microsoft · compulim · May 22, 2026 · May 22, 2026 · May 22, 2026 · May 22, 2026
@@ -423,6 +423,7 @@ Breaking changes in this release:
 - Fixed Fluent/Copilot typing indicator animation background color, in PR [#5770](https://github.com/microsoft/BotFramework-WebChat/pull/5770), by [@OEvgeny](https://github.com/OEvgeny)
 - Fixed `<AddFullBundle>` should not re-render when `attachment[ForScreenReader]Middleware` is updated without noticeable different (`iterateEquals`), by [@compulim](https://github.com/compulim), in PR [#5779](https://github.com/microsoft/BotFramework-WebChat/pull/5779)
 - Fixed send box should narrate `aria-label` prop, by [@compulim](https://github.com/compulim), in PR [#5805](https://github.com/microsoft/BotFramework-WebChat/pull/5805)
+- Fixed polymiddleware error should not propagate to React runtime if `<DebugProvider>` is not mounted, by [@compulim](https://github.com/compulim), in PR [#5833](https://github.com/microsoft/BotFramework-WebChat/pull/5833)
 
 ## [4.18.0] - 2024-07-10
 

@@ -0,0 +1,96 @@
+<!doctype html>
+<html lang="en-US">
+  <head>
+    <link href="/assets/index.css" rel="stylesheet" type="text/css" />
+    <script>
+      const warn = console.warn.bind(console);
+
+      // We need to fake the mock ourselves without using jest-mock.
+      // jest-mock is ESM and cannot mock before Web Chat.
+      console.warn = (...args) => {
+        console.warn.mock.calls.push(args);
+
+        warn(...args);
+      };
+
+      console.warn._isMockFunction = true;
+      console.warn.getMockName = () => 'warn';
+      console.warn.mock = { calls: [] };
+    </script>
+    <script type="importmap">
+      {
+        "imports": {
+          "react": "https://esm.sh/react@18.3.1",
+          "react-dom": "https://esm.sh/react-dom@18.3.1",
+          "react-dom/": "https://esm.sh/react-dom@18.3.1/"
+        }
+      }
+    </script>
+    <script crossorigin="anonymous" src="/test-harness.js"></script>
+    <script crossorigin="anonymous" src="/test-page-object.js"></script>
+    <script type="module">
+      import React from 'react';
+
+      window.React = React;
+    </script>
+    <script crossorigin="anonymous" defer src="/__dist__/webchat-es5.js"></script>
+    <script crossorigin="anonymous" defer src="/__dist__/botframework-webchat-debug-theme.development.js"></script>
+  </head>
+  <body>
+    <main id="webchat"></main>
+    <script type="module">
+      import React, { createElement } from 'react';
+      import { createRoot } from 'react-dom/client';
+
+      run(async function () {
+        const {
+          testHelpers: { createStore },
+          WebChat: { ReactWebChat }
+        } = window;
+
+        const { directLine, store } = testHelpers.createDirectLineEmulator();
+
+        // GIVEN: Web Chat is being rendered without <DebugProvider>.
+        createRoot(document.getElementById('webchat')).render(createElement(ReactWebChat, { directLine, store }));
+
+        await pageConditions.uiConnected();
+
+        await directLine.emulateIncomingActivity({
+          attachments: [
+            {
+              contentType: 'application/vnd.microsoft.card.adaptive',
+              content: {
+                // WHEN: We want to render a failing Adaptive Cards, adding "*" here to fail the renderer.
+                type: '*AdaptiveCard*',
+                $schema: 'http://adaptivecards.io/schemas/adaptive-card.json',
+                version: '1.5',
+                body: [
+                  {
+                    text: 'Hello, World!',
+                    type: 'TextBlock'
+                  }
+                ]
+              }
+            }
+          ],
+          type: 'message'
+        });
+
+        // THEN: Should have "render Adaptive Cards" error message.
+        expect(console.warn).toHaveBeenCalledWith(
+          expect.stringContaining('Failed to render Adaptive Cards.'),
+          expect.anything()
+        );
+
+        // THEN: Should not show RCoR error as we have injected catchall.
+        expect(console.warn).not.toHaveBeenCalledWith(
+          expect.stringContaining('the request has fall through all middleware, set "fallbackComponent" as a catchall'),
+          expect.anything()
+        );
+
+        // THEN: Should not show red box.
+        await host.snapshot('local');
+      });
+    </script>
+  </body>
+</html>
@@ -1,5 +1,5 @@
 import { validateProps } from '@msinternal/botframework-webchat-react-valibot';
-import React, { memo, useMemo } from 'react';
+import React, { memo, useCallback, useMemo } from 'react';
 import { object, pipe, readonly, string, unknown, type InferInput } from 'valibot';
 
 import templatePolymiddleware, {
@@ -18,7 +18,7 @@ const {
   Provider: ErrorBoxPolymiddlewareProvider,
   Proxy,
   reactComponent: errorBoxComponent,
-  useBuildRenderCallback: useBuildRenderErrorBoxCallback
+  useBuildRenderCallback
 } = templatePolymiddleware<{ readonly error: unknown; readonly where: string }, { readonly children?: never }>(
   'ErrorBox'
 );
@@ -41,15 +41,27 @@ const ErrorBoxPolymiddlewareProxyPropsSchema = pipe(
 
 type ErrorBoxPolymiddlewareProxyProps = Readonly<InferInput<typeof ErrorBoxPolymiddlewareProxyPropsSchema>>;
 
+// If no error box is defined, do not fallthrough into RCoR and it would error out. Render nothing instead.
+const NullComponent = () => null;
+
 // A friendlier version than the organic <Proxy>.
 const ErrorBoxPolymiddlewareProxy = memo(function ErrorBoxPolymiddlewareProxy(props: ErrorBoxPolymiddlewareProxyProps) {
   const { error, where } = validateProps(ErrorBoxPolymiddlewareProxyPropsSchema, props);
 
   const request = useMemo(() => ({ error, where }), [error, where]);
 
-  return <Proxy request={request} />;
+  return <Proxy fallbackComponent={NullComponent} request={request} />;
 });
 
+const useBuildRenderErrorBoxCallback: typeof useBuildRenderCallback = () => {
+  const buildRenderCallback = useBuildRenderCallback();
+
+  return useCallback(
+    (request, options) => buildRenderCallback(request, Object.freeze({ fallbackComponent: NullComponent, ...options })),
+    [buildRenderCallback]
+  );
+};
+
 export {
   createErrorBoxPolymiddleware,
   errorBoxComponent,