Qwen3.8-27B-AWQ-4.85bpw

Qwen3.8-27B — AWQ 4.85bpw (MLX / oMLX)

AWQ-calibrated quantization of Qwen/Qwen3.8-27B (27.8B dense, 64 layers, hybrid GatedDeltaNet + full attention, 256K context, MTP head, SigLIP-style vision tower) for Apple Silicon via oMLX.

16.83 GB · BPW 4.85 · 3.3× smaller than bf16

4-bit on the MLP, which is 62% of the weights, raised selectively elsewhere where measurement said it mattered. That averages out to 4.85 bits per weight. Bit widths came from an importance matrix collected on this model, not from another model's recipe. Details below.

Nothing is quantized to 3-bit: oMLX's qwen35_prefill kernels expose q2/q4/q5/q6/q8 but not q3, so 3-bit anywhere in the MLP would silently drop prefill to a slow path.

IMPORTANT IF USING SPEC PEFILL

If running a Speculative decoder, prefer using the Qwen 3.5 2B bf16 model if ram suffices, else use the Qwen 3.5 0.8b Bf16 Model BF16 will have the greatest speed and accuracy at the cost of higher ram, 8 bit is also acceptable, but at the negligible size of the model it is recommended to use the bf16 variant. Use acceptance rate of 40% for 2B, 50% for 0.8b for Maximum accuracy.

Quality

Measured against the bf16 original on identical prompts, via oMLX's own omlx.eval harness.

Four questions out of 564. The aggregate difference (89.2% vs 89.9%) is well inside noise for these sample sizes.

Quantizing from bf16 at ~4.8 BPW is a very different regime from squeezing an already-4-bit checkpoint to ~2.9 BPW, where the same pipeline costs 17 MMLU points. There is real headroom here and the recipe spends it carefully.

Speed

M5 Max, 128 GB, oMLX, MTP enabled, pp N / tg 128.

Against the bf16 original: 3.9× generation (55–65 vs 14 tok/s), prefill at parity, and a third of the memory. At 64k context this build uses 32.6 GB where bf16 needs 68.5 GB.

MTP

Native mtp_enabled works and is worth turning on: 1.8–2.1× generation, 88.7% draft acceptance, 3.05 tokens per backbone forward, with the drafter costing about 1% of backbone time. (MTP is a distinct option from speculative decoding with an external draft model — they accelerate different phases.)

Fixed 2026-08-16 — re-download if you pulled this before that date. The head shipped in raw-HF norm convention while the backbone was already converted to MLX's, so mtp.layers.0.input_layernorm averaged 0.0361 instead of ~1.036 and two norms were negative. Drafts stopped matching the target and acceptance collapsed. On a loader with no compensation this made MTP slower than no speculation at all (22.4 vs 24.6 tok/s); repaired, the same setup runs 43.7 tok/s. Inside oMLX the damage was partly masked — its norm_repair shifts any head norm averaging below 0.5, which caught 3 of the 7 and left q_norm (0.78), k_norm (0.79) and mtp.norm (1.25) raw; repairing those three measured +9% decode (median 53.3 vs 48.8 tok/s, pp 4096 / tg 128, three runs each, no overlap between the groups).

Only model-00005-of-00005.safetensors changed. Benchmarks in this card are unaffected: they were run with MTP off, and MTP is rejection-verified, so a degraded head costs acceptance rate but never output correctness.

The MTP head is quantized (8-bit attention, 6-bit fc, 4-bit MLP). This is safe: drafts are verified by the target model through rejection sampling, so drafter error costs acceptance rate, never output correctness. Leaving it at bf16 inside an otherwise quantized model measurably halves throughput, since it then sits outside oMLX's quantized verify path.

Draft acceptance at depth 3 was 96%, so mtp_num_draft_tokens above the default of 3 may be worth trying.

Two things to know before using this

1. oMLX only. The checkpoint keeps its mtp.* weights so the MTP head can bind. Stock mlx_vlm sees those keys, flips its should_shift_norm_weights heuristic, and applies a +1.0 offset to layernorms that already have it. The model still loads and still produces fluent text, but the text is garbage. oMLX patches sanitize to gate that on conv1d layout instead, which is correct here. Do not load this outside oMLX.

2. Prefill needs an environment variable until jundot/omlx#2657 ships. oMLX routes 4-bit gs128 MLP matmuls to a native kernel above 2048 tokens, but that kernel's speed comes from the NAX tensor-unit path, which is gated to group_size 64. At gs128 it demotes to a slower path:

OMLX_QWEN35_Q4_MLP_MIN_TOKENS=999999999
OMLX_QWEN35_Q4_LINEAR_MIN_TOKENS=999999999

Without it, prefill at 4k drops from 931 to 513 tok/s (TTFT 4.4s → 8.0s). The speed table above was measured with it set.

Sampling

The chat template takes reasoning_effort with values xhigh (default), medium, or low, and raises on anything else. It also accepts enable_thinking and preserve_thinking. Pass them via chat_template_kwargs.

For long-context work, enable KV cache quantization.

How it was built

Sequential AWQ over the dense MLP: each layer is calibrated on activations produced by the already-quantized layers above it, so accumulated error is accounted for. Everything outside the MLP is RTN at the bit widths above, where RTN is close to lossless.

Calibration was 192 prompts × 1024 tokens of coding, tool-use and agentic conversations, re-rendered through Qwen's own chat template with a 50/50 think/nothink split. Rendering matters: calibrating on raw text rather than templated conversations cost 27 MMLU points on an earlier model in this line.

Bit allocation came from an importance matrix collected over the same corpus, scored by participation ratio (what fraction of input channels actually carry the energy):

Attention q/k/v is 135× more concentrated than down_proj, so it gets 8-bit despite being only 4% of the weights, while down_proj gets no premium despite being 20%. This is the opposite of what a DeepSeek-V4 recipe would suggest, where down_proj is the concentrated tensor. Transferring that intuition would have spent bits on the family that needed them least.

Caveats

• Vision is not measured. Calibration was text-only, so the tower never ran and its 8-bit assignment is a conservative guess rather than a measurement. 27 linear_fc2 modules stay at bf16 because their input dim (4304) is not divisible by any supported group size.
• Shard filenames are inconsistent (00001-of-00004 through 00005-of-00005). The index is authoritative and loading is unaffected.