diff --git a/packages/svelte/tests/runtime-runes/samples/async-branch-commit-order/_config.js b/packages/svelte/tests/runtime-runes/samples/async-branch-commit-order/_config.js
new file mode 100644
index 0000000000..11c3bce995
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-branch-commit-order/_config.js
@@ -0,0 +1,27 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+const buttons = '<button>a</button><button>b</button><button>shift</button>';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [a, b, shift] = target.querySelectorAll('button');
+
+		// batch A waits on its key; batch B registers its key right away
+		a.click();
+		await tick();
+		b.click();
+		await tick();
+
+		// the older batch commits first
+		shift.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `<p>0</p><p>1</p>${buttons}`);
+
+		// the newer batch commits with its own branch
+		shift.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `<p>1</p><p>B</p>${buttons}`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-branch-commit-order/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-branch-commit-order/main.svelte
new file mode 100644
index 0000000000..d0b1a57053
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-branch-commit-order/main.svelte
@@ -0,0 +1,24 @@
+<script>
+	let a = $state(0);
+	let b = $state(0);
+	let pend = false;
+
+	const deferred = [];
+
+	function delay(value) {
+		if (!pend) return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+
+	const k = $derived(b === 0 ? await delay(a) : 'B');
+</script>
+
+<p>{await delay(b)}</p>
+
+{#key k}
+	<p>{k}</p>
+{/key}
+
+<button onclick={() => { pend = true; a = 1; }}>a</button>
+<button onclick={() => { pend = true; b = 1; }}>b</button>
+<button onclick={() => deferred.shift()()}>shift</button>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-derived-assign-keeps-maybe-dirty/_config.js b/packages/svelte/tests/runtime-runes/samples/async-derived-assign-keeps-maybe-dirty/_config.js
new file mode 100644
index 0000000000..ae9ef996f6
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-derived-assign-keeps-maybe-dirty/_config.js
@@ -0,0 +1,39 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+const buttons =
+	'<button>s</button><button>u</button><button>d</button><button>resolve a</button><button>resolve b</button><button>resolve c</button>';
+
+export default test({
+	mode: ['client'],
+	async test({ assert, target, logs }) {
+		await tick();
+		const [s, u, d, resolve_a, resolve_b, resolve_c] = target.querySelectorAll('button');
+
+		// batch A: `d` becomes 10, but is not evaluated yet (the `<span>` is deferred)
+		s.click();
+		await tick();
+
+		// batch B
+		u.click();
+		await tick();
+
+		// batch C overrides `d` while A and B are pending. This must not mark `d` as
+		// clean, because the other batches haven't seen its new value (10) yet
+		d.click();
+		await tick();
+
+		// B sees A's change (but not C's), so `d` is 10 in its world
+		resolve_b.click();
+		await tick();
+		assert.equal(logs[0], 'probe 10');
+
+		resolve_a.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>1</p><p>0</p><span>10</span><i>10</i>`);
+
+		resolve_c.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>1</p><p>1</p><span>99</span><i>99</i>`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-derived-assign-keeps-maybe-dirty/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-derived-assign-keeps-maybe-dirty/main.svelte
new file mode 100644
index 0000000000..08c00a4911
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-derived-assign-keeps-maybe-dirty/main.svelte
@@ -0,0 +1,42 @@
+<script>
+	let s = $state(0);
+	let u = $state(0);
+	let w = $state(0);
+	let show = $state(true);
+	let d = $derived(s * 10);
+
+	const pending = new Map();
+
+	function delay(name, value) {
+		if (!value) return value;
+		return new Promise((resolve) => pending.set(name, () => resolve(value)));
+	}
+
+	function probe(value) {
+		console.log(`probe ${value}`);
+		return value;
+	}
+</script>
+
+<button onclick={() => s++}>s</button>
+<button onclick={() => u++}>u</button>
+<button
+	onclick={() => {
+		d = 99;
+		w++;
+	}}>d</button
+>
+<button onclick={() => pending.get('a')?.()}>resolve a</button>
+<button onclick={() => pending.get('b')?.()}>resolve b</button>
+<button onclick={() => pending.get('c')?.()}>resolve c</button>
+
+<p>{await delay('a', s)}</p>
+<p>{await delay('c', w)}</p>
