fossui

The Fresh, Minimal Flutter UI kit.

Themed from one source. Inspired by coss.com/ui, Cal.com's design system.

fossui is a set of 30+ components for developers tired of every Flutter app
looking like Material. It drops into any app (MaterialApp, CupertinoApp, or a
bare WidgetsApp) and reads its own theme first, not replacing yours. The look
comes from coss.com/ui, Cal.com's design system: clean,
neutral, superellipse corners. One import, one theme, light and dark out of the
box.

[!IMPORTANT]
Unofficial and independent. Not affiliated with or endorsed by Cal.com, Inc.
or coss.com. See NOTICE for attribution.

Why fossui

• A look that isn't Material. A neutral, understated aesthetic with
  superellipse corners, drawn from the coss/Cal.com design language.
• Framework-agnostic. Built on package:flutter/widgets.dart, with no
  platform channels and no FossApp wrapper. The widgets work under any app
  shell.
• Reads your theme, not the other way around. Components resolve
  context.fossTheme before falling back to Material, so they keep their look
  inside an existing app.
• Themed from one source. A single FossThemeData holds every semantic
  token: color, type, radius, spacing, shadow, motion. Reskin the whole app,
  light and dark, in one call.
• Light on dependencies. One runtime dependency and no bundled icon package.
  A worst-case app that imports nearly every component adds about 384 KB: roughly
  310 KB of Dart code, which tree-shakes down to what you actually use, plus the
  Geist font (74 KB installed, ~35 KB over the wire). Pass your own icons through
  plain Widget slots.
• Preview-rich docs. Every component's API doc renders a live light and dark
  preview, not just text, and the same preview shows on hover in your IDE. Each
  one states plainly what it does and does not do.
• Accessible by default. Semantics, focus, and touch targets are built into
  each component.

Install

dependencies:
  fossui: ^0.1.1

Or from the command line:

flutter pub add fossui

Browse the package on pub.dev/packages/fossui.

Quick start

Register the theme once, then use the widgets anywhere.

import 'package:flutter/material.dart';
import 'package:fossui/fossui.dart';

void main() => runApp(
      MaterialApp(
        theme: FossThemeData.light.toThemeData(),
        darkTheme: FossThemeData.dark.toThemeData(),
        home: Scaffold(
          body: Center(
            child: FossButton(
              onPressed: () {},
              child: const Text('Get started'),
            ),
          ),
        ),
      ),
    );

No wrapper lock-in. FossTheme drops into any app, and context.fossTheme
resolves identically under MaterialApp, CupertinoApp, or a bare
WidgetsApp.

See example/ for a runnable app.

Theming

The defaults give fossui its look, but nothing is locked. Read tokens through
one accessor:

final theme = context.fossTheme;
final color = theme.colors.primary;
final radius = theme.radii.md;

To reskin the app, layer a FossThemeSpec over a base theme. Every field is
optional, and anything you leave unset keeps the default.

MaterialApp(
  theme: FossThemeData.light.retheme(
    const FossThemeSpec(primary: Color(0xFF16A34A), radius: 22),
  ).toThemeData(),
  darkTheme: FossThemeData.dark.retheme(
    const FossThemeSpec(primary: Color(0xFF51F0A8), radius: 22),
  ).toThemeData(),
);

Components

The library covers input, feedback, overlays, and layout:

See the components roadmap
for what is shipped and what is planned, and the
component checklist
for the bar each one clears.

Icons

Icon slots accept a plain Widget, so any icon set works: Lucide, Material
Icons, Cupertino, SVGs, or your own. The package pulls in no icon dependency of
its own. Examples and docs use Lucide
as the documented companion.

Platforms

With no platform channels, fossui runs anywhere Flutter does: Android, iOS, web,
macOS, Windows, and Linux are all supported.

AI-native

fossui runs an MCP server so your AI coding assistant writes fossui code against
the real component API instead of guessing prop names and enum values. It works
with any MCP client (Claude Code, Cursor, VS Code, and more) over one endpoint:

https://mcp.fossui.org

Per-client setup and the full tool list are in the
AI-native docs.

Blog

• Introducing fossui: a minimal Flutter UI kit
• Migrating from Material to fossui
• Minimal Flutter UI kits compared: fossui vs Material (2026)

More at fossui.org/blogs.

Ecosystem

• Documentation: fossui.org
• Blog: fossui.org/blogs
• Live gallery: play.fossui.org
• AI-native (MCP): fossui.org/docs/ai-native
• Package: pub.dev/packages/fossui

Development

This package pins its Flutter SDK with fvm:

fvm install          # uses .fvmrc (Flutter 3.41.9)
fvm flutter pub get
fvm flutter test

The test suite runs on every change and holds full line coverage; browse the
live report at coverage.fossui.org.

Contributions are welcome. Start with CONTRIBUTING.md and the
Code of Conduct.

Acknowledgements

The look and component API are drawn from coss.com/ui, the
Cal.com design system, with ideas from Base UI
and shadcn/ui. Full attribution is in
NOTICE.

Support

If fossui is useful to you, two clicks go a long way:

• ⭐ Star fossui on GitHub
• 👍 Like fossui on pub.dev
• ✉️ Questions or feedback: [email protected]

Star History