Bonsai-2 27B — W4A16 (community conversion)

W4A16 GPTQ conversion of Prism ML's Bonsai-2 27B (the 1.72-bpw ternary retrain of Qwen3.8-27B) into a vLLM-loadable checkpoint, so the weights can run the full vLLM feature set — including native DFlash2 speculative decoding in vLLM ≥ 0.28.0 — outside the llama.cpp-only ternary format. Community conversion, not an official Prism ML release.

Provenance

The source is the F16.gguf from prism-ml/Ternary-Bonsai-2-27B-gguf @ 6ed5e12bf84b7a63069882c91dd9e9218647d17b, whose weights are stored in a Hadamard-rotated, v-grouped, delta-normed layout for llama.cpp's custom kernels. Conversion inverts that layout deterministically:

1. Hadamard unrotation — W_hf = W_stored @ blockdiag(H_sylv·diag(s_b)/√1024) per 1024-block of the input dim; signs from the file's own prism.hadamard.* manifest.
2. GDN v-layout reorder — the 48 v-heads move from rep-major (3×16) to group-major (16×3) in attn_qkv (v segment), attn_gate, ssm_alpha, ssm_beta, ssm_a, ssm_dt, ssm_conv1d (v part) and ssm_out (columns), mirroring the Bonsai-demo runtime.py reorder().
3. Delta norms — attn_norm, post_attention_norm, output_norm, attn_q_norm, attn_k_norm are stored as g − 1 in the GGUF; restored to g (the runtime adds the 1 inside its fused RMSNorm+rotate kernel).
4. Grafts — model.visual.* and mtp.* are absent from the ternary GGUF and are copied verbatim (bf16) from Qwen/Qwen3.8-27B. The MTP module is what DFlash2 speculative decoding drafts with.

Verified: NLL 1.169 on a held-out sentence (base Qwen3.8-27B: 1.518 through the same harness), coherent greedy generation, structural index match.

Quantization

GPTQ W4A16 via llmcompressor 0.13 (pack-quantized compressed-tensors): 256 samples × 1024 tokens of open_platypus, symmetric group-128. Then the syv-ai single-user prepare chain: int8 (group-128) lm_head, embed_tokens and MTP module in place, plus the 40960-token draft head (mtp.draft_lm_head, 213 MB) for DFlash2 speculative decoding. Per-layer GPTQ round-trip relative errors 0.6–0.8%. The vision tower stays bf16.

Serving

vLLM ≥ 0.28.0 (DFlash2 is native there; no patch needed for the drafter):

vllm serve /path/to/Bonsai-2-27B-W4A16 \
  --speculative-config '{"method":"dflash2","method_config":{"draft_tokens":7}}'

or the syv-ai single-user launcher (SPEC=dflash2 CTX=fast), which also enables lookup-augmented drafting.

Measured on 1× NVIDIA CMP 170HX (SM80, 180 W): 155.7 tok/s single-stream decode (256-token cohort), 251.6 tok/s on a 900-token cohort (adaptive draft depth), prefill 1876 tok/s @ ~6.6k prompt, DFlash2 acceptance 4.17 tokens/draft (59.6% per position) with the base-calibrated drafter at zero recalibration. Full receipts, protocol and the ternary-llama.cpp comparison lane: PixelML/club-170hx PR #49 and #47.

Notes and limitations

• Quality benchmarks are the upstream Bonsai-2 numbers (84.78 thinking-mode average, community-reported) plus GPTQ error on top; no independent eval suite was run for this conversion.
• The checkpoint keeps Qwen3.8-27B's vision tower; the ternary retrain covers the language model only.
• mtp.* and model.visual.* are required at serve time for DFlash2 and image input respectively — do not strip them.
• If you serve the DFlash2 drafter (syvai/Qwen3.8-27B-DFlash2-W4A16), note it was calibrated on the base model's hidden states; acceptance against this retrain measured 59.6% per position with no recalibration.