diff --git a/backend/app/main.py b/backend/app/main.py
index a41e62e..d4ee9c2 100644
--- a/backend/app/main.py
+++ b/backend/app/main.py
@@ -1,7 +1,7 @@
 from contextlib import asynccontextmanager
 from pathlib import Path
 
-from fastapi import FastAPI
+from fastapi import FastAPI, HTTPException
 from fastapi.responses import FileResponse
 from fastapi.staticfiles import StaticFiles
 
@@ -28,9 +28,19 @@ async def healthz():
     return {"status": "ok"}
 
 
-app.mount("/assets", StaticFiles(directory=FRONTEND_DIST / "assets"), name="frontend-assets")
+app.mount(
+    "/assets",
+    StaticFiles(directory=FRONTEND_DIST / "assets", check_dir=False),
+    name="frontend-assets",
+)
 
 
 @app.get("/{full_path:path}")
 async def serve_spa(full_path: str):
-    return FileResponse(FRONTEND_DIST / "index.html")
+    index_file = FRONTEND_DIST / "index.html"
+    if not index_file.exists():
+        raise HTTPException(
+            status_code=503,
+            detail="Frontend not built. Run `npm run build` in frontend/ and restart.",
+        )
+    return FileResponse(index_file)
diff --git a/backend/tests/test_frontend_serving.py b/backend/tests/test_frontend_serving.py
index 4037837..a2f591d 100644
--- a/backend/tests/test_frontend_serving.py
+++ b/backend/tests/test_frontend_serving.py
@@ -14,3 +14,13 @@ async def test_auth_routes_still_work_alongside_spa_fallback(client):
         "/auth/register", json={"username": "frontendcheck", "password": "spookyspooky"}
     )
     assert response.status_code == 201
+
+
+@pytest.mark.asyncio
+async def test_missing_frontend_build_returns_clear_error(client, monkeypatch, tmp_path):
+    import app.main as main_module
+
+    monkeypatch.setattr(main_module, "FRONTEND_DIST", tmp_path)
+    response = await client.get("/some/route")
+    assert response.status_code == 503
+    assert "npm run build" in response.json()["detail"]