diff --git a/AGENTS.md b/AGENTS.md
index c8b31cd..083247a 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -31,12 +31,13 @@ Project constraints (do not regress):
   MariaDB/Redis come from docker-compose.yml.
 - Deployment will be Docker-based (compose); do not add systemd/cron unit
   files for scheduling — use the container setup for timers/workers.
-- Same-origin and cross-origin frontends both work: the SPA uses relative
-  URLs unless built with VITE_API_BASE, the backend allows extra origins
-  via CORS_ALLOWED_ORIGINS (plus CSRF_TRUSTED_ORIGINS for the admin), and
-  API media URLs are absolute (built from the request host), so signed
-  files load cross-origin too. TRUST_PROXY_HEADERS=true is required behind
-  a TLS-terminating proxy.
+- Same-origin and cross-origin frontends both work: a production build asks
+  for the backend origin on first start (/setup, stored in localStorage;
+  DEFAULT_BACKEND_URL in frontend/src/lib/backend.ts is the prefilled
+  default), the backend allows extra origins via CORS_ALLOWED_ORIGINS (plus
+  CSRF_TRUSTED_ORIGINS for the admin), and API media URLs are absolute
+  (built from the request host), so signed files load cross-origin too.
+  TRUST_PROXY_HEADERS=true is required behind a TLS-terminating proxy.
 - No server-side media processing: the home server cannot handle it.
   Compression/optimization runs client-side (WebCodecs + WASM in a worker)
   and the server only applies the result via POST /api/files/J-x/optimize/.
diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index af201da..d14869e 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -23,10 +23,12 @@ import OnlinePage from "@/features/online/OnlinePage";
 import PoolDetailPage from "@/features/pools/PoolDetailPage";
 import PoolsPage from "@/features/pools/PoolsPage";
 import SimilarPage from "@/features/similar/SimilarPage";
+import { SetupPage } from "@/features/setup/SetupPage";
 import StatsPage from "@/features/stats/StatsPage";
 import UploadPage from "@/features/upload/UploadPage";
 import UsersPage from "@/features/users/UsersPage";
 import { isAgeVerified, markAgeVerified } from "@/lib/age";
+import { needsBackendSetup } from "@/lib/backend";
 import { useAuth } from "@/store/auth";
 
 function RequireAuth({ children }: { children: ReactNode }) {
@@ -52,6 +54,11 @@ export default function App() {
     void loadUser();
   }, [loadUser]);
 
+  // Production builds ask where the API lives before anything else can work.
+  if (needsBackendSetup()) {
+    return <SetupPage />;
+  }
+
   if (!ageVerified) {
     return (
       <AgeGate
@@ -145,6 +152,7 @@ export default function App() {
       </Route>
       <Route path="/login" element={<LoginPage />} />
       <Route path="/register" element={<RegisterPage />} />
+      <Route path="/setup" element={<SetupPage />} />
     </Routes>
   );
 }
diff --git a/frontend/src/features/account/AccountPage.tsx b/frontend/src/features/account/AccountPage.tsx
index 134327b..9ec6fc5 100644
--- a/frontend/src/features/account/AccountPage.tsx
+++ b/frontend/src/features/account/AccountPage.tsx
@@ -14,6 +14,7 @@ import { useE621 } from "@/store/e621";
 import { toast } from "@/store/toasts";
 
 import { AvatarCard } from "./AvatarCard";
+import { BackendCard } from "./BackendCard";
 import { PreferencesCard } from "./PreferencesCard";
 
