pybricks · dlech · Jan 25, 2026 · Jan 25, 2026 · Jan 25, 2026 · Jan 25, 2026
diff --git a/src/alerts/UnexpectedErrorAlert.tsx b/src/alerts/UnexpectedErrorAlert.tsx
@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-// Copyright (c) 2022-2023 The Pybricks Authors
+// Copyright (c) 2022-2026 The Pybricks Authors
 
 import './UnexpectedErrorAlert.scss';
 import {
@@ -29,7 +29,11 @@ const UnexpectedErrorAlert: React.FunctionComponent<UnexpectedErrorAlertProps> =
 
     return (
         <>
-            <p>{i18n.translate('message')}</p>
+            <p>
+                {i18n.translate('message', {
+                    copyErrorMessage: i18n.translate('copyErrorMessage'),
+                })}
+            </p>
             <p>{error.message}</p>
             {error.stack && (
                 <>

diff --git a/src/alerts/translations/en.json b/src/alerts/translations/en.json
@@ -1,5 +1,5 @@
 {
-    "message": "An unexpected error occurred. Please consider reporting this so we can make a better error message.",
+    "message": "An unexpected error occurred. Please consider reporting this so we can make a better error message. Click the {copyErrorMessage} button below to copy the technical details and paste it in the bug report. Also include the operating system you are using, the web browser you are using and the actions you performed that led to this error.",
     "technicalInfo": "Expand for detailed technical information",
     "copyErrorMessage": "Copy Error Message",
     "reportBug": "Report Bug"

diff --git a/src/usb/alerts/AccessDenied.test.tsx b/src/usb/alerts/AccessDenied.test.tsx
@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2025-2026 The Pybricks Authors
+
+import { Toast } from '@blueprintjs/core';
+import { act } from '@testing-library/react';
+import React from 'react';
+import { testRender } from '../../../test';
+import { accessDenied } from './AccessDenied';
+
+it('should dismiss when close is clicked', async () => {
+    const callback = jest.fn();
+    const toast = accessDenied(callback, undefined as never);
+
+    const [user, message] = testRender(<Toast {...toast} />);
+
+    await act(() => user.click(message.getByRole('button', { name: /close/i })));
+
+    expect(callback).toHaveBeenCalledWith('dismiss');
+});
diff --git a/src/usb/alerts/AccessDenied.tsx b/src/usb/alerts/AccessDenied.tsx
@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2025-2026 The Pybricks Authors
+
+import { Intent } from '@blueprintjs/core';
+import { Error } from '@blueprintjs/icons';
+import React from 'react';
+import type { CreateToast } from '../../toasterTypes';
+import { isLinux } from '../../utils/os';
+import { useI18n } from './i18n';
+
+const AccessDenied: React.FunctionComponent = () => {
+    const i18n = useI18n();
+    return (
+        <>
+            <p>{i18n.translate('accessDenied.message')}</p>
+            {isLinux() && <p>{i18n.translate('accessDenied.linuxSuggestion')}</p>}
+        </>
+    );
+};
+
+export const accessDenied: CreateToast = (onAction) => ({
+    message: <AccessDenied />,
+    icon: <Error />,
+    intent: Intent.DANGER,
+    onDismiss: () => onAction('dismiss'),
+});
diff --git a/src/usb/alerts/AlreadyInUse.test.tsx b/src/usb/alerts/AlreadyInUse.test.tsx
@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2025-2026 The Pybricks Authors
+
+import { Toast } from '@blueprintjs/core';
+import { act } from '@testing-library/react';
+import React from 'react';
+import { testRender } from '../../../test';
+import { alreadyInUse } from './AlreadyInUse';
+
+it('should dismiss when close is clicked', async () => {
+    const callback = jest.fn();
+    const toast = alreadyInUse(callback, undefined as never);
+
+    const [user, message] = testRender(<Toast {...toast} />);
+
+    await act(() => user.click(message.getByRole('button', { name: /close/i })));
+
+    expect(callback).toHaveBeenCalledWith('dismiss');
+});
diff --git a/src/usb/alerts/AlreadyInUse.tsx b/src/usb/alerts/AlreadyInUse.tsx
@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2025-2026 The Pybricks Authors
+
+import { Intent } from '@blueprintjs/core';
+import { Error } from '@blueprintjs/icons';
+import React from 'react';
+import type { CreateToast } from '../../toasterTypes';
+import { useI18n } from './i18n';
+
+const AlreadyInUse: React.FunctionComponent = () => {
+    const i18n = useI18n();
+    return (
+        <>
+            <p>{i18n.translate('alreadyInUse.message')}</p>
+        </>
+    );
+};
+
+export const alreadyInUse: CreateToast = (onAction) => ({
+    message: <AlreadyInUse />,
+    icon: <Error />,
+    intent: Intent.DANGER,
+    onDismiss: () => onAction('dismiss'),
+});
diff --git a/src/usb/alerts/index.ts b/src/usb/alerts/index.ts
@@ -1,12 +1,16 @@
 // SPDX-License-Identifier: MIT
-// Copyright (c) 2025 The Pybricks Authors
+// Copyright (c) 2025-2026 The Pybricks Authors
 
+import { accessDenied } from './AccessDenied';
+import { alreadyInUse } from './AlreadyInUse';
 import { newPybricksProfile } from './NewPybricksProfile';
 import { noWebUsb } from './NoWebUsb';
 import { oldFirmware } from './OldFirmware';
 
