diff --git a/src/kimi_cli/web/app.py b/src/kimi_cli/web/app.py
index 013ddfa6a0..152b17d098 100644
--- a/src/kimi_cli/web/app.py
+++ b/src/kimi_cli/web/app.py
@@ -16,7 +16,7 @@
 from fastapi.middleware.gzip import GZipMiddleware
 from fastapi.staticfiles import StaticFiles
 from starlette.datastructures import MutableHeaders
-from starlette.responses import HTMLResponse
+from starlette.responses import HTMLResponse, Response
 from starlette.types import ASGIApp, Message, Receive, Scope, Send
 
 from kimi_cli import logger
@@ -100,6 +100,22 @@ async def _send_with_cache_headers(message: Message) -> None:
         await self.app(scope, receive, _send_with_cache_headers)
 
 
+class _WebStaticFiles(StaticFiles):
+    """Serve JavaScript assets with a browser-safe MIME type."""
+
+    def file_response(
+        self,
+        full_path: str | os.PathLike[str],
+        stat_result: os.stat_result,
+        scope: Scope,
+        status_code: int = 200,
+    ) -> Response:
+        response = super().file_response(full_path, stat_result, scope, status_code)
+        if Path(full_path).suffix.lower() == ".js":
+            response.headers["content-type"] = "text/javascript; charset=utf-8"
+        return response
+
+
 def _get_private_addresses(addresses: list[str]) -> list[str]:
     """Filter addresses to only include private IPs."""
     return [ip for ip in addresses if is_private_ip(ip)]
@@ -220,7 +236,7 @@ async def health_probe() -> dict[str, Any]:  # pyright: ignore[reportUnusedFunct
 
     # Mount static files as fallback (must be last)
     if STATIC_DIR.exists():
-        application.mount("/", StaticFiles(directory=STATIC_DIR, html=True), name="static")
+        application.mount("/", _WebStaticFiles(directory=STATIC_DIR, html=True), name="static")
 
     return application
 
diff --git a/tests/web/test_static_mime.py b/tests/web/test_static_mime.py
new file mode 100644
index 0000000000..6d05778b3a
--- /dev/null
+++ b/tests/web/test_static_mime.py
@@ -0,0 +1,36 @@
+from __future__ import annotations
+
+import mimetypes
+from pathlib import Path
+
+from fastapi.testclient import TestClient
+
+from kimi_cli.web import app as web_app
+
+
+def test_web_static_js_uses_javascript_mime_when_system_mapping_is_plain_text(
+    monkeypatch,
+    tmp_path: Path,
+) -> None:
+    static_dir = tmp_path / "static"
+    assets_dir = static_dir / "assets"
+    assets_dir.mkdir(parents=True)
+    (assets_dir / "index-test.js").write_text("console.log('ok')\n", encoding="utf-8")
+    (static_dir / "index.html").write_text(
+        '<script type="module" src="/assets/index-test.js"></script>',
+        encoding="utf-8",
+    )
+
+    monkeypatch.setattr(web_app, "STATIC_DIR", static_dir)
+
+    original_js_mime = mimetypes.guess_type("index-test.js")[0]
+    mimetypes.add_type("text/plain", ".js")
+    try:
+        with TestClient(web_app.create_app()) as client:
+            response = client.get("/assets/index-test.js")
+    finally:
+        if original_js_mime is not None:
+            mimetypes.add_type(original_js_mime, ".js")
+
+    assert response.status_code == 200
+    assert response.headers["content-type"].split(";")[0] == "text/javascript"