fix(conversion): preserve source generation config

Signed-off-by: yaoyu-33 <yaoyu.094@gmail.com>

Author: yaoyu-33

src/megatron/bridge/models/hf_pretrained/base.py

@@ -211,7 +211,28 @@ def save_artifacts(
         for name in self.OPTIONAL_ARTIFACTS:
             artifact = getattr(self, name, None)
             if artifact is not None and hasattr(artifact, "save_pretrained"):
-                artifact.save_pretrained(save_path)
+                try:
+                    artifact.save_pretrained(save_path)
+                except ValueError:
+                    if name != "generation_config":
+                        raise
+
+                    source_path = original_source_path or getattr(self, "model_name_or_path", None)
+                    if source_path is None:
+                        raise
+
+                    copied_files = self._copy_custom_modeling_files(
+                        source_path=source_path,
+                        target_path=save_path,
+                        file_patterns=["generation_config.json"],
+                    )
+                    if "generation_config.json" not in copied_files:
+                        raise
+
+                    logger.warning(
+                        "Generation config validation failed during export; preserving the source "
+                        "generation_config.json unchanged"
+                    )
 
         # Download/copy additional files if specified
         if additional_files:

tests/unit_tests/models/hf_pretrained/test_base.py

@@ -20,6 +20,7 @@
 from pathlib import Path
 from unittest.mock import Mock, patch
 
+import pytest
 from transformers.configuration_utils import PretrainedConfig
 
 
@@ -274,6 +275,42 @@ def test_save_artifacts_without_model_name_or_path():
         print("✅ test_save_artifacts_without_model_name_or_path passed")
 
 
+def test_save_artifacts_preserves_source_generation_config_after_validation_failure():
+    """Test invalid loaded generation configs are copied unchanged from the source."""
+    with tempfile.TemporaryDirectory() as tmp_dir:
+        tmp_path = Path(tmp_dir)
+        source_dir = tmp_path / "source"
+        source_dir.mkdir()
+        generation_config = '{"do_sample": false, "top_p": 0.95}\n'
+        (source_dir / "generation_config.json").write_text(generation_config)
+
+        target_dir = tmp_path / "target"
+        base = MockPreTrainedBase(model_name_or_path=str(source_dir))
+        base._config = Mock()
+        base._generation_config = Mock()
+        base._generation_config.save_pretrained.side_effect = ValueError("invalid generation config")
+
+        base.save_artifacts(target_dir)
+
+        assert (target_dir / "generation_config.json").read_text() == generation_config
+
+
+def test_save_artifacts_reraises_generation_config_error_without_source_file():
+    """Test generation config validation errors remain fatal without a source artifact."""
+    with tempfile.TemporaryDirectory() as tmp_dir:
+        tmp_path = Path(tmp_dir)
+        source_dir = tmp_path / "source"
+        source_dir.mkdir()
+
+        base = MockPreTrainedBase(model_name_or_path=str(source_dir))
+        base._config = Mock()
+        base._generation_config = Mock()
+        base._generation_config.save_pretrained.side_effect = ValueError("invalid generation config")
+
+        with pytest.raises(ValueError, match="invalid generation config"):
+            base.save_artifacts(tmp_path / "target")
+
+
 def test_copy_handles_permission_errors():
     """Test that copy failures are handled gracefully."""
     with tempfile.TemporaryDirectory() as tmp_dir: