diff --git a/src/google/adk/plugins/save_files_as_artifacts_plugin.py b/src/google/adk/plugins/save_files_as_artifacts_plugin.py
index d92d9a7a54..8944f8b6af 100644
--- a/src/google/adk/plugins/save_files_as_artifacts_plugin.py
+++ b/src/google/adk/plugins/save_files_as_artifacts_plugin.py
@@ -31,6 +31,10 @@
 # capabilities.
 _MODEL_ACCESSIBLE_URI_SCHEMES = {'gs', 'https', 'http'}
 
+# Maximum file size for inline_data (20MB as per Gemini API documentation)
+# https://ai.google.dev/gemini-api/docs/files
+_MAX_INLINE_DATA_SIZE_BYTES = 20 * 1024 * 1024  # 20 MB
+
 
 class SaveFilesAsArtifactsPlugin(BasePlugin):
   """A plugin that saves files embedded in user messages as artifacts.
@@ -81,8 +85,28 @@ async def on_user_message_callback(
         continue
 
       try:
-        # Use display_name if available, otherwise generate a filename
+        # Check file size before processing
         inline_data = part.inline_data
+        file_size = len(inline_data.data) if inline_data.data else 0
+
+        if file_size > _MAX_INLINE_DATA_SIZE_BYTES:
+          # File exceeds the inline_data limit
+          file_size_mb = file_size / (1024 * 1024)
+          limit_mb = _MAX_INLINE_DATA_SIZE_BYTES / (1024 * 1024)
+          error_message = (
+              f'File size ({file_size_mb:.2f} MB) exceeds the maximum allowed'
+              f' size for inline uploads ({limit_mb:.0f} MB). Please use the'
+              f' Files API to upload files larger than {limit_mb:.0f} MB. See'
+              ' https://ai.google.dev/gemini-api/docs/files for more'
+              ' information.'
+          )
+          logger.error(error_message)
+          # Replace with error message part
+          new_parts.append(types.Part(text=f'[Upload Error: {error_message}]'))
+          modified = True
+          continue
+
+        # Use display_name if available, otherwise generate a filename
         file_name = inline_data.display_name
         if not file_name:
           file_name = f'artifact_{invocation_context.invocation_id}_{i}'
diff --git a/tests/unittests/plugins/test_save_files_as_artifacts.py b/tests/unittests/plugins/test_save_files_as_artifacts.py
index 66ab08098c..a1cf6cbdd2 100644
--- a/tests/unittests/plugins/test_save_files_as_artifacts.py
+++ b/tests/unittests/plugins/test_save_files_as_artifacts.py
@@ -303,3 +303,127 @@ def test_plugin_name_default(self):
     """Test that plugin has correct default name."""
     plugin = SaveFilesAsArtifactsPlugin()
     assert plugin.name == "save_files_as_artifacts_plugin"
+
+  @pytest.mark.asyncio
+  async def test_file_size_exceeds_limit(self):
+    """Test that files exceeding 20MB limit show error message."""
+    # Create a file larger than 20MB (20 * 1024 * 1024 bytes)
+    large_file_data = b"x" * (21 * 1024 * 1024)  # 21 MB
+    inline_data = types.Blob(
+        display_name="large_file.pdf",
+        data=large_file_data,
+        mime_type="application/pdf",
+    )
+
+    user_message = types.Content(parts=[types.Part(inline_data=inline_data)])
+
+    result = await self.plugin.on_user_message_callback(
+        invocation_context=self.mock_context, user_message=user_message
+    )
+
+    # Should not try to save the artifact
+    self.mock_context.artifact_service.save_artifact.assert_not_called()
+
+    # Should return modified content with error message
+    assert result is not None
+    assert len(result.parts) == 1
+    assert result.parts[0].text is not None
+    assert "[Upload Error:" in result.parts[0].text
+    assert "21.00 MB" in result.parts[0].text
+    assert "20 MB" in result.parts[0].text
+    assert "Files API" in result.parts[0].text
+
+  @pytest.mark.asyncio
+  async def test_file_size_at_limit(self):
+    """Test that files exactly at 20MB limit are processed successfully."""
+    # Create a file exactly 20MB (20 * 1024 * 1024 bytes)
+    file_data = b"x" * (20 * 1024 * 1024)  # Exactly 20 MB
+    inline_data = types.Blob(
+        display_name="max_size_file.pdf",
+        data=file_data,
+        mime_type="application/pdf",
+    )
+
+    user_message = types.Content(parts=[types.Part(inline_data=inline_data)])
+
+    result = await self.plugin.on_user_message_callback(
+        invocation_context=self.mock_context, user_message=user_message
+    )
+
+    # Should save the artifact since it's at the limit
+    self.mock_context.artifact_service.save_artifact.assert_called_once()
+    assert result is not None
+    assert len(result.parts) == 2
+    assert result.parts[0].text == '[Uploaded Artifact: "max_size_file.pdf"]'
+    assert result.parts[1].file_data is not None
+
+  @pytest.mark.asyncio
+  async def test_file_size_just_over_limit(self):
+    """Test that files just over 20MB limit show error message."""
+    # Create a file just over 20MB
+    large_file_data = b"x" * (20 * 1024 * 1024 + 1)  # 20 MB + 1 byte
+    inline_data = types.Blob(
+        display_name="slightly_too_large.pdf",
+        data=large_file_data,
+        mime_type="application/pdf",
+    )
+
+    user_message = types.Content(parts=[types.Part(inline_data=inline_data)])
+
+    result = await self.plugin.on_user_message_callback(
+        invocation_context=self.mock_context, user_message=user_message
+    )
+
+    # Should not try to save the artifact
+    self.mock_context.artifact_service.save_artifact.assert_not_called()
+
+    # Should return error message
+    assert result is not None
+    assert len(result.parts) == 1
+    assert "[Upload Error:" in result.parts[0].text
+
+  @pytest.mark.asyncio
+  async def test_mixed_file_sizes(self):
+    """Test processing multiple files with mixed sizes."""
+    # Small file (should succeed)
+    small_file_data = b"x" * (5 * 1024 * 1024)  # 5 MB
+    small_inline_data = types.Blob(
+        display_name="small.pdf",
+        data=small_file_data,
+        mime_type="application/pdf",
+    )
+
+    # Large file (should fail)
+    large_file_data = b"x" * (25 * 1024 * 1024)  # 25 MB
+    large_inline_data = types.Blob(
+        display_name="large.pdf",
+        data=large_file_data,
+        mime_type="application/pdf",
+    )
+
+    user_message = types.Content(
+        parts=[
+            types.Part(inline_data=small_inline_data),
+            types.Part(inline_data=large_inline_data),
+        ]
+    )
+
+    result = await self.plugin.on_user_message_callback(
+        invocation_context=self.mock_context, user_message=user_message
+    )
+
+    # Should only save the small file
+    self.mock_context.artifact_service.save_artifact.assert_called_once_with(
+        app_name="test_app",
+        user_id="test_user",
+        session_id="test_session",
+        filename="small.pdf",
+        artifact=user_message.parts[0],
+    )
+
+    # Should return both success and error messages
+    assert result is not None
+    assert len(result.parts) == 3  # [success placeholder, file_data, error]
+    assert '[Uploaded Artifact: "small.pdf"]' in result.parts[0].text
+    assert result.parts[1].file_data is not None
+    assert "[Upload Error:" in result.parts[2].text