From 9d0edf720ef9ad8d08294119af5a3ff534a90596 Mon Sep 17 00:00:00 2001
From: Julia <julia.cavazos-england@entegrata.com>
Date: Tue, 17 Mar 2026 18:57:53 -0400
Subject: [PATCH] fix: better error for TS annotations in snippets without
 lang="ts"

---
 .changeset/gentle-dots-dream.md               |  5 +++++
 .../src/compiler/phases/1-parse/index.js      |  5 +++--
 .../src/compiler/phases/1-parse/state/tag.js  | 20 +++++++++++++++----
 .../snippet-ts-annotation-no-lang/_config.js  |  9 +++++++++
 .../snippet-ts-annotation-no-lang/main.svelte |  3 +++
 5 files changed, 36 insertions(+), 6 deletions(-)
 create mode 100644 .changeset/gentle-dots-dream.md
 create mode 100644 packages/svelte/tests/compiler-errors/samples/snippet-ts-annotation-no-lang/_config.js
 create mode 100644 packages/svelte/tests/compiler-errors/samples/snippet-ts-annotation-no-lang/main.svelte

diff --git a/.changeset/gentle-dots-dream.md b/.changeset/gentle-dots-dream.md
new file mode 100644
index 0000000000..d3987705ca
--- /dev/null
+++ b/.changeset/gentle-dots-dream.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+Improve error message when TypeScript syntax is used in snippet parameters without `lang="ts"`
diff --git a/packages/svelte/src/compiler/phases/1-parse/index.js b/packages/svelte/src/compiler/phases/1-parse/index.js
index 81adbbb555..5c2426fac6 100644
--- a/packages/svelte/src/compiler/phases/1-parse/index.js
+++ b/packages/svelte/src/compiler/phases/1-parse/index.js
@@ -177,10 +177,11 @@ export class Parser {
 
 	/**
 	 * @param {any} err
+	 * @param {string} [suffix]
 	 * @returns {never}
 	 */
-	acorn_error(err) {
-		e.js_parse_error(err.pos, err.message.replace(regex_position_indicator, ''));
+	acorn_error(err, suffix = '') {
+		e.js_parse_error(err.pos, err.message.replace(regex_position_indicator, '') + suffix);
 	}
 
 	/**
diff --git a/packages/svelte/src/compiler/phases/1-parse/state/tag.js b/packages/svelte/src/compiler/phases/1-parse/state/tag.js
index d9518c726f..255e190619 100644
--- a/packages/svelte/src/compiler/phases/1-parse/state/tag.js
+++ b/packages/svelte/src/compiler/phases/1-parse/state/tag.js
@@ -390,16 +390,28 @@ function open(parser) {
 		const prelude = parser.template.slice(0, params_start).replace(/\S/g, ' ');
 		const params = parser.template.slice(params_start, parser.index);
 
-		let function_expression = matched
-			? /** @type {ArrowFunctionExpression} */ (
+		let function_expression;
+
+		if (matched) {
+			try {
+				function_expression = /** @type {ArrowFunctionExpression} */ (
 					parse_expression_at(
 						prelude + `${params} => {}`,
 						parser.root.comments,
 						parser.ts,
 						params_start
 					)
-				)
-			: { params: [] };
+				);
+			} catch (/** @type {any} */ err) {
+				const suffix =
+					!parser.ts && parser.template[err.pos] === ':'
+						? ` (did you forget to add \`lang="ts"\`?)`
+						: '';
+				parser.acorn_error(err, suffix);
+			}
+		} else {
+			function_expression = { params: [] };
+		}
 
 		parser.allow_whitespace();
 		parser.eat('}', true);
diff --git a/packages/svelte/tests/compiler-errors/samples/snippet-ts-annotation-no-lang/_config.js b/packages/svelte/tests/compiler-errors/samples/snippet-ts-annotation-no-lang/_config.js
new file mode 100644
index 0000000000..b56e835f96
--- /dev/null
+++ b/packages/svelte/tests/compiler-errors/samples/snippet-ts-annotation-no-lang/_config.js
@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'js_parse_error',
+		message: 'Unexpected token (did you forget to add `lang="ts"`?)',
+		position: [18, 18]
+	}
+});
diff --git a/packages/svelte/tests/compiler-errors/samples/snippet-ts-annotation-no-lang/main.svelte b/packages/svelte/tests/compiler-errors/samples/snippet-ts-annotation-no-lang/main.svelte
new file mode 100644
index 0000000000..514e077bbc
--- /dev/null
+++ b/packages/svelte/tests/compiler-errors/samples/snippet-ts-annotation-no-lang/main.svelte
@@ -0,0 +1,3 @@
+{#snippet bar(text: string)}
+	{text}
+{/snippet}