From 26180d5f072a450bdf037fdab16b2f6fe7222307 Mon Sep 17 00:00:00 2001
From: Fedor Nezhivoi <f.nezhivoi@corp.vk.com>
Date: Tue, 24 Jun 2025 10:36:55 +0700
Subject: [PATCH 1/3] fix: lift unsafe_state_mutation constrained for SvelteSet
 and SvelteMap created inside the derived

---
 packages/svelte/src/reactivity/map.js         |  6 ++---
 packages/svelte/src/reactivity/set.js         |  6 ++---
 .../side-effect-derived-array/_config.js      | 23 ++++++++++++++++
 .../side-effect-derived-array/main.svelte     | 25 +++++++++++++++++
 .../side-effect-derived-map/_config.js        | 23 ++++++++++++++++
 .../side-effect-derived-map/main.svelte       | 27 +++++++++++++++++++
 .../side-effect-derived-object/_config.js     | 23 ++++++++++++++++
 .../side-effect-derived-object/main.svelte    | 25 +++++++++++++++++
 .../side-effect-derived-primitive/_config.js  | 23 ++++++++++++++++
 .../side-effect-derived-primitive/main.svelte | 25 +++++++++++++++++
 .../side-effect-derived-set/_config.js        | 23 ++++++++++++++++
 .../side-effect-derived-set/main.svelte       | 27 +++++++++++++++++++
 12 files changed, 250 insertions(+), 6 deletions(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/side-effect-derived-array/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/side-effect-derived-array/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/side-effect-derived-map/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/side-effect-derived-map/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/side-effect-derived-object/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/side-effect-derived-object/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/side-effect-derived-primitive/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/side-effect-derived-primitive/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/side-effect-derived-set/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/side-effect-derived-set/main.svelte

diff --git a/packages/svelte/src/reactivity/map.js b/packages/svelte/src/reactivity/map.js
index eed163dbf29b..cd2fac163fc6 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 fd22014cb3f6..8a656c2bc14a 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 000000000000..5ad6f57e311f
--- /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 000000000000..6468dbebc9b7
--- /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 000000000000..5ad6f57e311f
--- /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 000000000000..bdd5ccb75c91
--- /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 000000000000..5ad6f57e311f
--- /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 000000000000..2174bde59f60
--- /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 000000000000..5ad6f57e311f
--- /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 000000000000..21de8b9d91c9
--- /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 000000000000..5ad6f57e311f
--- /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 000000000000..8564f6e7c48e
--- /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}
+

From 3c3255fdb98e78b02f52e465a263e581886a14ce Mon Sep 17 00:00:00 2001
From: Fedor Nezhivoi <f.nezhivoi@corp.vk.com>
Date: Tue, 24 Jun 2025 10:42:46 +0700
Subject: [PATCH 2/3] add a changeset

---
 .changeset/fair-bats-visit.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/fair-bats-visit.md

diff --git a/.changeset/fair-bats-visit.md b/.changeset/fair-bats-visit.md
new file mode 100644
index 000000000000..272dbeddc8fe
--- /dev/null
+++ b/.changeset/fair-bats-visit.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+lift unsafe_state_mutation constrained for SvelteSet and SvelteMap created inside the derived

From a78460781192b90768686cde2e03da2f04fe981a Mon Sep 17 00:00:00 2001
From: Fedor Nezhivoi <f.nezhivoi@corp.vk.com>
Date: Tue, 24 Jun 2025 11:32:28 +0700
Subject: [PATCH 3/3] fix: lift unsafe_state_mutation constrained for
 SvelteDate, SvelteURL and SvelteURLSearchParams created inside the derived

---
 .changeset/fair-bats-visit.md                 |  2 +-
 packages/svelte/src/reactivity/date.js        |  4 +--
 .../src/reactivity/url-search-params.js       |  4 +--
 packages/svelte/src/reactivity/url.js         | 18 ++++++-------
 .../side-effect-derived-date/_config.js       | 23 ++++++++++++++++
 .../side-effect-derived-date/main.svelte      | 27 +++++++++++++++++++
 .../_config.js                                | 23 ++++++++++++++++
 .../main.svelte                               | 27 +++++++++++++++++++
 .../side-effect-derived-url/_config.js        | 23 ++++++++++++++++
 .../side-effect-derived-url/main.svelte       | 27 +++++++++++++++++++
 10 files changed, 164 insertions(+), 14 deletions(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/side-effect-derived-date/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/side-effect-derived-date/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/side-effect-derived-url-search-params/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/side-effect-derived-url-search-params/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/side-effect-derived-url/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/side-effect-derived-url/main.svelte

diff --git a/.changeset/fair-bats-visit.md b/.changeset/fair-bats-visit.md
index 272dbeddc8fe..bc0a42639594 100644
--- a/.changeset/fair-bats-visit.md
+++ b/.changeset/fair-bats-visit.md
@@ -2,4 +2,4 @@
 'svelte': patch
 ---
 
