diff --git a/__tests__/unit/services/warm-reader-is-browser-only.test.ts b/__tests__/unit/services/warm-reader-is-browser-only.test.ts
new file mode 100644
index 000000000..5125b4fc4
--- /dev/null
+++ b/__tests__/unit/services/warm-reader-is-browser-only.test.ts
@@ -0,0 +1,52 @@
+/**
+ * @jest-environment node
+ *
+ * Warming the reader's id must do NOTHING on the server.
+ *
+ * `warmCurrentUserId` is called at the top of every timeline read, and timeline
+ * reads happen during server rendering too. Unguarded, it started an
+ * unconditional `/auth/v1/user` request from the server using the BROWSER
+ * Supabase client — which carries no request cookies, so it can only fail, and
+ * it fails while the route is rendering.
+ *
+ * That wedged the server render of every feed page. /timeline, /community and
+ * /dashboard sat on their route-level loading.tsx skeleton forever on a fresh
+ * page load, while navigating to the same route from inside the app worked,
+ * because that path never server-renders. Nothing appeared in the console and
+ * every check was green; the skeleton is near-white on white, so it read as a
+ * blank page.
+ *
+ * This test runs in the `node` environment on purpose. Under jsdom `window`
+ * exists, the guard is never exercised, and this file would pass while the
+ * server path stayed broken — which is exactly how the bug shipped.
+ */
+
+const getUser = jest.fn();
+
+jest.mock('@/lib/supabase/browser', () => ({
+  __esModule: true,
+  default: {
+    auth: {
+      getUser: (...a: unknown[]) => getUser(...a),
+      onAuthStateChange: () => ({ data: { subscription: { unsubscribe: () => {} } } }),
+    },
+  },
+}));
+
+import { warmCurrentUserId } from '@/services/supabase/auth/session';
+
+describe('warmCurrentUserId on the server', () => {
+  it('has no window to speak of', () => {
+    // Guard the premise: if this environment ever gains a window, the test
+    // below stops testing anything and must not silently pass.
+    expect(typeof window).toBe('undefined');
+  });
+
+  it('does not touch the network', async () => {
+    warmCurrentUserId();
+    // Let any promise it might have started settle.
+    await new Promise(resolve => setTimeout(resolve, 0));
+
+    expect(getUser).not.toHaveBeenCalled();
+  });
+});
diff --git a/src/services/supabase/auth/session.ts b/src/services/supabase/auth/session.ts
index 6d67888e9..1a3f51fb1 100644
--- a/src/services/supabase/auth/session.ts
+++ b/src/services/supabase/auth/session.ts
@@ -149,8 +149,28 @@ export async function getCurrentUserId(): Promise<string | null> {
  * For callers that know they will need it later and can overlap the round-trip
  * with work that does not depend on it — fetching a feed, say. Returns nothing,
  * so it cannot be mistaken for the id itself.
+ *
+ * BROWSER ONLY, and the guard is load-bearing. This is called at the top of
+ * every timeline read, and timeline reads happen during server rendering too.
+ * Without the guard it started an unconditional `/auth/v1/user` request from
+ * the server using the BROWSER client — which has no request cookies, so it
+ * can only ever fail, and it does so while the route is rendering.
+ *
+ * That is not a wasted request, it is an outage: it wedged the server render
+ * of every feed page. /timeline, /community and /dashboard stayed on their
+ * route-level loading.tsx skeleton forever on a fresh page load, while
+ * navigating to the very same route from inside the app worked, because that
+ * path never server-renders. A near-white skeleton on a white page reads as a
+ * blank screen, so the symptom was "the timeline is empty", with nothing in
+ * the console and every check green.
+ *
+ * Deliberately silent on the server rather than throwing: this is an
+ * optimisation, and an optimisation must never be the reason a page fails.
  */
 export function warmCurrentUserId(): void {
+  if (typeof window === 'undefined') {
+    return;
+  }
   void getCurrentUserId();
 }