diff --git a/packages/@aws-cdk/toolkit-lib/lib/api/cloud-assembly/stack-assembly.ts b/packages/@aws-cdk/toolkit-lib/lib/api/cloud-assembly/stack-assembly.ts
index e92fac7ac..9f3a74f1a 100644
--- a/packages/@aws-cdk/toolkit-lib/lib/api/cloud-assembly/stack-assembly.ts
+++ b/packages/@aws-cdk/toolkit-lib/lib/api/cloud-assembly/stack-assembly.ts
@@ -140,7 +140,7 @@ async function includeDownstreamStacks(
 
     for (const [id, stack] of allStacks) {
       // Select this stack if it's not selected yet AND it depends on a stack that's in the selected set
-      if (!selectedStacks.has(id) && (stack.dependencies || []).some(dep => selectedStacks.has(dep.id))) {
+      if (!selectedStacks.has(id) && (stack.dependencies || []).some(dep => selectedStacks.has(dep.hierarchicalId))) {
         selectedStacks.set(id, stack);
         added.push(id);
         madeProgress = true;
diff --git a/packages/@aws-cdk/toolkit-lib/test/api/cloud-assembly/stack-selection-expand.test.ts b/packages/@aws-cdk/toolkit-lib/test/api/cloud-assembly/stack-selection-expand.test.ts
new file mode 100644
index 000000000..25dbb4d86
--- /dev/null
+++ b/packages/@aws-cdk/toolkit-lib/test/api/cloud-assembly/stack-selection-expand.test.ts
@@ -0,0 +1,69 @@
+import { ExpandStackSelection, StackSelectionStrategy } from '../../../lib/api/cloud-assembly';
+import { Toolkit } from '../../../lib/toolkit';
+import { TestIoHost } from '../../_helpers';
+import type { TestStackArtifact } from '../../_helpers/test-cloud-assembly-source';
+import { TestCloudAssemblySource } from '../../_helpers/test-cloud-assembly-source';
+
+const ioHost = new TestIoHost();
+const toolkit = new Toolkit({ ioHost });
+
+beforeEach(() => {
+  ioHost.notifySpy.mockClear();
+});
+
+// Regression test: a stack's hierarchicalId (used to key the internal
+// selection maps) is `manifest.displayName ?? id`. When a stack has an
+// explicit displayName that differs from its artifact id (as happens for
+// stacks inside nested assemblies/stages), `includeDownstreamStacks` used
+// to look up dependents by the dependency's raw `id` instead of its
+// `hierarchicalId`, so it never found a match and silently failed to pull
+// in downstream stacks - even though the symmetric `includeUpstreamStacks`
+// correctly used hierarchicalId all along.
+describe('downstream/upstream stack expansion with a non-trivial displayName', () => {
+  const DEPENDENCY: TestStackArtifact = {
+    stackName: 'DependencyStack',
+    displayName: 'CustomDisplayName',
+  };
+  const DOWNSTREAM: TestStackArtifact = {
+    stackName: 'DownstreamStack',
+    depends: ['DependencyStack'],
+  };
+
+  test('DOWNSTREAM expansion pulls in a stack that depends on the selected stack', async () => {
+    // GIVEN
+    const cx = new TestCloudAssemblySource({
+      stacks: [DEPENDENCY, DOWNSTREAM],
+    });
+
+    // WHEN: select only the dependency by its hierarchicalId (displayName), expand downstream
+    const stacks = await toolkit.list(cx, {
+      stacks: {
+        patterns: ['CustomDisplayName'],
+        strategy: StackSelectionStrategy.PATTERN_MATCH,
+        expand: ExpandStackSelection.DOWNSTREAM,
+      },
+    });
+
+    // THEN: the dependent stack must be included, not just the matched one
+    expect(stacks.map(s => s.id).sort()).toEqual(['CustomDisplayName', 'DownstreamStack']);
+  });
+
+  test('UPSTREAM expansion pulls in the stack a selected stack depends on', async () => {
+    // GIVEN
+    const cx = new TestCloudAssemblySource({
+      stacks: [DEPENDENCY, DOWNSTREAM],
+    });
+
+    // WHEN: select only the downstream stack, expand upstream
+    const stacks = await toolkit.list(cx, {
+      stacks: {
+        patterns: ['DownstreamStack'],
+        strategy: StackSelectionStrategy.PATTERN_MATCH,
+        expand: ExpandStackSelection.UPSTREAM,
+      },
+    });
+
+    // THEN: the dependency stack must be included
+    expect(stacks.map(s => s.id).sort()).toEqual(['CustomDisplayName', 'DownstreamStack']);
+  });
+});