Ui component architect

ui-component-architect

Decide the shape of a component tree before the markup is written. Picks composition over inheritance, names the slot contract, draws the controlled/uncontrolled axis, and stops prop bags from growing into god-components. Stack-agnostic — the same lens applies to Blade, Livewire, React, or Vue trees. Pair with existing-ui-audit first; never invent components that already exist.

When to use

• A new component / screen is being designed and the boundary between parent and children is unclear.
• An existing component has > 10 props, conditional rendering trees nested ≥ 3 deep, or a variant-prop with > 4 values.
• A primitive (button, card, dialog) is being added to the design system and its API will be reused across teams.
• German triggers: "wie schneide ich die Komponente?", "Slots oder Props?", "controlled oder uncontrolled?".

Do NOT use when:

• The component is a one-off, used in one place, and unlikely to be reused — over-architecting hurts more than it helps.
• The question is styling, not shape — route to tailwind-engineer.
• The audit step has not run — route to existing-ui-audit first; reusing beats inventing.

Componentization threshold — ≥4 repeats AND real state

Extract a component only when an element repeats ~4× or more AND carries real props/state. One or two repeats, or a repeat with no varying props, is not a component — inline it. A long single-file body is normal, not a smell; premature extraction (a "component" used once, or a pure-markup fragment split for tidiness) adds indirection without reuse and is the more common mistake than under-splitting. Meet BOTH conditions before you extract.

Procedure

1. Inspect prior art, state the responsibility in one sentence

Review existing components in the codebase for the same responsibility — extend rather than rebuild when a match is found. If none exists, write the new component's purpose: "Renders a labelled input with inline error and hint." If the sentence has an "and" joining two unrelated jobs, the component is two components. Reject the draft and split before continuing.

2. Pick composition over inheritance

Rules of thumb:

Pattern	When
Compound components (Card.Header, Card.Body)	Multiple slots with order semantics
Children + named slots	One main child, plus 1–2 optional regions
Render props / function-as-children	Caller controls rendering of internal state
Polymorphic (as prop)	Same shell, different semantic element
Inheritance / class-extension	Almost never — last resort for legacy adapters

Composition trades verbose call-sites for a tiny, stable component. Inheritance trades short call-sites for ABI fragility.

3. Draw the controlled / uncontrolled axis

For every piece of state (open, value, selected, expanded), pick:

• Controlled — caller passes value + onChange. Caller owns state. Use when state must sync across siblings or persist.
• Uncontrolled — component owns state internally; caller reads via ref or onChange callback. Use for ephemeral state local to the component.
• Controlled with default — both APIs supported via defaultValue + optional value. The most flexible, also the most code; reserve for design-system primitives.

Mixing controlled / uncontrolled in the same prop without a default is the single largest source of "why doesn't my component update?" tickets.

4. Cap the prop API

Prop budget per component:

Tier	Cap
Primitive (Button, Input)	≤ 6 props + ...rest to underlying element
Composite (Card, Dialog)	≤ 8 props; prefer slots for variants
Page section / feature shell	≤ 4 props; everything else via context

Over-budget triggers a refactor: extract a config object, push state into context, or split into compound parts.

5. Name the slot contract

For every slot, document: required vs optional, expected element type or component, default rendering when absent, accessibility implications (does the slot become the accessible name?). Slots without contracts become "stuff a div in there and pray".

Output format

Return:

1. Responsibility + composition pick — single-sentence purpose, chosen pattern (compound / slots / render-props / polymorphic) with the one-line trade-off.
2. State + prop API — controlled / uncontrolled / both per state piece, prop list with type and purpose, slot inventory with a11y notes.
3. Anti-case list — the combinations the component refuses to support (the explicit "no" surface that callers can rely on).

Concrete shape:

Component:        <Name>
Responsibility:   <one sentence — reject if "and" joins two jobs>
Pattern:          <compound | slots | render-props | polymorphic>
State:            value=<controlled|uncontrolled|both>; open=<...>; ...
Props (≤ tier):   [name: type — purpose]
Slots:            [name: required? default? a11y note]
Children:         <count, kind>
Anti-cases:       <combinations the component refuses to support>

Gotcha

• variant props with > 4 values are usually two components in a trench coat — split when the rendering branches diverge.
• as polymorphism is cheap in TypeScript when typed via generics, expensive without — the type cost is invisible in plain JS.
• Compound components share state via context; nesting two compound trees of the same family in one parent silently crosses contexts — namespace the context per instance or refuse the nesting.
• "Render props" + memoization fight; if the function changes every render, the child re-renders too. Stabilize via useCallback or hoist.

Cross-task design memory

Before proposing a component, check the project root for DESIGN.md (written by design-system-capture). If present, apply its captured visual conventions (radius/shadow/motion/spacing) to the new component rather than deriving them from scratch. Flag any new design decision (e.g., a first-use of a new elevation tier) for capture: "Suggest adding to DESIGN.md: [decision]."

Do NOT

• Do NOT design a component without running existing-ui-audit first. Reinventing primitives is the #1 source of design drift.
• Do NOT use inheritance when composition fits — class-extension hierarchies in UI age badly.
• Do NOT ship a "kitchen-sink" prop (config={...}) just to dodge the prop cap — that hides the API instead of taming it.
• Do NOT push the architecture into the tracker as code AC — output is a design note for refinement, not implementation steps.