diff --git a/apps/mobile-shell/scripts/check-config.mjs b/apps/mobile-shell/scripts/check-config.mjs
index d580360d3..a4accea7d 100644
--- a/apps/mobile-shell/scripts/check-config.mjs
+++ b/apps/mobile-shell/scripts/check-config.mjs
@@ -2104,6 +2104,20 @@ if (
 ) {
   throw new Error('mobile shell image export must reject empty and oversized bytes');
 }
+const exportTextFileBody = extractFunctionBody(filesSource, 'exportTextFile');
+const exportTextMimeValidationIndex = exportTextFileBody.indexOf(
+  'HOST_BRIDGE_TEXT_MIME_TYPE_SET.has(mimeType as HostBridgeTextMimeType)',
+);
+const exportTextSharingAvailabilityIndex = exportTextFileBody.indexOf(
+  'await assertMobileFileSharingAvailable()',
+);
+if (
+  exportTextMimeValidationIndex < 0 ||
+  exportTextSharingAvailabilityIndex < 0 ||
+  exportTextMimeValidationIndex > exportTextSharingAvailabilityIndex
+) {
+  throw new Error('mobile shell text export must validate MIME before native sharing checks');
+}
 
 for (const [wrapperName, fileCall] of [
   ['exportMobileHostBridgeTextFile', 'ok(request, await exportTextFile(request.payload))'],
@@ -2126,6 +2140,7 @@ for (const snippet of [
   'exportTextFile({',
   'exportImageFile({',
   'rejects every export before cache writes when system sharing is unavailable',
+  'rejects invalid text export MIME before touching native sharing',
   'const exportCases = [',
   'expect(writtenFiles).toHaveLength(0)',
   'importTextFile()',
diff --git a/apps/mobile-shell/src/host-bridge/files.test.ts b/apps/mobile-shell/src/host-bridge/files.test.ts
index 1d6c8deda..a202f40db 100644
--- a/apps/mobile-shell/src/host-bridge/files.test.ts
+++ b/apps/mobile-shell/src/host-bridge/files.test.ts
@@ -185,6 +185,23 @@ describe('mobile HostBridge file actions', () => {
     expect(shareAsyncMock).not.toHaveBeenCalled();
   });
 
+  test('rejects invalid text export MIME before touching native sharing', async () => {
+    await expect(
+      exportTextFile({
+        content: '泥巴AI',
+        fileName: '创作记录',
+        mimeType: 'application/octet-stream',
+      }),
+    ).rejects.toMatchObject({
+      code: 'invalid_request',
+      message: 'mimeType must be an allowed text type',
+    });
+
+    expect(shareAvailableMock).not.toHaveBeenCalled();
+    expect(writtenFiles).toHaveLength(0);
+    expect(shareAsyncMock).not.toHaveBeenCalled();
+  });
+
   test('maps native sharing availability failures to stable export errors', async () => {
     shareAvailableMock.mockRejectedValueOnce(new Error('expo sharing crashed'));
 
diff --git a/apps/mobile-shell/src/host-bridge/files.ts b/apps/mobile-shell/src/host-bridge/files.ts
index c0c5fd51f..c05252064 100644
--- a/apps/mobile-shell/src/host-bridge/files.ts
+++ b/apps/mobile-shell/src/host-bridge/files.ts
@@ -109,8 +109,6 @@ export async function exportTextFile(
     throw invalidRequest('content exceeds file export size limit');
   }
 
-  await assertMobileFileSharingAvailable();
-
   const fileName = normalizeHostBridgeExportFileName(exportPayload?.fileName);
   const rawMimeType = exportPayload?.mimeType;
   const mimeType =
@@ -120,6 +118,9 @@ export async function exportTextFile(
   if (!HOST_BRIDGE_TEXT_MIME_TYPE_SET.has(mimeType as HostBridgeTextMimeType)) {
     throw invalidRequest('mimeType must be an allowed text type');
   }
+
+  await assertMobileFileSharingAvailable();
+
   try {
     const file = new File(Paths.cache, fileName);
     file.write(content);