fix(core): fix nested subgraph rendering for 3+ level nesting in draw_mermaid

libs/core/langchain_core/runnables/graph_mermaid.py

@@ -236,15 +236,24 @@ def add_subgraph(edges: list[Edge], prefix: str) -> None:
     # Add empty subgraphs (subgraphs with no internal edges)
     if with_styles:
         for prefix, subgraph_node in subgraph_nodes.items():
-            if ":" not in prefix and prefix not in seen_subgraphs:
-                mermaid_graph += f"\tsubgraph {prefix}\n"
+            parts = prefix.split(":")
+            if all(part in seen_subgraphs for part in parts):
+                continue
 
-                # Add nodes that belong to this subgraph
-                for key, node in subgraph_node.items():
-                    mermaid_graph += render_node(key, node)
+            # Open subgraph blocks for each nesting level
+            opened = []
+            for part in parts:
+                if part not in seen_subgraphs:
+                    mermaid_graph += f"\tsubgraph {part}\n"
+                    seen_subgraphs.add(part)
+                    opened.append(part)
+
+            # Add nodes that belong to this subgraph
+            for key, node in subgraph_node.items():
+                mermaid_graph += render_node(key, node)
 
+            for _ in opened:
                 mermaid_graph += "\tend\n"
-                seen_subgraphs.add(prefix)
 
     # Add custom styles for nodes
     if with_styles:

libs/core/tests/unit_tests/runnables/__snapshots__/test_graph.ambr

@@ -2002,6 +2002,29 @@
 
   '''
 # ---
+# name: test_nested_empty_subgraph_mermaid[mermaid]
+  '''
+  ---
+  config:
+    flowchart:
+      curve: linear
+  ---
+  graph TD;
+  	__start__([<p>__start__</p>]):::first
+  	__end__([<p>__end__</p>]):::last
+  	__start__ --> parent\3achild\3agrandchild;
+  	parent\3achild\3agrandchild --> __end__;
+  	subgraph parent
+  	subgraph child
+  	parent\3achild\3agrandchild(grandchild)
+  	end
+  	end
+  	classDef default fill:#f2f0ff,line-height:1.2
+  	classDef first fill-opacity:0
+  	classDef last fill:#bfb6fc
+
+  '''
+# ---
 # name: test_parallel_subgraph_mermaid[mermaid]
   '''
   ---

libs/core/tests/unit_tests/runnables/test_graph.py

@@ -472,6 +472,39 @@ def test_single_node_subgraph_mermaid(snapshot: SnapshotAssertion) -> None:
     assert graph.draw_mermaid() == snapshot(name="mermaid")
 
 
+def test_nested_empty_subgraph_mermaid(snapshot: SnapshotAssertion) -> None:
+    """Test 3+ level nested subgraphs with no internal edges render correctly."""
+    empty_data = BaseModel
+    nodes = {
+        "__start__": Node(
+            id="__start__", name="__start__", data=empty_data, metadata=None
+        ),
+        "parent:child:grandchild": Node(
+            id="parent:child:grandchild",
+            name="grandchild",
+            data=empty_data,
+            metadata=None,
+        ),
+        "__end__": Node(id="__end__", name="__end__", data=empty_data, metadata=None),
+    }
+    edges = [
+        Edge(
+            source="__start__",
+            target="parent:child:grandchild",
+            data=None,
+            conditional=False,
+        ),
+        Edge(
+            source="parent:child:grandchild",
+            target="__end__",
+            data=None,
+            conditional=False,
+        ),
+    ]
+    graph = Graph(nodes, edges)
+    assert graph.draw_mermaid() == snapshot(name="mermaid")
+
+
 def test_runnable_get_graph_with_invalid_input_type() -> None:
     """Test that error isn't raised when getting graph with invalid input type."""