The base ships a multi-token prediction (MTP) head (mtp_num_hidden_layers: 1) that lets the
model self-speculate: it drafts the next token(s) and verifies in one pass, cutting decode
latency without a separate draft model.
That head sits on the acceptance-rate critical path. Quantizing it to 4-bit would push its drafts
away from what the full-precision main model predicts, the verifier would reject more of them, and
the speedup would shrink. So the recipe ignores re:.*mtp.* and keeps the head at bf16 (it is
tiny next to the 27B backbone). The 4-bit backbone does the heavy lifting; the full-precision MTP
head keeps acceptance high. The vision tower (re:.*visual.*) is kept at bf16 for the same
fidelity reason.
Fidelity
Near-lossless versus the bf16 source, ~20 GB vs 55.6 GB bf16 (~36%). GPTQ error compensation and an MSE observer keep the drop from bf16 minimal; the header lists the full characteristics and Quantization covers the recipe.
Quickstart
NVFP4 is auto-detected from config.json (compressed-tensors); no quantization flag needed.
--reasoning-parser qwen3 splits the block into .
FP4 (E2M1), group_size=16, tensor_group, FP8 (E4M3) group scales, shared across fused layers
Activations
FP4, dynamic per-group, FP8 (E4M3) scales
Quantized
all language-model Linear layers
Kept in bf16
MTP head (mtp.*), vision tower (model.visual.*), lm_head
Untouched
gated delta-net Conv1d and SSM params (A_log, dt_bias), never Linear
The model is loaded with AutoModelForImageTextToText so the vision tower is quantized-and-kept
(bf16) rather than dropped; the MTP head, which lives outside the transformers model graph, is
re-attached at bf16 from the base. GPTQ is a quantization-time cost only; inference speed and
format are identical to plain round-to-nearest NVFP4, but it chooses better 4-bit values.
About the base model
DavidAU's Fable-Fusion-711 is a 27B Qwen3.5-family (qwen3_5) vision-language model with thinking
mode, a 256K context window, and an MTP head, already run through Heretic uncensoring.
64 decoder layers; hybrid gated delta-net linear attention plus full attention; dense MLP; vision tower.
MTP head (mtp_num_hidden_layers: 1) for speculative decoding.