Bern MCP server

Why Bern

Who uses Bern

Built for AI

Writing code: what the AI forgets, the compiler remembers

AI rarely gets interface code wrong; it leaves things out. A feature is added without an entry in the compact layout, a new string is written in one language only, a callback sidesteps the commands. Elsewhere these wait for a user to find them; in Bern they do not compile. The stricter the rules, the harder it is for an AI to hand over half a job:

Errors are written for people and for agents alike: a code, a reason and a location, so an agent can fix them and build again on its own. Two real runs on the showcase: the first with an English translation deleted, the second, after restoring it, with the compact layout's "Change Clip" menu removed:

$ bern check
error[B102]: Missing translation
Missing language: en
Required languages: zh-Hans, en
  --> showcase-ui/src/pages.rs:13:31

$ bern check
error[B201]: Feature missing in adaptive layout
Contract: MediaFeature
Feature: Clip
compact: missing
  --> showcase-ui/src/media.rs:14:5

Every build also writes a full feature coverage report per target (features.json) that lists what is covered, exempt or only manually guaranteed.

Using the app: meaning, not pixels

In most frameworks, letting an AI operate a desktop app means screenshots, recognition, coordinates and synthetic clicks. A Bern app ships with an MCP server: every control has a stable name and role, the agent reads structure, its actions take the same code path as a user's click, and the app answers with what actually happened.

$ showcase ui inspect                 # the agent reads meaning, not pixels
[{ "target": "controls_page.sync", "role": "switch", "label": "Sync", "value": "false", "enabled": true, … }, …]

$ showcase ui set-bool --target controls_page.sync --value true

$ showcase window close --window settings#1
{ "outcome": "ConfirmationRequired", "message": "Do you want to save the display name?",
  "decisions": ["Save", "Discard", "Cancel"], "confirmation": "settings#1:c1", … }

$ showcase window confirm-close --window settings#1 --confirmation settings#1:c1 --decision Save
{ "outcome": "Rejected", "reason": "save_failed: invalid_name: a display name has 1 to 40 characters …" }

The same operations are MCP tools for agents (ui_inspect, ui_activate, window_close, …), and each business command is a tool too (for example showcase_set_display_name). When a button is disabled, blocked by a dialog or missing, the agent gets a clear error code rather than a click that does nothing.

Measured

• Subject: the release showcase (6 pages, a main and a settings window, a video player), measured on 2026-10-11, median of at least 3 runs.
• macOS: Apple M4, 16 GB, macOS 27.0.1. Memory is the physical footprint from footprint, the measure of Activity Monitor's Memory column.
• Windows: Intel Core i5-13490F, 32 GB, Windows 11 25H2. Memory is the private working set, the measure of Task Manager's Memory column.
• Launch time: from starting the process until the command line sees a visible native window, including engine start and the local control channel.
• The Windows numbers are the WinUI 3 front end (measured with Windows App Runtime 1.8; builds now use 2.x). Without the runtime, the same program falls back to the system XAML: 0.54 s, 19 MB. Neither needs .NET or the VC++ runtime; the app folder holds the executable and a 400 KB bootstrapper DLL.

More

• Video: Video plays on AVFoundation, Windows Media Foundation, GStreamer, Media3 on Android or HTML media, with a command-driven control bar and players that outlive a page.
• Async queries: Query subscriptions with loading, empty, failure and refresh states; only the latest answer is shown.
• Adaptive layouts: layouts for compact, medium, expanded and large, chosen by the space a view actually gets; expressions read the window's size too, for margins that grow with it.
• Typed navigation: sidebars, push navigation with typed routes, tabs, toolbars and menus.
• Feature contracts: declare which features a component must cover in each layout; the compiler proves the coverage.
• Bundled resources: files declared in Bern.toml, checked at build time and located by name at runtime.
• Images and layout: Image shows bundled pictures, HDR ones (PQ and HLG PNGs, JPEGs with a gain map) in HDR where the display can, and an undeclared one fails to compile; Grid adds and drops columns with the width, Stack layers views with nine alignments, Progress shows a bar or indeterminate progress.
• Style overrides and motion: controls keep the platform look by default; for one design everywhere, style! declares app-wide overrides, spring motion, appear and disappear transitions, staggered entrances and shared elements, and an override a target cannot show fails the build (on all five front ends).
• Drawing and placing by fraction: Canvas draws shapes, data curves and check patterns in semantic colors, and Text can carry a highlight mark; Rail puts elements at fractions that come from state, for timelines, ticks on a progress bar or a fill that grows.
• Media app components: a sidebar footer, toolbar search with results, an inspector, toasts, split buttons and popover buttons holding any content, buttons with content that reveal more on hover, shortcuts, picking and dropping files, scroll snapping, pinch and drag to set a value, on all five front ends.
• Reading the files users hand you: for a file the user picked, dropped or opened with the app, the engine gets its name and bytes with MediaSource::file_name() and open(), the same for a local path, an Android content:// document or a page's blob: file.
• Modular apps: an app is a shell and modules, each with its own protocol, engine, interface and translations, switchable at runtime; bern new --module adds one, so several people and AIs can work side by side (verified on macOS and Android).

