From ffa8cdebe80f9fe09d2688d069c6a2767119ffcd Mon Sep 17 00:00:00 2001
From: Nic <162764842+Nic-Polumeyv@users.noreply.github.com>
Date: Fri, 10 Jul 2026 19:28:16 -0400
Subject: [PATCH] fix: don't notify SvelteMap key trackers when an existing
 key's value is set

---
 .changeset/major-maps-agree.md             |  5 ++++
 packages/svelte/src/reactivity/map.js      | 18 ++++++++-----
 packages/svelte/src/reactivity/map.test.ts | 30 ++++++++++++++++++++++
 3 files changed, 46 insertions(+), 7 deletions(-)
 create mode 100644 .changeset/major-maps-agree.md

diff --git a/.changeset/major-maps-agree.md b/.changeset/major-maps-agree.md
new file mode 100644
index 0000000000..be414365ea
--- /dev/null
+++ b/.changeset/major-maps-agree.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't notify `SvelteMap` key trackers when an existing key's value is set
diff --git a/packages/svelte/src/reactivity/map.js b/packages/svelte/src/reactivity/map.js
index 48d06a05a7..fcfc4c2089 100644
--- a/packages/svelte/src/reactivity/map.js
+++ b/packages/svelte/src/reactivity/map.js
@@ -161,19 +161,23 @@ export class SvelteMap extends Map {
 		var sources = this.#sources;
 		var s = sources.get(key);
 		var prev_res = super.get(key);
+		var existed = super.has(key);
 		var res = super.set(key, value);
 		var version = this.#version;
 
 		if (s === undefined) {
-			s = this.#source(0);
+			// if the key existed but was never read individually, nothing tracks it and the key set is unchanged
+			if (!existed) {
+				s = this.#source(0);
 
-			if (DEV) {
-				tag(s, `SvelteMap get(${label(key)})`);
-			}
+				if (DEV) {
+					tag(s, `SvelteMap get(${label(key)})`);
+				}
 
-			sources.set(key, s);
-			set(this.#size, super.size);
-			increment(version);
+				sources.set(key, s);
+				set(this.#size, super.size);
+				increment(version);
+			}
 		} else if (prev_res !== value) {
 			increment(s);
 
diff --git a/packages/svelte/src/reactivity/map.test.ts b/packages/svelte/src/reactivity/map.test.ts
index 8bb6f72f7b..fd0779562e 100644
--- a/packages/svelte/src/reactivity/map.test.ts
+++ b/packages/svelte/src/reactivity/map.test.ts
@@ -307,3 +307,33 @@ test('not invoking reactivity when value is not in the map after changes', () =>
 test('Map.instanceOf', () => {
 	assert.equal(new SvelteMap() instanceof Map, true);
 });
+
+test('map.keys() is not notified when an existing unread key is set to a new value', () => {
+	const map = new SvelteMap([
+		[1, 'a'],
+		[2, 'b']
+	]);
+
+	const log: any = [];
+
+	const cleanup = effect_root(() => {
+		render_effect(() => {
+			log.push(Array.from(map.keys()));
+		});
+	});
+
+	flushSync(() => {
+		map.set(1, 'c');
+	});
+
+	flushSync(() => {
+		map.set(3, 'd');
+	});
+
+	assert.deepEqual(log, [
+		[1, 2],
+		[1, 2, 3]
+	]);
+
+	cleanup();
+});