diff --git a/packages/svelte/src/reactivity/map.js b/packages/svelte/src/reactivity/map.js
index eed163dbf2..cd2fac163f 100644
--- a/packages/svelte/src/reactivity/map.js
+++ b/packages/svelte/src/reactivity/map.js
@@ -1,6 +1,6 @@
 /** @import { Source } from '#client' */
 import { DEV } from 'esm-env';
-import { set, source } from '../internal/client/reactivity/sources.js';
+import { set, source, state } from '../internal/client/reactivity/sources.js';
 import { label, tag } from '../internal/client/dev/tracing.js';
 import { get } from '../internal/client/runtime.js';
 import { increment } from './utils.js';
@@ -54,8 +54,8 @@ import { increment } from './utils.js';
 export class SvelteMap extends Map {
 	/** @type {Map<K, Source<number>>} */
 	#sources = new Map();
-	#version = source(0);
-	#size = source(0);
+	#version = state(0);
+	#size = state(0);
 
 	/**
 	 * @param {Iterable<readonly [K, V]> | null | undefined} [value]
diff --git a/packages/svelte/src/reactivity/set.js b/packages/svelte/src/reactivity/set.js
index fd22014cb3..8a656c2bc1 100644
--- a/packages/svelte/src/reactivity/set.js
+++ b/packages/svelte/src/reactivity/set.js
@@ -1,6 +1,6 @@
 /** @import { Source } from '#client' */
 import { DEV } from 'esm-env';
-import { source, set } from '../internal/client/reactivity/sources.js';
+import { source, set, state } from '../internal/client/reactivity/sources.js';
 import { label, tag } from '../internal/client/dev/tracing.js';
 import { get } from '../internal/client/runtime.js';
 import { increment } from './utils.js';
@@ -48,8 +48,8 @@ var inited = false;
 export class SvelteSet extends Set {
 	/** @type {Map<T, Source<boolean>>} */
 	#sources = new Map();
-	#version = source(0);
-	#size = source(0);
+	#version = state(0);
+	#size = state(0);
 
 	/**
 	 * @param {Iterable<T> | null | undefined} [value]
diff --git a/packages/svelte/tests/runtime-runes/samples/side-effect-derived-array/_config.js b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-array/_config.js
new file mode 100644
index 0000000000..5ad6f57e31
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-array/_config.js
@@ -0,0 +1,23 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true,
+		runes: true
+	},
+
+	test({ assert, target }) {
+		const [button1, button2] = target.querySelectorAll('button');
+
+		assert.throws(() => {
+			button1?.click();
+			flushSync();
+		}, /state_unsafe_mutation/);
+
+		assert.doesNotThrow(() => {
+			button2?.click();
+			flushSync();
+		});
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/side-effect-derived-array/main.svelte b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-array/main.svelte
new file mode 100644
index 0000000000..6468dbebc9
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-array/main.svelte
@@ -0,0 +1,25 @@
+<script>
+	let visibleExternal = $state(false);
+	let external = $state([]);
+	const throws = $derived.by(() => {
+		external.push(1);
+		return external;
+	});
+
+	let visibleInternal = $state(false)
+	const works = $derived.by(() => {
+		let internal = $state([]);
+		internal.push(1);
+		return internal;
+	});
+</script>
+
+<button onclick={() => (visibleExternal = true)}>external</button>
+{#if visibleExternal}
+	{throws}
+{/if}
+<button onclick={() => (visibleInternal = true)}>internal</button>
+{#if visibleInternal}
+	{works}
+{/if}
+
diff --git a/packages/svelte/tests/runtime-runes/samples/side-effect-derived-map/_config.js b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-map/_config.js
new file mode 100644
index 0000000000..5ad6f57e31
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-map/_config.js
@@ -0,0 +1,23 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true,
+		runes: true
+	},
+
+	test({ assert, target }) {
+		const [button1, button2] = target.querySelectorAll('button');
+
+		assert.throws(() => {
+			button1?.click();
+			flushSync();
+		}, /state_unsafe_mutation/);
+
+		assert.doesNotThrow(() => {
+			button2?.click();
+			flushSync();
+		});
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/side-effect-derived-map/main.svelte b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-map/main.svelte
new file mode 100644
index 0000000000..bdd5ccb75c
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-map/main.svelte
@@ -0,0 +1,27 @@
+<script>
+	import { SvelteMap } from 'svelte/reactivity';
+
+	let visibleExternal = $state(false);
+	let external = new SvelteMap();
+	const throws = $derived.by(() => {
+		external.set(1, 1);
+		return external;
+	});
+
+	let visibleInternal = $state(false);
+	const works = $derived.by(() => {
+		let internal = new SvelteMap();
+		internal.set(1, 1);
+		return internal;
+	});
+</script>
+
+<button onclick={() => (visibleExternal = true)}>external</button>
