slint

Slint Development Skill

Use this skill when building, debugging, or reviewing applications that use Slint, a declarative GUI toolkit for native user interfaces across desktop, embedded, mobile, and web platforms.

When to Use This Skill

Use this skill when the task involves:

• Writing or debugging .slint files
• Integrating Slint with Rust, C++, JavaScript, or Python
• Investigating layout, binding, rendering, or event-handling issues
• Enabling the Slint MCP server for runtime inspection and UI debugging
• Explaining or reviewing Slint-specific code patterns

How to Help

When using this skill:

• Prefer idiomatic Slint patterns over manual UI workarounds
• Match guidance to the user's language binding and Slint version
• Watch for common pitfalls such as binding loops, missing layout constraints, and type mismatches
• Suggest the MCP server when runtime inspection or interaction would make debugging easier
• Prefer solutions that preserve Slint's declarative and reactive model

The .slint Language

Slint UIs are written in .slint markup files. The language is declarative and reactive.

Project Setup

Rust

# Cargo.toml
[dependencies]
slint = "1.x"

[build-dependencies]
slint-build = "1.x"

// build.rs
fn main() {
    slint_build::compile("ui/main.slint").unwrap();
}

// main.rs
slint::include_modules!();

fn main() -> Result<(), slint::PlatformError> {
    let app = MainWindow::new()?;
    // Set up callbacks, models, etc.
    app.run()
}

C++

Use CMake with FetchContent or find_package:

find_package(Slint)
slint_target_sources(my_app ui/main.slint)

Node.js

const slint = require("slint-ui");
const app = new slint.MainWindow();
app.run();

Python

import slint
# Load .slint files dynamically

Debugging Slint Applications

Common Issues

1. Binding loops: A property depends on itself through a chain of bindings. The compiler warns about these. Break the cycle by introducing an intermediate property or restructuring.

2. Elements not visible: Check width, height (may be 0 if not in a layout), visible, opacity, and parent clipping.

3. Layout sizing: Elements outside layouts need explicit width/height. Inside layouts, they get sized automatically. Use preferred-width, min-width, max-width to constrain.

4. Type mismatches: length and int/float are different types. Use 1px * my_int to convert, or my_length / 1px to get a number.

5. Performance: Use ListView (not for in ScrollView) for long lists because it virtualizes. Use image-rendering: pixelated only when needed. Avoid deeply nested opacity or clip layers.

Debug Helpers

• debug("message", expression) prints to stderr at runtime
• SLINT_DEBUG_PERFORMANCE=refresh_lazy,console enables performance diagnostics
• Run with SLINT_BACKEND=winit-skia or other backend variants for testing

MCP Server for AI-Assisted Debugging

Slint includes an embedded MCP (Model Context Protocol) server that lets you inspect and interact with a running Slint application in real time. The server provides tools for exploring the UI tree, taking screenshots, clicking elements, dragging, typing, and more.

Once enabled, an AI coding assistant can connect to the MCP endpoint to inspect and interact with the running UI.

Enabling the MCP Server

Important: The MCP server is exposed through the internal crate i-slint-backend-selector, not the public slint crate. This internal crate does not follow semver and must be pinned to the exact Slint version using =. If the project uses slint = "1.16.0", then the backend selector must use version = "=1.16.0". A version mismatch will cause build failures.

Step 1: Add the i-slint-backend-selector crate with the mcp feature to the project's Cargo.toml, pinned to the exact same version as the slint crate:

[dependencies]
slint = "1.16.0"
i-slint-backend-selector = { version = "=1.16.0", features = ["mcp"] }

If the project is part of a workspace that depends on Slint from a path (e.g. working within the Slint repo itself), use the workspace reference instead and enable the feature via --features i-slint-backend-selector/mcp on the cargo command line.

Step 2: Build with SLINT_EMIT_DEBUG_INFO=1 so that element IDs and source locations are preserved in the compiled output. Without this, elements will lack the debug metadata needed for meaningful introspection. Set SLINT_MCP_PORT to an available port when running:

SLINT_EMIT_DEBUG_INFO=1 SLINT_MCP_PORT=9315 cargo run -p my-app

Step 3: Connect to the running application's MCP server at http://localhost:9315/mcp using Streamable HTTP transport and use the available tools to inspect and interact with the UI.

Version Requirements

The MCP server uses internal Slint APIs (i-slint-backend-selector), so the available features depend on the Slint version:

Slint Version	MCP Support
< 1.16.0	Not available
>= 1.16.0	Full MCP server with i-slint-backend-selector mcp feature

The i-slint-backend-selector crate does not follow semver. It must be pinned to the exact Slint version with =. MCP features and tools may change between Slint releases without notice.

When to Suggest MCP

Suggest enabling the MCP server when the user is:

• Debugging layout or visual issues
• Trying to understand the runtime element hierarchy
• Testing interactions programmatically
• Verifying accessibility properties
• Diagnosing event handling problems

Documentation Reference

Full documentation for the latest version is at https://slint.dev/docs. Key sections:

• Language guide: concepts, syntax, and coding patterns
• Reference: elements, properties, types, and standard widgets
• Language integrations: Rust, C++, Node.js, and Python API docs
• Tutorials: step-by-step guides for each language

For a specific Slint version, the documentation can be found at https://releases.slint.dev/<version>/docs, for example https://releases.slint.dev/1.15.1/docs.