ComfyUI KSampler error: trace the cause
Use the root exception to distinguish sampling, models, memory and custom nodes.
Recognize the message
KSampler; IndexError: list index out of range
KSampler may appear in a traceback because sampling is where incompatible inputs become visible. Its name alone is not the root cause, and an IndexError does not identify a universal sampler setting to change.
Work through the evidence
- Save the complete traceback, final exception, node ID, model family and graph. Inspect frames just before the failure, especially extension code or conditioning adapters.
- Use a minimal core graph for the same installed model. Compare conditioning, latent input, VAE and optional extensions; remove only optional additions in a copied workflow.
- If the minimal graph runs, restore one addition at a time. If it also fails, investigate the exact exception: allocation, dimensions, missing files or an extension import each needs a different path.
Verify the result
The exact original exception stops recurring on a small representative job. Record the change and runtime environment; a structurally clean export alone is insufficient.
What this does not prove
Changing the seed, increasing steps or switching samplers without evidence may conceal the trigger while leaving incompatible components in place. Do not label a case solved until a real run verifies it.
Inspect before changing your workflow
The log inspector works in your browser. The workflow checker reads exported graph structure. Neither sends files automatically or executes nodes. Preserve the original, inspect a copy and record the actual test result in Casebook.
Inspect your log ↗ · Check the graph ↗ · Record a recovery case ↗Before asking for help
Include the first full exception, the failing stage, software versions, model family and the smallest reproducible graph. Redact API keys, user paths and private prompts. A summary such as “KSampler broken” removes the context that distinguishes one failure from another.
Prepare a redacted copy ↗Related investigations
ComfyUI CUDA out of memory · ComfyUI missing custom nodes · ComfyUI model not found · ComfyUI custom node import failed
Source and scope
These are OnBeg’s diagnostic checklists, informed by official ComfyUI troubleshooting. They are not universal fixes and do not replace model- or extension-specific instructions.