 const BASE_URL_OPTIONS = [
@@ -215,6 +216,7 @@ export default function AccountPage() {
         <>
           <AvatarCard />
           <PreferencesCard />
+          <BackendCard />
         </>
       ) : null}
       {loading && !credentials ? (
diff --git a/frontend/src/features/account/BackendCard.tsx b/frontend/src/features/account/BackendCard.tsx
new file mode 100644
index 0000000..f1836a0
--- /dev/null
+++ b/frontend/src/features/account/BackendCard.tsx
@@ -0,0 +1,27 @@
+import { Link } from "react-router-dom";
+
+import { linkButtonClass } from "@/components/ui";
+import { getBackendUrl } from "@/lib/backend";
+
+export function BackendCard() {
+  const backend = getBackendUrl();
+
+  return (
+    <section className="rounded-lg border border-ctp-surface0 bg-ctp-base p-5">
+      <h2 className="text-sm font-semibold text-ctp-subtext1">
+        Backend connection
+      </h2>
+      <p className="mt-1 text-xs text-ctp-overlay0">
+        Where this app sends its API requests. Changing it signs you out.
+      </p>
+      <p className="mt-3 break-all font-mono text-xs text-ctp-subtext0">
+        {backend || `${window.location.origin} (same origin)`}
+      </p>
+      <div className="mt-3">
+        <Link to="/setup" className={linkButtonClass}>
+          Change backend
+        </Link>
+      </div>
+    </section>
+  );
+}
diff --git a/frontend/src/features/setup/SetupPage.tsx b/frontend/src/features/setup/SetupPage.tsx
new file mode 100644
index 0000000..cd8c2d0
--- /dev/null
+++ b/frontend/src/features/setup/SetupPage.tsx
@@ -0,0 +1,167 @@
+import { Cable, CheckCircle2, XCircle } from "lucide-react";
+import { useState } from "react";
+
+import { Button, Spinner, inputClass } from "@/components/ui";
+import { cn } from "@/lib/cn";
+import {
+  DEFAULT_BACKEND_URL,
+  getBackendUrl,
+  getStoredBackendUrl,
+  normalizeBackendUrl,
+  setBackendUrl,
+} from "@/lib/backend";
+import { queryClient } from "@/lib/queryClient";
+import { setToken } from "@/lib/api";
+
+type TestResult = { ok: boolean; text: string } | null;
+
+async function testBackend(base: string): Promise<TestResult> {
+  const controller = new AbortController();
+  const timeout = window.setTimeout(() => controller.abort(), 5_000);
+  try {
+    const response = await fetch(`${base}/health`, {
+      signal: controller.signal,
+    });
+    const data = (await response.json().catch(() => null)) as {
+      status?: string;
+      database?: boolean;
+    } | null;
+    if (response.ok) {
+      const database =
+        data?.database === true
+          ? "database ok"
+          : data?.database === false
+            ? "database unreachable"
+            : "database unknown";
+      return { ok: true, text: `Connected — ${data?.status ?? "ok"}, ${database}.` };
+    }
+    return {
+      ok: false,
+      text: `The backend answered HTTP ${response.status}.`,
+    };
+  } catch {
+    return {
+      ok: false,
+      text:
+        "Could not reach the backend. Check the URL, and if it lives on " +
+        "another domain add this site to CORS_ALLOWED_ORIGINS there.",
+    };
+  } finally {
+    window.clearTimeout(timeout);
+  }
+}
+
+/**
+ * First-start (production) and on-demand screen that stores where the API
+ * lives. Saving reloads the app so every request uses the new origin.
+ */
+export function SetupPage() {
+  const stored = getStoredBackendUrl();
+  const [value, setValue] = useState(stored ?? DEFAULT_BACKEND_URL);
+  const [testing, setTesting] = useState(false);
+  const [result, setResult] = useState<TestResult>(null);
+
+  async function runTest() {
+    const base = normalizeBackendUrl(value);
+    setTesting(true);
+    setResult(await testBackend(base));
+    setTesting(false);
+  }
+
+  function save(next: string) {
+    const normalized = normalizeBackendUrl(next);
+    if (normalized !== getBackendUrl()) {
+      // The token belongs to the previous backend.
+      setToken(null);
+      queryClient.clear();
+    }
+    setBackendUrl(normalized);
+    window.location.replace("/");
+  }
+
+  return (
+    <div className="flex min-h-screen items-center justify-center bg-ctp-crust px-4">
+      <div className="w-full max-w-lg">
+        <div className="flex items-center gap-3">
+          <span className="rounded bg-ctp-mauve px-2.5 py-1.5 font-mono text-sm font-bold tracking-wide text-ctp-crust">
+            J621
+          </span>
+          <span className="font-mono text-[11px] uppercase tracking-widest text-ctp-overlay0">
+            Self-hosted media archive
+          </span>
+        </div>
+
+        <div className="mt-6 rounded-lg border border-ctp-surface0 bg-ctp-base p-6">
+          <div className="flex items-start gap-3">
+            <Cable className="mt-0.5 h-5 w-5 shrink-0 text-ctp-mauve" />
+            <h1 className="text-lg font-semibold leading-snug text-ctp-text">
+              Where is your backend?
+            </h1>
+          </div>
+
+          <p className="mt-3 text-sm leading-relaxed text-ctp-subtext0">
+            Enter the origin this app should call for its API. Leave it blank
+            when the app and the API are served from the same domain.
+          </p>
+
+          <label className="mt-5 flex flex-col gap-1.5">
+            <span className="text-xs font-medium uppercase tracking-wide text-ctp-overlay1">
+              Backend URL
+            </span>
+            <input
+              className={cn(inputClass, "font-mono")}
+              placeholder="https://j621.example.com — blank for this server"
+              value={value}
+              onChange={(event) => {
+                setValue(event.target.value);
+                setResult(null);
+              }}
+              autoFocus
+              spellCheck={false}
+            />
+          </label>
+
+          {result ? (
+            <p
+              className={cn(
+                "mt-3 flex items-start gap-2 rounded-md border px-3 py-2 text-sm",
+                result.ok
+                  ? "border-ctp-green/30 bg-ctp-green/10 text-ctp-green"
+                  : "border-ctp-red/30 bg-ctp-red/10 text-ctp-red",
+              )}
+            >
+              {result.ok ? (
+                <CheckCircle2 className="mt-0.5 h-4 w-4 shrink-0" />
+              ) : (
+                <XCircle className="mt-0.5 h-4 w-4 shrink-0" />
+              )}
+              {result.text}
+            </p>
+          ) : null}
+
+          <div className="mt-6 flex flex-wrap items-center gap-3">
+            <Button onClick={() => save(value)}>Save and continue</Button>
+            <Button
+              variant="secondary"
+              disabled={testing}
+              onClick={() => void runTest()}
+            >
+              {testing ? <Spinner className="h-3.5 w-3.5" /> : null}
+              {testing ? "Testing…" : "Test connection"}
+            </Button>
+            {value.trim() ? (
+              <Button variant="ghost" onClick={() => save("")}>
+                Use this server
+              </Button>
+            ) : null}
+          </div>
+
+          <p className="mt-4 text-xs leading-relaxed text-ctp-overlay0">
+            Stored in this browser only. Changing the backend signs you out of
+            the previous one.
+          </p>
+        </div>
+      </div>
+    </div>
+  );
+}
diff --git a/frontend/src/lib/api.ts b/frontend/src/lib/api.ts
index e551eea..fe90d3d 100644
--- a/frontend/src/lib/api.ts
+++ b/frontend/src/lib/api.ts
@@ -1,18 +1,12 @@
 import type { TempUpload } from "@/lib/types";
+import { getBackendUrl } from "@/lib/backend";
 
