fix(scheduler): reject step ranges that produce empty timestep slices

nodes_sampler.py

@@ -2753,6 +2753,28 @@ def INPUT_TYPES(s):
     CATEGORY = "WanVideoWrapper"
     EXPERIMENTAL = True
 
+    @classmethod
+    def VALIDATE_INPUTS(cls, steps, start_step, end_step, **kwargs):
+        # Run at prompt-queue time, before any node executes — this prevents
+        # an upstream sampler from loading the model and burning GPU time on
+        # work that a downstream sampler with an empty timestep slice will
+        # then crash on (UnboundLocalError: 'callback_latent' / 'noise_pred').
+        if start_step >= steps:
+            return (
+                f"start_step ({start_step}) must be < steps ({steps}); "
+                f"the requested range would produce an empty timestep slice "
+                f"and the sampler would crash mid-graph."
+            )
+        if end_step != -1:
+            if end_step > steps:
+                return f"end_step ({end_step}) must be <= steps ({steps})."
+            if end_step <= start_step:
+                return (
+                    f"end_step ({end_step}) must be > start_step ({start_step}); "
+                    f"the requested range would produce an empty timestep slice."
+                )
+        return True
+
     def process(self, scheduler, steps, start_step, end_step, shift, unique_id, sigmas=None, enhance_hf=False):
         sample_scheduler, timesteps, start_idx, end_idx = get_scheduler(
             scheduler, steps, start_step, end_step, shift, device, sigmas=sigmas, log_timesteps=True, enhance_hf=enhance_hf)

wanvideo/schedulers/__init__.py

@@ -174,8 +174,22 @@ def get_scheduler(scheduler, steps, start_step, end_step, shift, device, transfo
         sample_scheduler.sigmas = torch.cat([timesteps / 1000, torch.zeros(1, device=timesteps.device)])
 
     steps = len(timesteps)
+    # Reject ranges that produce an empty timestep slice — otherwise the
+    # sampler's main loop never executes and downstream code (e.g. callbacks
+    # referenced after the loop) crashes with a confusing UnboundLocalError.
+    if isinstance(start_step, int) and start_step >= steps:
+        raise ValueError(
+            f"start_step ({start_step}) must be < steps ({steps}); "
+            f"the requested range would produce an empty timestep slice"
+        )
+    if isinstance(end_step, int) and end_step != -1 and end_step > steps:
+        raise ValueError(
+            f"end_step ({end_step}) must be <= steps ({steps})"
+        )
     if (isinstance(start_step, int) and end_step != -1 and start_step >= end_step) or (not isinstance(start_step, int) and start_step != -1 and end_step >= start_step):
-        raise ValueError("start_step must be less than end_step")
+        raise ValueError(
+            f"start_step ({start_step}) must be less than end_step ({end_step})"
+        )
 
     # Determine start and end indices for slicing
     start_idx = 0