+{#if visibleExternal}
+	{throws}
+{/if}
+<button onclick={() => (visibleInternal = true)}>internal</button>
+{#if visibleInternal}
+	{works}
+{/if}
+
diff --git a/packages/svelte/tests/runtime-runes/samples/side-effect-derived-object/_config.js b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-object/_config.js
new file mode 100644
index 0000000000..5ad6f57e31
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-object/_config.js
@@ -0,0 +1,23 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true,
+		runes: true
+	},
+
+	test({ assert, target }) {
+		const [button1, button2] = target.querySelectorAll('button');
+
+		assert.throws(() => {
+			button1?.click();
+			flushSync();
+		}, /state_unsafe_mutation/);
+
+		assert.doesNotThrow(() => {
+			button2?.click();
+			flushSync();
+		});
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/side-effect-derived-object/main.svelte b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-object/main.svelte
new file mode 100644
index 0000000000..2174bde59f
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-object/main.svelte
@@ -0,0 +1,25 @@
+<script>
+	let visibleExternal = $state(false);
+	let external = $state({ v: 1 });
+	const throws = $derived.by(() => {
+		external.v = 2;
+		return external;
+	});
+
+	let visibleInternal = $state(false)
+	const works = $derived.by(() => {
+		let internal = $state({ v: 1 });
+		internal.v = 2;
+		return internal;
+	});
+</script>
+
+<button onclick={() => (visibleExternal = true)}>external</button>
+{#if visibleExternal}
+	{throws}
+{/if}
+<button onclick={() => (visibleInternal = true)}>internal</button>
+{#if visibleInternal}
+	{works}
+{/if}
+
diff --git a/packages/svelte/tests/runtime-runes/samples/side-effect-derived-primitive/_config.js b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-primitive/_config.js
new file mode 100644
index 0000000000..5ad6f57e31
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-primitive/_config.js
@@ -0,0 +1,23 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true,
+		runes: true
+	},
+
+	test({ assert, target }) {
+		const [button1, button2] = target.querySelectorAll('button');
+
+		assert.throws(() => {
+			button1?.click();
+			flushSync();
+		}, /state_unsafe_mutation/);
+
+		assert.doesNotThrow(() => {
+			button2?.click();
+			flushSync();
+		});
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/side-effect-derived-primitive/main.svelte b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-primitive/main.svelte
new file mode 100644
index 0000000000..21de8b9d91
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-primitive/main.svelte
@@ -0,0 +1,25 @@
+<script>
+	let visibleExternal = $state(false);
+	let external = $state(1);
+	const throws = $derived.by(() => {
+		external = 2;
+		return external;
+	});
+
+	let visibleInternal = $state(false);
+	const works = $derived.by(() => {
+		let internal = $state(1);
+		internal = 2;
+		return internal;
+	});
+</script>
+
+<button onclick={() => (visibleExternal = true)}>external</button>
+{#if visibleExternal}
+	{throws}
+{/if}
+<button onclick={() => (visibleInternal = true)}>internal</button>
+{#if visibleInternal}
+	{works}
+{/if}
+
diff --git a/packages/svelte/tests/runtime-runes/samples/side-effect-derived-set/_config.js b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-set/_config.js
new file mode 100644
index 0000000000..5ad6f57e31
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-set/_config.js
@@ -0,0 +1,23 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true,
+		runes: true
+	},
+
+	test({ assert, target }) {
+		const [button1, button2] = target.querySelectorAll('button');
+
+		assert.throws(() => {
+			button1?.click();
+			flushSync();
+		}, /state_unsafe_mutation/);
+
+		assert.doesNotThrow(() => {
+			button2?.click();
+			flushSync();
+		});
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/side-effect-derived-set/main.svelte b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-set/main.svelte
new file mode 100644
index 0000000000..8564f6e7c4
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-set/main.svelte
@@ -0,0 +1,27 @@
+<script>
+	import { SvelteSet } from 'svelte/reactivity';
+
+	let visibleExternal = $state(false);
+	let external = new SvelteSet();
+	const throws = $derived.by(() => {
+		external.add(1);
+		return external;
+	})
+
+	let visibleInternal = $state(false);
+	const works = $derived.by(() => {
+		let internal = new SvelteSet();
+		internal.add(1);
+		return internal;
+	})
+</script>
+
+<button onclick={() => (visibleExternal = true)}>external</button>
+{#if visibleExternal}
+	{throws}
+{/if}
+<button onclick={() => (visibleInternal = true)}>internal</button>
+{#if visibleInternal}
+	{works}
+{/if}
+