---
name: composition-patterns
description: Use when writing, reviewing, or refactoring React component APIs that risk boolean prop proliferation, renderHeader/renderFooter-style slots, prop drilling, trapped local state, context-provider architecture, compound components, explicit variants, or React 19 ref/context composition patterns.
---

# Composition Patterns

## Overview

Apply scalable React composition patterns from Vercel's `composition-patterns`
skill, adapted for this Next.js/FSD repository. Prefer explicit composed APIs
over configuration-heavy components, and keep reusable UI parts decoupled from
state implementations.

## Workflow

1. Identify whether the component is becoming a mode switcher.
   Look for multiple boolean props, conditional branches that choose whole UI
   regions, `renderHeader`/`renderFooter` props, or impossible prop
   combinations.
2. Split behavior into explicit variants when the rendered structure differs.
   Prefer names such as `ThreadComposer`, `EditComposer`, or
   `ForwardComposer` over one `Composer` with `isThread`, `isEditing`, and
   `isForwarding`.
3. Extract shared pieces into compound components when consumers need to
   arrange the parts themselves. Use `Root`/`Provider`, `Frame`, `Header`,
   `Body`, `Footer`, `Trigger`, `Content`, `Action`, or domain-specific names
   that fit the existing component.
4. Lift shared state into a provider boundary when siblings or custom outer UI
   need the same state/actions. Keep visual nesting separate from state access:
   components only need to be inside the provider, not inside the same DOM box.
5. Define a context contract with `state`, `actions`, and `meta` when multiple
   providers can drive the same UI. UI subcomponents consume the contract, while
   providers decide whether state comes from local hooks, server-synced data,
   forms, or feature-specific stores.
6. In this React 19 repo, pass `ref` as a normal prop and use `use(Context)` for
   context reads where the surrounding codebase allows it. Preserve existing
   conventions if a nearby component has not migrated yet.

## Decision Rules

- Do not add a boolean prop to control a large behavior branch until checking
  whether an explicit variant or child composition would make the state space
  clearer.
- Use children for static structure composition. Keep render props for cases
  where the parent must pass item data, measurements, or callback-local state
  back into the child.
- Keep providers as the only layer that knows a concrete state implementation.
  Subcomponents should not import feature stores or synchronization hooks unless
  they are provider components.
- Avoid prop drilling through compound components. Put shared state/actions in a
  typed context and expose narrow subcomponents.
- Keep FSD boundaries intact: shared compound UI belongs under `src/shared/ui`;
  feature-specific variants and providers belong in the appropriate feature,
  entity, widget, or page layer.

## Reference

Read `references/rules.md` when you need examples, a review checklist, or the
upstream rule inventory. It summarizes all files from:
`https://github.com/vercel-labs/agent-skills/tree/main/skills/composition-patterns`
at commit `ce3e64e468f8fa09a2d075d102771838061fdac0`.
