diff --git a/.changeset/nasty-friends-crash.md b/.changeset/nasty-friends-crash.md
new file mode 100644
index 0000000000..5895f3752a
--- /dev/null
+++ b/.changeset/nasty-friends-crash.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: handle asnyc updates within pending boundary
diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index c1ee4f3f52..d8989ef03d 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -146,8 +146,18 @@ export function async_derived(fn, label, location) {
 		if (should_suspend) {
 			var decrement_pending = increment_pending();
 
-			deferreds.get(batch)?.reject(STALE_REACTION);
-			deferreds.delete(batch); // delete to ensure correct order in Map iteration below
+			if (/** @type {Boundary} */ (parent.b).is_rendered()) {
+				deferreds.get(batch)?.reject(STALE_REACTION);
+				deferreds.delete(batch); // delete to ensure correct order in Map iteration below
+			} else {
+				// While the boundary is still showing pending, a new run supersedes all older in-flight runs
+				// for this async expression. Cancel eagerly so resolution cannot commit stale values.
+				for (const d of deferreds.values()) {
+					d.reject(STALE_REACTION);
+				}
+				deferreds.clear();
+			}
+
 			deferreds.set(batch, d);
 		}
 
diff --git a/packages/svelte/tests/runtime-runes/samples/async-boundary-update-while-pending/_config.js b/packages/svelte/tests/runtime-runes/samples/async-boundary-update-while-pending/_config.js
new file mode 100644
index 0000000000..e444aa8f9b
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-boundary-update-while-pending/_config.js
@@ -0,0 +1,51 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [shift, increment] = target.querySelectorAll('button');
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>shift</button>
+				<button>increment</button>
+				loading
+			`
+		);
+
+		increment.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>shift</button>
+				<button>increment</button>
+				loading
+			`
+		);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>shift</button>
+				<button>increment</button>
+				loading
+			`
+		);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>shift</button>
+				<button>increment</button>
+				1
+			`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-boundary-update-while-pending/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-boundary-update-while-pending/main.svelte
new file mode 100644
index 0000000000..c5a32dc4b9
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-boundary-update-while-pending/main.svelte
@@ -0,0 +1,19 @@
+<script>
+	let queue = [];
+
+	function push(value) {
+		const deferred = Promise.withResolvers();
+		queue.push(() => deferred.resolve(value));
+		return deferred.promise;
+	}
+
+	let count = $state(0);
+</script>
+
+<button onclick={() => queue.shift()()}>shift</button>
+<button onclick={() => count++}>increment</button>
+
+<svelte:boundary>
+	{await push(count)}
+	{#snippet pending()}loading{/snippet}
+</svelte:boundary>