+<!-- inside a branch, so that B's traversal doesn't reach (and stash) the effect reading `d` -->
+{#if show}
+	<span>{d}</span>
+{/if}
+
+{#if await delay('b', u)}
+	<i>{await probe(d)}</i>
+{/if}
diff --git a/packages/svelte/tests/runtime-runes/samples/async-derived-equal-real-value-hidden/_config.js b/packages/svelte/tests/runtime-runes/samples/async-derived-equal-real-value-hidden/_config.js
new file mode 100644
index 0000000000..d319831f2e
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-derived-equal-real-value-hidden/_config.js
@@ -0,0 +1,33 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+const buttons =
+	'<button>x</button><button>s</button><button>resolve a</button><button>resolve b</button>';
+
+export default test({
+	mode: ['client'],
+	async test({ assert, target }) {
+		await tick();
+		const [x, s, resolve_a, resolve_b] = target.querySelectorAll('button');
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>0:5</p><p>5</p>`);
+
+		// batch A: `t` becomes truthy (pending)
+		x.click();
+		await tick();
+
+		// batch B: `d` becomes 0 (pending)
+		s.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>0:5</p><p>5</p>`);
+
+		// In A's world, `d` is recomputed to 0. That's the real value, but not the value
+		// A sees for it (the one from before B's change), so A has to record it
+		resolve_a.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>1:0</p><p>5</p>`);
+
+		resolve_b.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>1:0</p><p>0</p>`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-derived-equal-real-value-hidden/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-derived-equal-real-value-hidden/main.svelte
new file mode 100644
index 0000000000..9fe34a5e12
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-derived-equal-real-value-hidden/main.svelte
@@ -0,0 +1,25 @@
+<script>
+	let x = $state(0);
+	let s = $state(5);
+
+	const pending = new Map();
+
+	function delay(name, value, initial) {
+		if (value === initial) return value;
+		return new Promise((resolve) => pending.set(name, () => resolve(value)));
+	}
+
+	const t = $derived(await delay('a', x, 0));
+	const d = $derived(t ? 0 : s);
+</script>
+
+<button onclick={() => x++}>x</button>
+<button onclick={() => (s = 0)}>s</button>
+<button onclick={() => pending.get('a')?.()}>resolve a</button>
+<button onclick={() => pending.get('b')?.()}>resolve b</button>
+
+{#if d >= 0}
+	<p>{t}:{d}</p>
+{/if}
+
+<p>{await delay('b', s, 5)}</p>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-derived-previous-effect-batch/_config.js b/packages/svelte/tests/runtime-runes/samples/async-derived-previous-effect-batch/_config.js
new file mode 100644
index 0000000000..8f021032e0
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-derived-previous-effect-batch/_config.js
@@ -0,0 +1,33 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+// Batch B (x) commits while an earlier batch A (y) is still pending, but B stays around because
+// of the async work in the new boundary. While B flushes its effects, an effect creates a new batch C,
+// and `d` is computed for the first time afterwards (so it's captured in both B and C). When A
+// commits later, it sees B's previous values, which must be consistent (`d` must be `x * 2`).
+export default test({
+	async test({ assert, target, logs }) {
+		await tick();
+		const [y, x, shift] = target.querySelectorAll('button');
+		assert.deepEqual(logs, [0, 0, 0]);
+
+		// A
+		y.click();
+		await tick();
+
+		// B
+		x.click();
+		await tick();
+		assert.deepEqual(logs, [0, 0, 0, 0, 1, 2]);
+
+		// A commits
+		shift.click();
+		await tick();
+
+		for (let i = 0; i < logs.length; i += 3) {
+			assert.equal(logs[i + 2], logs[i + 1] * 2, `torn read: ${logs.slice(i, i + 3)}`);
+		}
+
+		assert.deepEqual(logs.slice(-3), [1, 1, 2]);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-derived-previous-effect-batch/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-derived-previous-effect-batch/main.svelte
new file mode 100644
index 0000000000..5cd0876884
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-derived-previous-effect-batch/main.svelte
@@ -0,0 +1,38 @@
+<script>
+	import { untrack } from 'svelte';
+
+	let x = $state(0);
+	let y = $state(0);
+	let z = $state(0);
+	let d = $derived(x * 2);
+
+	const queue = [];
+	function push(v) {
+		return new Promise((r) => queue.push(() => r(v)));
+	}
+
+	// creates a new batch while the batch that changed `x` flushes its effects...
