diff --git a/lib/__tests__/server-plugin-list.test.ts b/lib/__tests__/server-plugin-list.test.ts
new file mode 100644
index 000000000..cf8e216a3
--- /dev/null
+++ b/lib/__tests__/server-plugin-list.test.ts
@@ -0,0 +1,55 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+const mocks = vi.hoisted(() => ({ apiFetch: vi.fn() }));
+vi.mock('@/lib/browser-navigation', async (orig) => ({
+  ...(await orig<typeof import('@/lib/browser-navigation')>()),
+  apiFetch: (...args: unknown[]) => mocks.apiFetch(...args),
+}));
+vi.mock('@/lib/plugin-loader', () => ({
+  loadPlugin: vi.fn().mockResolvedValue(undefined),
+  deactivatePlugin: vi.fn(),
+  setPluginStoreAccessor: vi.fn(),
+  setupAutoDisable: vi.fn(),
+  setSandboxLocale: vi.fn(),
+}));
+
+import { fetchServerPluginList } from '@/lib/server-plugin-list';
+import { usePluginStore } from '@/stores/plugin-store';
+import { useThemeStore } from '@/stores/theme-store';
+
+const pluginListCalls = () => mocks.apiFetch.mock.calls.filter(([url]) => url === '/api/plugins');
+
+beforeEach(() => {
+  mocks.apiFetch.mockReset();
+  mocks.apiFetch.mockImplementation(async () => new Response(JSON.stringify({ plugins: [], themes: [] })));
+});
+
+describe('the server plugin list', () => {
+  it('is fetched once when the plugin and theme stores sync at the same time', async () => {
+    usePluginStore.setState({ plugins: [], initialized: false });
+
+    // What the mail view does on mount.
+    await Promise.all([
+      usePluginStore.getState().initializePlugins(),
+      useThemeStore.getState().syncServerThemes(),
+    ]);
+
+    expect(pluginListCalls()).toHaveLength(1);
+    expect(usePluginStore.getState().initialized).toBe(true);
+  });
+
+  it('is fetched again by a later call', async () => {
+    const first = await fetchServerPluginList();
+    const second = await fetchServerPluginList();
+
+    expect(pluginListCalls()).toHaveLength(2);
+    expect(first.data).toEqual({ plugins: [], themes: [] });
+    expect(second.ok).toBe(true);
+  });
+
+  it('passes a refused request on without a body', async () => {
+    mocks.apiFetch.mockImplementation(async () => new Response('{"error":"Not authenticated"}', { status: 401 }));
+
+    await expect(fetchServerPluginList()).resolves.toEqual({ ok: false, status: 401, data: null });
+  });
+});
diff --git a/lib/server-plugin-list.ts b/lib/server-plugin-list.ts
new file mode 100644
index 000000000..4b1c1a04a
--- /dev/null
+++ b/lib/server-plugin-list.ts
@@ -0,0 +1,27 @@
+// GET /api/plugins, shared by the plugin store and the theme store.
+//
+// Both sync from the same list when the app starts, and each used to fetch
+// it: two copies of a response that carries every plugin's translations
+// (hundreds of kB compressed) on every page load. Calls that overlap share
+// one request; a later call (after signing in, say) fetches again.
+
+import { apiFetch } from '@/lib/browser-navigation';
+
+export interface ServerPluginListResult {
+  ok: boolean;
+  status: number;
+  /** The parsed body when `ok`, otherwise null. */
+  data: unknown;
+}
+
+let inFlight: Promise<ServerPluginListResult> | null = null;
+
+export function fetchServerPluginList(): Promise<ServerPluginListResult> {
+  if (!inFlight) {
+    inFlight = (async () => {
+      const res = await apiFetch('/api/plugins');
+      return { ok: res.ok, status: res.status, data: res.ok ? await res.json() : null };
+    })().finally(() => { inFlight = null; });
+  }
+  return inFlight;
+}
diff --git a/stores/plugin-store.ts b/stores/plugin-store.ts
index 8de589761..dc399547e 100644
--- a/stores/plugin-store.ts
+++ b/stores/plugin-store.ts
@@ -22,6 +22,7 @@ import { get as getActivePlugin } from '@/lib/plugin-sandbox/registry';
 import { coerceSetting, effectiveSettings } from '@/lib/plugin-settings';
 import { usePolicyStore } from '@/stores/policy-store';
 import { apiFetch } from '@/lib/browser-navigation';
+import { fetchServerPluginList } from '@/lib/server-plugin-list';
 import { IMPLICIT_PERMISSIONS } from '@/lib/plugin-types';
 import type { Permission } from '@/lib/plugin-types';
 import { IS_LITE } from '@/lib/lite';
@@ -569,12 +570,12 @@ async function syncServerPlugins(
   set: (partial: Partial<PluginStoreState> | ((state: PluginStoreState) => Partial<PluginStoreState>)) => void,
 ): Promise<'signed-out' | void> {
   try {
-    const res = await apiFetch('/api/plugins');
+    const res = await fetchServerPluginList();
     if (res.status === 401) return 'signed-out';
     if (!res.ok) return;
 
-    const data: { plugins: ServerPluginInfo[] } = await res.json();
-    if (!data.plugins || !Array.isArray(data.plugins)) return;
+    const data = res.data as { plugins: ServerPluginInfo[] };
+    if (!data?.plugins || !Array.isArray(data.plugins)) return;
 
     const serverPlugins = data.plugins;
     const serverPluginIds = new Set(serverPlugins.map(p => p.id));
diff --git a/stores/theme-store.ts b/stores/theme-store.ts
index 23bbc9b9b..5dad5661a 100644
--- a/stores/theme-store.ts
+++ b/stores/theme-store.ts
@@ -14,6 +14,7 @@ import { extractTheme } from '@/lib/plugin-validator';
 import { BUILTIN_THEMES } from '@/lib/builtin-themes';
 import { usePolicyStore } from '@/stores/policy-store';
 import { apiFetch } from '@/lib/browser-navigation';
+import { fetchServerPluginList } from '@/lib/server-plugin-list';
 import { themeHooks } from '@/lib/plugin-hooks';
 import { syncThemeColorMeta } from '@/lib/theme-color-meta';
 import { IS_LITE } from '@/lib/lite';
@@ -368,11 +369,11 @@ export const useThemeStore = create<ThemeState>()(
 
         themeSyncPromise = (async () => {
           try {
-            const res = await apiFetch('/api/plugins');
+            const res = await fetchServerPluginList();
             if (!res.ok) return;
 
-            const data: { themes: ServerThemeInfo[] } = await res.json();
-            if (!data.themes || !Array.isArray(data.themes)) return;
+            const data = res.data as { themes: ServerThemeInfo[] };
+            if (!data?.themes || !Array.isArray(data.themes)) return;
 
             const serverThemes = data.themes;