From a23599a196380ce7fc3fb374dac4daee173776b7 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Sun, 13 Jul 2025 18:20:29 -0400
Subject: [PATCH] fix: don't show adjusted error messages in boundaries
 (#16360)

---
 .changeset/large-balloons-agree.md            |  5 ++
 .../src/internal/client/error-handling.js     | 47 ++++++++++++-------
 .../samples/error-boundary-3/_config.js       |  2 +-
 .../samples/error-boundary-3/main.svelte      |  6 +--
 4 files changed, 38 insertions(+), 22 deletions(-)
 create mode 100644 .changeset/large-balloons-agree.md

diff --git a/.changeset/large-balloons-agree.md b/.changeset/large-balloons-agree.md
new file mode 100644
index 0000000000..9355336862
--- /dev/null
+++ b/.changeset/large-balloons-agree.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't show adjusted error messages in boundaries
diff --git a/packages/svelte/src/internal/client/error-handling.js b/packages/svelte/src/internal/client/error-handling.js
index 378f7408ef..b12f21adfc 100644
--- a/packages/svelte/src/internal/client/error-handling.js
+++ b/packages/svelte/src/internal/client/error-handling.js
@@ -7,14 +7,16 @@ import { BOUNDARY_EFFECT, EFFECT_RAN } from './constants.js';
 import { define_property, get_descriptor } from '../shared/utils.js';
 import { active_effect } from './runtime.js';
 
+const adjustments = new WeakMap();
+
 /**
  * @param {unknown} error
  */
 export function handle_error(error) {
 	var effect = /** @type {Effect} */ (active_effect);
 
-	if (DEV && error instanceof Error) {
-		adjust_error(error, effect);
+	if (DEV && error instanceof Error && !adjustments.has(error)) {
+		adjustments.set(error, get_adjustments(error, effect));
 	}
 
 	if ((effect.f & EFFECT_RAN) === 0) {
@@ -48,21 +50,19 @@ export function invoke_error_boundary(error, effect) {
 		effect = effect.parent;
 	}
 
+	if (error instanceof Error) {
+		apply_adjustments(error);
+	}
+
 	throw error;
 }
 
-/** @type {WeakSet<Error>} */
-const adjusted_errors = new WeakSet();
-
 /**
  * Add useful information to the error message/stack in development
  * @param {Error} error
  * @param {Effect} effect
  */
-function adjust_error(error, effect) {
-	if (adjusted_errors.has(error)) return;
-	adjusted_errors.add(error);
-
+function get_adjustments(error, effect) {
 	const message_descriptor = get_descriptor(error, 'message');
 
 	// if the message was already changed and it's not configurable we can't change it
@@ -78,17 +78,28 @@ function adjust_error(error, effect) {
 		context = context.p;
 	}
 
-	define_property(error, 'message', {
-		value: error.message + `\n${component_stack}\n`
-	});
+	return {
+		message: error.message + `\n${component_stack}\n`,
+		stack: error.stack
+			?.split('\n')
+			.filter((line) => !line.includes('svelte/src/internal'))
+			.join('\n')
+	};
+}
+
+/**
+ * @param {Error} error
+ */
+function apply_adjustments(error) {
+	const adjusted = adjustments.get(error);
+
+	if (adjusted) {
+		define_property(error, 'message', {
+			value: adjusted.message
+		});
 
-	if (error.stack) {
-		// Filter out internal modules
 		define_property(error, 'stack', {
-			value: error.stack
-				.split('\n')
-				.filter((line) => !line.includes('svelte/src/internal'))
-				.join('\n')
+			value: adjusted.stack
 		});
 	}
 }
diff --git a/packages/svelte/tests/runtime-runes/samples/error-boundary-3/_config.js b/packages/svelte/tests/runtime-runes/samples/error-boundary-3/_config.js
index 040e13676e..06da8f667c 100644
--- a/packages/svelte/tests/runtime-runes/samples/error-boundary-3/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/error-boundary-3/_config.js
@@ -9,6 +9,6 @@ export default test({
 		flushSync();
 
 		assert.deepEqual(logs, ['error caught']);
-		assert.htmlEqual(target.innerHTML, `<div>Fallback!</div><button>+</button>`);
+		assert.htmlEqual(target.innerHTML, `<div>oh no!</div><button>+</button>`);
 	}
 });
diff --git a/packages/svelte/tests/runtime-runes/samples/error-boundary-3/main.svelte b/packages/svelte/tests/runtime-runes/samples/error-boundary-3/main.svelte
index bad84666c0..bc7fe072c4 100644
--- a/packages/svelte/tests/runtime-runes/samples/error-boundary-3/main.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/error-boundary-3/main.svelte
@@ -1,6 +1,6 @@
 <script>
 	function throw_error() {
-		throw new Error('test')
+		throw new Error('oh no!')
 	}
 
 	let count = $state(0);
@@ -9,8 +9,8 @@
 <svelte:boundary onerror={(e) => console.log('error caught')}>
 	{count > 0 ? throw_error() : null}
 
-	{#snippet failed()}
-		<div>Fallback!</div>
+	{#snippet failed(e)}
+		<div>{e.message}</div>
 	{/snippet}
 </svelte:boundary>