diff --git a/craft/src/state/GlobalSectionsContext.test.tsx b/craft/src/state/GlobalSectionsContext.test.tsx
index c30c1b5..53de5ae 100644
--- a/craft/src/state/GlobalSectionsContext.test.tsx
+++ b/craft/src/state/GlobalSectionsContext.test.tsx
@@ -16,6 +16,7 @@ import { GuidedStyles } from '../panels/right/GuidedStyles';
 import { ContextMenu } from '../panels/context-menu/ContextMenu';
 import { SectionLibraryModal, openSectionLibrary, closeSectionLibrary } from '../panels/sections/SectionLibraryModal';
 import { Canvas } from '../editor/Canvas';
+import { useApplyAiResponse } from '../utils/apply-ai-response';
 
 /**
  * #57 global sections against the REAL Craft editor + PageProvider +
@@ -28,12 +29,14 @@ let pages: ReturnType<typeof usePages>;
 let globals: ReturnType<typeof useGlobalSections>;
 let extras: ReturnType<typeof useProjectExtras>;
 let guard: ReturnType<typeof useSaveGuard>;
+let applyAi: ReturnType<typeof useApplyAiResponse>;
 
 const Probe: React.FC = () => {
   pages = usePages();
   globals = useGlobalSections();
   extras = useProjectExtras();
   guard = useSaveGuard();
+  applyAi = useApplyAiResponse();
   return null;
 };
 
@@ -306,6 +309,55 @@ describe('#57 global sections', () => {
     expect(h!.query.history.canUndo()).toBe(false);
   });
 
+  test('Sitesmith page-scope replace is refused while a master is open: no cross-page rewrite', async () => {
+    mount();
+    const sec = addPromo();
+    let libraryId = '';
+    h!.act(() => { libraryId = globals.makeGlobal(sec, 'Promo')!; });
+    h!.act(() => { pages.addPage('About', 'about'); });
+    await flush();
+    let inst2 = '';
+    h!.act(() => { inst2 = globals.insertInstance(libraryId)!; });
+    const aiPage = {
+      name: 'AI page',
+      tree: {
+        type: { resolvedName: 'Container' }, props: {},
+        nodes: [{ type: { resolvedName: 'Section' }, props: {}, nodes: [
+          { type: { resolvedName: 'Heading' }, props: { text: 'AI takeover' } },
+        ] }],
+      },
+    } as any;
+
+    h!.act(() => { pages.editGlobal(libraryId); });
+    await flush();
+    const zoneBefore = h!.getSerialized();
+    let r: any;
+    await h!.act(async () => { r = await applyAi({ type: 'replace', scope: 'page', pages: [aiPage] } as any); });
+    await flush();
+    expect(r.ok).toBe(false);
+    expect(r.message).toMatch(/global section/i);
+    expect(pages.isEditingGlobal).toBe(true);
+    expect(h!.getSerialized()).toBe(zoneBefore);
+    expect(pages.pages.map((p) => p.name)).toEqual(['Home', 'About']);
+
+    h!.act(() => { globals.finishEditingMaster(); });
+    await flush();
+    expect(lib()[libraryId].rev).toBe(1);
+    const p2 = live();
+    expect(instanceRoots(p2)).toEqual([inst2]);
+    expect(JSON.stringify(p2)).not.toContain('AI takeover');
+    expect(JSON.stringify(p2)).toContain('Welcome');
+    const p1 = pages.pages[0].craftState!;
+    expect(p1).not.toContain('AI takeover');
+    expect(instanceMeta((JSON.parse(p1) as SNodes)[sec])!.rev).toBe(1);
+
+    // control: the same response on a normal page still replaces it
+    await h!.act(async () => { r = await applyAi({ type: 'replace', scope: 'page', pages: [aiPage] } as any); });
+    await flush();
+    expect(r.ok).toBe(true);
+    expect(h!.getSerialized()).toContain('AI takeover');
+  });
+
   test('remove is refused while instances exist, allowed once none are left', () => {
     mount();
     const sec = addPromo();
diff --git a/craft/src/state/PageContext.tsx b/craft/src/state/PageContext.tsx
index 62462dc..4855637 100644
--- a/craft/src/state/PageContext.tsx
+++ b/craft/src/state/PageContext.tsx
@@ -83,7 +83,8 @@ interface PageContextValue {
   setActivePageIdDirect: (pageId: string) => void;
   /** AI helpers — replace entire site or page with a new tree */
   replaceAllPages: (pages: { name: string; tree: SerializedTreeNode }[]) => void;
-  replaceCurrentPage: (page: { name: string; tree: SerializedTreeNode }) => void;
+  /** False (nothing changed) while a global-section master is on the canvas (#57). */
+  replaceCurrentPage: (page: { name: string; tree: SerializedTreeNode }) => boolean;
   setHeader: (tree: SerializedTreeNode) => void;
   setFooter: (tree: SerializedTreeNode) => void;
   siteDesign: SiteDesign;
@@ -267,7 +268,7 @@ const PageContext = createContext<PageContextValue>({
   setPagesCraftState: () => {},
   setActivePageIdDirect: () => {},
   replaceAllPages: () => {},
-  replaceCurrentPage: () => {},
+  replaceCurrentPage: () => false,
   setHeader: () => {},
   setFooter: () => {},
   siteDesign: {} as SiteDesign,
@@ -889,9 +890,13 @@ export const PageProvider: React.FC<{ children: ReactNode }> = ({ children }) =>
    * AI helper: replace the current page's tree.
    * Deserializes the new tree into the live Craft.js canvas and persists it.
    */
-  const replaceCurrentPage = useCallback((page: { name: string; tree: SerializedTreeNode }) => {
-    const craftState = treeToCraftState(page.tree);
+  const replaceCurrentPage = useCallback((page: { name: string; tree: SerializedTreeNode }): boolean => {
     const currentId = activePageIdRef.current;
+    // #57: a master zone is not a page. Loading a page tree here would make
+    // its first section the new master on Done, and the sync would then
+    // rewrite every instance on every page with it. Refuse instead.
+    if (isGlobalZoneId(currentId)) return false;
+    const craftState = treeToCraftState(page.tree);
     if (currentId === HEADER_ID) {
       setHeaderPage((prev) => ({ ...prev, name: page.name, craftState }));
     } else if (currentId === FOOTER_ID) {
@@ -902,6 +907,7 @@ export const PageProvider: React.FC<{ children: ReactNode }> = ({ children }) =>
       );
     }
     loadState(craftState, EMPTY_CANVAS, { keepHistory: true });
+    return true;
   }, [loadState]);
 
   /**
diff --git a/craft/src/utils/apply-ai-response.ts b/craft/src/utils/apply-ai-response.ts
index 1b2b555..0fe283e 100644
--- a/craft/src/utils/apply-ai-response.ts
+++ b/craft/src/utils/apply-ai-response.ts
@@ -65,7 +65,12 @@ export function useApplyAiResponse() {
       }
 
       if (resp.scope === 'page') {
-        pages.replaceCurrentPage(resp.pages[0]);
+        if (!pages.replaceCurrentPage(resp.pages[0])) {
+          return {
+            ok: false,
+            message: 'You are editing a global section. Click Done, then ask again to replace the page.',
+          };
+        }
         return { ok: true };
       }