+	$effect(() => {
+		if (x) untrack(() => z++);
+	});
+
+	// ...before `d` is computed for the first time in that batch
+	$effect(() => {
+		console.log(y, x, d);
+	});
+</script>
+
+<button onclick={() => y++}>y</button>
+<button onclick={() => x++}>x</button>
+<button onclick={() => queue.shift()()}>shift</button>
+
+<p>{y ? await push(y) : y}</p>
+
+{#if x}
+	<svelte:boundary>
+		<p>{await push(x)}</p>
+		{#snippet pending()}
+			<p>loading</p>
+		{/snippet}
+	</svelte:boundary>
+{/if}
diff --git a/packages/svelte/tests/runtime-runes/samples/async-eager-batch-no-merge/_config.js b/packages/svelte/tests/runtime-runes/samples/async-eager-batch-no-merge/_config.js
new file mode 100644
index 0000000000..f90497846c
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-eager-batch-no-merge/_config.js
@@ -0,0 +1,44 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+const buttons =
+	'<button>page</button><button>filter</button><button>shift</button><button>pop</button>';
+
+// The eager batch that re-runs `load` for the new `$state.eager(filter)` value computes it
+// with the old `page`, because eager batches don't see other batches' changes. Therefore it
+// must not be merged into the (pending) `page` batch — else its result would resolve that
+// batch's `load(2, ...)` and be shown next to `page: 2`
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [page, filter, shift, pop] = target.querySelectorAll('button');
+
+		// B1: page = 2 -> load(2, 'a') pending
+		page.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>page: 1</p><p>result: 1/a</p>`);
+
+		// filter = 'b' -> eager batch re-runs load(1, 'b'), pending
+		filter.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>page: 1</p><p>result: 1/a</p>`);
+
+		// resolve load(1, 'b') -> the eager batch commits on its own
+		pop.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>page: 1</p><p>result: 1/b</p>`);
+
+		// resolve load(2, 'a') -> B1 commits (showing B1's own result is fine, as is
+		// waiting for load(2, 'b'), as long as the result matches the page)
+		shift.click();
+		await tick();
+		const [p1, p2] = target.querySelectorAll('p');
+		assert.equal(p1.textContent, 'page: 2');
+		assert.match(p2.textContent ?? '', /^result: 2\//);
+
+		// load(2, 'b') resolves
+		shift.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>page: 2</p><p>result: 2/b</p>`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-eager-batch-no-merge/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-eager-batch-no-merge/main.svelte
new file mode 100644
index 0000000000..008fabe2e5
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-eager-batch-no-merge/main.svelte
@@ -0,0 +1,20 @@
+<script>
+	let page = $state(1);
+	let filter = $state('a');
+
+	const deferred = [];
+
+	function load(page, filter) {
+		const value = `${page}/${filter}`;
+		if (value === '1/a') return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+<button onclick={() => page++}>page</button>
+<button onclick={() => (filter = 'b')}>filter</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>
+
+<p>page: {page}</p>
+<p>result: {await load(page, $state.eager(filter))}</p>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/Child.svelte b/packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/Child.svelte
new file mode 100644
index 0000000000..6f48eaf129
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/Child.svelte
@@ -0,0 +1,9 @@
+<script>
+	let { big } = $props();
+
+	$effect(() => {
+		console.log(`effect ${big}`);
+	});
+</script>
+
+<p>child</p>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/_config.js b/packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/_config.js
new file mode 100644
index 0000000000..4fd60a98ce
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/_config.js
@@ -0,0 +1,37 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	mode: ['client'],
+	async test({ assert, target, logs, raf }) {
+		await tick();
+		const [x, s, toggle, shift] = target.querySelectorAll('button');
+		assert.deepEqual(logs, ['effect false']);
+
+		// keep an earlier batch pending, so that the next one commits while time travelling
+		x.click();
+		await tick();
+
+		// `big` stays `false`, so the effect must not re-run
+		s.click();
+		await tick();
