Qwen3.8-27B-heretic-ara-NVFP4

NVFP4 (W4A16) quantization of trohrbaugh/Qwen3.8-27B-heretic-ara — the ARA-abliterated Qwen3.8-27B — with the model's MTP speculative-decoding head preserved and wired for vLLM. 20 GB on disk versus 52 GB for the BF16 source.

Built for and benchmarked on NVIDIA GB10 / DGX Spark (128 GB unified memory, aarch64), where NVFP4 is the sweet spot: the platform is bandwidth-limited (~273 GB/s), so weight size dominates decode speed, and NVFP4 gets a native CUTLASS kernel on sm_121. In our testing this quant beat an FP8 build of the same model by ~29% — the opposite of results published for datacenter GPUs, and the reason this repo exists.

What was done

• Quantized with nvidia-modelopt 0.45.0 (W4A16_NVFP4_CFG).
• Vision tower left unquantized (model.visual* excluded) — image and video input verified working after conversion.
• MTP head left unquantized and wired in. The abliterated release ships the same MTP tensors as the stock model; this repo adds the "mtp*" entries to exclude_modules in config.json / hf_quant_config.json and maps the 15 mtp.* tensors (in model_mtp.safetensors) into the safetensors index, so vLLM can use --speculative-config with it.
• Tokenizer, chat template, and processor configs are unchanged from the upstream abliterated release.

Serving with vLLM

The known-good command (this is what produced the numbers below):

vllm serve <model-path> \
  --max-model-len 131072 \
  --gpu-memory-utilization 0.40 \
  --kv-cache-dtype fp8 \
  --enable-prefix-caching \
  --max-num-seqs 4 \
  --tool-call-parser qwen3_coder --enable-auto-tool-choice \
  --reasoning-parser qwen3

To enable MTP speculative decoding (read the stability section first):

  --speculative-config '{"method":"mtp","num_speculative_tokens":3}'

Notes:

• Tool-call dialect is qwen3_coder, not qwen3_xml — the chat template emits <tool_call><function=NAME><parameter=KEY>…. The template also uses <think>…</think>, hence the qwen3 reasoning parser. vLLM validates both names before loading weights, so a mismatch fails fast.
• Native context is 262,144; we serve 131,072 because KV cache is the dominant memory cost. Do not go below ~131k if you front an agentic coding client — large system prompts plus a 32k output reservation overflow a 65,536 window.
• On unified-memory platforms, vLLM sizes its pool from memory free at launch, not total — anything already resident silently shrinks your KV cache. Measured footprints on GB10: ~27 GiB at --gpu-memory-utilization 0.25, ~43 GiB at 0.40 (both at 131k context).

Performance (GB10, vLLM)

	this quant	stock Qwen3.8-27B NVFP4
Single-stream decode, MTP off	12.6 tok/s	—
Single-stream decode, MTP on (k=3)	19.1 tok/s	23.6 tok/s
Aggregate, 4 concurrent, MTP off	33.7 tok/s	30.8 tok/s
Aggregate, 4 concurrent, MTP on	52.3 tok/s	—

The stock model is faster single-stream with MTP because its draft head was trained on the weights it ships with; here the head drafts for abliterated weights it never saw, which costs acceptance rate. In exchange: no refusals, and better concurrency.

MTP stability — read before enabling

Full transparency on the one incident: on first real-world deployment, MTP crashed the engine once — CUBLAS_STATUS_INTERNAL_ERROR in the MTP head's BF16 GEMM, triggered when a new request prefilled while other requests were decoding with draft tokens in flight. Multi-client front-ends (e.g. open-webui's background title/tag requests racing a chat) produce exactly that pattern.

Since then it has not reproduced: three deliberate reproduction attempts (synthetic mixed prefill+decode load, staggered-arrival load, and adversarial interactive use) plus a 4-hour monitored soak under real multi-client traffic — zero crashes, zero container restarts.

Recommendation: enable MTP together with a container restart policy (--restart unless-stopped or equivalent) so a recurrence costs a reload, not an outage. For strictly single-client use it has been trouble-free. If you need hard guarantees, serve without --speculative-config — you keep the concurrency advantage (33.7 tok/s at c=4) either way. Untried mitigations if you do hit it: num_speculative_tokens: 1, --enforce-eager.

Intended use and limitations

This is an abliterated ("uncensored") derivative: refusal behavior has been removed by the upstream ARA process. It will comply with requests the base model would decline. You are responsible for the legality and consequences of what you generate with it, and for adding whatever guardrails your deployment requires. Do not expose it unauthenticated to the public internet.

Quantization was calibrated for general use; task-critical accuracy should be validated against the BF16 upstream for your workload. The base model's knowledge, biases, and failure modes carry over.

Lineage and licensing

Qwen/Qwen3.8-27B (Apache-2.0) → ARA abliteration by trohrbaugh → NVFP4 conversion + MTP wiring (this repo). A Q6_K GGUF of the same abliterated weights is available at mradermacher/Qwen3.8-27B-heretic-ara-GGUF.

Licensed Apache-2.0, same as the base model (LICENSE included).