Qwen3.5-27B AWQ-4bit (calibrated — v2 thinking-aware)

v2 (2026-04-19): re-calibrated with thinking-aware data, replaces v1. v1 (Open-Platypus calibration) silently broke <think> termination — the model emitted unbounded reasoning tokens even on trivial questions like "What is the capital of France?". v2 fixes it; in-place update so existing users get the correction automatically. Old commit retained on the v1-broken-thinking git tag for reproducibility.

TL;DR

Why this exists

The default AWQ calibration recipes (Open-Platypus, ShareGPT, etc.) have no <think> traces in the assistant turns. When you quantize with that data, the model never sees a </think> followed by an answer in calibration, so it loses the ability to terminate the thinking block. Result: validate_capabilities.py basic test ("What is the capital of France? Answer in one word.") returns empty content because all 2048 generated tokens live inside an unclosed <think> block — SGLang's --reasoning-parser qwen3 strips those into reasoning_content and you get back nothing in content.

This checkpoint was calibrated with a thinking-aware mixed dataset:

• 50% a-m-team/AM-Thinking-v1-Distilled — Qwen3-verified <think>...</think> traces
• 25% AI-MO/NuminaMath-CoT — math reasoning
• 25% HuggingFaceH4/ultrachat_200k — general dialogue

with tokenizer.apply_chat_template(..., enable_thinking=True) so the <think>...</think> structure appears in every render. 256 samples × 1024 tokens, GPTQ via llmcompressor (CPU, ~6h on AMD Ryzen 9 7900), then converted to native AWQ format.

Sampling — IMPORTANT

Do NOT use temperature=0 (greedy decode) — Qwen3-family models loop on greedy: "Paris\n</think>\nParis\n</think>...". Use the model's recommended sampling, which SGLang picks up automatically via sampling_defaults='model':

temperature=0.7
top_p=0.95
top_k=20

Validator confirms temperature=0.6 with chat_template_kwargs={"enable_thinking": true} produces clean output.

Validation results

validate_capabilities.py --skip-vision --skip-video (from the calibration repo):

[PASS] basic     finish=stop answer='paris'  (45 reasoning tokens, was BROKEN on original)
[~]   thinking  reasoning_seen answer_ok    (model derives correct $0.05 for the
                                              ball-and-bat puzzle in ~400 reasoning
                                              tokens at temp=0.6; verbose at temp=0.7
                                              — bumped validator budget to 4096 tok)

Thinking on simple QA terminates in tens of tokens. Hard reasoning (multi-step math) the model is verbose at recommended sampling — the answer ends up inside the reasoning block before finish_reason=stop. This is much better than the original AWQ where the model never terminated even on trivial questions.

Architecture

• Base: Qwen/Qwen3.5-27B (multimodal Qwen3_5ForConditionalGeneration, 48 layers, hybrid DeltaNet + full-attention)
• Quantization: AWQ 4-bit, group_size=128
• Excluded from quantization (kept BF16): lm_head, DeltaNet in_proj_a/in_proj_b (recurrent state — INT4 destroys it), vision tower
• Format: native AWQ (qweight + scales + qzeros). Loadable by SGLang's AWQ Triton + HIP GEMV kernels.
• Vision weights preserved: model-vision.safetensors carries the BF16 vision tower + preprocessor_config.json is included so multimodal inference still works.

Files

SGLang launch (RDNA4)

MODEL=mattbucci/Qwen3.5-27B-AWQ-4bit-calibrated python -m sglang.launch_server \
    --model-path $MODEL \
    --tensor-parallel-size 2 \
    --dtype float16 \
    --kv-cache-dtype fp8_e4m3 \
    --context-length 262144 \
    --quantization awq \
    --reasoning-parser qwen3 \
    --disable-cuda-graph \
    --disable-custom-all-reduce \
    --disable-overlap-schedule \
    --attention-backend triton

Or via the reference launcher: MODEL=$MODEL ./scripts/launch.sh qwen35

Performance (2x AMD R9700, RDNA4, ROCm 7.2)

Single-user, FP8 KV cache, --disable-cuda-graph:

Dense DeltaNet hybrid — bandwidth-bound at short context, full-attention layers dominate at long. For 256K agent workloads, prefer Qwen3.6-35B-A3B MoE (12-13 tok/s @ 256K).

Calibration repo

All scripts, validator, and benchmark code: github.com/mattbucci/2x-R9700-RDNA4-GFX1201-sglang-inference — see scripts/quantize/quantize_qwen35_thinking_aware.py and scripts/quantize/calibration_datasets.py.

License

Apache 2.0, inherited from the base model.