+		assert.deepEqual(logs, ['effect false']);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			'<button>x</button><button>s</button><button>toggle</button><button>shift</button><p>1</p><div><p>child</p></div>'
+		);
+
+		// outro, then reintro before the outro finished: the branch is resumed
+		toggle.click();
+		await tick();
+		raf.tick(50);
+		toggle.click();
+		await tick();
+		raf.tick(200);
+
+		assert.deepEqual(logs, ['effect false']);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/main.svelte
new file mode 100644
index 0000000000..30d68f614b
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-resume-no-rerun/main.svelte
@@ -0,0 +1,29 @@
+<script>
+	import { fade } from 'svelte/transition';
+	import Child from './Child.svelte';
+
+	let x = $state(0);
+	let s = $state(0);
+	let show = $state(true);
+	const big = $derived(s > 100);
+
+	const queued = [];
+
+	function push(v) {
+		if (v === 0) return v;
+		return new Promise((resolve) => queued.push(() => resolve(v)));
+	}
+</script>
+
+<button onclick={() => x++}>x</button>
+<button onclick={() => s++}>s</button>
+<button onclick={() => (show = !show)}>toggle</button>
+<button onclick={() => queued.shift()?.()}>shift</button>
+
+<p>{await push(x)}</p>
+
+{#if show}
+	<div transition:fade={{ duration: 100 }}>
+		<Child {big} />
+	</div>
+{/if}
diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-write-keeps-batch-derived/_config.js b/packages/svelte/tests/runtime-runes/samples/async-effect-write-keeps-batch-derived/_config.js
new file mode 100644
index 0000000000..fb0b6e8a4f
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-write-keeps-batch-derived/_config.js
@@ -0,0 +1,33 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+const buttons = '<button>x</button><button>t</button><button>shift</button>';
+
+// While batch A (x) commits, a pre effect writes to `s`. That marks `label` (via `big`) as
+// maybe dirty, but it doesn't actually change. `label` must still show the value from A's
+// point of view, not the one from the later, still-pending batch B (t).
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [x, t, shift] = target.querySelectorAll('button');
+
+		// A
+		x.click();
+		await tick();
+
+		// B
+		t.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>0</p><p>0</p><p>0 false-0</p>`);
+
+		// A commits, B is still pending
+		shift.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>1</p><p>0</p><p>1 false-0</p>`);
+
+		// B commits
+		shift.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>1</p><p>1</p><p>1 false-1</p>`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-write-keeps-batch-derived/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-effect-write-keeps-batch-derived/main.svelte
new file mode 100644
index 0000000000..1791827eae
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-write-keeps-batch-derived/main.svelte
@@ -0,0 +1,32 @@
+<script>
+	import { untrack } from 'svelte';
+
+	let x = $state(0);
+	let t = $state(0);
+	let s = $state(0);
+
+	// `s` changes, but `big` doesn't
+	let big = $derived(s > 100);
+	let label = $derived(`${big}-${t}`);
+
+	const queue = [];
+	function push(v) {
+		if (!v) return v;
+		return new Promise((r) => queue.push(() => r(v)));
+	}
+
+	$effect.pre(() => {
+		if (x) untrack(() => s++);
+	});
+</script>
+
+<button onclick={() => x++}>x</button>
+<button onclick={() => t++}>t</button>
+<button onclick={() => queue.shift()()}>shift</button>
+
+<p>{await push(x)}</p>
+<p>{await push(t)}</p>
+<!-- evaluates `label` during traversal of B -->
+{#key label}{/key}
+
+<p>{x} {label}</p>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-fork-commit-new-reader/_config.js b/packages/svelte/tests/runtime-runes/samples/async-fork-commit-new-reader/_config.js
new file mode 100644
index 0000000000..a9e860bec7
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-fork-commit-new-reader/_config.js
@@ -0,0 +1,35 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+const buttons =
+	'<button>u</button><button>fork</button><button>cond</button><button>commit</button><button>shift</button>';
+
+// The template effect is deferred by the fork (it depends on `s` via `big`), but before the
+// fork is committed, it re-runs in the real world and starts reading `s` directly.
+// Committing the fork (while another batch is pending) must still update it.
