Svelte 5 Development Skill

When to Activate

Activate this skill when:

• Creating Svelte 5 components
• Working with SvelteKit routing
• Implementing runes ($state, $derived, $effect)
• Building forms with actions
• Setting up SvelteKit projects

Quick Commands

npx sv create my-app      # Create SvelteKit project
cd my-app && pnpm install
pnpm dev               # Start dev server (localhost:5173)
pnpm build             # Build for production

Runes Quick Reference

Rune	Purpose	Example
$state	Reactive state	let count = $state(0)
$derived	Computed values	let doubled = $derived(count * 2)
$effect	Side effects	$effect(() => console.log(count))
$props	Component props	let { name } = $props()
$bindable	Two-way binding	let { value = $bindable() } = $props()

Reactive State ($state)

<script>
	let count = $state(0);
	let user = $state({ name: "Alice", age: 30 });
	let items = $state(["apple", "banana"]);
</script>

<button onclick={() => count++}>
	Clicked {count} times
</button>

<button onclick={() => user.age++}>
	{user.name} is {user.age}
</button>

Deep reactivity: Objects and arrays update automatically on mutation.

Computed Values ($derived)

<script>
	let count = $state(0);
	let doubled = $derived(count * 2);

	// For complex computations
	let summary = $derived.by(() => {
		const total = items.length;
		const done = items.filter((i) => i.done).length;
		return `${done}/${total}`;
	});
</script>

Side Effects ($effect)

<script>
	let count = $state(0);

	$effect(() => {
		console.log(`Count is ${count}`);
		document.title = `Count: ${count}`;

		// Cleanup function
		return () => {
			console.log("Cleanup");
		};
	});
</script>

Component Props ($props)

<!-- Button.svelte -->
<script>
	let { label, disabled = false, onclick } = $props();
</script>

<button {disabled} {onclick}>{label}</button>

TypeScript Props

<script lang="ts">
	interface Props {
		label: string;
		disabled?: boolean;
		onclick?: () => void;
	}

	let { label, disabled = false, onclick }: Props = $props();
</script>

SvelteKit File Conventions

File	Purpose
+page.svelte	Page component
+page.server.js	Server-only load/actions
+layout.svelte	Shared layout
+server.js	API endpoints
+error.svelte	Error boundary

Data Loading

// src/routes/posts/+page.server.js
export async function load({ fetch }) {
	const response = await fetch("/api/posts");
	return { posts: await response.json() };
}

<!-- src/routes/posts/+page.svelte -->
<script>
	let { data } = $props();
</script>

{#each data.posts as post}
	<article>
		<h2>{post.title}</h2>
	</article>
{/each}

Form Actions

Type-Safe Form Validation (Rootwork Pattern)

Use parseFormData() for type-safe form handling at trust boundaries:

// src/routes/login/+page.server.ts
import { fail, redirect } from "@sveltejs/kit";
import { parseFormData } from "@autumnsgrove/lattice/server";
import { z } from "zod";

const LoginSchema = z.object({
	email: z.string().email("Valid email required"),
	password: z.string().min(8, "Password must be at least 8 characters"),
});

export const actions = {
	default: async ({ request, cookies }) => {
		const formData = await request.formData();
		const result = parseFormData(formData, LoginSchema);

		if (!result.success) {
			const firstError = Object.values(result.errors).flat()[0];
			return fail(400, { error: firstError || "Invalid input" });
		}

		const { email, password } = result.data;

		// Authenticate with validated, typed data
		const token = await authenticate(email, password);
		if (!token) {
			return fail(401, { error: "Invalid credentials" });
		}

		cookies.set("session", token, { path: "/" });
		throw redirect(303, "/dashboard");
	},
};

Benefits:

• Type-safe: result.data is fully typed as LoginSchema
• Structured errors: result.errors is a map of field → messages
• No as casts at trust boundaries

<!-- +page.svelte -->
<script>
	import { enhance } from "$app/forms";
	let { form } = $props();
</script>

<form method="POST" use:enhance>
	<input name="email" value={form?.email ?? ""} />
	{#if form?.missing}
		<p class="error">Email required</p>
	{/if}
	<button>Submit</button>
</form>

