Fix for case-insensitive alias matching in InitSettingsSource

pydantic_settings/sources/base.py

@@ -258,27 +258,44 @@ def __init__(
         settings_cls: type[BaseSettings],
         init_kwargs: dict[str, Any],
         nested_model_default_partial_update: bool | None = None,
+        case_sensitive: bool | None = None,
     ):
+        super().__init__(settings_cls)
+
+        self.case_sensitive = case_sensitive if case_sensitive is not None else self.config.get('case_sensitive', False)
+
         self.init_kwargs = {}
-        init_kwarg_names = set(init_kwargs.keys())
+        init_kwargs_processed = set()
+
+        init_kwargs_lookup: dict[str, str]
+        if not self.case_sensitive:
+            init_kwargs_lookup = {k.lower(): k for k in init_kwargs}
+        else:
+            init_kwargs_lookup = {k: k for k in init_kwargs}
+
         for field_name, field_info in settings_cls.model_fields.items():
-            alias_names, *_ = _get_alias_names(field_name, field_info)
-            init_kwarg_name = init_kwarg_names & set(alias_names)
-            if init_kwarg_name:
-                preferred_alias = alias_names[0]
-                init_kwarg_names -= init_kwarg_name
-                self.init_kwargs[preferred_alias] = init_kwargs[init_kwarg_name.pop()]
-        self.init_kwargs.update({key: val for key, val in init_kwargs.items() if key in init_kwarg_names})
+            canonical_aliases, _ = _get_alias_names(field_name, field_info, case_sensitive=True)
+            preferred_alias = canonical_aliases[0]
+
+            match_aliases, _ = _get_alias_names(field_name, field_info, case_sensitive=self.case_sensitive)
+            for alias in match_aliases:
+                original_kwarg_key = init_kwargs_lookup.get(alias)
+                if original_kwarg_key is not None and original_kwarg_key not in init_kwargs_processed:
+                    self.init_kwargs[preferred_alias] = init_kwargs[original_kwarg_key]
+                    init_kwargs_processed.add(original_kwarg_key)
+                    break
+
+        for original_key, value in init_kwargs.items():
+            if original_key not in init_kwargs_processed:
+                self.init_kwargs[original_key] = value
 
-        super().__init__(settings_cls)
         self.nested_model_default_partial_update = (
             nested_model_default_partial_update
             if nested_model_default_partial_update is not None
             else self.config.get('nested_model_default_partial_update', False)
         )
 
     def get_field_value(self, field: FieldInfo, field_name: str) -> tuple[Any, str, bool]:
-        # Nothing to do here. Only implement the return statement to make mypy happy
         return None, '', False
 
     def __call__(self) -> dict[str, Any]:

tests/test_settings.py

@@ -871,6 +871,44 @@ class Settings(BaseSettings):
     ]
 
 
+def test_init_settings_source_extra_fields_case_sensitive(monkeypatch):
+    class CaseSensitiveSettings(BaseSettings):
+        foo: str = Field(..., alias='FOO')
+        model_config = SettingsConfigDict(case_sensitive=True, extra='allow')
+
+    # Test case-sensitive with missing alias
+    monkeypatch.setattr(os, 'environ', value={})
+    init_kwargs = {'Foo': 'wrong_value', 'extra_field': 'extra_value'}
+    with pytest.raises(ValidationError) as exc_info:
+        CaseSensitiveSettings(**init_kwargs)
+    assert exc_info.value.errors(include_url=False) == [
+        {
+            'type': 'missing',
+            'loc': ('FOO',),
+            'msg': 'Field required',
+            'input': init_kwargs,
+        }
+    ]
+
+    # Test case-sensitive with correct alias and extra field
+    monkeypatch.setattr(os, 'environ', value={})
+    init_kwargs = {'FOO': 'foo_value', 'extra_field': 'extra_value'}
+    settings = CaseSensitiveSettings(**init_kwargs)
+    assert settings.foo == 'foo_value'
+    assert settings.__pydantic_extra__ == {'extra_field': 'extra_value'}
+
+    class CaseInsensitiveSettings(BaseSettings):
+        foo: str = Field(..., alias='FOO')
+        model_config = SettingsConfigDict(case_sensitive=False, extra='allow')
+
+    # Test case-insensitive with extra field
+    monkeypatch.setattr(os, 'environ', value={})
+    init_kwargs = {'Foo': 'foo_value', 'extra_field': 'extra_value'}
+    settings = CaseInsensitiveSettings(**init_kwargs)
+    assert settings.foo == 'foo_value'
+    assert settings.__pydantic_extra__ == {'extra_field': 'extra_value'}
+
+
 @pytest.mark.parametrize('env_nested_delimiter', [None, ''])
 def test_case_sensitive_no_nested_delimiter(monkeypatch, env_nested_delimiter):
     class Subsettings(BaseSettings):