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [u, f, cond, commit, shift] = target.querySelectorAll('button');
+
+		// unrelated pending batch
+		u.click();
+		await tick();
+
+		f.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>0</p><p>false</p>`);
+
+		cond.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>0</p><p>0</p>`);
+
+		commit.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>0</p><p>1</p>`);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>1</p><p>1</p>`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-fork-commit-new-reader/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-fork-commit-new-reader/main.svelte
new file mode 100644
index 0000000000..a52d6b4dfd
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-fork-commit-new-reader/main.svelte
@@ -0,0 +1,25 @@
+<script>
+	import { fork } from 'svelte';
+
+	let s = $state(0);
+	let cond = $state(false);
+	let u = $state(0);
+	let big = $derived(s > 100);
+
+	const queue = [];
+	function push(v) {
+		if (!v) return v;
+		return new Promise((r) => queue.push(() => r(v)));
+	}
+
+	let f;
+</script>
+
+<button onclick={() => u++}>u</button>
+<button onclick={() => (f = fork(() => s++))}>fork</button>
+<button onclick={() => (cond = true)}>cond</button>
+<button onclick={() => f.commit()}>commit</button>
+<button onclick={() => queue.shift()()}>shift</button>
+
+<p>{await push(u)}</p>
+<p>{cond ? s : big}</p>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-merge-blocking-pending/_config.js b/packages/svelte/tests/runtime-runes/samples/async-merge-blocking-pending/_config.js
new file mode 100644
index 0000000000..9cd8eba0b3
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-merge-blocking-pending/_config.js
@@ -0,0 +1,34 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+const buttons = '<button>x</button><button>toggle</button><button>pop</button>';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [x, toggle, pop] = target.querySelectorAll('button');
+
+		// X is deferred on its own await
+		x.click();
+		await tick();
+		// Y is deferred on the await inside the new branch
+		toggle.click();
+		await tick();
+		// Z hides that branch again, merges into Y, which merges into X
+		toggle.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `<p>n = 0</p><p>x = 0</p>${buttons}`);
+
+		// the branch's await resolves while it is hidden
+		pop.click();
+		await tick();
+		// W shows the branch again and merges into X
+		toggle.click();
+		await tick();
+
+		// nothing is pending any more, so X commits
+		pop.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `<p>n = 4</p><p>x = 1</p><p>cond: e</p>${buttons}`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-merge-blocking-pending/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-merge-blocking-pending/main.svelte
new file mode 100644
index 0000000000..d4cbd8c5b6
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-merge-blocking-pending/main.svelte
@@ -0,0 +1,29 @@
+<script>
+	let n = $state(0);
+	let x = $state(0);
+	let cond = $state(false);
+	let pend = false;
+
+	const deferred = [];
+
+	function delay(value) {
+		if (!pend) return value;
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+<svelte:boundary>
+	<p>n = {n}</p>
+	<p>x = {await delay(x)}</p>
+	{#if cond}
+		<p>cond: {await delay('e')}</p>
+	{/if}
+
+	{#snippet pending()}
+		<p>loading</p>
+	{/snippet}
+</svelte:boundary>
+
+<button onclick={() => { pend = true; n++; x = 1; }}>x</button>
+<button onclick={() => { n++; cond = !cond; }}>toggle</button>
+<button onclick={() => deferred.pop()()}>pop</button>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/Item.svelte b/packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/Item.svelte
new file mode 100644
index 0000000000..7e30dd0125
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/Item.svelte
@@ -0,0 +1,9 @@
+<script>
+	let { item } = $props();
+	$effect(() => {
+		console.log('mount ' + item);
+		return () => console.log('destroy ' + item);
+	});
+</script>
+
+<span>{item}</span>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/_config.js b/packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/_config.js
new file mode 100644
index 0000000000..9bd1abce5c
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/_config.js
@@ -0,0 +1,47 @@
+import { flushSync, tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	mode: ['client'],
+	async test({ assert, target, logs }) {
+		const [a, push, clear, shift] = target.querySelectorAll('button');
+
+		shift.click();
+		await tick();
+		assert.deepEqual(logs, ['mount 1', 'mount 2', 'mount 3']);
+
+		// batch X: a=1, pending
+		a.click();
+		await tick();
+
+		// batch Y: list changes; the each block reads `a`, so Y merges into X.
