fix: don't define error.message if it's not configurable

.changeset/unlucky-plants-jump.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't define `error.message` if it's not configurable

packages/svelte/src/internal/client/error-handling.js

@@ -3,7 +3,7 @@ import { DEV } from 'esm-env';
 import { FILENAME } from '../../constants.js';
 import { is_firefox } from './dom/operations.js';
 import { BOUNDARY_EFFECT, EFFECT_RAN } from './constants.js';
-import { define_property } from '../shared/utils.js';
+import { define_property, get_descriptor } from '../shared/utils.js';
 import { active_effect } from './runtime.js';
 
 /**
@@ -63,6 +63,12 @@ function adjust_error(error, effect) {
 	if (adjusted_errors.has(error)) return;
 	adjusted_errors.add(error);
 
+	const message_descriptor = get_descriptor(error, 'message');
+
+	// if the message was already changed and it's not configurable we can't change it
+	// or it will throw a different error swallowing the original error
+	if (message_descriptor && !message_descriptor.configurable) return;
+
 	var indent = is_firefox ? '  ' : '\t';
 	var component_stack = `\n${indent}in ${effect.fn?.name || '<unknown>'}`;
 	var context = effect.ctx;

packages/svelte/tests/runtime-runes/samples/non-configurable-errors/_config.js

@@ -0,0 +1,8 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+	error: 'test'
+});

packages/svelte/tests/runtime-runes/samples/non-configurable-errors/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	class CustomError extends Error {
+	  constructor() {
+	    super();
+	    Object.defineProperty(this, "message", {
+			value: "test"
+		});
+	  }
+	}
+	throw new CustomError()
+</script>