diff --git a/packages/core/src/client/inject/runtime.test.ts b/packages/core/src/client/inject/runtime.test.ts
new file mode 100644
index 00000000..d653aaf5
--- /dev/null
+++ b/packages/core/src/client/inject/runtime.test.ts
@@ -0,0 +1,53 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { startDevTools } from './runtime'
+
+const mocks = vi.hoisted(() => ({
+  getDevToolsRpcClient: vi.fn(
+    (_options: { baseURL: string[] }) => new Promise(() => {}),
+  ),
+}))
+
+vi.mock('@vitejs/devtools-kit/client', () => ({
+  CLIENT_CONTEXT_KEY: '__VITE_DEVTOOLS_CLIENT_CONTEXT__',
+  getDevToolsRpcClient: mocks.getDevToolsRpcClient,
+}))
+
+vi.mock('@vueuse/core', () => ({
+  useLocalStorage: vi.fn(),
+}))
+
+vi.mock('../webcomponents/state/context', () => ({
+  createDocksContext: vi.fn(),
+}))
+
+describe('injected DevTools runtime', () => {
+  beforeEach(() => {
+    const hostWindow = {
+      addEventListener: vi.fn(),
+    } as unknown as Window
+    Object.defineProperty(hostWindow, 'parent', { value: hostWindow })
+    vi.stubGlobal('window', hostWindow)
+
+    startDevTools('normal')
+  })
+
+  afterEach(() => {
+    vi.unstubAllGlobals()
+    mocks.getDevToolsRpcClient.mockClear()
+  })
+
+  it('preserves the host-relative mount path as the primary base', () => {
+    const options = mocks.getDevToolsRpcClient.mock.calls[0]![0]
+
+    expect(options.baseURL).toHaveLength(2)
+    expect(options.baseURL[0]).toBe('/__devtools/')
+  })
+
+  it('adds the Vite module origin as the fallback base', () => {
+    const options = mocks.getDevToolsRpcClient.mock.calls[0]![0]
+
+    expect(options.baseURL[1]).toBe(
+      new URL('/__devtools/', import.meta.url).href,
+    )
+  })
+})
diff --git a/packages/core/src/client/inject/runtime.ts b/packages/core/src/client/inject/runtime.ts
index df483659..0a5d2c52 100644
--- a/packages/core/src/client/inject/runtime.ts
+++ b/packages/core/src/client/inject/runtime.ts
@@ -50,10 +50,15 @@ async function mountDock(): Promise<void> {
   if (dockEl)
     return
 
-  // Inject runs in the user's host page, so `document.baseURI` points at
-  // the user's app — not at the Vite DevTools mount. Pass the mount path
-  // explicitly so the connection meta lookup hits `/__devtools/__connection.json`.
-  const rpc = await getDevToolsRpcClient({ baseURL: DEVTOOLS_MOUNT_PATH })
+  // Prefer the host page's origin for backwards compatibility. When the page
+  // is served by another backend, fall back to the Vite origin that loaded
+  // this module.
+  const rpc = await getDevToolsRpcClient({
+    baseURL: [
+      DEVTOOLS_MOUNT_PATH,
+      new URL(DEVTOOLS_MOUNT_PATH, import.meta.url).href,
+    ],
+  })
 
   const state = useLocalStorage<DockPanelStorage>(
     'vite-devtools-dock-state',