From 7e83327400eeaa7e054935439792b696c282c1cf Mon Sep 17 00:00:00 2001
From: cui fliter <imcusg@gmail.com>
Date: Tue, 6 Oct 2026 13:22:22 +0800
Subject: [PATCH] fix: avoid tracking SvelteURLSearchParams during URL updates
 (#18791)

### Before submitting the PR, please make sure you do the following

- [x] It's really useful if your PR references an issue where it is
discussed ahead of time. In many cases, features are absent for a
reason. For large changes, please create an RFC:
https://github.com/sveltejs/rfcs
- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [ ] This message body should clearly illustrate what problems it
solves.
- [ ] Ideally, include a test that fails without this PR but passes with
it.
- [ ] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).

### Tests and linting

- [x] Run the tests with `pnpm test` and lint the project with `pnpm
lint`


---


Avoids creating an unintended reactive dependency when
`SvelteURLSearchParams` synchronizes its owning `SvelteURL`.

`#update_url()` now uses `super.toString()` instead of the reactive
`toString()` method. URL synchronization only needs native serialization
and should not subscribe the current reaction to the params version
signal.


Before this change, code such as:

```js
render_effect(() => {
	url.searchParams.set('foo', 'bar');
});
```


could cause the effect to run twice:

1. set() calls #update_url()
2. #update_url() calls reactive toString()
3. toString() subscribes the effect to #version
4. set() increments #version
5. The effect is scheduled again

Signed-off-by: cuishuang <imcusg@gmail.com>
---
 .changeset/fuzzy-urls-rest.md                  |  5 +++++
 .../svelte/src/reactivity/url-search-params.js |  2 +-
 packages/svelte/src/reactivity/url.test.ts     | 18 ++++++++++++++++++
 3 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/fuzzy-urls-rest.md

diff --git a/.changeset/fuzzy-urls-rest.md b/.changeset/fuzzy-urls-rest.md
new file mode 100644
index 0000000000..c49215278e
--- /dev/null
+++ b/.changeset/fuzzy-urls-rest.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: avoid tracking SvelteURLSearchParams during URL updates
diff --git a/packages/svelte/src/reactivity/url-search-params.js b/packages/svelte/src/reactivity/url-search-params.js
index d745bb6b08..0b87820bfc 100644
--- a/packages/svelte/src/reactivity/url-search-params.js
+++ b/packages/svelte/src/reactivity/url-search-params.js
@@ -42,7 +42,7 @@ export class SvelteURLSearchParams extends URLSearchParams {
 		if (!this.#url || this.#updating) return;
 		this.#updating = true;
 
-		const search = this.toString();
+		const search = super.toString();
 		this.#url.search = search && `?${search}`;
 
 		this.#updating = false;
diff --git a/packages/svelte/src/reactivity/url.test.ts b/packages/svelte/src/reactivity/url.test.ts
index 72518d730a..4f597526be 100644
--- a/packages/svelte/src/reactivity/url.test.ts
+++ b/packages/svelte/src/reactivity/url.test.ts
@@ -115,6 +115,24 @@ test('url.searchParams', () => {
 	cleanup();
 });
 
+test('url.searchParams writes do not create a reactive dependency', () => {
+	const url = new SvelteURL('https://svelte.dev');
+	let runs = 0;
+
+	const cleanup = effect_root(() => {
+		render_effect(() => {
+			runs += 1;
+			url.searchParams.set('foo', 'bar');
+		});
+	});
+
+	flushSync();
+
+	assert.equal(runs, 1);
+
+	cleanup();
+});
+
 test('url.searchParams.set updates url when duplicate values collapse to the same joined string', () => {
 	const url = new SvelteURL('https://svelte.dev?a=ab&a=c');
 	const log: any = [];