DPMSolverMultistepScheduler out-of-bounds error when using beta_schedule="squaredcos_cap_v2" + use_karras_sigmas=True

[scp3125]

Describe the bug

Hi, I might be misunderstanding something here — I’m not very experienced with submitting issues to Diffusers yet, so please feel free to correct me if I’ve overlooked anything.

When using DPMSolverMultistepScheduler with the combination:

DPMSolverMultistepScheduler(
 beta_schedule="squaredcos_cap_v2",
 use_karras_sigmas=True,
)

calling set_timesteps(20) produces duplicate timesteps, which causes the multistep solver’s internal index tracking to drift, eventually leading to an index out of bounds error during sampling.

Reproduction

import diffusers
import torch
from diffusers import DPMSolverMultistepScheduler

print("Diffusers version:", diffusers.__version__)
print("Torch version:", torch.__version__)

scheduler = DPMSolverMultistepScheduler(
 beta_schedule="squaredcos_cap_v2",
 use_karras_sigmas=True,
)

scheduler.set_timesteps(20)

print("Timesteps:", scheduler.timesteps)

latents = torch.randn(1, 4, 64, 64)
noise_pred = torch.randn_like(latents)

for i, t in enumerate(scheduler.timesteps):
 print(f"Step {i}: t={t.item()}")
 try:
 result = scheduler.step(noise_pred, t, latents)
 latents = result.prev_sample
 except Exception as e:
 print(f"ERROR: {e}")
 break

Logs

Diffusers version: 0.35.2
Torch version: 2.9.0+cu128
Timesteps: tensor([998, 998, 998, 998, 998, 998, 998, 998, 998, 997, 996, 994, 989, 978,
 949, 867, 623, 221, 33, 0])
Step 0: t=998
Step 1: t=998
Step 2: t=998
Step 3: t=998
Step 4: t=998
Step 5: t=998
Step 6: t=998
Step 7: t=998
Step 8: t=998
Step 9: t=997
Step 10: t=996
Step 11: t=994
Step 12: t=989
Step 13: t=978
Step 14: t=949
Step 15: t=867
Step 16: t=623
Step 17: t=221
Step 18: t=33
Step 19: t=0
ERROR: index 21 is out of bounds for dimension 0 with size 21

System Info

• 🤗 Diffusers version: 0.35.2
• Platform: Linux-6.8.0-88-generic-x86_64-with-glibc2.39
• Running on Google Colab?: No
• Python version: 3.12.3
• PyTorch version (GPU?): 2.9.0+cu128 (True)
• Flax version (CPU?/GPU?/TPU?): not installed (NA)
• Jax version: not installed
• JaxLib version: not installed
• Huggingface_hub version: 0.35.3
• Transformers version: 4.57.1
• Accelerate version: 1.10.1
• PEFT version: not installed
• Bitsandbytes version: not installed
• Safetensors version: 0.6.2
• xFormers version: not installed
• Accelerator: NVIDIA GeForce RTX 4070 Ti SUPER, 16376 MiB
• Using GPU in script?:
• Using distributed or parallel set-up in script?:

Who can help?

No response

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[yiyixuxu]

hey @scp3125

Do you have an actual use case for squaredcos_cap_v2 + use_karras_sigmas? I'm not entirely sure why squaredcos_cap_v2 was excluded from rounding in the scheduler code. I can dig into the git commits history, but any change here would require extensive testing from our end, so I want to make sure there's a real use case first.
Thanks!

[scp3125]

hey @scp3125

Do you have an actual use case for squaredcos_cap_v2 + use_karras_sigmas? I'm not entirely sure why squaredcos_cap_v2 was excluded from rounding in the scheduler code. I can dig into the git commits history, but any change here would require extensive testing from our end, so I want to make sure there's a real use case first. Thanks!

Yes, this is a real inference use case.

import diffusers
import torch
from diffusers import DPMSolverMultistepScheduler, StableDiffusionPipeline

print("Diffusers:", diffusers.__version__)
print("Torch:", torch.__version__)

device = "cuda" if torch.cuda.is_available() else "cpu"

pipe = StableDiffusionPipeline.from_pretrained(
 "stable-diffusion-v1-5/stable-diffusion-v1-5",
 requires_safety_checker=False,
).to(device)
pipe.set_progress_bar_config(disable=True)

pipe.scheduler = DPMSolverMultistepScheduler.from_config(
 pipe.scheduler.config,
 beta_schedule="squaredcos_cap_v2",
 use_karras_sigmas=True,
)

pipe.scheduler.set_timesteps(20, device=device)
print("Timesteps:", pipe.scheduler.timesteps)

out = pipe(
 prompt=["an astronaut riding a horse on mars"],
 num_inference_steps=20,
)
print("OK, got", len(out.images), "images")

Output:

Diffusers: 0.35.2
Torch: 2.8.0
Loading pipeline components...: 100%|█████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 7/7 [00:00<00:00, 29.48it/s]
Timesteps: tensor([998, 998, 998, 998, 998, 998, 998, 998, 998, 997, 996, 994, 989, 978,
 949, 867, 623, 221, 33, 0])
Traceback (most recent call last):
 File "/Users/py-test/./src/demo/main.py", line 25, in <module>
 out = pipe(
 ^^^^^
 File "/Users/py-test/.venv/lib/python3.12/site-packages/torch/utils/_contextlib.py", line 120, in decorate_context
 return func(*args, **kwargs)
 ^^^^^^^^^^^^^^^^^^^^^
 File "/Users/py-test/.venv/lib/python3.12/site-packages/diffusers/pipelines/stable_diffusion/pipeline_stable_diffusion.py", line 1061, in __call__
 latents = self.scheduler.step(noise_pred, t, latents, **extra_step_kwargs, return_dict=False)[0]
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/Users/py-test/.venv/lib/python3.12/site-packages/diffusers/schedulers/scheduling_dpmsolver_multistep.py", line 1112, in step
 prev_sample = self.multistep_dpm_solver_second_order_update(self.model_outputs, sample=sample, noise=noise)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/Users/py-test/.venv/lib/python3.12/site-packages/diffusers/schedulers/scheduling_dpmsolver_multistep.py", line 835, in multistep_dpm_solver_second_order_update
 self.sigmas[self.step_index + 1],
 ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
IndexError: index 21 is out of bounds for dimension 0 with size 21

Version:

- 🤗 Diffusers version: 0.35.2
- Platform: macOS-26.2-arm64-arm-64bit
- Running on Google Colab?: No
- Python version: 3.12.11
- PyTorch version (GPU?): 2.8.0 (False)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Huggingface_hub version: 0.35.3
- Transformers version: 4.57.3
- Accelerate version: 1.10.1
- PEFT version: 0.18.0
- Bitsandbytes version: not installed
- Safetensors version: 0.6.2
- xFormers version: not installed
- Accelerator: Apple M4 Pro
- Using GPU in script?: <fill in>
- Using distributed or parallel set-up in script?: <fill in>

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the contributing guidelines are likely to be ignored.