Overview
Many apparent ComfyUI failures are actually environment or model-placement problems. If the interface opens but Queue Prompt produces a checkpoint error, rebuilding the graph will not solve the issue. The model must first exist in the correct directory and be selectable by the checkpoint loader.
Model choice also affects all later decisions. Resolution, step count, and even the usefulness of certain extensions depend on the model family. A Lightning checkpoint, for example, is designed to produce an image in far fewer steps than a conventional setup. Selecting the model first prevents the rest of the workflow from being tuned around the wrong assumptions.