UI Component Router

UI Component Router

One decision, made once: which component system serves this need. Never hand-roll a lookalike of a component a system already ships, and never mix component systems in one project.

The sources

Decision order

Stop at the first match:

0. Did the user explicitly choose a source — an instruction or stated preference ("use antd for this dialog", "add the shadcn table", "pull it from 21st.dev")? → that source. Explicit user choice beats every generic rule below — a chosen system is never re-routed to a "better" one. Naming candidates is not choosing: a question that merely mentions sources ("should we use shadcn or antd here?") selects nothing — it runs through the generic order below and the router answers with its pick and the step that chose it. When the chosen source differs from the project's existing component system, the ask itself is the explicit say-so the one-system rule requires, but announce the mix (name both systems and the consequence) before adding. Honoring is bounded by feasibility: when the chosen source's output cannot run in the project's stack (a React-emitting source in a Vue/Svelte project; shadcn without Tailwind), do not add unusable code and do not silently reroute either — report the incompatibility and hand the decision back. Say-so licenses mixing systems; it cannot make incompatible code run.

1. Does the project already have a component system? Existing components/ui/ with shadcn conventions → shadcn. antd in package.json → Ant Design. Another system (MUI, Chakra, Mantine, project-internal design system) → use that system; this router adds nothing and must not introduce a second one. The existing stack wins over everything below — a "better" component from another system is never a reason to mix. When that system does not cover the need (a specialized component or section it simply doesn't ship), the route is a declared custom build following that system's conventions and primitives — the coverage gap changes the build, never the system; steps 2–3 stay closed to a project that already has one. Steps 2–3 are stack-gated: every source below emits React code (shadcn additionally assumes Tailwind), so they apply only where that code can run. A non-React stack (Vue, Svelte, …) with no component system of its own skips them entirely and lands on step 4 — an incompatible source is never a match, however well the need's shape fits.

2. Anything the shadcn registry ships, in a React/Tailwind project — standard primitives (button, dialog, table, form, tabs, dropdown, toast…) and registry blocks (login/signup, sidebars, dashboard shells, calendars…)? → shadcn. For any need that isn't an obvious primitive — a section, a pattern, a "something like X" search — this step includes an operative check, not an assumption: look it up in the registry first — the shadcn-ui MCP listing when it answers, otherwise the public registry index — and only a miss falls through to step 3. The lookup is free; step 3 is not. The add itself goes through npx shadcn@latest add <name> — the free path that works in any session.

3. A need in a React project that the registry lookup in step 2 missed — a full section or pattern (pricing section, hero, landing block…) or a community search ("something like X")? → magic-mcp. magic-mcp is metered — spend only for what the free paths can't produce; the step-2 lookup is a prerequisite for every branch of this step, and reaching it without that lookup is a routing error.

4. Nothing fits (non-React stack with no system, unique bespoke need)? → declared custom build: say explicitly that no source covers this, then build following the project's own conventions. A custom build is a stated decision, never a silent default.

(Ant Design has no generic step of its own: an antd project is caught by step 1, an explicit antd ask by step 0.)

Availability rule (inherited from browser-router)

No tool named here is assumed present. Confirm the chosen tool actually answers in this session before routing to it — an MCP can be unloaded, a key revoked, credits exhausted. A tool that doesn't answer is not a route.

Fallbacks apply only to routes the router chose (steps 1+). When the unavailable source was the user's explicit choice (step 0), there is no silent substitute at any layer: report the inability and get their say-so before rerouting — substituting changes the provenance the user asked for, and can smuggle in a system they never approved. Otherwise, fall back by layer:

• shadcn-ui MCP missing → the CLI path (npx shadcn@latest add) is the same registry and needs only npx; for source inspection, the registry is public — fetch the component page/source directly.
• magic-mcp missing or out of credits → in a React/Tailwind project, compose the section from shadcn primitives (free); a React project without Tailwind gets the declared custom build (step 4) instead — the stack gate binds fallbacks too, shadcn assumes Tailwind. Never substitute a different metered service without saying so.
• npx/network unavailable → say so. Deliver the component as a patch only when the registry source is actually readable — a cached copy, a vendored registry, an MCP that still answers. With no reachable source there is nothing to copy from: report the blockage rather than reconstructing the component from memory — that fabricates provenance and is the hand-rolled lookalike this router exists to prevent.

Design constraints (conditional design-dna link)

If a client DESIGN.md exists for the project (see the design-dna skill, when loaded — soft reference, no dependency), its Color/Typography/ Components sections override any source's default theme: shadcn's default palette, magic-mcp's generated styling, and antd's theme tokens are starting points to be themed, not the final look. Without a DESIGN.md, the source's defaults stand — but never invent a brand for a real client (that is design-dna's Extract job, not this router's).

Rules

• One system per project. The stack decision (step 1) is made once and respected forever after; adding a second component system needs the user's explicit say-so — an explicit ask for a named source (step 0) counts, the router's own initiative never does, and the mix is announced either way.
• Free before metered. shadcn CLI/registry and antd are free; magic-mcp spends credits — reach it only when the free paths don't cover the need.
• Route, don't build. This skill picks the source; the actual add / generate / import runs through that source's own workflow. Hand-rolling a copy of a registry component is a routing failure.
• Announce the pick. State the chosen source and the step that chose it before adding anything.

Out of scope

Installing or configuring the tools (each has its own docs), choosing a framework, page/brand design (see design-dna for brand enforcement), and non-component UI work (layout, routing, animation).