API Routes

// src/routes/api/posts/+server.js
import { json } from "@sveltejs/kit";

export async function GET({ url }) {
	const posts = await getPosts();
	return json(posts);
}

export async function POST({ request }) {
	const data = await request.json();
	const post = await createPost(data);
	return json(post, { status: 201 });
}

Common Pitfalls

Destructuring Breaks Reactivity

// ❌ Bad
let { count } = $state({ count: 0 });

// ✅ Good
let state = $state({ count: 0 });
state.count++;

Missing Keys in Each

<!-- ❌ Bad -->
{#each items as item}

<!-- ✅ Good -->
{#each items as item (item.id)}

Use Progressive Enhancement

<script>
  import { enhance } from '$app/forms';
</script>

<form method="POST" use:enhance>

Error Feedback (Signpost + Toast)

Toast is the primary client-side feedback for user actions:

import { toast } from "@autumnsgrove/lattice/ui";

// After successful action
toast.success("Post published!");

// After failed action
toast.error(err instanceof Error ? err.message : "Something went wrong");

// Async operations
toast.promise(apiRequest("/api/export", { method: "POST" }), {
	loading: "Exporting...",
	success: "Done!",
	error: "Export failed.",
});

// Multi-step flows
const id = toast.loading("Saving...");
// ... later
toast.dismiss(id);
toast.success("Saved!");

When NOT to use toast: form validation (use fail() + inline errors), page load failures (+error.svelte), persistent notices (use GroveMessages).

Server-side errors use Signpost codes:

import {
	API_ERRORS,
	buildErrorJson,
	throwGroveError,
	logGroveError,
} from "@autumnsgrove/lattice/errors";

// API route: return structured error
return json(buildErrorJson(API_ERRORS.RESOURCE_NOT_FOUND), { status: 404 });

// Page load: throw to +error.svelte
throwGroveError(404, SITE_ERRORS.PAGE_NOT_FOUND, "Engine");

Type-Safe Error Handling (Rootwork Guards)

Use isRedirect() and isHttpError() to safely handle caught exceptions in form actions:

import { isRedirect, isHttpError } from "@autumnsgrove/lattice/server";
import { fail } from "@sveltejs/kit";

export const actions = {
	default: async ({ request }) => {
		try {
			const formData = await request.formData();
			const result = parseFormData(formData, MySchema);

			if (!result.success) {
				return fail(400, { error: "Validation failed" });
			}

			// Process data
			const response = await processData(result.data);
			throw redirect(303, "/success");
		} catch (err) {
			// Redirects and HTTP errors must be re-thrown
			if (isRedirect(err)) throw err;
			if (isHttpError(err)) return fail(err.status, { error: err.body.message });

			// Unexpected errors
			console.error("Unexpected error:", err);
			return fail(500, { error: "Something went wrong" });
		}
	},
};

See AgentUsage/error_handling.md and AgentUsage/rootwork_type_safety.md for the full reference.

Project Structure

src/
├── lib/
│   ├── components/
│   └── server/
├── routes/
│   ├── +layout.svelte
│   ├── +page.svelte
│   └── api/
├── app.html
└── hooks.server.js

Grove-Specific: GroveTerm Components

When building Grove UI that includes nature-themed terminology, always use the GroveTerm component suite instead of hardcoding terms. This respects users' Grove Mode setting (standard terms by default, opt-in to Grove vocabulary).

<script lang="ts">
	import { GroveTerm, GroveSwap, GroveText } from "@autumnsgrove/lattice/ui";
	import groveTermManifest from "$lib/data/grove-term-manifest.json";
</script>

<!-- Interactive term with popup definition -->
<GroveTerm term="bloom" manifest={groveTermManifest} />

<!-- Silent swap (no popup, no underline) -->
<GroveSwap term="arbor" manifest={groveTermManifest} />

<!-- Parse [[term]] syntax in data strings -->
<GroveText
	content="Your [[bloom|posts]] live in your [[garden|blog]]."
	manifest={groveTermManifest}
/>

See libs/engine/src/lib/ui/components/ui/groveterm/ for component source and chameleon-adapt skill for full UI design guidance.

Related Resources

See AgentUsage/svelte5_guide.md for complete documentation including:

• Advanced rune patterns
• Snippets for reusable markup
• Performance optimization
• TypeScript integration