From 24de0e38c7d9557fcbd13b691b4cf2b80499de90 Mon Sep 17 00:00:00 2001
From: Serghei <43180231+xyrolle@users.noreply.github.com>
Date: Thu, 22 Oct 2026 16:02:01 +0300
Subject: [PATCH] fix: keep the case of Svelte file paths with
 --tsgo-experimental-api

---
 .changeset/tsgo-api-file-casing.md            |  6 +++++
 .../features/DiagnosticsProvider.ts           |  5 +++-
 .../features/diagnostics/index.test.ts        | 27 +++++++++++++++++++
 .../typescript-go/testfiles/Upper.svelte      |  3 +++
 .../typescript-go/testfiles/tsconfig.json     |  3 +++
 5 files changed, 43 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/tsgo-api-file-casing.md
 create mode 100644 packages/language-server/test/plugins/typescript-go/testfiles/Upper.svelte
 create mode 100644 packages/language-server/test/plugins/typescript-go/testfiles/tsconfig.json

diff --git a/.changeset/tsgo-api-file-casing.md b/.changeset/tsgo-api-file-casing.md
new file mode 100644
index 000000000..90b1e163f
--- /dev/null
+++ b/.changeset/tsgo-api-file-casing.md
@@ -0,0 +1,6 @@
+---
+'svelte-language-server': patch
+'svelte-check': patch
+---
+
+fix: report Svelte compiler and CSS diagnostics under the file's real path with `--tsgo-experimental-api` on case-insensitive file systems
diff --git a/packages/language-server/src/plugins/typescript-go/features/DiagnosticsProvider.ts b/packages/language-server/src/plugins/typescript-go/features/DiagnosticsProvider.ts
index 272d9c871..a9a855948 100644
--- a/packages/language-server/src/plugins/typescript-go/features/DiagnosticsProvider.ts
+++ b/packages/language-server/src/plugins/typescript-go/features/DiagnosticsProvider.ts
@@ -193,7 +193,10 @@ export class SvelteCheckTSGoDiagnosticsProvider implements DiagnosticsProvider {
     }
 
     getAllSvelteFiles(): string[] {
-        return Array.from(this.files.keys()).filter(isSvelteFilePath);
+        // not the keys, which are lowercase on case-insensitive file systems
+        return Array.from(this.files.values(), (snapshot) => snapshot.filePath).filter(
+            isSvelteFilePath
+        );
     }
 
     mapAndFilterDiagnostics(
diff --git a/packages/language-server/test/plugins/typescript-go/features/diagnostics/index.test.ts b/packages/language-server/test/plugins/typescript-go/features/diagnostics/index.test.ts
index ae4537d06..2b426ba6e 100644
--- a/packages/language-server/test/plugins/typescript-go/features/diagnostics/index.test.ts
+++ b/packages/language-server/test/plugins/typescript-go/features/diagnostics/index.test.ts
@@ -12,6 +12,7 @@ import { Document } from '../../../../../src/lib/documents';
 import ts from 'typescript';
 import { getPackageInfo } from '../../../../../src/importPackage';
 import assert from 'assert';
+import sinon from 'sinon';
 
 const isSvelte5Plus = Number(VERSION.split('.')[0]) >= 5;
 
@@ -40,6 +41,32 @@ describe('SvelteCheckTSGoDiagnosticsProvider', function () {
         workspaceDir: fixturesDir
     });
 
+    afterEach(() => sinon.restore());
+
+    it('returns Svelte file paths in their original case', async () => {
+        const syncApi = await import('@typescript/native-preview/unstable/sync');
+        const tsAst = await import('@typescript/native-preview/unstable/ast');
+        const tsconfig = path.join(__dirname, '../../testfiles/tsconfig.json');
+        // The file map lowercases its keys on case-insensitive file systems
+        sinon.stub(ts.sys, 'useCaseSensitiveFileNames').value(false);
+        const service = new SvelteCheckTSGoDiagnosticsProvider(
+            syncApi,
+            tsAst,
+            tsconfig,
+            'svelte2tsx',
+            (filePath, content) => new Document(pathToUrl(filePath), content)
+        );
+        sinon.restore();
+        try {
+            await service.getProject();
+            assert.deepStrictEqual(service.getAllSvelteFiles(), [
+                normalizePath(path.join(path.dirname(tsconfig), 'Upper.svelte'))
+            ]);
+        } finally {
+            service.dispose();
+        }
+    });
+
     after(() => {
         for (const provider of providerPool.values()) {
             provider.dispose();
diff --git a/packages/language-server/test/plugins/typescript-go/testfiles/Upper.svelte b/packages/language-server/test/plugins/typescript-go/testfiles/Upper.svelte
new file mode 100644
index 000000000..702431c1f
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript-go/testfiles/Upper.svelte
@@ -0,0 +1,3 @@
+<script lang="ts">
+    export const value = 1;
+</script>
diff --git a/packages/language-server/test/plugins/typescript-go/testfiles/tsconfig.json b/packages/language-server/test/plugins/typescript-go/testfiles/tsconfig.json
new file mode 100644
index 000000000..16a0f353b
--- /dev/null
+++ b/packages/language-server/test/plugins/typescript-go/testfiles/tsconfig.json
@@ -0,0 +1,3 @@
+{
+    "compilerOptions": { "strict": true, "types": [] }
+}