diff --git a/scripts/svelte-size-guards.ts b/scripts/svelte-size-guards.ts
new file mode 100644
index 00000000..2546ef58
--- /dev/null
+++ b/scripts/svelte-size-guards.ts
@@ -0,0 +1,22 @@
+import { normalizePath, type Plugin } from "vite";
+
+export function svelteSizeGuards(): Plugin {
+	return {
+		name: "podnotes-svelte-size-guards",
+		generateBundle(_options, bundle) {
+			for (const output of Object.values(bundle)) {
+				if (output.type !== "chunk") continue;
+				for (const [id, module] of Object.entries(output.modules)) {
+					if (
+						normalizePath(id).includes("/node_modules/clsx/") &&
+						module.renderedLength > 0
+					) {
+						this.error(
+							`${output.fileName} includes clsx (${id}); use class directives or a string template instead of class={expression}.`,
+						);
+					}
+				}
+			}
+		},
+	};
+}
diff --git a/tests/tooling/svelteSizeGuards.test.ts b/tests/tooling/svelteSizeGuards.test.ts
new file mode 100644
index 00000000..02c6e325
--- /dev/null
+++ b/tests/tooling/svelteSizeGuards.test.ts
@@ -0,0 +1,73 @@
+import { mkdirSync, mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import { build } from "vite";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+const root = fileURLToPath(new URL("../..", import.meta.url));
+
+describe("Svelte size guards in the production build", { timeout: 20_000 }, () => {
+	let dir: string;
+
+	beforeEach(() => {
+		const scratch = path.join(root, ".obsidian-e2e-artifacts");
+		mkdirSync(scratch, { recursive: true });
+		dir = mkdtempSync(path.join(scratch, "svelte-size-guards-"));
+		vi.stubEnv("NODE_ENV", "production");
+	});
+
+	afterEach(() => {
+		vi.unstubAllEnvs();
+		rmSync(dir, { recursive: true, force: true });
+	});
+
+	function buildComponent(source: string, extra = "") {
+		writeFileSync(path.join(dir, "Component.svelte"), source);
+		const entry = path.join(dir, "entry.js");
+		writeFileSync(
+			entry,
+			`export { default as Component } from "./Component.svelte";\n${extra}`,
+		);
+		return build({
+			configFile: path.join(root, "vite.config.ts"),
+			configLoader: "bundle",
+			mode: "production",
+			logLevel: "silent",
+			build: { write: false, lib: { entry } },
+		});
+	}
+
+	it.each([
+		["no runes syntax", "<p>Plain markup</p>"],
+		[
+			"a string class",
+			'<script>let { name } = $props();</script><p class="item {name}">Text</p>',
+		],
+		[
+			"a class directive",
+			"<script>let { active } = $props();</script><p class:active>Text</p>",
+		],
+	])("accepts a component with %s", async (_name, source) => {
+		await expect(buildComponent(source)).resolves.toBeDefined();
+	});
+
+	it("rejects legacy prop declarations", async () => {
+		await expect(
+			buildComponent("<script>export let value;</script><p>{value}</p>"),
+		).rejects.toThrow(/Cannot use.*export let.*runes mode/);
+	});
+
+	it("rejects a dynamic class that retains clsx after minification", async () => {
+		await expect(
+			buildComponent(
+				"<script>let { classes } = $props();</script><p class={classes}>Text</p>",
+			),
+		).rejects.toThrow(/main\.js.*includes clsx/);
+	});
+
+	it("rejects a direct clsx dependency", async () => {
+		await expect(
+			buildComponent("<p>Plain markup</p>", 'export { clsx } from "clsx";'),
+		).rejects.toThrow(/main\.js.*includes clsx/);
+	});
+});
diff --git a/vite.config.ts b/vite.config.ts
index 585beee1..80522b7c 100644
--- a/vite.config.ts
+++ b/vite.config.ts
@@ -7,6 +7,7 @@ import { createBuildArtifactPlugin } from "./scripts/build-artifacts.ts";
 import { stripCssComments } from "./scripts/css-comments.ts";
 import { compactCss } from "./scripts/compact-css.ts";
 import { svelteWithoutHydration } from "./scripts/svelte-without-hydration.ts";
+import { svelteSizeGuards } from "./scripts/svelte-size-guards.ts";
 import { syntaxFloor } from "./scripts/syntax-floor.ts";
 
 // Obsidian's Android app sets no WebView minimum and its own code is ES2020, and its
@@ -68,6 +69,7 @@ export default defineConfig(({ mode }) => {
 					},
 				},
 				compilerOptions: {
+					runes: true,
 					css: "injected",
 					discloseVersion: false,
 					// Svelte skips a style whose id is already in the document, and Obsidian
@@ -77,6 +79,7 @@ export default defineConfig(({ mode }) => {
 			}),
 			// Development builds keep their source maps, which this transform does not update.
 			isProd && svelteWithoutHydration(),
+			svelteSizeGuards(),
 			syntaxFloor(),
 			createBuildArtifactPlugin({ isProduction: isProd, outDir, rootDir }),
 		],