diff --git a/.claude/rules/sveltekit.md b/.claude/rules/sveltekit.md
index 6e6ca32bb..c04634b8b 100644
--- a/.claude/rules/sveltekit.md
+++ b/.claude/rules/sveltekit.md
@@ -25,6 +25,20 @@ All internal navigation must use `resolve()` from `$app/paths`:
 - **With search/hash**: `resolve(pathname + search + hash)` (concatenate inside, not outside)
 - **External**: no `resolve()`, use `rel="noreferrer external"`
 
+## HTTP Cache Headers (`setHeaders`)
+
+**Never set `s-maxage` / `stale-while-revalidate` on a route whose content varies by session.**
+Shared caches key on URL + method + the headers named by `Vary` (RFC 9111), so without
+`Vary: Cookie` an anonymous response is served to logged-in users (#3862).
+
+- Setting the header only when logged out controls the write side, not delivery of an
+  already-cached entry to a request with a session cookie.
+- `Vary: Cookie` matches the whole header; with `_ga` present every returning visitor gets a
+  unique key and the cache barely hits.
+- No `s-maxage` means no CDN caching, so `private` / `no-store` are unnecessary (YAGNI).
+- Estimate the saved invocations first. At this site's traffic the `/problems` header saved
+  single-digit dollars a month and cost two weeks of a production bug.
+
 ## Form Data Validation
 
 `formData.get()` returns `string | File | null`. Always validate before casting:
diff --git a/e2e/problems_cache.spec.ts b/e2e/problems_cache.spec.ts
index 1c28c9b4d..6301d3c86 100644
--- a/e2e/problems_cache.spec.ts
+++ b/e2e/problems_cache.spec.ts
@@ -2,8 +2,13 @@ import { expect, test } from '@playwright/test';
 
 import { loginAsUser } from './helpers/auth';
 
+// Regression guard for #3862. A shared cache keys on URL + method + the headers named by
+// Vary (RFC 9111), so an anonymous response cached without `Vary: Cookie` is also served to
+// logged-in users — showing them the signed-out page. /problems must not opt into the shared
+// cache at all, for anonymous and logged-in responses alike.
+
 test.describe('anonymous /problems response', () => {
-  test('is cache-eligible (cache-control set, no set-cookie)', async ({ page }) => {
+  test('is not shared-cached (no CDN cache-control directive)', async ({ page }) => {
     const response = await page.goto('/problems');
 
     if (!response) {
@@ -12,17 +17,11 @@ test.describe('anonymous /problems response', () => {
 
     const headers = response.headers();
 
-    // Local dev server (pnpm preview) is not behind Vercel edge, so s-maxage is visible.
-    expect(headers['cache-control']).toContain('public');
-    expect(headers['cache-control']).toContain('max-age=0');
-    expect(headers['cache-control']).toContain('s-maxage=300');
-    expect(headers['cache-control']).toContain('stale-while-revalidate=600');
-
-    // set-cookie makes a response ineligible for CDN caching.
-    // The auth layer must not attach a session cookie to anonymous requests.
-    // If this assertion fails, verify the session handling does not set cookies
-    // on unauthenticated requests.
-    expect(headers['set-cookie']).toBeUndefined();
+    // Local dev server (pnpm preview) is not behind Vercel edge, so s-maxage would be
+    // visible here if it were re-introduced.
+    expect(headers['cache-control'] ?? '').not.toContain('public');
+    expect(headers['cache-control'] ?? '').not.toContain('s-maxage');
+    expect(headers['cache-control'] ?? '').not.toContain('stale-while-revalidate');
   });
 });
 
@@ -37,7 +36,6 @@ test.describe('logged-in /problems response', () => {
 
     const headers = response.headers();
 
-    // Personalized responses must never be shared-cached.
     expect(headers['cache-control'] ?? '').not.toContain('public');
     expect(headers['cache-control'] ?? '').not.toContain('s-maxage');
   });
diff --git a/src/routes/problems/+page.server.ts b/src/routes/problems/+page.server.ts
index 7e333f936..1551d2b35 100644
--- a/src/routes/problems/+page.server.ts
+++ b/src/routes/problems/+page.server.ts
@@ -11,7 +11,7 @@ import { voteAbsoluteGradeSchema } from '$features/votes/zod/schema';
 import { BAD_REQUEST } from '$lib/constants/http-response-status-codes';
 
