fix: Omit program slot ID when not specified.

We don't have a UI for it, so we should assume no slot is chosen rather than default to the first slot.

This allows the user to select the slot on the hub without a front end UI.

This is compatible with the existing protocol.

src/ble-pybricks-service/actions.ts

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-// Copyright (c) 2021-2024 The Pybricks Authors
+// Copyright (c) 2021-2025 The Pybricks Authors
 //
 // Actions for Bluetooth Low Energy Pybricks service
 
@@ -85,11 +85,13 @@ export const sendLegacyStartReplCommand = createAction((id: number) => ({
  *
  * @since Pybricks Profile v1.4.0
  */
-export const sendStartUserProgramCommand = createAction((id: number, slot: number) => ({
-    type: 'blePybricksServiceCommand.action.sendStartUserProgram',
-    id,
-    slot,
-}));
+export const sendStartUserProgramCommand = createAction(
+    (id: number, slot: number | null) => ({
+        type: 'blePybricksServiceCommand.action.sendStartUserProgram',
+        id,
+        slot,
+    }),
+);
 
 /**
  * Action that requests to write user program metadata.

src/ble-pybricks-service/protocol.ts

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-// Copyright (c) 2020-2024 The Pybricks Authors
+// Copyright (c) 2020-2025 The Pybricks Authors
 //
 // Definitions related to the Pybricks Bluetooth low energy GATT service.
 
@@ -114,12 +114,20 @@ export function createStopUserProgramCommand(): Uint8Array {
  * Parameters:
  * - slot: Program identifier (one byte). Slots 0--127 are reserved for
  *   downloaded user programs. Slots 128--255 are for builtin user programs.
+ *   If null, the hub will start the program slot selected on the hub.
  *
  * @since Pybricks Profile v1.4.0
  */
 export function createStartUserProgramCommand(
-    slot: number | BuiltinProgramId,
+    slot: number | BuiltinProgramId | null,
 ): Uint8Array {
+    // Omit optional slot id to start currently active slot.
+    if (slot === null) {
+        const msg = new Uint8Array(1);
+        msg[0] = CommandType.StartUserProgram;
+        return msg;
+    }
+
     const msg = new Uint8Array(2);
     msg[0] = CommandType.StartUserProgram;
     msg[1] = slot;

src/hub/actions.ts

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-// Copyright (c) 2020-2024 The Pybricks Authors
+// Copyright (c) 2020-2025 The Pybricks Authors
 
 import { createAction } from '../actions';
 import { FileFormat } from '../ble-pybricks-service/protocol';
@@ -26,7 +26,7 @@ export const downloadAndRun = createAction(
         fileFormat: FileFormat | null,
         useLegacyDownload: boolean,
         useLegacyStartUserProgram: boolean,
-        slot: number,
+        slot: number | null,
     ) => ({
         type: 'hub.action.downloadAndRun',
         fileFormat,

src/hub/sagas.test.ts

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-// Copyright (c) 2020-2024 The Pybricks Authors
+// Copyright (c) 2020-2025 The Pybricks Authors
 
 import { AsyncSaga } from '../../test';
 import { alertsShowAlert } from '../alerts/actions';
@@ -38,7 +38,7 @@ describe('downloadAndRun', () => {
 
         saga.updateState({ editor: { isReady: true } });
 
-        saga.put(downloadAndRun(FileFormat.Mpy5, true, true, 0));
+        saga.put(downloadAndRun(FileFormat.Mpy5, true, true, null));
 
         // first, it gets the value from the current editor
         const editorValueAction = await saga.take();

src/toolbar/buttons/run/RunButton.test.tsx

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-// Copyright (c) 2022-2024 The Pybricks Authors
+// Copyright (c) 2022-2025 The Pybricks Authors
 
 import { act, cleanup } from '@testing-library/react';
 import React from 'react';
@@ -43,7 +43,7 @@ test.each([
                 FileFormat.MultiMpy6,
                 legacyDownload,
                 legacyStartUserProgram,
-                0,
+                null,
             ),
         );
     },

src/toolbar/buttons/run/RunButton.tsx

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-// Copyright (c) 2020-2024 The Pybricks Authors
+// Copyright (c) 2020-2025 The Pybricks Authors
 
 import React from 'react';
 import { useDispatch } from 'react-redux';
@@ -48,7 +48,7 @@ const RunButton: React.FunctionComponent<RunButtonProps> = ({ id }) => {
                         preferredFileFormat,
                         useLegacyDownload,
                         useLegacyStartUserProgram,
-                        0, // No slot UI yet
+                        null, // No slot UI yet, downloading to active hub slot
                     ),
                 )
             }