fix: add async parameter to svelte-autofixer

Author: paoloricciuti

.changeset/icy-queens-greet.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/mcp': patch
+---
+
+fix: add `async` parameter to `svelte-autofixer`

packages/mcp-server/src/mcp/autofixers/add-autofixers-issues.ts

@@ -8,14 +8,15 @@ export function add_autofixers_issues(
 code: string,
 desired_svelte_version: number,
 filename = 'Component.svelte',
+async = false,
 ) {
 const parsed = parse(code, filename);
 
 // Run each autofixer separately to avoid interrupting logic flow
 for (const autofixer of Object.values(autofixers)) {
 walk(
 parsed.ast as unknown as Node,
-{ output: content, parsed, desired_svelte_version },
+{ output: content, parsed, desired_svelte_version, async },
 autofixer,
 );
 }

packages/mcp-server/src/mcp/autofixers/add-compile-issues.ts

@@ -7,6 +7,7 @@ export function add_compile_issues(
 code: string,
 desired_svelte_version: number,
 filename = 'Component.svelte',
+async = false,
 ) {
 let compile = compile_component;
 const extension = extname(filename);
@@ -27,6 +28,7 @@ export function add_compile_issues(
 filename: filename || 'Component.svelte',
 generate: false,
 runes: desired_svelte_version >= 5,
+experimental: { async },
 });
 
 for (const warning of compilation_result.warnings) {

packages/mcp-server/src/mcp/autofixers/add-eslint-issues.ts

@@ -51,7 +51,7 @@ function base_config(svelte_config: Config): ESLint.Options['baseConfig'] {
 ];
 }
 
-function get_linter(version: number) {
+function get_linter(version: number, async = false) {
 if (version < 5) {
 return (svelte_4_linter ??= new ESLint({
 overrideConfigFile: true,
@@ -67,6 +67,7 @@ function get_linter(version: number) {
 baseConfig: base_config({
 compilerOptions: {
 runes: true,
+experimental: { async },
 },
 }),
 }));
@@ -77,8 +78,9 @@ export async function add_eslint_issues(
 code: string,
 desired_svelte_version: number,
 filename = 'Component.svelte',
+async = false,
 ) {
-const eslint = get_linter(desired_svelte_version);
+const eslint = get_linter(desired_svelte_version, async);
 const results = await eslint.lintText(code, { filePath: filename || './Component.svelte' });
 
 for (const message of results[0]?.messages ?? []) {

packages/mcp-server/src/mcp/autofixers/visitors/index.ts

@@ -7,6 +7,7 @@ export type AutofixerState = {
 output: { issues: string[]; suggestions: string[] };
 parsed: ParseResult;
 desired_svelte_version: number;
+async?: boolean;
 };
 
 export type Autofixer = Visitors<Node | AST.SvelteNode, AutofixerState>;

packages/mcp-server/src/mcp/handlers/tools/svelte-autofixer.test.ts

@@ -8,7 +8,12 @@ function request<const T>(request: T) {
 return request;
 }
 
-async function autofixer_tool_call(code: string, is_error = false, desired_svelte_version = 5) {
+async function autofixer_tool_call(
+code: string,
+is_error = false,
+desired_svelte_version = 5,
+async = false,
+) {
 const result = await server.receive({
 jsonrpc: '2.0',
 id: 2,
@@ -19,6 +24,7 @@ async function autofixer_tool_call(code: string, is_error = false, desired_svelt
 code,
 desired_svelte_version,
 filename: 'App.svelte',
+async,
 },
 },
 });
@@ -65,6 +71,61 @@ describe('svelte-autofixer tool', () => {
 );
 });
 
+it('should error out if async is true with a version less than 5', async () => {
+const content = await autofixer_tool_call(
+`<script>
+$state count = 0;
+</script>`,
+true,
+4,
+true,
+);
+expect(content.isError).toBeTruthy();
+expect(content.content[0]).toBeDefined();
+expect(content.content[0].text).toBe(
+'The async option can only be used with Svelte version 5 or higher.',
+);
+});
+
+it('should not add suggestion/issues if async is true and await is used in the template/derived', async () => {
+const content = await autofixer_tool_call(
+`<script>
+import { slow_double } from './utils.js';
+let count = $state(0);
+let double = $derived(await slow_double(count));
+</script>
+
+{double}
+{await slow_double(count)}`,
+false,
+5,
+true,
+);
+expect(content.issues).toHaveLength(0);
+expect(content.suggestions).toHaveLength(0);
+expect(content.require_another_tool_call_after_fixing).toBeFalsy();
+});
+
+it('should add suggestion/issues if async is false and await is used in the template/derived', async () => {
+const content = await autofixer_tool_call(
+`<script>
+import { slow_double } from './utils.js';
+let count = $state(0);
+let double = $derived(await slow_double(count));
+</script>
+
+{double}
+{await slow_double(count)}`,
+false,
+5,
+);
+expect(content.issues.length).toBeGreaterThanOrEqual(1);
+expect(content.issues).toEqual(
+expect.arrayContaining([expect.stringContaining('experimental_async')]),
+);
+expect(content.require_another_tool_call_after_fixing).toBeTruthy();
+});
+
 it('should add suggestions for css invalid identifier', async () => {
 const content = await autofixer_tool_call(`<script>
 let my_color = $state('red');

packages/mcp-server/src/mcp/handlers/tools/svelte-autofixer.ts

@@ -21,6 +21,12 @@ export function svelte_autofixer(server: SvelteMcp) {
 'The desired svelte version...if possible read this from the package.json of the user project, otherwise use some hint from the wording (if the user asks for runes it wants version 5). Default to 5 in case of doubt.',
 ),
 ),
+async: v.pipe(
+v.optional(v.boolean()),
+v.description(
+'If true the code is an async component/module and might use await in the markup or top-level awaits in the script tag. If possible check the svelte.config.js/svelte.config.ts to check if the option is enabled otherwise asks the user if they prefer using it or not. You can only use this option if the version is 5.',
+),
+),
 filename: v.pipe(
 v.optional(v.string()),
 v.description(
@@ -45,6 +51,7 @@ export function svelte_autofixer(server: SvelteMcp) {
 code,
 filename: filename_or_path,
 desired_svelte_version: desired_svelte_version_unchecked,
+async,
 }) => {
 if (server.ctx.sessionId && server.ctx.custom?.track) {
 await server.ctx.custom?.track?.(server.ctx.sessionId, 'svelte-autofixer');
@@ -71,6 +78,18 @@ export function svelte_autofixer(server: SvelteMcp) {
 
 const desired_svelte_version = parsed_version.output;
 
+if (async && +desired_svelte_version < 5) {
+return {
+isError: true,
+content: [
+{
+type: 'text',
+text: `The async option can only be used with Svelte version 5 or higher.`,
+},
+],
+};
+}
+
 const content: {
 issues: string[];
 suggestions: string[];
@@ -82,11 +101,11 @@ export function svelte_autofixer(server: SvelteMcp) {
 
 const filename = filename_or_path ? basename(filename_or_path) : 'Component.svelte';
 
-add_compile_issues(content, code, +desired_svelte_version, filename);
+add_compile_issues(content, code, +desired_svelte_version, filename, async);
 
-add_autofixers_issues(content, code, +desired_svelte_version, filename);
+add_autofixers_issues(content, code, +desired_svelte_version, filename, async);
 
-await add_eslint_issues(content, code, +desired_svelte_version, filename);
+await add_eslint_issues(content, code, +desired_svelte_version, filename, async);
 } catch (e: unknown) {
 const error = e as Error & { start?: { line: number; column: number } };
 content.issues.push(