 // 一覧表ページは、ログインしていなくても閲覧できるようにする
-export async function load({ locals, url, setHeaders }) {
+export async function load({ locals, url }) {
   const session = await locals.auth.validate();
   const params = await url.searchParams;
 
@@ -21,24 +21,13 @@ export async function load({ locals, url, setHeaders }) {
 
   // Degrade gracefully if vote stats are unavailable — the problems page must remain accessible.
   let voteResults = new Map();
-  let voteStatsOk = true;
 
   try {
     voteResults = await getVoteGradeStatistics();
   } catch (error) {
-    voteStatsOk = false;
     console.error('Failed to load vote statistics:', error);
   }
 
-  // Anonymous responses are identical for all users and contain no per-user
-  // answer state, so they are safe to cache at the CDN. Logged-in responses
-  // are personalized and must never be shared-cached.
-  // Skip caching a degraded response (vote stats failed) to avoid pinning a
-  // broken page at the edge for the full TTL.
-  if (session === null && voteStatsOk) {
-    setHeaders({ 'Cache-Control': 'public, max-age=0, s-maxage=300, stale-while-revalidate=600' });
-  }
-
   const taskResults = (
     tagIds
       ? await task_crud.getTasksWithTagIds(tagIds, session?.user.userId)
diff --git a/src/routes/problems/page_server.test.ts b/src/routes/problems/page_server.test.ts
index 28ee6d845..f69682584 100644
--- a/src/routes/problems/page_server.test.ts
+++ b/src/routes/problems/page_server.test.ts
@@ -60,33 +60,25 @@ beforeEach(() => {
 });
 
 describe('load() cache-control behaviour', () => {
-  describe('sets cache-control', () => {
-    test('anonymous users get a public shared-cache header when vote stats succeed', async () => {
+  // Shared caches key on URL + method + headers named by Vary only (RFC 9111), so an
+  // anonymous response cached without `Vary: Cookie` is served to logged-in users too.
+  describe('never sets a shared-cache header — shared caches ignore cookies without Vary (#3862)', () => {
+    test('anonymous users', async () => {
       const event = createMockEvent({ session: null });
 
       await load(event);
 
-      expect(event.setHeaders).toHaveBeenCalledOnce();
-      const headerArg = event.setHeaders.mock.calls[0][0] as Record<string, string>;
-      expect(headerArg['Cache-Control']).toBe(
-        'public, max-age=0, s-maxage=300, stale-while-revalidate=600',
-      );
+      expect(event.setHeaders).not.toHaveBeenCalled();
     });
 
-    test('anonymous users with tagIds also get a public shared-cache header', async () => {
+    test('anonymous users with tagIds', async () => {
       const event = createMockEvent({ session: null, tagIds: 'abc,dp' });
 
       await load(event);
 
-      expect(event.setHeaders).toHaveBeenCalledOnce();
-      const headerArg = event.setHeaders.mock.calls[0][0] as Record<string, string>;
-      expect(headerArg['Cache-Control']).toBe(
-        'public, max-age=0, s-maxage=300, stale-while-revalidate=600',
-      );
+      expect(event.setHeaders).not.toHaveBeenCalled();
     });
-  });
 
-  describe('does not set cache-control', () => {
     test('logged-in users — personalized response must never be shared-cached', async () => {
       const event = createMockEvent({ session: LOGGED_IN_SESSION });
 
@@ -95,7 +87,7 @@ describe('load() cache-control behaviour', () => {
       expect(event.setHeaders).not.toHaveBeenCalled();
     });
 
-    test('degraded response when vote stats fail — avoids pinning a broken page at the CDN', async () => {
+    test('degraded response when vote stats fail', async () => {
       mockGetVoteGradeStatistics.mockRejectedValue(new Error('DB timeout'));
       const event = createMockEvent({ session: null });