astro-islands

Also covers transition:persist for preserving component state across View Transitions, and prop serialization rules for data crossing the server/client boundary. Does not cover installing the underlying UI framework itself (astro-integrations) or Server Actions used inside islands (astro-actions) — those are separate skills.

Astro Islands Expert

Partial hydration architecture: zero JS by default, selective interactivity via directives.

Agent Workflow (MANDATORY)

Before ANY implementation, spawn 3 agents in parallel, one Agent call each with a name:

1. fuse-ai-pilot:explore-codebase - Analyze existing components and hydration patterns
2. fuse-ai-pilot:research-expert - Verify latest Islands docs via Context7/Exa
3. mcp__context7__query-docs - Get client directive and server:defer examples

After implementation, run fuse-ai-pilot:sniper for validation.

---

Overview

When to Use

• Adding interactive React/Vue/Svelte/Solid components to Astro pages
• Deferring dynamic server content without blocking page load
• Persisting component state during View Transitions
• Optimizing Time to Interactive with lazy hydration

Why Islands Architecture

Concept	Benefit
Zero JS by default	Maximum performance, minimal payload
Selective hydration	Only interactive components ship JS
server:defer	Dynamic server content without SSR blocking
client:visible	Lazy-load below-fold components
transition:persist	State survives page navigation

---

Client Directives

Directive	When JS Loads	Use Case
client:load	Immediately on page load	Critical interactive UI
client:idle	After requestIdleCallback	Non-critical UI
client:visible	When component enters viewport	Below-fold components
client:media="(query)"	When media query matches	Responsive components
client:only="framework"	Client-only, no SSR	Components using browser APIs

Server Islands

server:defer renders the component on the server after the page loads:

• Uses slot="fallback" for placeholder content
• Ideal for personalized or auth-gated content
• Does not block initial page render
• Requires a server adapter

---

Reference Guide

Need	Reference
Architecture overview	overview.md
Client directive details	client-directives.md
server:defer patterns	server-islands.md
transition:persist	transitions.md
View Transitions (complete)	view-transitions.md
Prop serialization rules	prop-serialization.md
Interactive component	templates/interactive-island.md
Server island with fallback	templates/server-island.md

---

Best Practices

1. Default to no directive — Ship zero JS unless interactivity is required
2. Prefer client:visible — Defer below-fold components automatically
3. client:only for browser APIs — localStorage, window, canvas
4. server:defer for personalized content — Avatars, prices, auth state
5. transition:persist — Preserve media players or forms during navigation