Qwen3.6-27B-MTP-IQ4_XS-GGUF

Qwen3.6-27B with NextN/MTP (Multi-Token Prediction) speculative-decoding head, quantized to IQ4_XS for single-GPU 24GB inference.

What this is

The published Qwen3.6 family ships with native NextN MTP heads embedded in the safetensors. Most public GGUFs strip these. This conversion preserves them, producing a single GGUF that:

• Loads as qwen35moe_mtp arch in a patched llama.cpp
• Serves at ~2× decode speed vs. the same trunk without MTP
• Fits in ~15 GB VRAM at IQ4_XS + q4/q4 KV
• Native 262K context

Build pipeline

Source: Qwen/Qwen3.6-27B (HF safetensors, with NextN tensors).

1. Clone llama.cpp at the crucible-mtp branch on llama.cpp (patched) (Aman Gupta's MTP fork) — adds LLM_ARCH_QWEN35MOE_MTP + the NextN draft path.
2. Run convert_hf_to_gguf.py against the HF repo. Produces a BF16 GGUF with arch qwen35moe_mtp and the NextN tensors fused in.
3. Quantize to IQ4_XS via llama-quantize.

python convert_hf_to_gguf.py /path/to/Qwen3.6-27B \
    --outfile Qwen3.6-27B-MTP-bf16.gguf
llama-quantize Qwen3.6-27B-MTP-bf16.gguf \
    Qwen3.6-27B-MTP-IQ4_XS.gguf IQ4_XS

Optimal serving config (RTX 3090 Ti, 24 GB)

Cherry-pick PRs #20819 + #20822 for cross-process KV-slot save/restore (we use these for sub-second resume on long contexts).

llama-server \
  -m Qwen3.6-27B-MTP-IQ4_XS.gguf \
  -ngl 999 -fa on \
  --spec-type mtp --spec-draft-n-max 4 \
  --no-mmap \
  --ctx-size 262144 \
  --batch-size 1024 --ubatch-size 512 \
  -ctk q4_0 -ctv q4_0 \
  --parallel 1 --kv-unified \
  --ctx-checkpoints 8 --checkpoint-every-n-tokens 2048 \
  --cache-ram -1 --cache-idle-slots \
  --metrics --jinja

Why these flags:

• --spec-type mtp: enables NextN-head draft path (this is the whole point of the MTP variant).
• --spec-draft-n-max 4: empirically the sweet spot — beyond that, accept rate drops faster than draft count grows.
• --no-mmap: required for KV-slot persistence + measured ~no perf hit on this rig.
• -ctk q4_0 -ctv q4_0: dense KV cache fits 262K context inside 24 GB without spilling.
• --parallel 1: MTP path currently only supports n_parallel=1 upstream.

What NOT to set:

• -ot (expert offload) — defeats the GPU-resident speedup.
• -ctk q8_0 at full 262K ctx — overflows VRAM during warmup.

Performance (RTX 3090 Ti, 350 W power limit)

Measured 2026-05-06 at short-context inference, persistence + MTP on:

Metric	Value
Decode tok/s (short ctx, no thinking)	100.3 (live measured 2026-05-06, n=4 spec)
Decode tok/s (longer ramp 4K–256K ctx, mean)	70–73
Draft accept rate (n=4)	86.6%
Speedup vs same trunk without MTP	2.92× (33 → 97 t/s on identical workload)
KV slot restore (typical 50 K–200 K ctx)	0.16–0.36 s
Cold load (model → ready)	~5–6 s

Memory footprint at 262 K ctx: ~17 GB (model) + ~5 GB (KV q4/q4) + scratch = ~22.5 GB used, ~1.5 GB headroom on a 24 GB card.

Tokenizer

Inherits Qwen3.6 tokenizer (248,320 vocab, qwen35 pre-tokenizer). Same chat template as upstream Qwen3.6-Instruct. If your runtime errors on Jinja Exception: System message must be at the beginning, use the loosened template at: https://huggingface.co/localweights/qwen36-loose-jinja (single line edit removing the strict-position assertion).

License

Apache 2.0 (inherited from Qwen3.6).

Provenance

Built on Crucible: 9950X / 96 GB DDR5 / RTX 3090 Ti. Same pipeline used for the sibling Qwen3.6-35B-A3B-MTP-IQ4_XS-GGUF repo.