 const TOKEN_KEY = "j621.token";
 
-/**
- * Backend origin when the SPA is served from somewhere else (build-time
- * `VITE_API_BASE`, e.g. `https://j621-api.example.com`). Empty means
- * same-origin, which is the default and needs no configuration.
- */
-const API_BASE = String(import.meta.env.VITE_API_BASE ?? "").replace(/\/+$/, "");
-
 /** Absolutise a backend path; external/absolute URLs pass through. */
 export function apiUrl(path: string): string {
   if (!path.startsWith("/")) return path;
-  return `${API_BASE}${path}`;
+  return `${getBackendUrl()}${path}`;
 }
 
 export function getToken(): string | null {
diff --git a/frontend/src/lib/backend.ts b/frontend/src/lib/backend.ts
new file mode 100644
index 0000000..13491ec
--- /dev/null
+++ b/frontend/src/lib/backend.ts
@@ -0,0 +1,48 @@
+/**
+ * Where the J621 API lives.
+ *
+ * A production build asks for this on first start (`/setup`) and remembers the
+ * answer per browser, so one build works same-origin AND against a backend on
+ * another domain. Same-origin is the empty string.
+ *
+ * Self-hosters: set DEFAULT_BACKEND_URL to the origin where your API is
+ * reachable to prefill the setup screen — the equivalent of the matrix.org
+ * default in Matrix clients. Leave it empty to default to "this server".
+ */
+export const DEFAULT_BACKEND_URL = "";
+
+const STORAGE_KEY = "j621.backend";
+
+/** Trim slashes, add a scheme when missing (http for localhost, else https). */
+export function normalizeBackendUrl(value: string): string {
+  const trimmed = value.trim().replace(/\/+$/, "");
+  if (!trimmed) return "";
+  if (/^https?:\/\//i.test(trimmed)) return trimmed;
+  const local = /^(localhost|127\.0\.0\.1|\[::1\])(?::|\/|$)/i.test(trimmed);
+  return `${local ? "http" : "https"}://${trimmed}`;
+}
+
+/** null = the user never chose; "" = same-origin. */
+export function getStoredBackendUrl(): string | null {
+  const raw = window.localStorage.getItem(STORAGE_KEY);
+  return raw === null ? null : normalizeBackendUrl(raw);
+}
+
+/** Current API origin; empty string means same-origin. */
+export function getBackendUrl(): string {
+  return getStoredBackendUrl() ?? DEFAULT_BACKEND_URL;
+}
+
+export function setBackendUrl(value: string): void {
+  window.localStorage.setItem(STORAGE_KEY, normalizeBackendUrl(value));
+}
+
+/**
+ * Production builds must be told where the API is on first start; the dev
+ * server keeps defaulting to the same-origin Vite proxy (visit /setup to
+ * change it manually).
+ */
+export function needsBackendSetup(): boolean {
+  if (getStoredBackendUrl() !== null) return false;
+  return import.meta.env.PROD;
+}