Qwen3-VL 4B as a ComfyUI text encoder — W4A8 (and a W4A4 you should not take)

The text encoder of the Krea 2 pipeline in ComfyUI (qwen3vl_4b, Comfy-Org's single-file repack of Qwen3-VL-4B-Instruct), re-encoded into ComfyUI's native per-layer quantization formats. After the Krea 2 DiT drops to 7.5 GiB, this 8.27 GiB encoder is the largest file in the chain — which is why it was the next thing measured.

Source: qwen3vl_4b_bf16.safetensors, 8,875,719,384 B. The 252 layers are exactly 36 decoder blocks × 7 Linears (model.language_model.layers.N.{q,k,v,o}_proj, {gate,up,down}_proj); the vision tower, embeddings, norms and lm_head are preserved byte-for-byte. Backend recorded in the sidecars: comfy_kitchen.backends.cuda. Method, tools and the full log: https://github.com/JoaoZaokk/comfy-quant-bench (bench/krea2_suite.md, section 8).

What it costs, measured against the BF16 original

Conditioning error (relative RMSE of the encoder's output against the BF16 encoder on the same prompt) and encode time, median of 5 encodes per cell, each arm in its own process, RTX 3090. Two prompt lengths, because the answer depends on it.

• W4A8 is 2.53x more faithful than W4A4 at the short prompt (0.1438 against 0.3637) for 0.22 GiB more, and running its math quantized costs it nothing measurable (0.14381 against 0.14382) while making it 2.09x faster. W4A4's quantized math is a real trade: 1.26x faster for 1.38x worse. Cosine with BF16 at 76 tokens, W4A8: 0.9896.
• The error grows with the prompt — 0.1438 at 76 tokens, 0.5839 at 1220 (cosine 0.8456) — and so does the speed win (2.09x → 3.05x against dequantized, 2.71x against BF16). The format is cheapest exactly where it is least accurate. The 1220-token prompt is synthetic and repetitive (nine clauses repeated six times), so the short-prompt column is the better founded of the two.
• "Quantized math" needs a patched ComfyUI. Stock ComfyUI runs every text encoder through the dequantized path behind two locks (comfy/sd.py forces float32 compute on every CLIP object and comfy/sd1_clip.py hardcodes full_precision_mm=True), so on stock ComfyUI these files save memory and not time — the "dequantized math" rows above are what you get. The method repo carries the patch that releases both locks only for checkpoints that really carry quantized layers (patches/comfyui_text_encoder_quantized_math.patch, opt-out flag --disable-quantized-text-encoder); a non-quantized encoder is untouched by it (control arm: 0 quantized layers, force_cast_weights True in both arms).
• Speed measurements here were noisy on the first pass and were re-done: the first encode of any arm is 2.8–3.5x its median (warm-up), and the BF16 reference measured 1.54x apart between two single invocations. Everything above is a median of five with the reference repeated like the arms.

Using it

CLIPLoader (or the Krea 2 template's loader) with clip_type for Krea 2, pointing at the file in ComfyUI/models/text_encoders/. Needs the converter's qwen3vl profile lineage only at conversion time; at load time ComfyUI reads the per-layer metadata itself. Krea 2 chain measured end to end with this encoder in bench/krea2_suite.md.

What is NOT covered

• Conditioning distance is not image quality: no image was rendered for this card; the Krea 2 DiT measurements in the sibling repo used the BF16 encoder.
• One card (RTX 3090), two prompts, one seed-free measurement (encoders are deterministic).
• No text-encoder LoRA, no vision input (the vision tower is preserved but was not exercised).

Credits

• Qwen — Qwen3-VL-4B-Instruct, Apache 2.0.
• Comfy-Org — the single-file repack ComfyUI loads.
• ComfyUI / comfy-kitchen — the asym_w4a8_int8 and convrot_w4a4 formats and the CUDA kernels that execute them.