diff --git a/packages/void/src/pages/protocol.ts b/packages/void/src/pages/protocol.ts
index e1a72e701..ba12f1994 100644
--- a/packages/void/src/pages/protocol.ts
+++ b/packages/void/src/pages/protocol.ts
@@ -109,6 +109,12 @@ function appendVary(headers: Headers, name: string): void {
   }
 }
 
+function varyPagesResponse(c: CloudContext): void {
+  const headers = new Headers(c.res.headers);
+  appendVary(headers, 'X-VoidPages');
+  c.header('Vary', headers.get('Vary')!);
+}
+
 function escapeHtmlAttr(value: string): string {
   return value.replaceAll('&', '&amp;').replaceAll('"', '&quot;').replaceAll('<', '&lt;');
 }
@@ -464,6 +470,9 @@ export async function handlePageGet(
 
   // Redirect/error responses from the loader are returned before rendering
   return runLoader(handler, c, async (props) => {
+    if (renderMode !== 'island') {
+      varyPagesResponse(c);
+    }
     // Read at render time so per-route middleware can set them before next()
     const shared = (c.get('shared') as Record<string, unknown> | undefined) ?? {};
     const pageHead = headFn ? headFn(c, props) : undefined;
@@ -538,9 +547,6 @@ export async function handlePageGet(
       }
       // Stream NDJSON: first line is PageObject, subsequent lines are deferred groups
       c.header('X-VoidPages', 'true');
-      const headers = new Headers(c.res.headers);
-      appendVary(headers, 'X-VoidPages');
-      c.header('Vary', headers.get('Vary')!);
       return streamDeferredResponse(
         JSON.stringify(pageObj) + '\n',
         deferredMap,
@@ -652,6 +658,7 @@ export async function handlePageMutation(
   async function respondWithPage(errors: Record<string, string> | null): Promise<Response> {
     try {
       return await runLoader(getHandler, c, async (props) => {
+        varyPagesResponse(c);
         const pageObj = buildMutationPageObj(stripDeferred(props), errors);
         if (isPagesRequest(c)) {
           return pageJsonResponse(c, pageObj);
diff --git a/packages/void/test/unit/pages-protocol.test.ts b/packages/void/test/unit/pages-protocol.test.ts
index e4d259002..e7ed05d26 100644
--- a/packages/void/test/unit/pages-protocol.test.ts
+++ b/packages/void/test/unit/pages-protocol.test.ts
@@ -67,26 +67,64 @@ const mockRenderFn = vi.fn(async (pageObj: RenderPage) => `<html>${pageObj.compo
 const mockGetAssetTags = vi.fn(async () => ({ css: '', preloads: '', body: '' }));
 
 describe('handlePageGet', () => {
-  it.each([false, true])(
-    'preserves loader Vary headers on Pages JSON (deferred=%s)',
-    async (deferred) => {
-      const c = await createTestContext('GET', '/users', { headers: { 'x-voidpages': 'true' } });
-      const response = await handlePageGet(
-        (ctx) => {
-          ctx.header('Vary', 'Accept-Language');
-          return deferred ? { users: defer(async () => []) } : { users: [] };
-        },
-        undefined,
-        c,
-        'users',
-        mockRenderFn,
-        mockGetAssetTags,
-      );
-      expect(response.headers.get('X-VoidPages')).toBe('true');
-      expect(response.headers.get('Vary')).toBe('Accept-Language, X-VoidPages');
-      await response.text();
-    },
-  );
+  describe.each([
+    { name: 'HTML', spa: false, deferred: false, contentType: 'text/html; charset=utf-8' },
+    { name: 'JSON', spa: true, deferred: false, contentType: 'application/json' },
+    { name: 'streamed HTML', spa: false, deferred: true, contentType: 'text/html; charset=utf-8' },
+    { name: 'streamed NDJSON', spa: true, deferred: true, contentType: 'application/x-ndjson' },
+  ])('$name cache negotiation', ({ spa, deferred, contentType }) => {
+    it.each([
+      { existing: undefined, expected: 'X-VoidPages' },
+      { existing: 'Accept-Language', expected: 'Accept-Language, X-VoidPages' },
+      { existing: 'Accept-Language, x-voidpages', expected: 'Accept-Language, x-voidpages' },
+      { existing: '*', expected: '*' },
+    ])(
+      'sets Vary without losing or duplicating existing values ($existing)',
+      async ({ existing, expected }) => {
+        const app = new Hono<CloudEnv>();
+        app.get('/users', (c) =>
+          handlePageGet(
+            (ctx) => {
+              if (existing) {
+                ctx.header('Vary', existing);
+              }
+              return deferred ? { users: defer(async () => []) } : { users: [] };
+            },
+            undefined,
+            c,
+            'users',
+            mockRenderFn,
+            mockGetAssetTags,
+          ),
+        );
+        const response = await app.request('/users', {
+          headers: spa ? { 'x-voidpages': 'true' } : {},
+        });
+        // Consume streams even if a header assertion fails.
+        await response.text();
+        expect(response.headers.get('Content-Type')).toBe(contentType);
+        expect(response.headers.get('X-VoidPages')).toBe(spa ? 'true' : null);
+        expect(response.headers.get('Vary')).toBe(expected);
+      },
+    );
+  });
+
+  it.each([false, true])('does not vary HTML-only island pages (deferred=%s)', async (deferred) => {
+    const c = await createTestContext('GET', '/users', { headers: { 'x-voidpages': 'true' } });
+    const response = await handlePageGet(
+      () => (deferred ? { users: defer(async () => []) } : { users: [] }),
+      undefined,
+      c,
+      'users',
+      mockRenderFn,
+      mockGetAssetTags,
+      undefined,
+      { renderMode: 'island' },
+    );
+    await response.text();
+    expect(response.headers.get('Content-Type')).toBe('text/html; charset=utf-8');
+    expect(response.headers.get('Vary')).toBeNull();
+  });
 
   it('returns JSON page object when X-VoidPages header present', async () => {
     const c = await createTestContext('GET', '/users', {
@@ -523,6 +561,26 @@ describe('handlePageGet', () => {
 });
 
 describe('handlePageMutation', () => {
+  it.each([false, true])('varies re-rendered action pages by X-VoidPages (spa=%s)', async (spa) => {
+    const c = await createTestContext('POST', '/users', {
+      headers: spa ? { 'x-voidpages': 'true' } : {},
+    });
+    const response = await handlePageMutation(
+      () => undefined,
+      (ctx) => {
+        ctx.header('Vary', 'Accept-Language');
+        return { users: [] };
+      },
+      undefined,
+      c,
+      'users',
+      mockRenderFn,
+      mockGetAssetTags,
+    );
+    await response.text();
+    expect(response.headers.get('Vary')).toBe('Accept-Language, X-VoidPages');
+  });
+
   it.each<Record<string, string>>([
     { origin: 'https://attacker.test' },
     { origin: 'http://sibling.localhost', 'sec-fetch-site': 'same-site' },