diff --git a/.changeset/quick-lies-pull.md b/.changeset/quick-lies-pull.md
new file mode 100644
index 0000000..7dbf8e9
--- /dev/null
+++ b/.changeset/quick-lies-pull.md
@@ -0,0 +1,5 @@
+---
+'@hono/vite-dev-server': patch
+---
+
+fix: set `navigator.userAgent` correctly
diff --git a/packages/dev-server/src/adapter/cloudflare.ts b/packages/dev-server/src/adapter/cloudflare.ts
index f7fcdec..6a4892f 100644
--- a/packages/dev-server/src/adapter/cloudflare.ts
+++ b/packages/dev-server/src/adapter/cloudflare.ts
@@ -16,11 +16,18 @@ export const cloudflareAdapter: (options?: CloudflareAdapterOptions) => Promise<
   proxy ??= await getPlatformProxy(options?.proxy)
   // Cache API provided by `getPlatformProxy` currently do nothing.
   Object.assign(globalThis, { caches: proxy.caches })
-  Object.assign(globalThis, {
-    navigator: {
+  if (typeof globalThis.navigator === 'undefined') {
+    // @ts-expect-error not typed well
+    globalThis.navigator = {
       userAgent: 'Cloudflare-Workers',
-    },
-  })
+    }
+  } else {
+    Object.defineProperty(globalThis.navigator, 'userAgent', {
+      value: 'Cloudflare-Workers',
+      writable: false,
+    })
+  }
+
   return {
     env: proxy.env,
     executionContext: proxy.ctx,