[Bug] `QwenImagePipeline` silently disables CFG when passing `negative_prompt_embeds` if mask is `None` (which `encode_prompt` returns by default)

[Sunhill666]

Describe the bug

In QwenImagePipeline, when users manually pre-compute prompt embeddings to optimize memory usage (e.g., placing the encoder and transformer on different GPUs), Classifier-Free Guidance (CFG) is silently disabled if negative_prompt_embeds_mask is set to None.

However, encode_prompt explicitly converts an all-ones mask to None as an optimization. This creates a logical contradiction where the pipeline's own encoder returns a valid state (None) that the __call__ method subsequently rejects, causing CFG to fail with a warning.

Reproduction

If you manually extract embeddings and pass them to the pipeline:

# 1. Manually encode prompts
pos_embeds, pos_mask = pipeline.encode_prompt("A photo of a cat")
neg_embeds, neg_mask = pipeline.encode_prompt("bad quality") 
# Note: neg_mask is often `None` here because `encode_prompt` optimizes `prompt_embeds_mask.all() -> None`

# 2. Pass them to the pipeline
image = pipeline(
 prompt_embeds=pos_embeds,
 prompt_embeds_mask=pos_mask,
 negative_prompt_embeds=neg_embeds,
 negative_prompt_embeds_mask=neg_mask, # This passes None
 true_cfg_scale=4.0
).images[0]

Output Warning:

true_cfg_scale is passed as 4.0, but classifier-free guidance is not enabled since no negative_prompt is provided.

Root Cause Analysis:
In pipeline_qwenimage.py:

1. The has_neg_prompt check in __call__ requires the mask to NOT be None:

has_neg_prompt = negative_prompt is not None or (
 negative_prompt_embeds is not None and negative_prompt_embeds_mask is not None
)

2. But encode_prompt intentionally sets the mask to None if it's full:

if prompt_embeds_mask is not None:
 # ... (reshape logic)
 if prompt_embeds_mask.all():
 prompt_embeds_mask = None # <--- Here!

Because neg_mask becomes None, has_neg_prompt evaluates to False, and do_true_cfg is set to False.

Expected behavior:
The presence of negative_prompt_embeds alone should be sufficient to trigger has_neg_prompt = True. The negative_prompt_embeds_mask being None should simply mean "no masking is required" (all valid), which is consistent with the behavior of encode_prompt.

The check in __call__ should probably be relaxed to:

has_neg_prompt = negative_prompt is not None or negative_prompt_embeds is not None

Temporary Workaround:
Users currently have to manually fake an all-ones mask before passing it to the transformer:

if neg_mask is None:
 neg_mask = torch.ones(neg_embeds.shape[:2], dtype=torch.long, device=device)

Logs

System Info

• 🤗 Diffusers version: 0.37.1
• Platform: Linux-6.6.87.2-microsoft-standard-WSL2-x86_64-with-glibc2.39
• Running on Google Colab?: No
• Python version: 3.10.20
• PyTorch version (GPU?): 2.11.0+cu130 (True)
• Flax version (CPU?/GPU?/TPU?): not installed (NA)
• Jax version: not installed
• JaxLib version: not installed
• Huggingface_hub version: 1.8.0
• Transformers version: 5.4.0
• Accelerate version: 1.13.0
• PEFT version: not installed
• Bitsandbytes version: not installed
• Safetensors version: 0.7.0
• xFormers version: not installed
• Accelerator: NVIDIA GeForce RTX 3060, 12288 MiB
• Using GPU in script?: Yes
• Using distributed or parallel set-up in script?: Yes

Who can help?

@yiyixuxu @DN6

[asomoza]

thanks @Sunhill666 you're right this is an issue and it should be sufficient with just checking the negative_prompt_embeds , do you want to open a PR to fix it?

[Sunhill666]

@asomoza Sure, i will open a PR later.