microsoft/add-fallback-commands

Add Fallback Commands

Fallback commands are shown in Command Palette when no other results match the user's query. They enable your extension to act as a catch-all handler — perfect for calculators, web search, command execution, file path opening, and more.

When to Use This Skill

• Adding search functionality that responds to any user input
• Creating a calculator that evaluates expressions as the user types
• Building a web search that triggers on unmatched queries
• Opening files or URLs typed directly into the palette
• Executing shell commands from the search bar

How Fallback Commands Work

1. User types a query in Command Palette
2. If no top-level commands match, CmdPal asks extensions for fallback results
3. Your extension's FallbackCommands() provides items that respond to the query
4. The fallback items can be static (always shown) or dynamic (filtered by query)

Quick Start: Static Fallback

Override FallbackCommands() in your CommandProvider:

public partial class MyCommandsProvider : CommandProvider
{
    private readonly ICommandItem[] _commands;
    private readonly FallbackCommandItem[] _fallbacks;

    public MyCommandsProvider()
    {
        DisplayName = "Web Search";
        Icon = new IconInfo("\uE721"); // Search icon

        var searchPage = new WebSearchPage();
        _commands = [new CommandItem(searchPage) { Title = DisplayName }];
        _fallbacks = [new FallbackCommandItem(searchPage) { Title = "Search the web" }];
    }

    public override ICommandItem[] TopLevelCommands() => _commands;
    public override IFallbackCommandItem[] FallbackCommands() => _fallbacks;
}

Dynamic Fallback with DynamicListPage

For fallbacks that filter results based on the query, use DynamicListPage:

internal sealed partial class WebSearchPage : DynamicListPage
{
    private string _query = string.Empty;

    public WebSearchPage()
    {
        Icon = new IconInfo("\uE721");
        Title = "Web Search";
        Name = "Search";
        PlaceholderText = "Type to search...";
    }

    public override void UpdateSearchText(string oldSearch, string newSearch)
    {
        _query = newSearch;
        RaiseItemsChanged();
    }

    public override IListItem[] GetItems()
    {
        if (string.IsNullOrWhiteSpace(_query))
            return [];

        return [
            new ListItem(new OpenUrlCommand($"https://www.google.com/search?q={Uri.EscapeDataString(_query)}"))
            {
                Title = $"Search Google for \"{_query}\"",
                Icon = new IconInfo("\uE721"),
            },
            new ListItem(new OpenUrlCommand($"https://www.bing.com/search?q={Uri.EscapeDataString(_query)}"))
            {
                Title = $"Search Bing for \"{_query}\"",
                Icon = new IconInfo("\uE721"),
            },
        ];
    }
}

Responsive Fallback with Cancellation

For expensive operations (API calls, file searches), use cancellation to stay responsive:

internal sealed partial class SmartSearchPage : DynamicListPage
{
    private CancellationTokenSource? _cts;
    private IListItem[] _results = [];

    public override void UpdateSearchText(string oldSearch, string newSearch)
    {
        // Cancel any in-flight search
        _cts?.Cancel();
        _cts = new CancellationTokenSource();
        var token = _cts.Token;

        _ = Task.Run(async () =>
        {
            // Debounce: wait for user to stop typing
            await Task.Delay(300, token);
            if (token.IsCancellationRequested) return;

            // Perform search
            _results = await SearchAsync(newSearch, token);
            RaiseItemsChanged();
        }, token);
    }

    public override IListItem[] GetItems() => _results;

    private async Task<IListItem[]> SearchAsync(string query, CancellationToken token)
    {
        // Your search logic here
        // Check token.IsCancellationRequested periodically
        return [];
    }
}

Real-World Examples (from built-in extensions)

| Extension | Fallback Behavior | |-----------|------------------| | Apps | Search installed applications by name | | Calc | Evaluate mathematical expressions directly | | Shell | Execute command-line commands | | WebSearch | Search the web with configured engine | | Indexer | Open files by path | | TimeDate | Parse time/date queries | | WindowsSettings | Jump to Windows Settings pages | | WinGet | Search WinGet packages | | WindowWalker | Find and switch to open windows |

Key Points

• FallbackCommands() returns IFallbackCommandItem[] (not ICommandItem[])
• Use FallbackCommandItem wrapper (not CommandItem)
• Wrap a DynamicListPage for query-reactive results
• Cancel previous searches when new input arrives
• Keep fallback responses fast — users expect instant results
• Use PlaceholderText on your page to guide users

Documentation

• Extension samples
• Extensibility overview