Restore init kwarg names before returning final state dictionary.

pydantic_settings/main.py

@@ -35,6 +35,7 @@
     YamlConfigSettingsSource,
     get_subcommand,
 )
+from .sources.utils import _get_alias_names
 
 T = TypeVar('T')
 
@@ -444,13 +445,35 @@ def _settings_build_values(
 
             # Strip any default values not explicity set before returning final state
             state = {key: val for key, val in state.items() if key not in defaults or defaults[key] != val}
+            self._settings_restore_init_kwarg_names(self.__class__, init_kwargs, state)
 
             return state
         else:
             # no one should mean to do this, but I think returning an empty dict is marginally preferable
             # to an informative error and much better than a confusing error
             return {}
 
+    @staticmethod
+    def _settings_restore_init_kwarg_names(
+        settings_cls: type[BaseSettings], init_kwargs: dict[str, Any], state: dict[str, Any]
+    ) -> None:
+        """
+        Restore the init_kwarg key names to the final merged state dictionary.
+        """
+        if init_kwargs and state:
+            state_kwarg_names = set(state.keys())
+            init_kwarg_names = set(init_kwargs.keys())
+            for field_name, field_info in settings_cls.model_fields.items():
+                alias_names, *_ = _get_alias_names(field_name, field_info)
+                matchable_names = set(alias_names)
+                include_name = settings_cls.model_config.get('populate_by_name', False)
+                if include_name:
+                    matchable_names.add(field_name)
+                init_kwarg_name = init_kwarg_names & matchable_names
+                state_kwarg_name = state_kwarg_names & matchable_names
+                if init_kwarg_name and state_kwarg_name:
+                    state[init_kwarg_name.pop()] = state.pop(state_kwarg_name.pop())
+
     @staticmethod
     def _settings_warn_unused_config_keys(sources: tuple[object, ...], model_config: SettingsConfigDict) -> None:
         """

tests/test_settings.py

@@ -29,6 +29,7 @@
     Tag,
     ValidationError,
     field_validator,
+    model_validator,
 )
 from pydantic import (
     dataclasses as pydantic_dataclasses,
@@ -723,6 +724,25 @@ class Example(BaseSettings):
     env.set('PREFIX_SURNAME', 'smith')
     assert Example(name='john', PREFIX_SURNAME='doe').model_dump() == {'name': 'john', 'last_name': 'doe'}
 
+    class Settings(BaseSettings):
+        NAME: str = Field(
+            default='',
+            validation_alias=AliasChoices('NAME', 'OLD_NAME'),
+        )
+
+        @model_validator(mode='before')
+        def check_for_deprecated_attributes(cls, data: Any) -> Any:
+            if isinstance(data, dict):
+                old_keys = {k for k in data.keys() if k.startswith('OLD_')}
+                assert not old_keys
+            return data
+
+    s = Settings(NAME='foo')
+    s.model_dump() == {'NAME': 'foo'}
+
+    with pytest.raises(ValidationError, match="Assertion failed, assert not {'OLD_NAME'}"):
+        Settings(OLD_NAME='foo')
+
 
 def test_init_kwargs_alias_resolution_deterministic():
     class Example(BaseSettings):