fix: `defaultValue`/`defaultChecked` in spread attributes

packages/svelte/src/internal/client/dom/elements/attributes.js

@@ -472,6 +472,16 @@ function set_attributes(
 						element.__value = null;
 					}
 				}
+			} else if (is_default && renderer != null) {
+				// Route through the renderer-aware abstraction so custom renderers
+				// see defaultValue/defaultChecked as proper attributes
+				if (name === 'defaultValue') {
+					set_element_default_value(element, value);
+				} else {
+					set_element_default_checked(element, value);
+				}
+				// remove it from attributes's cache
+				if (name in attributes) attributes[name] = UNINITIALIZED;
 			} else if (
 				is_default ||
 				(setters.includes(name) && (is_custom_element || typeof value !== 'string'))

packages/svelte/tests/custom-renderers/samples/default-value-spread/_config.js

@@ -0,0 +1,79 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target, dispatch_event }) {
+		const inputs = target.children.filter(
+			(/** @type {any} */ n) => n.type === 'element' && n.name === 'input'
+		);
+		const button = target.children.find(
+			(/** @type {any} */ n) => n.type === 'element' && n.name === 'button'
+		);
+
+		assert.equal(inputs.length, 4);
+		assert.ok(button);
+
+		// Input 1: direct defaultValue attribute
+		const input_direct_val = inputs[0];
+		// Input 2: defaultValue via spread
+		const input_spread_val = inputs[1];
+		// Input 3: direct defaultChecked attribute (with `checked` alongside for consistent codegen)
+		const input_direct_chk = inputs[2];
+		// Input 4: defaultChecked via spread
+		const input_spread_chk = inputs[3];
+
+		// --- Initial state ---
+
+		// Direct defaultValue should appear as a renderer attribute
+		assert.equal(input_direct_val.attributes['value'], 'fixed');
+		assert.equal(input_direct_val.attributes['defaultValue'], 'default_val');
+
+		// Spread defaultValue should produce the SAME result as direct defaultValue.
+		// BUG: without fix, spread bypasses the renderer and writes element.defaultValue directly
+		// so the attribute won't exist on the object-based renderer node
+		assert.equal(input_spread_val.attributes['value'], 'fixed');
+		assert.equal(
+			input_spread_val.attributes['defaultValue'],
+			'default_val',
+			'defaultValue via spread should go through renderer.setAttribute, not element.defaultValue'
+		);
+
+		// Direct defaultChecked should appear as a renderer attribute
+		assert.equal(input_direct_chk.attributes['type'], 'checkbox');
+		assert.equal(input_direct_chk.attributes['checked'], '');
+		assert.equal(input_direct_chk.attributes['defaultChecked'], '');
+
+		// Spread defaultChecked should also go through the renderer API.
+		// set_element_default_checked treats true as a boolean attribute (empty string).
+		assert.equal(input_spread_chk.attributes['type'], 'checkbox');
+		assert.equal(
+			input_spread_chk.attributes['defaultChecked'],
+			'',
+			'defaultChecked via spread should go through renderer.setAttribute, not element.defaultChecked'
+		);
+
+		// --- After update ---
+		dispatch_event(button, 'click');
+		flushSync();
+
+		// Direct defaultValue should update
+		assert.equal(input_direct_val.attributes['defaultValue'], 'new_default');
+
+		// Spread defaultValue should also update via renderer
+		assert.equal(
+			input_spread_val.attributes['defaultValue'],
+			'new_default',
+			'updated defaultValue via spread should go through renderer.setAttribute'
+		);
+
+		// Direct defaultChecked should be removed (false)
+		assert.equal(input_direct_chk.attributes['defaultChecked'], undefined);
+
+		// Spread defaultChecked should also be removed via renderer
+		assert.equal(
+			input_spread_chk.attributes['defaultChecked'],
+			undefined,
+			'updated defaultChecked=false via spread should go through renderer.removeAttribute'
+		);
+	}
+});

packages/svelte/tests/custom-renderers/samples/default-value-spread/main.svelte

@@ -0,0 +1,16 @@
+<script>
+	let default_val = $state("default_val");
+	let default_chk = $state(true);
+	let spread_val = $derived({ defaultValue: default_val });
+	let spread_chk = $derived({ defaultChecked: default_chk });
+</script>
+
+<input value="fixed" defaultValue={default_val} />
+<input value="fixed" {...spread_val} />
+<input type="checkbox" checked defaultChecked={default_chk} />
+<input type="checkbox" checked {...spread_chk} />
+
+<button onclick={() => {
+	default_val = "new_default";
+	default_chk = false;
+}}>update</button>