How it works

flowchart LR
    subgraph app["Your application (Rust)"]
        P["Protocol<br/>types · state · commands"]
        U["UI declarations<br/>component + ui!"]
        E["Engine<br/>plain Rust"]
    end
    P --> C{{"bern build<br/>compiler + IR"}}
    U --> C
    C --> S["SwiftUI<br/>macOS · iOS"]
    C --> W["WinUI 3<br/>Windows"]
    C --> G["GTK 4<br/>Linux"]
    C --> A["Jetpack Compose<br/>Android"]
    C --> B["DOM + WASM<br/>Web"]
    E <--> K[["Command catalog"]]
    K <--> N["Native UI"]
    K <--> L["App CLI"]
    K <--> J["JSON channel"]
    K <--> M["MCP server"]

1. Protocol – #[bern::protocol] declares value types, the read-only state and the command and query traits.
2. Engine – implements those traits in ordinary Rust; it never sees a window, a control or a transport.
3. Interface – #[component] functions whose body is a ui! declaration. The compiler reads them into a shared intermediate representation, and each target module generates native code from it. Adding a target never touches the parser, the IR or your business logic.

A taste of Bern

1 · Protocol: types, state and commands

#[bern::protocol(namespace = "counter", version = 1)]
pub mod counter {
    use bern::prelude::*;

    #[state]
    pub struct State {
        pub count: i64,
    }

    #[commands]
    pub trait Commands {
        /// Add to the count.
        fn add(&mut self, by: i64) -> Result<(), CommandError>;
    }
}

2 · Engine: plain Rust, no UI

use bern::CommandError;

pub struct Counter {
    state: counter::State,
}

impl Default for Counter {
    fn default() -> Self {
        Self { state: counter::State { count: 0 } }
    }
}

impl counter::Commands for Counter {
    fn add(&mut self, by: i64) -> Result<(), CommandError> {
        self.state.count += by;
        Ok(())
    }
}

impl counter::Engine for Counter {
    fn state(&self) -> &counter::State {
        &self.state
    }
}

3 · Interface: a ui! declaration

#[component]
pub fn counter_view(app: counter::Client) -> impl View {
    ui! {
        Form {
            Text(content: tr! {
                "zh-Hans" => "已计数 {n} 次",
                "en" => "Counted {n} times";
                n: app.count
            })
            Button(
                id: add,
                label: tr! { "zh-Hans" => "加一", "en" => "Add one" },
                prominence: prominent
            ) => app.add(1)
        }
    }
}

4 · Everyone drives the same engine

counter ui inspect
counter ui activate --target counter_view.add
counter counter add --by 5
counter-mcp   # the same catalog for AI agents

Bern.toml declares languages, theme, sources, bundled resources and targets. See the full showcase: a sidebar app with text styles, buttons, switches, sliders, text fields, an engine-backed search, list-to-detail navigation, video, and a settings window with tabs and a close confirmation. The planner is a modular app with style overrides, motion and Canvas. The gallery shows the media app components: a poster wall that enters in turn, a timeline placed by fraction, a volume popover, margins and layouts that follow the four sizes, and a dropped video whose name, format and size the engine reads from the file.

Platforms

Implementation notes: macOS · iOS · tvOS · Windows · Linux · Web and WebUI · Android; the next platforms are in the adaptation specs.

Omitting the UI technology picks the platform default; an unavailable default fails loudly instead of silently switching to another stack.

Windows and iOS each have two front ends

These two platforms span more system versions than one UI framework reaches, so the same declaration generates two front ends for each. By default both go into one package and the app chooses as it starts, by the system it finds. Neither the user nor the app's code has to do anything.

Platform	Front end	Name in frontends	Runs on
Windows	WinUI 3	winui3	Systems with Windows App Runtime 2, with Mica and the Windows 11 look
Windows	The system XAML, through XAML Islands	xaml-islands	Every other Windows 10 and 11, with no runtime to install
iOS	SwiftUI	swiftui	iOS 17 and later; every Apple target shares this front end
iOS	SwiftUI for iOS 16	swiftui-16	iOS 16, rewritten from the same generated code

To keep only one, name it:

[targets.windows]
frontends = ["winui3"]   # asks to install the runtime on a system without it

[targets.ios]
frontends = ["swiftui"]  # iOS 17 and later

With both iOS front ends, packages = "separate" makes one .ipa of each. The Windows and iOS implementation notes give the rules of the choice, and what is reduced on iOS 16.

Compared with other cross-platform frameworks

Every framework answers the same question: how does one codebase become the interface on each platform? The answers differ, and so do the trade-offs. What follows is based on each project's official documentation; size and memory vary a lot with versions and settings, so the comparison stays structural. Bern's own numbers are in Measured.

The first two routes give you system controls: look, accessibility, input methods and dark mode come from the system, at the cost of controls that differ a little between platforms. The other two draw the same interface everywhere, at the cost of keeping up with each platform's design and behavior yourself, or of shipping a browser.

The chart is a sketch by architecture, not a ranking: the horizontal axis is whether UI elements are the system's controls or drawn by the framework or a web page; the vertical axis is how much runtime the app carries (a browser engine, a JS engine, a rendering engine, a language runtime).

Technology

Platforms and capabilities

• Mobile: iOS and Android. React Native's desktop versions are maintained by Microsoft; uni-app also covers mini programs and HarmonyOS; Bern's Android target runs on devices and makes packages, its iOS target builds simulator apps, and iOS device packages are planned.
• Native look: the controls are the platform's own, with look, accessibility and input methods from the system. Qt Widgets and uni-app x come close and count as partly.
• Built-in MCP: an MCP server and command line that come with the framework and act on the running interface. Other frameworks can be automated from outside through accessibility APIs or test tools such as Playwright and Flutter Driver.
• Compile-time checks: a missing translation, a feature without an entry in some layout, or an entry not registered to a feature is a compiler error (see Built for AI). Other frameworks mostly find these with linters, translation tools or tests.
• Maturity: Electron and Qt are very mature; Flutter, React Native, uni-app, Tauri and Avalonia are mature; Dioxus is developing quickly; GPUI powers the Zed editor while its general-purpose components are early; Bern is an early preview.