Extensions

Modern AI applications evolve too quickly for a fixed collection of hard-coded screens. New model providers, tools, modalities and workflows arrive continuously, and enterprises need to disable capabilities, replace UI and integrate their own systems without maintaining a fork.

AI Chat solves this with a shared extension architecture spanning the server and the browser. Everything above the hosting layer is an extension, including the core conversation UI.

Built-in extensions​

What an extension can contribute​

Each extension can register only what it needs:

• ServiceStack routes and APIs, under /{RoutePrefix}/ext/{name}/
• Model tools and tool groups
• UI components and pages
• Sidebar and toolbar actions
• Chat request, tool, approval, status, response and error filters
• Import maps and client dependencies
• Per-user files and preferences
• Database tables and background workers
• Startup and shutdown lifecycle hooks

The entire UI is assembled from registered Vue components, so an extension can add a new component or deliberately replace an existing building block by registering the same component name.

Disabling extensions​

services.AddPlugin(new ChatFeature {
    DisableExtensions = ["computer", "publish", "katex"],
});

DisableExtensions removes the capability from the server and the UI together - a disabled extension registers no routes, no tools and no components.

The same list can be set in App_Data/chat/llms.json, which is merged with the programmatic list:

{
  "disable_extensions": ["computer"]
}

An extension can also disable itself. gemini does so when no Gemini API key is configured, voice when neither ffmpeg nor Mistral is available, and pdf when the typst CLI isn't on PATH. InstalledExtensionNames reports what actually loaded:

var installed = feature.InstalledExtensionNames;   // ["app","agents","tools",...]

Configuring an extension​

ChatFeature exposes each built-in extension as a typed property, so configuration reads as one object graph:

services.AddPlugin(new ChatFeature {
    Tools    = { EnableCodeExecution = true, EnableFilesystemTools = true },
    ApiTools = { IncludeTags = ["CoffeeShop"] },
    Mcp      = { ToolGroups = ["api_tools"] },
    Publish  = { Enabled = true },
});

AssertExtension<T>() resolves any of them, including extensions you add yourself:

var bookings = feature.AssertExtension<BookingToolsExtension>();

Install order​

Extensions install in the order they appear in ChatFeature.Extensions. The built-in list ends with CustomExtension, so your App's own extension sees everything the built-ins registered - letting it override a tool, replace a UI component or reconfigure a group after the fact.

Adding your own extension appends it before that final custom extension:

services.AddPlugin(new ChatFeature {
    Extensions = {
        new BookingToolsExtension(),
    },
});

To run before a built-in instead, replace the list outright or insert at an index in Setup.

Lifecycle​

An Install that throws logs the failure and skips that extension rather than failing App startup. Shutdown handlers are independent - one throwing doesn't skip the rest.

Static assets​

An extension with a chat/ext/{name}/ folder of embedded resources gets its files served automatically at /{RoutePrefix}/ext/{name}/{path}, and one containing an index.mjs is registered in the /ext list so the Chat UI dynamically imports it on load.

Nothing under chat/custom/** is ever overwritten by an upstream UI sync, which makes it the place to put your own UI. See Custom Extensions.