diff --git a/.changeset/volatile-each-key.md b/.changeset/volatile-each-key.md
new file mode 100644
index 0000000000..674bce9bec
--- /dev/null
+++ b/.changeset/volatile-each-key.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: detect and error on non-idempotent each block keys in dev mode
diff --git a/documentation/docs/98-reference/.generated/client-errors.md b/documentation/docs/98-reference/.generated/client-errors.md
index 8601a728a7..7fccac5808 100644
--- a/documentation/docs/98-reference/.generated/client-errors.md
+++ b/documentation/docs/98-reference/.generated/client-errors.md
@@ -62,6 +62,14 @@ Keyed each block has duplicate key at indexes %a% and %b%
 Keyed each block has duplicate key `%value%` at indexes %a% and %b%
 ```
 
+### each_key_volatile
+
+```
+Keyed each block has key that is not idempotent — the key for item at index %index% was `%a%` but is now `%b%`. Keys must be the same each time for a given item
+```
+
+The key expression in a keyed each block must return the same value when called multiple times for the same item. Using expressions like `[item.a, item.b]` creates a new array each time, which will never be equal to itself. Instead, use a primitive value or create a stable key like `item.a + '-' + item.b`.
+
 ### effect_in_teardown
 
 ```
diff --git a/packages/svelte/messages/client-errors/errors.md b/packages/svelte/messages/client-errors/errors.md
index bedf6db0a5..3f20cb989d 100644
--- a/packages/svelte/messages/client-errors/errors.md
+++ b/packages/svelte/messages/client-errors/errors.md
@@ -42,6 +42,12 @@ See the [migration guide](/docs/svelte/v5-migration-guide#Components-are-no-long
 
 > Keyed each block has duplicate key `%value%` at indexes %a% and %b%
 
+## each_key_volatile
+
+> Keyed each block has key that is not idempotent — the key for item at index %index% was `%a%` but is now `%b%`. Keys must be the same each time for a given item
+
+The key expression in a keyed each block must return the same value when called multiple times for the same item. Using expressions like `[item.a, item.b]` creates a new array each time, which will never be equal to itself. Instead, use a primitive value or create a stable key like `item.a + '-' + item.b`.
+
 ## effect_in_teardown
 
 > `%rune%` cannot be used inside an effect cleanup function
diff --git a/packages/svelte/src/internal/client/dom/blocks/each.js b/packages/svelte/src/internal/client/dom/blocks/each.js
index 25f7cf91eb..7ae02d073c 100644
--- a/packages/svelte/src/internal/client/dom/blocks/each.js
+++ b/packages/svelte/src/internal/client/dom/blocks/each.js
@@ -250,6 +250,14 @@ export function each(node, flags, get_collection, get_key, render_fn, fallback_f
 			var value = array[index];
 			var key = get_key(value, index);
 
+			if (DEV) {
+				// Check that the key function is idempotent (returns the same value when called twice)
+				var key_again = get_key(value, index);
+				if (key !== key_again) {
+					e.each_key_volatile(String(index), String(key), String(key_again));
+				}
+			}
+
 			var item = first_run ? null : items.get(key);
 
 			if (item) {
diff --git a/packages/svelte/src/internal/client/errors.js b/packages/svelte/src/internal/client/errors.js
index 34f1d85540..d60c2dd280 100644
--- a/packages/svelte/src/internal/client/errors.js
+++ b/packages/svelte/src/internal/client/errors.js
@@ -147,6 +147,25 @@ export function each_key_duplicate(a, b, value) {
 	}
 }
 
+/**
+ * Keyed each block has key that is not idempotent — the key for item at index %index% was `%a%` but is now `%b%`. Keys must be the same each time for a given item
+ * @param {string} index
+ * @param {string} a
+ * @param {string} b
+ * @returns {never}
+ */
+export function each_key_volatile(index, a, b) {
+	if (DEV) {
+		const error = new Error(`each_key_volatile\nKeyed each block has key that is not idempotent — the key for item at index ${index} was \`${a}\` but is now \`${b}\`. Keys must be the same each time for a given item\nhttps://svelte.dev/e/each_key_volatile`);
+
+		error.name = 'Svelte error';
+
+		throw error;
+	} else {
+		throw new Error(`https://svelte.dev/e/each_key_volatile`);
+	}
+}
+
 /**
  * `%rune%` cannot be used inside an effect cleanup function
  * @param {string} rune
diff --git a/packages/svelte/tests/runtime-runes/samples/each-key-volatile/_config.js b/packages/svelte/tests/runtime-runes/samples/each-key-volatile/_config.js
new file mode 100644
index 0000000000..87d4bf45c7
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/each-key-volatile/_config.js
@@ -0,0 +1,11 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+
+	mode: ['client'],
+
+	error: 'each_key_volatile'
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/each-key-volatile/main.svelte b/packages/svelte/tests/runtime-runes/samples/each-key-volatile/main.svelte
new file mode 100644
index 0000000000..689c257cfa
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/each-key-volatile/main.svelte
@@ -0,0 +1,10 @@
+<script>
+	let things = $state([
+		{ group: 'a', id: 1 },
+		{ group: 'b', id: 2 }
+	]);
+</script>
+
+{#each things as thing ([thing.group, thing.id])}
+	<p>{thing.group}-{thing.id}</p>
+{/each}