From f160d0f9ab982ebeaf9f1a184a96033e785d3fdf Mon Sep 17 00:00:00 2001
From: darshanr0107 <darshan@mermaidchart.com>
Date: Tue, 13 Jan 2026 17:21:33 +0530
Subject: [PATCH 1/2] fix: improve anchor node handling for clusters with
 external connections on-behalf-of: @Mermaid-Chart <hello@mermaidchart.com>

---
 .../dagre/mermaid-graphlib.js                 | 46 +++++++++++++++++++
 1 file changed, 46 insertions(+)

diff --git a/packages/mermaid/src/rendering-util/layout-algorithms/dagre/mermaid-graphlib.js b/packages/mermaid/src/rendering-util/layout-algorithms/dagre/mermaid-graphlib.js
index f0e5cd5ed9b..e9ce3c14615 100644
--- a/packages/mermaid/src/rendering-util/layout-algorithms/dagre/mermaid-graphlib.js
+++ b/packages/mermaid/src/rendering-util/layout-algorithms/dagre/mermaid-graphlib.js
@@ -247,6 +247,15 @@ export const adjustClustersAndEdges = (graph, depth) => {
     if (parent !== id && clusterDb.has(parent) && !clusterDb.get(parent).externalConnections) {
       clusterDb.get(id).id = parent;
     }
+    const hasDirectOutgoingEdge = graph.edges().some((edge) => edge.v === id);
+    if (nonClusterChild && clusterDb.get(id)?.externalConnections && hasDirectOutgoingEdge) {
+      // Check if the current anchor is invalid
+      isNodeInExtractableCluster(graph, nonClusterChild, id);
+      const safeAnchor = findSafeAnchorNode(graph, id, graph.parent(nonClusterChild));
+      if (safeAnchor) {
+        clusterDb.get(id).id = safeAnchor;
+      }
+    }
   }
 
   graph.edges().forEach(function (e) {
@@ -411,3 +420,40 @@ const sorter = (graph, nodes) => {
 };
 
 export const sortNodesByHierarchy = (graph) => sorter(graph, graph.children());
+
+/** Checks if a node is inside a cluster that will be extracted (has no external connections). */
+const isNodeInExtractableCluster = (graph, node, rootId) => {
+  let parent = graph.parent(node);
+
+  while (parent && parent !== rootId) {
+    const cluster = clusterDb.get(parent);
+    if (cluster && !cluster.externalConnections) {
+      return true;
+    }
+    parent = graph.parent(parent);
+  }
+
+  return false;
+};
+
+/** Finds an alternative anchor node for a cluster that is not inside an extractable cluster. */
+const findSafeAnchorNode = (graph, clusterId, excludedCluster) => {
+  const children = graph.children(clusterId) ?? [];
+
+  for (const child of children) {
+    if (child === excludedCluster || isDescendant(child, excludedCluster)) {
+      continue;
+    }
+
+    const candidate = findNonClusterChild(child, graph, clusterId);
+    if (!candidate || candidate === child) {
+      continue;
+    }
+
+    if (!isNodeInExtractableCluster(graph, candidate, clusterId)) {
+      return candidate;
+    }
+  }
+
+  return null;
+};

From 4e4e6c4a108d834dd0f643b08deb89159e0eca94 Mon Sep 17 00:00:00 2001
From: darshanr0107 <darshan@mermaidchart.com>
Date: Tue, 13 Jan 2026 18:25:04 +0530
Subject: [PATCH 2/2] fix: add test for rendering nested subgraphs with edges
 from clusters on-behalf-of: @Mermaid-Chart <hello@mermaidchart.com>

---
 .changeset/brave-radios-live.md               |  5 +++
 .../rendering/flowchart-v2.spec.js            | 31 +++++++++++++++++++
 2 files changed, 36 insertions(+)
 create mode 100644 .changeset/brave-radios-live.md

diff --git a/.changeset/brave-radios-live.md b/.changeset/brave-radios-live.md
new file mode 100644
index 00000000000..8f24a2324b1
--- /dev/null
+++ b/.changeset/brave-radios-live.md
@@ -0,0 +1,5 @@
+---
+'mermaid': patch
+---
+
+fix: Prevent crash when flowchart node shape is undefined
diff --git a/cypress/integration/rendering/flowchart-v2.spec.js b/cypress/integration/rendering/flowchart-v2.spec.js
index cd3676fbf3d..dbc6657a0b0 100644
--- a/cypress/integration/rendering/flowchart-v2.spec.js
+++ b/cypress/integration/rendering/flowchart-v2.spec.js
@@ -1211,4 +1211,35 @@ class link myClass
 `
     );
   });
+
+  it('V2 - 18: should render nested subgraphs with edge from cluster containing extractable subgraph', () => {
+    imgSnapshotTest(
+      `flowchart TB
+    subgraph asub
+        aa
+    end
+   a
+    subgraph bsub
+        subgraph csub
+            subgraph dsub
+                da
+            end
+        end
+        subgraph esub
+            subgraph fsub
+                %%comment out subgraph below
+                subgraph gsub
+                    ga
+                    abc
+                    abcd
+                end
+            end
+        end
+    end
+    bsub-->a
+    da-->a
+      `,
+      { htmlLabels: true, flowchart: { htmlLabels: true }, securityLevel: 'loose' }
+    );
+  });
 });