+		// The each block registers commit/discard callbacks with Y
+		push.click();
+		await tick();
+
+		// X resolves and throws while processing (not handled by any boundary), so it is discarded
+		// rather than committed. Y's discard callbacks must still run
+		let error = null;
+		shift.click();
+		try {
+			for (let i = 0; i < 10; i++) {
+				await Promise.resolve();
+				flushSync();
+			}
+		} catch (e) {
+			error = e;
+		}
+		assert.match(/** @type {Error} */ (error)?.message, /^boom/);
+
+		logs.length = 0;
+		clear.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>a</button><button>push</button><button>clear</button><button>shift</button><p>a: 0</p>`
+		);
+		// the removed items must be destroyed, not kept around for the discarded batch Y
+		assert.deepEqual(logs, ['destroy 1', 'destroy 2', 'destroy 3']);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/main.svelte
new file mode 100644
index 0000000000..ea1bfd8594
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-merge-discard-after-error/main.svelte
@@ -0,0 +1,39 @@
+<script>
+	import Item from './Item.svelte';
+
+	let a = $state(0);
+	let list = $state([1, 2, 3]);
+
+	let queue = [];
+	function push(v) {
+		const d = Promise.withResolvers();
+		queue.push(() => d.resolve(v));
+		return d.promise;
+	}
+
+	function check(v) {
+		if (v === 1) throw new Error('boom');
+		return true;
+	}
+</script>
+
+<button onclick={() => a++}>a</button>
+<button onclick={() => list.push(4)}>push</button>
+<button onclick={() => (list = [])}>clear</button>
+<button onclick={() => queue.shift()?.()}>shift</button>
+
+<!-- no failed snippet/onerror: errors are not handled by the boundary -->
+<svelte:boundary>
+	{#if check(await push(a))}
+		<p>a: {a}</p>
+	{/if}
+
+	<!-- reads `a`, so batches changing `list` depend on (and merge into) a pending batch changing `a` -->
+	{#each list.slice(0, a + 10) as item (item)}
+		<Item {item} />
+	{/each}
+
+	{#snippet pending()}
+		<p>loading...</p>
+	{/snippet}
+</svelte:boundary>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-stale-read-many-sources/_config.js b/packages/svelte/tests/runtime-runes/samples/async-stale-read-many-sources/_config.js
new file mode 100644
index 0000000000..f5318b930a
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-stale-read-many-sources/_config.js
@@ -0,0 +1,39 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+const buttons = '<button>show</button><button>items</button><button>shift</button>';
+/** @param {number} offset */
+const list = (offset) => Array.from({ length: 2000 }, (_, i) => `${i + offset},`).join('');
+
+// When the branch is created in B1, the each block reads all 2000 items, each of which B2 holds
+// a newer value of. The each block must be scheduled to re-run in B2 once, not once per stale
+// value — otherwise B2 is flushed 2000 times (each flush being O(size of B2)), which takes seconds
+export default test({
+	mode: ['client'],
+	async test({ assert, target }) {
+		await tick();
+		const [show, items, shift] = target.querySelectorAll('button');
+
+		// B1: show = true -> pending
+		show.click();
+		await tick();
+
+		// B2: increment all items -> pending
+		items.click();
+		await tick();
+
+		// B1 resolves -> the branch is created, reading 2000 values that are stale relative to B2
+		const start = performance.now();
+		shift.click();
+		await tick();
+		const duration = performance.now() - start;
+
+		assert.ok(duration < 1000, `took ${duration}ms`);
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>0</p><div>${list(0)}</div>`);
+
+		// B2 resolves
+		shift.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>1</p><div>${list(1)}</div>`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-stale-read-many-sources/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-stale-read-many-sources/main.svelte
new file mode 100644
index 0000000000..f932241cc6
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-stale-read-many-sources/main.svelte
@@ -0,0 +1,23 @@
+<script>
+	let show = $state(false);
+	let items = $state(Array.from({ length: 2000 }, (_, i) => i));
+
+	const deferred = [];
+
+	function delay(v) {
+		if (!v) return v;
+		return new Promise((resolve) => deferred.push(() => resolve(v)));
+	}
+</script>
+
+<button onclick={() => (show = true)}>show</button>
+<button onclick={() => items.forEach((_, i) => items[i]++)}>items</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+
+<p>{await delay(items[0])}</p>
+
+{#if await delay(show)}
+	<div>
+		{#each items as item}{item},{/each}
+	</div>
+{/if}