diff --git a/mobile/src/session/mobile-file-language.ts b/mobile/src/session/mobile-file-language.ts
index 30e4656c0..03b368c94 100644
--- a/mobile/src/session/mobile-file-language.ts
+++ b/mobile/src/session/mobile-file-language.ts
@@ -10,6 +10,8 @@ function extname(filePath: string): string {
 const EXT_TO_LANGUAGE: Record<string, string> = {
   '.ts': 'typescript',
   '.tsx': 'typescript',
+  '.cts': 'typescript',
+  '.mts': 'typescript',
   '.js': 'javascript',
   '.jsx': 'javascript',
   '.mjs': 'javascript',
diff --git a/mobile/src/session/mobile-file-syntax.test.ts b/mobile/src/session/mobile-file-syntax.test.ts
index 595097425..f01a240e1 100644
--- a/mobile/src/session/mobile-file-syntax.test.ts
+++ b/mobile/src/session/mobile-file-syntax.test.ts
@@ -11,9 +11,12 @@ import type { MobileDiffLine } from './mobile-diff-lines'
 describe('mobile file syntax highlighting', () => {
   it('detects common source languages from file paths', () => {
     expect(detectMobileFileLanguage('src/App.tsx')).toBe('typescript')
+    expect(detectMobileFileLanguage('config/vitest.config.mts')).toBe('typescript')
+    expect(detectMobileFileLanguage('C:\\repo\\scripts\\postinstall.CTS')).toBe('typescript')
     expect(detectMobileFileLanguage('scripts/deploy.sh')).toBe('shell')
     expect(detectMobileFileLanguage('Dockerfile')).toBe('dockerfile')
     expect(resolveMobileSyntaxLanguage('src/App.tsx')).toBe('typescript')
+    expect(resolveMobileSyntaxLanguage('worktrees/feature/build.cts')).toBe('typescript')
     expect(resolveMobileSyntaxLanguage('Dockerfile')).toBe('plaintext')
   })
 
diff --git a/src/renderer/src/lib/language-detect.test.ts b/src/renderer/src/lib/language-detect.test.ts
index 0bb81a5c7..8fd253c97 100644
--- a/src/renderer/src/lib/language-detect.test.ts
+++ b/src/renderer/src/lib/language-detect.test.ts
@@ -47,6 +47,18 @@ describe('detectLanguage', () => {
     expect(detectLanguage('C:\\Users\\alice\\.codex\\LOG.JSONL')).toBe('jsonl')
   })
 
+  it('maps .cts/.mts files to the Monaco built-in typescript language id (case-insensitive)', () => {
+    expect(detectLanguage('config/vitest.config.mts')).toBe('typescript')
+    expect(detectLanguage('scripts/postinstall.cts')).toBe('typescript')
+    expect(detectLanguage('types/global.d.mts')).toBe('typescript')
+    expect(detectLanguage('C:\\repo\\config\\BUILD.MTS')).toBe('typescript')
+  })
+
+  it('keeps .mjs/.cjs on the Monaco built-in javascript language id', () => {
+    expect(detectLanguage('scripts/build.mjs')).toBe('javascript')
+    expect(detectLanguage('scripts/legacy.cjs')).toBe('javascript')
+  })
+
   it('keeps .json/.jsonc on the built-in json language and unknown on plaintext', () => {
     expect(detectLanguage('config/settings.json')).toBe('json')
     expect(detectLanguage('config/tsconfig.jsonc')).toBe('json')
diff --git a/src/renderer/src/lib/language-detect.ts b/src/renderer/src/lib/language-detect.ts
index fabf6c314..fc918dd43 100644
--- a/src/renderer/src/lib/language-detect.ts
+++ b/src/renderer/src/lib/language-detect.ts
@@ -14,6 +14,8 @@ const EXT_TO_LANGUAGE: Record<string, string> = {
   // is what gives .tsx/.jsx files syntax highlighting in the editor.
   '.ts': 'typescript',
   '.tsx': 'typescript',
+  '.cts': 'typescript',
+  '.mts': 'typescript',
   '.js': 'javascript',
   '.jsx': 'javascript',
   '.mjs': 'javascript',