 // gathers all of the alert creation functions for passing up to the top level
 export default {
+    accessDenied,
+    alreadyInUse,
     newPybricksProfile,
     noWebUsb,
     oldFirmware,

diff --git a/src/usb/alerts/translations/en.json b/src/usb/alerts/translations/en.json
@@ -4,6 +4,13 @@
         "suggestion": "Use a supported browser such as Google Chrome or Microsoft Edge.",
         "action": "More Info"
     },
+    "accessDenied": {
+        "message": "Access to the USB device was denied.",
+        "linuxSuggestion": "On Linux, ensure that you have the correct udev rules installed."
+    },
+    "alreadyInUse": {
+        "message": "This hub is already in use by another application."
+    },
     "oldFirmware": {
         "message": "A new firmware version is available for this hub. Please install the latest version to use all new features.",
         "flashFirmware": {

diff --git a/src/usb/sagas.ts b/src/usb/sagas.ts
@@ -128,7 +128,9 @@ function* handleUsbConnectPybricks(hotPlugDevice?: USBDevice): Generator {
                 return;
             }
 
-            // TODO: show unexpected error message to user here
+            yield* put(
+                alertsShowAlert('alerts', 'unexpectedError', { error: reqDeviceErr }),
+            );
             console.error('Failed to request USB device:', reqDeviceErr);
             yield* put(usbDidFailToConnectPybricks());
             yield* cleanup();
@@ -154,7 +156,25 @@ function* handleUsbConnectPybricks(hotPlugDevice?: USBDevice): Generator {
                 continue;
             }
 
-            // TODO: show error message to user here
+            // Only show error to the user if they initiated the connection.
+            if (hotPlugDevice === undefined) {
+                if (openErr.name === 'SecurityError') {
+                    // Known causes:
+                    // - Linux without proper udev rules to allow access to USB devices
+                    // - Trying to access a device on a host machine when the USB
+                    //   device is shared with a VM guest OS.
+                    // Other suspected causes:
+                    // - Issues with permissions in containerized apps (e.g. Snaps on Ubuntu)
+                    yield* put(alertsShowAlert('usb', 'accessDenied'));
+                } else {
+                    yield* put(
+                        alertsShowAlert('alerts', 'unexpectedError', {
+                            error: openErr,
+                        }),
+                    );
+                }
+            }
+
             console.error('Failed to open USB device:', openErr);
             yield* put(usbDidFailToConnectPybricks());
             yield* cleanup();
@@ -168,7 +188,7 @@ function* handleUsbConnectPybricks(hotPlugDevice?: USBDevice): Generator {
 
     const [, selectErr] = yield* call(() => maybe(usbDevice.selectConfiguration(1)));
     if (selectErr) {
-        // TODO: show error message to user here
+        yield* put(alertsShowAlert('alerts', 'unexpectedError', { error: selectErr }));
         console.error('Failed to select USB device configuration:', selectErr);
         yield* put(usbDidFailToConnectPybricks());
         yield* cleanup();
@@ -205,7 +225,17 @@ function* handleUsbConnectPybricks(hotPlugDevice?: USBDevice): Generator {
         maybe(usbDevice.claimInterface(iface.interfaceNumber)),
     );
     if (claimErr) {
-        // TODO: show error message to user here
+        // Only show error to the user if they initiated the connection.
+        if (hotPlugDevice === undefined) {
+            if (claimErr.name === 'NetworkError') {
+                yield* put(alertsShowAlert('usb', 'alreadyInUse'));
+            } else {
+                yield* put(
+                    alertsShowAlert('alerts', 'unexpectedError', { error: claimErr }),
+                );
+            }
+        }
+
         console.error('Failed to claim USB interface:', claimErr);
         yield* put(usbDidFailToConnectPybricks());
         yield* cleanup();
@@ -229,7 +259,11 @@ function* handleUsbConnectPybricks(hotPlugDevice?: USBDevice): Generator {
         ),
     );
     if (fwVerError || fwVerResult?.status !== 'ok') {
-        // TODO: show error message to user here
+        yield* put(
+            alertsShowAlert('alerts', 'unexpectedError', {
+                error: fwVerError || ensureError(fwVerResult?.status),
+            }),
+        );
         console.error('Failed to get firmware version:', fwVerError);
         yield* put(usbDidFailToConnectPybricks());
         yield* cleanup();
@@ -285,7 +319,11 @@ function* handleUsbConnectPybricks(hotPlugDevice?: USBDevice): Generator {
         ),
     );
     if (nameError || nameResult?.status !== 'ok') {
-        // TODO: show error message to user here
+        yield* put(
+            alertsShowAlert('alerts', 'unexpectedError', {
+                error: nameError || ensureError(nameResult?.status),
+            }),
+        );
         console.error('Failed to get device name:', nameError);
         yield* put(usbDidFailToConnectPybricks());
         yield* cleanup();
@@ -312,7 +350,11 @@ function* handleUsbConnectPybricks(hotPlugDevice?: USBDevice): Generator {
         ),
     );
     if (swVerError || swVerResult?.status !== 'ok') {
-        // TODO: show error message to user here
+        yield* put(
+            alertsShowAlert('alerts', 'unexpectedError', {
+                error: swVerError || ensureError(swVerResult?.status),
+            }),
+        );
         console.error('Failed to get software version:', swVerError);
         yield* put(usbDidFailToConnectPybricks());
         yield* cleanup();
@@ -354,7 +396,11 @@ function* handleUsbConnectPybricks(hotPlugDevice?: USBDevice): Generator {
         ),
     );
     if (hubCapErr || hubCapResult?.status !== 'ok') {
-        // TODO: show error message to user here
+        yield* put(
+            alertsShowAlert('alerts', 'unexpectedError', {
+                error: hubCapErr || ensureError(hubCapResult?.status),
+            }),
+        );
         console.error('Failed to get hub capabilities:', hubCapErr);
         yield* put(usbDidFailToConnectPybricks());
         yield* cleanup();