From 6b16b7f73fe8223ad7ab8bc629c98ba6d581f5cf Mon Sep 17 00:00:00 2001
From: Nic Polumeyv <nicolas.polum@outlook.com>
Date: Tue, 25 Aug 2026 04:35:46 -0400
Subject: [PATCH] fix: keep defaultChecked on hydrated radio inputs with spread
 attributes (#18701)

radio buttons were missing in the type check - remove the check instead (aligns with how we do it in the compiler, see `has_default_value_attribute`). Strictly speaking not fully correct but pragmatic solution that doesn't have any impact in practise

Needed by sveltejs/kit#16926.
---
 .changeset/keep-radio-default-checked.md      |  5 ++++
 .../client/dom/elements/attributes.js         |  7 ++----
 .../_config.js                                | 23 +++++++++++++++++++
 .../main.svelte                               |  9 ++++++++
 4 files changed, 39 insertions(+), 5 deletions(-)
 create mode 100644 .changeset/keep-radio-default-checked.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/form-default-checked-radio-spread/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/form-default-checked-radio-spread/main.svelte

diff --git a/.changeset/keep-radio-default-checked.md b/.changeset/keep-radio-default-checked.md
new file mode 100644
index 0000000000..b3b32afda6
--- /dev/null
+++ b/.changeset/keep-radio-default-checked.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: keep `defaultChecked` on hydrated radio inputs with spread attributes
diff --git a/packages/svelte/src/internal/client/dom/elements/attributes.js b/packages/svelte/src/internal/client/dom/elements/attributes.js
index 3d71abcd10..69d4dd42a8 100644
--- a/packages/svelte/src/internal/client/dom/elements/attributes.js
+++ b/packages/svelte/src/internal/client/dom/elements/attributes.js
@@ -291,11 +291,8 @@ function set_attributes(
 	skip_warning = false
 ) {
 	if (hydrating && should_remove_defaults && element.nodeName === INPUT_TAG) {
-		var input = /** @type {HTMLInputElement} */ (element);
-		var attribute = input.type === 'checkbox' ? 'defaultChecked' : 'defaultValue';
-
-		if (!(attribute in next)) {
-			remove_input_defaults(input);
+		if (!('defaultValue' in next || 'defaultChecked' in next)) {
+			remove_input_defaults(/** @type {HTMLInputElement} */ (element));
 		}
 	}
 
diff --git a/packages/svelte/tests/runtime-runes/samples/form-default-checked-radio-spread/_config.js b/packages/svelte/tests/runtime-runes/samples/form-default-checked-radio-spread/_config.js
new file mode 100644
index 0000000000..3f3134ee7a
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/form-default-checked-radio-spread/_config.js
@@ -0,0 +1,23 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	mode: ['hydrate'],
+
+	async test({ assert, target }) {
+		const [a, b, reset] = target.querySelectorAll('input');
+
+		// let the deferred hydration cleanup run
+		await Promise.resolve();
+		flushSync();
+
+		b.checked = true;
+		reset.click();
+		await Promise.resolve();
+		flushSync();
+
+		assert.equal(a.defaultChecked, true);
+		assert.equal(a.checked, true);
+		assert.equal(b.checked, false);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/form-default-checked-radio-spread/main.svelte b/packages/svelte/tests/runtime-runes/samples/form-default-checked-radio-spread/main.svelte
new file mode 100644
index 0000000000..cb7ff0abd3
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/form-default-checked-radio-spread/main.svelte
@@ -0,0 +1,9 @@
+<script>
+	let spread = { defaultChecked: true, checked: true };
+</script>
+
+<form>
+	<input type="radio" name="option" value="a" {...spread} />
+	<input type="radio" name="option" value="b" />
+	<input type="reset" value="Reset" />
+</form>