diff --git a/nodes.py b/nodes.py
index 0255129..6ad6b33 100644
--- a/nodes.py
+++ b/nodes.py
@@ -15,6 +15,11 @@ VACE_MODES = [
 ]
 
 
+def _snap_4n1(n):
+    """Round up to nearest 4n+1 value (1, 5, 9, 13, ..., 77, 81, ...)."""
+    return ((n + 2) // 4) * 4 + 1
+
+
 def _create_solid_batch(count, height, width, color_value, device="cpu"):
     """Create a batch of solid-color frames (B, H, W, 3). Returns empty tensor if count <= 0."""
     if count <= 0:
@@ -30,7 +35,7 @@ class VACEMaskGenerator:
     OUTPUT_TOOLTIPS = (
         "Visual reference for VACE — source pixels where mask is black, grey (#7f7f7f) fill where mask is white.",
         "Mask sequence — black (0) = keep original, white (1) = generate. Per-frame for most modes; per-pixel for Video Inpaint.",
-        "Total frame count of the output sequence — wire directly to VACE encode.",
+        "Total frame count snapped to 4n+1 (1, 5, 9, …, 81, …) — wire directly to VACE encode.",
     )
     DESCRIPTION = """VACE Mask Generator — builds mask + control_frames sequences for all VACE generation modes.
 
@@ -121,6 +126,7 @@ If your source is longer, use VACE Source Prep upstream to trim it first."""
     def generate(self, source_clip, mode, target_frames, split_index, edge_frames, inpaint_mask=None, keyframe_positions=None):
         B, H, W, C = source_clip.shape
         dev = source_clip.device
+        target_frames = _snap_4n1(target_frames)
 
         modes_using_target = {"End Extend", "Pre Extend", "Middle Extend", "Edge Extend",
                               "Join Extend", "Bidirectional Extend", "Keyframe"}
@@ -215,7 +221,7 @@ If your source is longer, use VACE Source Prep upstream to trim it first."""
                     ctrl_parts.append(solid(step, GREY))
             mask = torch.cat(mask_parts, dim=0)
             control_frames = torch.cat(ctrl_parts, dim=0)
-            return (control_frames, mask, B + frames_to_generate)
+            return (control_frames, mask, _snap_4n1(B + frames_to_generate))
 
         elif mode == "Replace/Inpaint":
             if split_index >= B:
@@ -231,7 +237,7 @@ If your source is longer, use VACE Source Prep upstream to trim it first."""
             after = source_clip[end:]
             mask = torch.cat([solid(before.shape[0], BLACK), solid(length, WHITE), solid(after.shape[0], BLACK)], dim=0)
             control_frames = torch.cat([before, solid(length, GREY), after], dim=0)
-            return (control_frames, mask, B)
+            return (control_frames, mask, _snap_4n1(B))
 
         elif mode == "Video Inpaint":
             if inpaint_mask is None:
@@ -254,7 +260,7 @@ If your source is longer, use VACE Source Prep upstream to trim it first."""
             mask = m3
             grey = torch.full_like(source_clip, GREY)
             control_frames = source_clip * (1.0 - m3) + grey * m3
-            return (control_frames, mask, B)
+            return (control_frames, mask, _snap_4n1(B))
 
         elif mode == "Keyframe":
             if B > target_frames: