark sharpness for style fidelity. If you need maximum benchmark performance on agentic harnesses, use the base; if you want its capability with a transparent, well-structured reasoning trace in local use, use this.

Limitations

The base model's own benchmark figures are vendor self-reported and were not independently reproduced here.
Training ran on a 4-bit quantized base (QLoRA); the F16 merge cannot exceed 4-bit-base quality.
Requires very recent llama.cpp (2026-07-27+); ecosystem tools bundling older builds will not load it yet.

Provenance & licensing

Fine-tuned from Nanbeige/Nanbeige4.2-3B (Apache-2.0). Training data: Glint-Research/Fable-5-traces (AGPL-3.0) and Roman1111111/gpt5.5-terminal (MIT). Because those traces originate from third-party assistants, the providers' terms may apply to downstream training and distillation. If you plan to build on this model commercially, confirm your use aligns with those terms.

Citation

The recipe, evaluation methodology and failure analysis behind this model are documented in the tech report:

Aglawe, A. (2026). Agent-Trace Fine-Tuning of Small Language Models under Constrained Compute. Zenodo. doi:10.5281/zenodo.21676407

@misc{aglawe2026agenttrace,
  author    = {Aglawe, Ankit},
  title     = {Agent-Trace Fine-Tuning of Small Language Models under Constrained Compute},
  year      = {2026},
  publisher = {Zenodo},
  doi       = {10.5281/zenodo.21676407},
  url       = {https://doi.org/10.5281/zenodo.21676407}
}