filter virtual start node event for snippet draft run.

api/services/snippet_generate_service.py

@@ -80,6 +80,50 @@ class SnippetGenerateService:
     # Specific ID for the injected virtual Start node so it can be recognised
     _VIRTUAL_START_NODE_ID = "__snippet_virtual_start__"
 
+    @classmethod
+    def _is_virtual_start_event(cls, message: Mapping[str, Any] | str) -> bool:
+        """
+        Return True when *message* is a snippet-only virtual Start node event.
+
+        The virtual Start node is injected purely for snippet execution and is
+        not part of the persisted draft graph. Filter its node lifecycle events
+        out of the SSE stream so the frontend only receives nodes that exist on
+        the canvas.
+        """
+        if not isinstance(message, Mapping):
+            return False
+
+        if message.get("event") not in {"node_started", "node_finished"}:
+            return False
+
+        data = message.get("data")
+        if not isinstance(data, Mapping):
+            return False
+
+        return data.get("node_id") == cls._VIRTUAL_START_NODE_ID
+
+    @classmethod
+    def _filter_virtual_start_events(
+        cls,
+        response: Mapping[str, Any] | Generator[Mapping[str, Any] | str, None, None],
+    ) -> Mapping[str, Any] | Generator[Mapping[str, Any] | str, None, None]:
+        """
+        Drop snippet virtual Start node lifecycle events from stream responses.
+
+        Blocking responses are returned unchanged because they never expose the
+        injected node as a standalone event payload.
+        """
+        if isinstance(response, Mapping):
+            return response
+
+        def _stream() -> Generator[Mapping[str, Any] | str, None, None]:
+            for message in response:
+                if cls._is_virtual_start_event(message):
+                    continue
+                yield message
+
+        return _stream()
+
     @classmethod
     def generate(
         cls,
@@ -119,16 +163,18 @@ class SnippetGenerateService:
         # Adapt snippet to App-like interface for WorkflowAppGenerator
         app_proxy = _SnippetAsApp(snippet)
 
+        response = WorkflowAppGenerator().generate(
+            app_model=app_proxy,  # type: ignore[arg-type]
+            workflow=workflow,
+            user=user,
+            args=args,
+            invoke_from=invoke_from,
+            streaming=streaming,
+            call_depth=0,
+        )
+
         return WorkflowAppGenerator.convert_to_event_stream(
-            WorkflowAppGenerator().generate(
-                app_model=app_proxy,  # type: ignore[arg-type]
-                workflow=workflow,
-                user=user,
-                args=args,
-                invoke_from=invoke_from,
-                streaming=streaming,
-                call_depth=0,
-            )
+            cls._filter_virtual_start_events(response)
         )
 
     @classmethod