-lift unsafe_state_mutation constrained for SvelteSet and SvelteMap created inside the derived
+lift unsafe_state_mutation constraints for SvelteSet, SvelteMap, SvelteDate, SvelteURL and SvelteURLSearchParams created inside the derived
diff --git a/packages/svelte/src/reactivity/date.js b/packages/svelte/src/reactivity/date.js
index 4176f0ceec3f..8e2b5d41ab94 100644
--- a/packages/svelte/src/reactivity/date.js
+++ b/packages/svelte/src/reactivity/date.js
@@ -1,6 +1,6 @@
 /** @import { Source } from '#client' */
 import { derived } from '../internal/client/index.js';
-import { source, set } from '../internal/client/reactivity/sources.js';
+import { set, state } from '../internal/client/reactivity/sources.js';
 import { tag } from '../internal/client/dev/tracing.js';
 import { active_reaction, get, set_active_reaction } from '../internal/client/runtime.js';
 import { DEV } from 'esm-env';
@@ -40,7 +40,7 @@ var inited = false;
  * ```
  */
 export class SvelteDate extends Date {
-	#time = source(super.getTime());
+	#time = state(super.getTime());
 
 	/** @type {Map<keyof Date, Source<unknown>>} */
 	#deriveds = new Map();
diff --git a/packages/svelte/src/reactivity/url-search-params.js b/packages/svelte/src/reactivity/url-search-params.js
index c77ff9c8225c..389da7cdb67a 100644
--- a/packages/svelte/src/reactivity/url-search-params.js
+++ b/packages/svelte/src/reactivity/url-search-params.js
@@ -1,5 +1,5 @@
 import { DEV } from 'esm-env';
-import { source } from '../internal/client/reactivity/sources.js';
+import { state } from '../internal/client/reactivity/sources.js';
 import { tag } from '../internal/client/dev/tracing.js';
 import { get } from '../internal/client/runtime.js';
 import { get_current_url } from './url.js';
@@ -34,7 +34,7 @@ export const REPLACE = Symbol();
  * ```
  */
 export class SvelteURLSearchParams extends URLSearchParams {
-	#version = DEV ? tag(source(0), 'SvelteURLSearchParams version') : source(0);
+	#version = DEV ? tag(state(0), 'SvelteURLSearchParams version') : state(0);
 	#url = get_current_url();
 
 	#updating = false;
diff --git a/packages/svelte/src/reactivity/url.js b/packages/svelte/src/reactivity/url.js
index 56732a040247..dfede23f6ee2 100644
--- a/packages/svelte/src/reactivity/url.js
+++ b/packages/svelte/src/reactivity/url.js
@@ -1,5 +1,5 @@
 import { DEV } from 'esm-env';
-import { source, set } from '../internal/client/reactivity/sources.js';
+import { set, state } from '../internal/client/reactivity/sources.js';
 import { tag } from '../internal/client/dev/tracing.js';
 import { get } from '../internal/client/runtime.js';
 import { REPLACE, SvelteURLSearchParams } from './url-search-params.js';
@@ -40,14 +40,14 @@ export function get_current_url() {
  * ```
  */
 export class SvelteURL extends URL {
-	#protocol = source(super.protocol);
-	#username = source(super.username);
-	#password = source(super.password);
-	#hostname = source(super.hostname);
-	#port = source(super.port);
-	#pathname = source(super.pathname);
-	#hash = source(super.hash);
-	#search = source(super.search);
+	#protocol = state(super.protocol);
+	#username = state(super.username);
+	#password = state(super.password);
+	#hostname = state(super.hostname);
+	#port = state(super.port);
+	#pathname = state(super.pathname);
+	#hash = state(super.hash);
+	#search = state(super.search);
 	#searchParams;
 
 	/**
diff --git a/packages/svelte/tests/runtime-runes/samples/side-effect-derived-date/_config.js b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-date/_config.js
new file mode 100644
index 000000000000..5ad6f57e311f
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-date/_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-date/main.svelte b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-date/main.svelte
new file mode 100644
index 000000000000..a3c6aa629fec
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-date/main.svelte
@@ -0,0 +1,27 @@
+<script>
+	import { SvelteDate } from 'svelte/reactivity';
+
+	let visibleExternal = $state(false);
+	let external = new SvelteDate();
+	const throws = $derived.by(() => {
+		external.setTime(12345);
+		return external;
+	})
+
+	let visibleInternal = $state(false);
+	const works = $derived.by(() => {
+		let internal = new SvelteDate();
+		internal.setTime(12345);
+		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-url-search-params/_config.js b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-url-search-params/_config.js
new file mode 100644
index 000000000000..5ad6f57e311f
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-url-search-params/_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-url-search-params/main.svelte b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-url-search-params/main.svelte
new file mode 100644
index 000000000000..014a1e4e6dcf
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-url-search-params/main.svelte
@@ -0,0 +1,27 @@
+<script>
+	import { SvelteURLSearchParams } from 'svelte/reactivity';
+
+	let visibleExternal = $state(false);
+	let external = new SvelteURLSearchParams();
+	const throws = $derived.by(() => {
+		external.append('foo', 'bar')
+		return external;
+	})
+
+	let visibleInternal = $state(false);
+	const works = $derived.by(() => {
+		let internal = new SvelteURLSearchParams();
+		internal.append('foo', 'bar')
+		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-url/_config.js b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-url/_config.js
new file mode 100644
index 000000000000..5ad6f57e311f
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-url/_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-url/main.svelte b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-url/main.svelte
new file mode 100644
index 000000000000..69ead384c311
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/side-effect-derived-url/main.svelte
@@ -0,0 +1,27 @@
+<script>
+	import { SvelteURL } from 'svelte/reactivity';
+
+	let visibleExternal = $state(false);
+	let external = new SvelteURL('https://svelte.dev');
+	const throws = $derived.by(() => {
+		external.host = 'kit.svelte.dev'
+		return external;
+	})
+
+	let visibleInternal = $state(false);
+	const works = $derived.by(() => {
+		let internal = new SvelteURL('https://svelte.dev');
+		internal.host = 'kit.svelte.dev'
+		return internal;
+	})
+</script>
+
+<button onclick={() => (visibleExternal = true)}>external</button>
+{#if visibleExternal}
+	{throws}
+{/if}
+<button onclick={() => (visibleInternal = true)}>internal</button>
+{#if visibleInternal}
+	{works}
+{/if}
+