diff --git a/server/packages/sandbox-agent/src/router.rs b/server/packages/sandbox-agent/src/router.rs
index 7270336..59b89c3 100644
--- a/server/packages/sandbox-agent/src/router.rs
+++ b/server/packages/sandbox-agent/src/router.rs
@@ -235,9 +235,7 @@ pub fn build_router_with_state(shared: Arc<AppState>) -> (Router, Arc<AppState>)
         .nest("/opencode", opencode_router)
         .merge(opencode_root_router);
 
-    if ui::is_enabled() {
-        router = router.merge(ui::router());
-    }
+    router = router.merge(ui::router());
 
     let http_logging = match std::env::var("SANDBOX_AGENT_LOG_HTTP") {
         Ok(value) if value == "0" || value.eq_ignore_ascii_case("false") => false,
diff --git a/server/packages/sandbox-agent/src/ui.rs b/server/packages/sandbox-agent/src/ui.rs
index 3bb475f..c2c27ef 100644
--- a/server/packages/sandbox-agent/src/ui.rs
+++ b/server/packages/sandbox-agent/src/ui.rs
@@ -15,7 +15,10 @@ pub fn is_enabled() -> bool {
 
 pub fn router() -> Router {
     if !INSPECTOR_ENABLED {
-        return Router::new();
+        return Router::new()
+            .route("/ui", get(handle_not_built))
+            .route("/ui/", get(handle_not_built))
+            .route("/ui/*path", get(handle_not_built));
     }
     Router::new()
         .route("/ui", get(handle_index))
@@ -23,6 +26,18 @@ pub fn router() -> Router {
         .route("/ui/*path", get(handle_path))
 }
 
+async fn handle_not_built() -> Response {
+    let body = "Inspector UI was not included in this build.\n\n\
+                To enable it, build the frontend first:\n\n\
+                  cd frontend/packages/inspector && pnpm install && pnpm build\n\n\
+                Then rebuild sandbox-agent without SANDBOX_AGENT_SKIP_INSPECTOR.\n";
+    Response::builder()
+        .status(StatusCode::NOT_FOUND)
+        .header(header::CONTENT_TYPE, "text/plain; charset=utf-8")
+        .body(Body::from(body))
+        .unwrap()
+}
+
 async fn handle_index() -> Response {
     serve_path("")
 }