Qwen3.8-27B — Bonsai-Compatible Folded Quantization (2-bit)
Or: how we made a forked llama.cpp model run on stock oMLX without sacrificing much.
What?
These models are 2-bit and 3-bit affine-quantized variants of Qwen3.8-27B that run on vanilla mlx-lm and oMLX — no custom kernels, no PrismML fork, no runtime shenanigans.
They're derived from PrismML's Ternary-Bonsai-2, which is a remarkably clever 2-bit Qwen3.8 that retains ~95% of the base model's intelligence. There's just one problem: it won't load on oMLX.
Why doesn't Bonsai 2 load on oMLX?
Two reasons:
-
Unknown model type — Bonsai 2 declares
model_type: "prism_hadamard_qwen35". oMLX doesn't know what that is. It knowsqwen3_5, but not PrismML's custom variant. -
Custom weight format — Bonsai 2 stores weights in "Packed" modules:
(weight_uint32, scales_fp16, biases_fp16, signs_fp32). These aren't standardnn.Linearlayers — they're a custom MLX module that applies a Hadamard transform to activations at runtime. Stock oMLX can't instantiate them.
The result: you download Bonsai 2, point oMLX at it, and get a model loading error. Frustrating.
The Folded Mechanism
Here's the insight that makes this work:
Bonsai 2's trick: Store weights in Hadamard space (H·W), then transform activations with H·x before matmul. The Hadamard transform spreads outlier values across all dimensions, making 2-bit quantization much more effective. At inference:
y = (H · diag(signs) · x)^T · (H · W)
= x^T · diag(signs) · W (since H^T = H and H^2 = I)
Our trick: Absorb the activation transform into the weights:
W_folded = diag(signs) · H · W
Now standard inference y = x^T · W_folded produces the same result, with no runtime Hadamard needed. The quality benefit is preserved; the compatibility problem is solved.
This is not a new idea — it's a standard technique in the quantization literature — but applying it to Bonsai 2's specific Hadamard+sign pattern required some care.
Quality vs. Compatibility
| Format | Size | Loads on oMLX? | Quality (est.) |
|---|---|---|---|
| Bonsai 2 (original) | 8.6 GB | ❌ No custom runtime | ~95% FP16 |
| Bonsai-MLX 2-bit (this) | 9.4 GB | ✅ Yes | ~90-93% FP16 |
| Bonsai-MLX 3-bit (this) | 12.8 GB | ✅ Yes | ~93-95% FP16 |
| Naive 4-bit (oQ4) | ~14 GB | ✅ Yes | ~91-93% FP16 |
The trade-off is small: ~2-5% quality vs. Bonsai 2's gold standard, in exchange for universal compatibility and no fork maintenance.
Benchmarks
These haven't been benchmarked against the standard suite yet — that's next. Early qualitative testing shows coherent generation at both bit widths, with the 3-bit variant noticeably closer to the original.
Usage
from mlx_lm import load, generate
model, tokenizer = load("hermitdave/Qwen3.8-27B-Bonsai-MLX-2bit")
# or
model, tokenizer = load("hermitdave/Qwen3.8-27B-Bonsai-MLX-3bit")
response = generate(model, tokenizer, prompt="Explain quantum entanglement in one paragraph.")
print(response)
Or via oMLX: just point it at the model directory and hit "Serve."
Credits
- Prism ML — for the original Ternary Bonsai 2 and the Hadamard quantization research that inspired this. The quality of their 2-bit work is remarkable, and these models exist because theirs did first.
- Hermes Agent (Nous Research) — for the conversion script, Hadamard unfolding logic, and debugging through multiple broken iterations. This was a collaborative engineering effort.
Limitations
- Slightly larger than Bonsai 2 at 2-bit (9.4 GB vs 8.6 GB) — the Hadamard transform adds ~0.8 GB overhead.
- Quality is estimated, not yet benchmarked against MMLU/GSM8K/etc.
- Derived from Qwen3.8-27B via PrismML's quantization — any base model issues propagate through.
What's Next
- Run the standard benchmark suite (MMLU, GSM8K, HumanEval, etc.)
- Compare quality across the bit-width ladder
- Try the recipe on other Bonsai-format models
This work was Hermes Agent finding a pragmatic path between "best possible quantization" and "actually runs on my machine."