4 files changed, 190 insertions, 0 deletions
diff --git a/packages/integrations/svelte/src/client.svelte.ts b/packages/integrations/svelte/src/client.svelte.ts
new file mode 100644
index 000000000..102f99490
--- /dev/null
+++ b/packages/integrations/svelte/src/client.svelte.ts
@@ -0,0 +1,86 @@
+import { createRawSnippet, hydrate, mount, unmount } from 'svelte';
+
+const existingApplications = new WeakMap<HTMLElement, ReturnType<typeof createComponent>>();
+
+export default (element: HTMLElement) => {
+	return async (
+		Component: any,
+		props: Record<string, any>,
+		slotted: Record<string, any>,
+		{ client }: Record<string, string>,
+	) => {
+		if (!element.hasAttribute('ssr')) return;
+
+		let children = undefined;
+		let _$$slots: Record<string, any> | undefined = undefined;
+		let renderFns: Record<string, any> = {};
+
+		for (const [key, value] of Object.entries(slotted)) {
+			// Legacy slot support
+			_$$slots ??= {};
+			if (key === 'default') {
+				_$$slots.default = true;
+				children = createRawSnippet(() => ({
+					render: () => `<astro-slot>${value}</astro-slot>`,
+				}));
+			} else {
+				_$$slots[key] = createRawSnippet(() => ({
+					render: () => `<astro-slot name="${key}">${value}</astro-slot>`,
+				}));
+			}
+			// @render support for Svelte ^5.0
+			if (key === 'default') {
+				renderFns.children = createRawSnippet(() => ({
+					render: () => `<astro-slot>${value}</astro-slot>`,
+				}));
+			} else {
+				renderFns[key] = createRawSnippet(() => ({
+					render: () => `<astro-slot name="${key}">${value}</astro-slot>`,
+				}));
+			}
+		}
+
+		const resolvedProps = {
+			...props,
+			children,
+			$$slots: _$$slots,
+			...renderFns,
+		};
+		if (existingApplications.has(element)) {
+			existingApplications.get(element)!.setProps(resolvedProps);
+		} else {
+			const component = createComponent(Component, element, resolvedProps, client !== 'only');
+			existingApplications.set(element, component);
+			element.addEventListener('astro:unmount', () => component.destroy(), { once: true });
+		}
+	};
+};
+
+
+function createComponent(
+	Component: any,
+	target: HTMLElement,
+	props: Record<string, any>,
+	shouldHydrate: boolean,
+) {
+	let propsState = $state(props);
+	const bootstrap = shouldHydrate ? hydrate : mount;
+	if (!shouldHydrate) {
+		target.innerHTML = '';
+	}
+	const component = bootstrap(Component, { target, props: propsState });
+	return {
+		setProps(newProps: Record<string, any>) {
+			Object.assign(propsState, newProps);
+			// Remove props in `propsState` but not in `newProps`
+			for (const key in propsState) {
+				if (!(key in newProps)) {
+					delete propsState[key];
+				}
+			}
+		},
+		destroy() {
+			unmount(component);
+		},
+	};
+}
diff --git a/packages/integrations/svelte/src/context.ts b/packages/integrations/svelte/src/context.ts
new file mode 100644
index 000000000..833755044
--- /dev/null
+++ b/packages/integrations/svelte/src/context.ts
@@ -0,0 +1,26 @@
+import type { SSRResult } from 'astro';
+
+const contexts = new WeakMap<SSRResult, { currentIndex: number; readonly id: string }>();
+
+const ID_PREFIX = 's';
+
+function getContext(rendererContextResult: SSRResult) {
+	if (contexts.has(rendererContextResult)) {
+		return contexts.get(rendererContextResult);
+	}
+	const ctx = {
+		currentIndex: 0,
+		get id() {
+			return ID_PREFIX + this.currentIndex.toString();
+		},
+	};
+	contexts.set(rendererContextResult, ctx);
+	return ctx;
+}
+
+export function incrementId(rendererContextResult: SSRResult) {
+	const ctx = getContext(rendererContextResult)!;
+	const id = ctx.id;
+	ctx.currentIndex++;
+	return id;
+}
diff --git a/packages/integrations/svelte/src/server.ts b/packages/integrations/svelte/src/server.ts
new file mode 100644
index 000000000..4b0fccb3d
--- /dev/null
+++ b/packages/integrations/svelte/src/server.ts
@@ -0,0 +1,74 @@
+import type { AstroComponentMetadata } from 'astro';
+import { createRawSnippet } from 'svelte';
+import { render } from 'svelte/server';
+import { incrementId } from './context.js';
+import type { RendererContext } from './types.js';
+
+function check(Component: any) {
+	if (typeof Component !== 'function') return false;
+	// Svelte 5 generated components always accept a `$$payload` prop.
+	// This assumes that the SSR build does not minify it (which Astro enforces by default).
+	// This isn't the best check, but the only other option otherwise is to try to render the
+	// component, which is taxing. We'll leave it as a last resort for the future for now.
+	return Component.toString().includes('$$payload');
+}
+
+function needsHydration(metadata: AstroComponentMetadata) {
+	// Adjust how this is hydrated only when the version of Astro supports `astroStaticSlot`
+	return metadata.astroStaticSlot ? !!metadata.hydrate : true;
+}
+
+async function renderToStaticMarkup(
+	this: RendererContext,
+	Component: any,
+	props: Record<string, any>,
+	slotted: Record<string, any>,
+	metadata: AstroComponentMetadata,
+) {
+	const tagName = needsHydration(metadata) ? 'astro-slot' : 'astro-static-slot';
+
+	let children = undefined;
+	let $$slots: Record<string, any> | undefined = undefined;
+	let idPrefix;
+	if (this && this.result) {
+		idPrefix = incrementId(this.result);
+	}
+	const renderProps: Record<string, any> = {};
+	for (const [key, value] of Object.entries(slotted)) {
+		// Legacy slot support
+		$$slots ??= {};
+		if (key === 'default') {
+			$$slots.default = true;
+			children = createRawSnippet(() => ({
+				render: () => `<${tagName}>${value}</${tagName}>`,
+			}));
+		} else {
+			$$slots[key] = createRawSnippet(() => ({
+				render: () => `<${tagName} name="${key}">${value}</${tagName}>`,
+			}));
+		}
+		// @render support for Svelte ^5.0
+		const slotName = key === 'default' ? 'children' : key;
+		renderProps[slotName] = createRawSnippet(() => ({
+			render: () => `<${tagName}${key !== 'default' ? ` name="${key}"` : ''}>${value}</${tagName}>`,
+		}));
+	}
+
+	const result = render(Component, {
+		props: {
+			...props,
+			children,
+			$$slots,
+			...renderProps,
+		},
+		idPrefix,
+	});
+	return { html: result.body };
+}
+
+export default {
+	name: '@astrojs/svelte',
+	check,
+	renderToStaticMarkup,
+	supportsAstroStaticSlot: true,
+};
diff --git a/packages/integrations/svelte/src/types.ts b/packages/integrations/svelte/src/types.ts
new file mode 100644
index 000000000..86834336e
--- /dev/null
+++ b/packages/integrations/svelte/src/types.ts
@@ -0,0 +1,4 @@
+import type { SSRResult } from 'astro';
+export type RendererContext = {
+	result: SSRResult;
+};
+\ No newline at end of file