diff --git a/src/SEQTA.ts b/src/SEQTA.ts
index 219482b0..3450a32c 100644
--- a/src/SEQTA.ts
+++ b/src/SEQTA.ts
@@ -141,11 +141,6 @@ async function init() {
   }
 }
 
-function shouldBootKitten404(): boolean {
-  if (IsSEQTAPage || document.getElementById("container")) return false;
-  return /404|not found/i.test(document.title);
-}
-
 /** Hide the rebranded 404 before the kitten card mounts (removed after boot). */
 function inject404FlashHide(): void {
   if (document.getElementById("bsplus-404-flash-hide")) return;
@@ -159,13 +154,17 @@ function inject404FlashHide(): void {
 async function bootErrorPage() {
   if (IsSEQTAPage) return;
 
+  const { isSeqta404Page, mountErrorPageKitten } = await import(
+    "@/plugins/built-in/errorPageKitten"
+  );
+
   const storagePromise = browser.storage.local.get([
     "onoff",
     "plugin.error-page-kitten.settings",
   ]);
 
   const watchTitle = () => {
-    if (shouldBootKitten404()) inject404FlashHide();
+    if (isSeqta404Page()) inject404FlashHide();
   };
   watchTitle();
   if (document.readyState === "loading") {
@@ -179,7 +178,7 @@ async function bootErrorPage() {
   }
   document.removeEventListener("readystatechange", watchTitle);
 
-  if (!shouldBootKitten404()) return;
+  if (!isSeqta404Page()) return;
 
   const stored = await storagePromise;
   if ((stored.onoff ?? true) === false) return;
@@ -189,9 +188,6 @@ async function bootErrorPage() {
   if (kittenSettings?.enabled === false) return;
 
   try {
-    const { mountErrorPageKitten } = await import(
-      "@/plugins/built-in/errorPageKitten"
-    );
     mountErrorPageKitten();
     document.getElementById("bsplus-404-flash-hide")?.remove();
   } catch (error) {
diff --git a/src/plugins/built-in/errorPageKitten/index.test.ts b/src/plugins/built-in/errorPageKitten/index.test.ts
index f3144e3f..a677f288 100644
--- a/src/plugins/built-in/errorPageKitten/index.test.ts
+++ b/src/plugins/built-in/errorPageKitten/index.test.ts
@@ -2,8 +2,6 @@
  * @jest-environment jsdom
  */
 /// <reference types="jest" />
-import type { PluginAPI } from "@/plugins/core/types";
-
 jest.mock("@/lib/extensionAssetUrl", () => ({
   resolveExtensionAssetUrl: (url: string) =>
     `chrome-extension://test/${String(url).replace(/^\/+/, "")}`,
@@ -17,7 +15,10 @@ jest.mock("./styles.css?inline", () => ({
   default: "/* test */",
 }));
 
-import { mountErrorPageKitten } from "./index";
+import errorPageKittenPlugin, {
+  isSeqta404Page,
+  mountErrorPageKitten,
+} from "./index";
 
 const REBRANDED_404_BODY = `
   <div class="message">
@@ -43,18 +44,33 @@ describe("errorPageKitten", () => {
 
     const cleanup = mountErrorPageKitten();
 
-    const card = getCard();
-    expect(card).not.toBeNull();
+    expect(getCard()).not.toBeNull();
     expect(document.documentElement.classList).toContain("bsplus-kitten-404");
-    expect(card?.querySelector("h1")?.textContent).toBe("404 Not Found");
-    expect(document.title).toBe("404 Not Found");
-    expect(card?.querySelector(".kitten img")?.getAttribute("src")).toContain("kitten");
-    expect(
-      document.querySelector(".message")?.classList.contains("bsplus-kitten-404-hidden"),
-    ).toBe(true);
 
     cleanup();
     expect(getCard()).toBeNull();
-    expect(document.title).toBe("Page not found");
+  });
+
+  it("detects standalone 404 pages by .message and title", () => {
+    document.title = "Page not found";
+    document.body.innerHTML = REBRANDED_404_BODY;
+    expect(isSeqta404Page()).toBe(true);
+  });
+
+  it("does not detect the SPA", () => {
+    document.title = "SEQTA Learn";
+    document.body.innerHTML = '<div id="container"></div>';
+    expect(isSeqta404Page()).toBe(false);
+  });
+
+  it("plugin run is a no-op outside a 404 page", async () => {
+    document.title = "SEQTA Learn";
+    document.body.innerHTML = '<div id="container"></div>';
+
+    const cleanup = await errorPageKittenPlugin.run();
+
+    expect(getCard()).toBeNull();
+    expect(cleanup).toBeDefined();
+    cleanup?.();
   });
 });
diff --git a/src/plugins/built-in/errorPageKitten/index.ts b/src/plugins/built-in/errorPageKitten/index.ts
index 624a06b0..c99c7021 100644
--- a/src/plugins/built-in/errorPageKitten/index.ts
+++ b/src/plugins/built-in/errorPageKitten/index.ts
@@ -16,7 +16,14 @@ const CARD_HTML = `<h1>404 Not Found</h1>
 </div>
 <a href="http://www.seqta.com.au">SEQTA</a>`;
 
-/** Boot path in SEQTA.ts gates when this runs; keep render-only here. */
+/** Standalone SEQTA 404 document — not the SPA (#container). */
+export function isSeqta404Page(): boolean {
+  if (document.getElementById("container")) return false;
+  const heading = document.querySelector(".message > h1");
+  const text = heading?.textContent ?? document.title;
+  return /not found/i.test(text) || /404/.test(document.title);
+}
+
 export function mountErrorPageKitten(): () => void {
   const styleEl = document.createElement("style");
   styleEl.textContent = styles;
@@ -52,7 +59,7 @@ const errorPageKittenPlugin: Plugin = {
   disableToggle: true,
   defaultEnabled: true,
   styles,
-  run: async () => mountErrorPageKitten(),
+  run: async () => (isSeqta404Page() ? mountErrorPageKitten() : () => {}),
 };
 
 export default errorPageKittenPlugin;
diff --git a/src/plugins/core/manager.ts b/src/plugins/core/manager.ts
index 0719975f..9cc8d508 100644
--- a/src/plugins/core/manager.ts
+++ b/src/plugins/core/manager.ts
@@ -43,6 +43,9 @@ const PLUGIN_START_PHASES: readonly string[][] = [
   ["global-search"],
 ];
 
+/** Booted only from SEQTA.ts on standalone 404 documents — never auto-start in the SPA. */
+const PLUGIN_BOOT_ONLY_IDS = new Set(["error-page-kitten"]);
+
 /**
  * Singleton class responsible for the entire lifecycle of plugins.
  * This includes registration, starting, stopping, event dispatching,
@@ -263,7 +266,7 @@ export class PluginManager {
 
     const phasedIds = new Set(PLUGIN_START_PHASES.flat());
     const remainingIds = Array.from(this.plugins.keys()).filter(
-      (id) => !phasedIds.has(id),
+      (id) => !phasedIds.has(id) && !PLUGIN_BOOT_ONLY_IDS.has(id),
     );
     if (remainingIds.length > 0) {
       await this.startPluginPhase(remainingIds);