Rails UI is the product UI system for Rails SaaS, built for the AI era
Andy Leverenz
For years, Rails UI was described in terms of themes and components. That was accurate, and it also undersold it. What people actually used it for was shipping real products faster. So we made the positioning match reality: Rails UI is the product UI system for Rails SaaS builders, and we renamed themes to app kits to make that clear.
Here is what changed, why it changed, and how to get the most out of it.
The afternoon app problem
AI changed the economics of building a Rails app. You can describe a feature and get working code in minutes. Scaffolds, models, controllers, queries, tests: the mechanical parts are mostly solved, and they keep getting better. That is genuinely great, and you should lean on it.
But there is a gap between an app that runs and a product you can charge for. AI generates one screen at a time from a single prompt, with no memory of the previous screen and no sense of the whole. So you ship fast and then notice the drift:
- Spacing and components that do not match from page to page
- Empty, loading, error, and permission states that never got built
- A dashboard that looks nothing like the onboarding flow
- The unglamorous screens (auth, settings, billing, team management, transactional email) are half-done or missing
None of that is a model failing. It is the difference between typing code and exercising product judgment. The hard part was never the code. It is the thousand small decisions that make a hundred screens feel like one team built them.
What an app kit actually is
An app kit is not a skin or a color theme. It is a complete product surface for a kind of SaaS, built as one coherent system:
- Real, designed screens: dashboards, list and detail views, settings, onboarding, and billing
- Auth views and transactional email, styled to match
- The empty, loading, and error states that make a product feel finished
- A shared design language so screen twenty matches screen one
There are eight, each tuned to a use case: Hound (CRM and projects), Shepherd (property), Corgie (AI apps), Husky (finance), Boxer (agency and client portals), Collie (community and courses), Retriever (PaaS and servers), and Setter (hiring). You pick the one closest to what you are building, keep the screens that fit, and delete or restyle the rest. It is a starting point, as no app is truly finished.
Within every kit is the same Rails-native component library (forms, navigation, dialogs, tables, and more) and a form builder, so anything you add stays consistent with what shipped.
Rails UI and your AI
This is where the AI era makes Rails UI more useful, not less. Rails UI is plain ERB and Tailwind that lives in your repo, which is exactly what coding assistants read best.
The workflow is like so:
- Install the app kit closest to your product.
- Point Codex, Cursor, Claude, or your favorite LLM at your Rails UI files, or at our llms.txt.
- Ask for new screens in the style of an existing view, and the output inherits the same components, spacing, and states.
- Review against the kit for consistency.
A couple of prompts that work well once a kit is installed:
Add a settings page using the existing Rails UI components and Railsui::FormBuilder. Match the layout and spacing of an existing Rails UI screen.
Build empty, loading, and error states for this index view using the Rails UI card and button components already in this project.
Use AI for the logic. Use Rails UI for the product layer. The model moves fast, and the system keeps it coherent.
Why Rails-native still matters
Generic component libraries and framework-agnostic HTML make you pay a conversion tax: translate to ERB, wire up your own form helpers and JavaScript, then build auth and email yourself. Rails UI skips that. It is ERB partials, a form builder that understands your models, Stimulus controllers that work with Turbo out of the box, Devise views, and mailers. It is your code the moment it is installed, with no proprietary DSL and no lock-in.
What is new
- App kits naming across the product and docs
- A dedicated page on building with AI: https://railsui.com/ai
- A Start here guide that takes you from install to a polished screen fast: https://railsui.com/docs/start_here
- AI-friendly by default: llms.txt and plain ERB and Tailwind, your assistant can read
The takeaway
AI writes your Rails code. Rails UI makes it a product. Pick the app kit closest to what you are building and ship something that looks like a team made it, in a fraction of the time.
Explore the free app kits at https://railsui.com/kits and get full access at https://railsui.com/pricing.