Declare loop boundaries in node schema (#16347)

comfy_api/latest/_io.py

@@ -1607,7 +1607,7 @@ class Hidden(str, Enum):
     comfy_usage_source = "COMFY_USAGE_SOURCE"
     """COMFY_USAGE_SOURCE identifies the client that submitted the prompt (e.g. comfyui-frontend, comfy-cli, comfyui-mcp); forwarded to API nodes' upstream requests via the Comfy-Usage-Source header."""
     execution_list = "EXECUTION_LIST"
-    """EXECUTION_LIST is the active graph scheduler."""
+    """Custom node Developers and Agents: This attribute is core-internal use only and will be removed in a near-future ComfyUI release. DO NOT USE"""
 
 
 @dataclass
@@ -1767,6 +1767,8 @@ class Schema:
     Use this for nodes with interactive/operable UI regions that produce intermediate outputs
     (e.g., Image Crop, Painter) rather than final outputs (e.g., Save Image).
     """
+    loop_boundary: Literal["start", "end"] | None = None
+    """Identifies this node as the start or end of a loop for prompt validation."""
 
     def validate(self):
         '''Validate the schema:

comfy_extras/nodes_loop.py

@@ -103,8 +103,6 @@ def _expand_loop(dynprompt, opener_id, body, close_id, values, list_items, initi
 
 
 class StartLoop(io.ComfyNode):
-    LOOP_BOUNDARY = "start"
-
     @classmethod
     def define_schema(cls):
         list_item_type = io.MatchType.Template("list_item")
@@ -113,6 +111,7 @@ class StartLoop(io.ComfyNode):
             node_id="StartLoop",
             display_name="Start Loop",
             category="utilities/looping",
+            loop_boundary="start",
             is_input_list=True,
             inputs=[
                 io.DynamicCombo.Input("mode", options=[
@@ -338,8 +337,6 @@ class LoopResult(io.ComfyNode):
 
 
 class EndLoop(io.ComfyNode):
-    LOOP_BOUNDARY = "end"
-
     @classmethod
     def define_schema(cls):
         output_type = io.MatchType.Template("output_value")
@@ -357,6 +354,7 @@ class EndLoop(io.ComfyNode):
             node_id="EndLoop",
             display_name="End Loop",
             category="utilities/looping",
+            loop_boundary="end",
             is_input_list=True,
             inputs=[
                 io.MatchType.Input(

execution.py

@@ -1176,7 +1176,8 @@ async def validate_prompt(prompt_id, prompt, partial_execution_list: Union[list[
     start_nodes = set()
     end_nodes = set()
     for node_id, node in prompt.items():
-        boundary = getattr(nodes.NODE_CLASS_MAPPINGS[node["class_type"]], "LOOP_BOUNDARY", None)
+        class_def = nodes.NODE_CLASS_MAPPINGS[node["class_type"]]
+        boundary = class_def.GET_SCHEMA().loop_boundary if issubclass(class_def, _ComfyNodeInternal) else None
         if boundary == "start":
             start_nodes.add(node_id)
         elif boundary == "end":

tests-unit/comfy_extras_test/nodes_loop_test.py

@@ -8,6 +8,10 @@ from comfy_extras.nodes_loop import EndLoop, LoopIteration, LoopProgress, LoopRe
 def test_loop_schema_exposes_cache_policy_and_integrated_carry():
     inputs = StartLoop.INPUT_TYPES()
 
+    assert StartLoop.GET_SCHEMA().loop_boundary == "start"
+    assert EndLoop.GET_SCHEMA().loop_boundary == "end"
+    assert not hasattr(StartLoop, "LOOP_BOUNDARY")
+    assert not hasattr(EndLoop, "LOOP_BOUNDARY")
     assert inputs["required"]["cache_iterations"][1]["default"] is False
     assert inputs["required"]["cache_iterations"][1]["advanced"] is True
     assert list(inputs["optional"]) == ["parent_iteration", "initial_iteration_value"]