Qwen3.8-27B, MLX 6-bit for mlx-serve

Plain affine 6-bit (group size 64) MLX conversion of Qwen/Qwen3.8-27B, built for mlx-serve, the native Zig MLX server for Apple Silicon.

23.2 GB on disk, 6.50 bits per weight averaged over everything that is quantized. Text, vision, tool calling and thinking all work.

Allocation

Uniform 6-bit, group size 64, on every matmul-read weight — no imatrix calibration. Measured against imatrix-calibrated mixed-width builds at comparable sizes, a uniform width won: quantization error is convex in bits, so lifting every weight off the cliff beats spending extra bits on a subset. Calibration earns its keep below 4 bits, not here. Group size 64 over 128 was measured on the same held-out set — 95.5% top-1 and KL 0.056 against 93.4% and 0.088 — and costs nothing in decode (46.3 against 46.4 tok/s), so the 0.85 GB is worth paying.

Measured against the 4-bit and 8-bit builds

Top-1 agreement and mean KL(bf16 || pack) over 64 held-out windows x 32 sampled positions (2048 distributions), standard error 0.4-0.9 points. The repeated-call column is the longest run of consecutive identical tool calls over three 12-round agent loops against a scripted repository. Decode is M4 Max, temperature 0, code prompt, median of 6 across two passes in opposite boot order, from the server's own reported rate. The jump from 4-bit is the largest single step in the table: KL falls 5x, and the code slice goes from 78.9% to 92.6% top-1. It costs about 31% of decode speed, because MLX's 4-bit affine matmul is the width every fast path in the engine is specialised for.

Attention q/k/v/o, the GatedDeltaNet in_proj_a/in_proj_b gates and the whole MTP head are pinned at 4-bit/gs-64 — cheap insurance, and the MTP head is the decode lever.

Serving

mlx-serve --model ddalcu/Qwen3.8-27B-MLX-Serve-6bit --serve --kv-quant 4

• Which Mac. This build exists for the 24 GB gap: the 4-bit pack needs about 18 GB of weights resident and does not fit there, and the 8-bit one needs a 48 GB machine. 13.0 GB of weights plus a ~1.4 GB prefill working set at --prefill-chunk 512 leaves roughly 1.5 GB for the cache inside a 24 GB Mac's ~16 GB Metal working set, which is about 85k tokens at --kv-quant 4. A 16 GB Mac is NOT covered — the budget that fits one is around 8.6 GB, and at that size this model stops following instructions (measured: 46% top-1 agreement, and two of three agent runs made no tool calls at all). Those figures are computed from measurements taken on a 128 GB Mac, not from a run on a 24 GB one.
• --kv-quant 4 is half the point. At fp16 the cache is 64 KB per token here; at 4-bit it is 18 KB. mlx-serve sizes its context window accordingly.
• Thinking is on by default. "enable_thinking": false turns it off; depth is "reasoning_effort": "xhigh" | "medium" | "low" (Qwen3.8's own vocabulary).
• Tools use Qwen3.8's XML call format; mlx-serve parses, repairs and schema-coerces it into standard OpenAI tool_calls.
• Images work on the chat APIs. Send image_url content as usual.

Conversion

• Calibrated affine quantization on every matmul-read weight, widths per the table above.
• Kept bf16: mtp.fc and every norm, bias, conv and SSM state, and the whole vision tower.
• The MTP head ships with the model; mlx-serve finds it in the shards and loads it with no flags.

Same three raw-checkpoint fixes as the 4-bit and 8-bit builds: the delta-encoded norms get their +1 folded in, the depthwise conv1d is transposed to MLX's [C, K, 1] and the Conv3d patch embed is channels-last.

Weights, config, tokenizer and chat template are otherwise verbatim from the base repo.