diff --git a/src/env.test.ts b/src/env.test.ts
new file mode 100644
index 0000000..0c42d37
--- /dev/null
+++ b/src/env.test.ts
@@ -0,0 +1,136 @@
+import {afterEach, beforeEach, describe, expect, jest, test} from "@jest/globals";
+import {isBackground} from "./env";
+
+describe("isBackground", () => {
+    let originalBrowserDescriptor: PropertyDescriptor | undefined;
+    let originalChromeDescriptor: PropertyDescriptor | undefined;
+    let originalWindowDescriptor: PropertyDescriptor | undefined;
+    let originalLocationDescriptor: PropertyDescriptor | undefined;
+
+    beforeEach(() => {
+        originalBrowserDescriptor = Object.getOwnPropertyDescriptor(globalThis, "browser");
+        originalChromeDescriptor = Object.getOwnPropertyDescriptor(globalThis, "chrome");
+        originalWindowDescriptor = Object.getOwnPropertyDescriptor(globalThis, "window");
+        originalLocationDescriptor = Object.getOwnPropertyDescriptor(globalThis, "location");
+
+        delete (globalThis as any).browser;
+        delete (globalThis as any).chrome;
+        delete (globalThis as any).window;
+        delete (globalThis as any).location;
+    });
+
+    afterEach(() => {
+        restoreGlobalProperty("browser", originalBrowserDescriptor);
+        restoreGlobalProperty("chrome", originalChromeDescriptor);
+        restoreGlobalProperty("window", originalWindowDescriptor);
+        restoreGlobalProperty("location", originalLocationDescriptor);
+        jest.resetAllMocks();
+    });
+
+    const setGlobalProperty = (name: string, value: unknown): void => {
+        Object.defineProperty(globalThis, name, {
+            configurable: true,
+            value,
+            writable: true,
+        });
+    };
+
+    const restoreGlobalProperty = (name: string, descriptor: PropertyDescriptor | undefined): void => {
+        if (descriptor) {
+            Object.defineProperty(globalThis, name, descriptor);
+
+            return;
+        }
+
+        delete (globalThis as any)[name];
+    };
+
+    const setRuntime = (runtime: object | undefined): void => {
+        setGlobalProperty("chrome", runtime ? {runtime} : {});
+    };
+
+    const setWindow = (pathname: string): void => {
+        setGlobalProperty("window", {});
+        setGlobalProperty("location", {pathname});
+    };
+
+    test("returns false when the browser API is unavailable", () => {
+        expect(isBackground()).toBe(false);
+    });
+
+    test("returns false when runtime is unavailable", () => {
+        setRuntime(undefined);
+
+        expect(isBackground()).toBe(false);
+    });
+
+    test("returns false when runtime.id is unavailable", () => {
+        setRuntime({getManifest: jest.fn()});
+
+        expect(isBackground()).toBe(false);
+    });
+
+    test("returns false without throwing when runtime.getManifest is unavailable", () => {
+        setRuntime({id: "extension-id", getManifest: undefined});
+
+        expect(() => isBackground()).not.toThrow();
+        expect(isBackground()).toBe(false);
+    });
+
+    test("returns false when runtime.getManifest is not a function", () => {
+        setRuntime({id: "extension-id", getManifest: "manifest"});
+
+        expect(isBackground()).toBe(false);
+    });
+
+    test("identifies an MV3 service worker as background", () => {
+        setRuntime({
+            getManifest: jest.fn(() => ({
+                background: {service_worker: "service-worker.js"},
+                manifest_version: 3,
+            })),
+            id: "extension-id",
+        });
+
+        expect(isBackground()).toBe(true);
+    });
+
+    test("does not identify an MV3 extension document as background", () => {
+        setRuntime({
+            getManifest: jest.fn(() => ({
+                background: {service_worker: "service-worker.js"},
+                manifest_version: 3,
+            })),
+            id: "extension-id",
+        });
+        setWindow("/popup.html");
+
+        expect(isBackground()).toBe(false);
+    });
+
+    test("identifies an MV2 generated background page as background", () => {
+        setRuntime({
+            getManifest: jest.fn(() => ({
+                background: {scripts: ["background.js"]},
+                manifest_version: 2,
+            })),
+            id: "extension-id",
+        });
+        setWindow("/_generated_background_page.html");
+
+        expect(isBackground()).toBe(true);
+    });
+
+    test("does not identify a regular extension page as background", () => {
+        setRuntime({
+            getManifest: jest.fn(() => ({
+                background: {scripts: ["background.js"]},
+                manifest_version: 2,
+            })),
+            id: "extension-id",
+        });
+        setWindow("/popup.html");
+
+        expect(isBackground()).toBe(false);
+    });
+});
diff --git a/src/env.ts b/src/env.ts
index 91c429e..1fe926f 100644
--- a/src/env.ts
+++ b/src/env.ts
@@ -1,11 +1,23 @@
-import {getId, getManifest} from "./runtime";
+import {browser} from "./browser";
 
 export const isBackground = (): boolean => {
-    if (!getId()) {
+    let runtime: typeof chrome.runtime | undefined;
+
+    try {
+        runtime = browser()?.runtime;
+    } catch {
+        return false;
+    }
+
+    if (!runtime?.id) {
+        return false;
+    }
+
+    if (typeof runtime.getManifest !== "function") {
         return false;
     }
 
-    const manifest = getManifest();
+    const manifest = runtime.getManifest();
 
     if (!manifest.background) {
         return false;