From 5560b8a2e5aca802c071125c744f1fb2800eba0a Mon Sep 17 00:00:00 2001
From: Yifeng Lu <yifenglu@google.com>
Date: Thu, 12 Feb 2026 17:01:47 -0800
Subject: [PATCH] Fix Mime compatibility for text-based files with misdetected
 MIME types.

PiperOrigin-RevId: 869448798
---
 langfun/core/modalities/mime.py      | 17 +++++++-
 langfun/core/modalities/mime_test.py | 58 ++++++++++++++++++++++++++++
 2 files changed, 74 insertions(+), 1 deletion(-)

diff --git a/langfun/core/modalities/mime.py b/langfun/core/modalities/mime.py
index 0df0cc56..dcff3ae9 100644
--- a/langfun/core/modalities/mime.py
+++ b/langfun/core/modalities/mime.py
@@ -161,7 +161,18 @@ def is_compatible(
   def _is_compatible(self, mime_types: Iterable[str]):
     if not mime_types:
       return False
-    return self.mime_type in mime_types
+    if self.mime_type in mime_types:
+      return True
+    # Fallback: if text/plain is accepted, check if content is valid UTF-8.
+    # This handles files with misdetected MIME types (e.g., .ts detected as
+    # video/mp2t) that are actually text.
+    if 'text/plain' in mime_types:
+      try:
+        self.to_bytes().decode('utf-8')
+        return True
+      except Exception:  # pylint: disable=broad-exception-caught
+        pass
+    return False
 
   def make_compatible(
       self,
@@ -175,6 +186,10 @@ def make_compatible(
           f'MIME type {self.mime_type!r} cannot be converted to supported '
           f'types: {mime_types!r}.'
       )
+    # If compatibility was achieved via the UTF-8 text fallback (not exact MIME
+    # match), wrap content as text/plain so the LLM receives the correct type.
+    if self.mime_type not in mime_types and 'text/plain' in mime_types:
+      return Custom(mime='text/plain', content=self.to_bytes())
     return self._make_compatible(mime_types)
 
   def _make_compatible(
diff --git a/langfun/core/modalities/mime_test.py b/langfun/core/modalities/mime_test.py
index f37d0044..3666fcda 100644
--- a/langfun/core/modalities/mime_test.py
+++ b/langfun/core/modalities/mime_test.py
@@ -211,5 +211,63 @@ def test_binary_mime_type_raises_error(self):
       content.to_text()
 
 
+class TextCompatibilityTest(unittest.TestCase):
+
+  def test_exact_mime_match(self):
+    content = mime.Custom('text/plain', b'hello')
+    self.assertTrue(content.is_compatible('text/plain'))
+    result = content.make_compatible('text/plain')
+    self.assertIs(result, content)
+
+  def test_text_content_with_misdetected_mime_type(self):
+    ts_code = b'import type { Foo } from "bar";\nexport const x = 1;\n'
+    content = mime.Custom('video/mp2t', ts_code)
+    self.assertTrue(content.is_compatible(['text/plain', 'image/png']))
+    result = content.make_compatible(['text/plain', 'image/png'])
+    self.assertIsInstance(result, mime.Custom)
+    self.assertEqual(result.mime_type, 'text/plain')
+    self.assertEqual(result.to_bytes(), ts_code)
+
+  def test_text_mime_not_in_supported_list(self):
+    content = mime.Custom('text/x-typescript', b'const x: number = 1;\n')
+    self.assertTrue(content.is_compatible(['text/plain']))
+    result = content.make_compatible(['text/plain'])
+    self.assertEqual(result.mime_type, 'text/plain')
+
+  def test_application_text_compatible_with_text_plain(self):
+    content = mime.Custom('application/x-yaml', b'key: value\n')
+    self.assertTrue(content.is_compatible(['text/plain']))
+    result = content.make_compatible(['text/plain'])
+    self.assertEqual(result.mime_type, 'text/plain')
+
+  def test_binary_content_not_compatible_with_text_plain(self):
+    binary_data = bytes(range(256))
+    content = mime.Custom('application/octet-stream', binary_data)
+    self.assertFalse(content.is_compatible(['text/plain']))
+    with self.assertRaises(lf.ModalityError):
+      content.make_compatible(['text/plain'])
+
+  def test_no_fallback_without_text_plain_in_targets(self):
+    ts_code = b'const x = 1;\n'
+    content = mime.Custom('video/mp2t', ts_code)
+    self.assertFalse(content.is_compatible(['image/png', 'audio/wav']))
+    with self.assertRaises(lf.ModalityError):
+      content.make_compatible(['image/png', 'audio/wav'])
+
+  def test_make_compatible_preserves_content(self):
+    original = b'# Markdown\n\nHello world\n'
+    content = mime.Custom('text/markdown', original)
+    result = content.make_compatible(['text/plain'])
+    self.assertEqual(result.to_bytes(), original)
+    self.assertEqual(result.to_text(), '# Markdown\n\nHello world\n')
+
+  def test_unicode_content_compatible_with_text_plain(self):
+    unicode_bytes = 'こんにちは世界 🎉'.encode('utf-8')
+    content = mime.Custom('video/mp2t', unicode_bytes)
+    self.assertTrue(content.is_compatible(['text/plain']))
+    result = content.make_compatible(['text/plain'])
+    self.assertEqual(result.to_text(), 'こんにちは世界 🎉')
+
+
 if __name__ == '__main__':
   unittest.main()