diff --git a/ui/vitest.config.ts b/ui/vitest.config.ts
index 370ad40..dfc9ffa 100644
--- a/ui/vitest.config.ts
+++ b/ui/vitest.config.ts
@@ -2,6 +2,12 @@ import { defineConfig } from "vitest/config";
 import react from "@vitejs/plugin-react";
 import { resolve } from "path";
 
+// React 19's CJS entry picks production vs development at require-time off
+// NODE_ENV; the production build omits `act`. Force the test build before any
+// worker forks so callers with NODE_ENV=production (CI-adjacent shells, the
+// Paperclip agent harness) get the same result as bare `npm test`.
+process.env.NODE_ENV = "test";
+
 export default defineConfig({
   plugins: [react()],
   resolve: {