diff --git a/apps/server/src/database/helpers/page-hierarchy-cycle.spec.ts b/apps/server/src/database/helpers/page-hierarchy-cycle.spec.ts
new file mode 100644
index 000000000..cc8f82329
--- /dev/null
+++ b/apps/server/src/database/helpers/page-hierarchy-cycle.spec.ts
@@ -0,0 +1,48 @@
+import {
+  assertAcyclicPageTraversal,
+  PageHierarchyCycleError,
+  stripPageTraversalMetadata,
+} from './page-hierarchy-cycle';
+
+describe('page hierarchy cycle contract', () => {
+  it('does nothing when no row is marked as a cycle', () => {
+    expect(() =>
+      assertAcyclicPageTraversal(
+        [
+          { id: 'page-1', title: 'Root', isCycle: false },
+          { id: 'page-2', title: 'Child', isCycle: false },
+        ],
+        'page-1',
+      ),
+    ).not.toThrow();
+  });
+
+  it('throws PageHierarchyCycleError when a row is marked as a cycle', () => {
+    expect(() =>
+      assertAcyclicPageTraversal(
+        [{ id: 'page-1', title: 'Root', isCycle: true }],
+        'page-1',
+      ),
+    ).toThrow(PageHierarchyCycleError);
+  });
+
+  it('keeps the root page id on the error for safe logging', () => {
+    try {
+      assertAcyclicPageTraversal(
+        [{ id: 'page-1', title: 'Root', isCycle: true }],
+        'root-page',
+      );
+      throw new Error('expected a cycle error');
+    } catch (error) {
+      expect(error).toBeInstanceOf(PageHierarchyCycleError);
+      expect((error as PageHierarchyCycleError).rootPageId).toBe('root-page');
+      expect((error as PageHierarchyCycleError).code).toBe('PAGE_HIERARCHY_CYCLE');
+    }
+  });
+
+  it('removes traversal metadata without changing the public row fields', () => {
+    const row = { id: 'page-1', title: 'Root', isCycle: false };
+
+    expect(stripPageTraversalMetadata(row)).toEqual({ id: 'page-1', title: 'Root' });
+  });
+});
diff --git a/apps/server/src/database/helpers/page-hierarchy-cycle.ts b/apps/server/src/database/helpers/page-hierarchy-cycle.ts
new file mode 100644
index 000000000..847c9470f
--- /dev/null
+++ b/apps/server/src/database/helpers/page-hierarchy-cycle.ts
@@ -0,0 +1,28 @@
+export type CycleTrackedRow = {
+  isCycle: boolean;
+};
+
+export class PageHierarchyCycleError extends Error {
+  readonly code = 'PAGE_HIERARCHY_CYCLE';
+
+  constructor(readonly rootPageId: string) {
+    super('Cyclic page hierarchy detected');
+    this.name = 'PageHierarchyCycleError';
+  }
+}
+
+export function assertAcyclicPageTraversal<T extends CycleTrackedRow>(
+  rows: readonly T[],
+  rootPageId: string,
+): void {
+  if (rows.some((row) => row.isCycle)) {
+    throw new PageHierarchyCycleError(rootPageId);
+  }
+}
+
+export function stripPageTraversalMetadata<T extends CycleTrackedRow>(
+  row: T,
+): Omit<T, 'isCycle'> {
+  const { isCycle: _isCycle, ...publicRow } = row;
+  return publicRow;
+}