Change apply_update / _update_settings return type from set[str] to dict[str, Any]. The dict maps each changed field name to its pre-update value, enabling services to do granular diffing of complex settings objects. Existing call-site patterns ("field" in changed, if changed, iteration) work unchanged; set-difference sites use changed.keys() - {...}.

tests/test_settings.py

@@ -100,7 +100,8 @@ class TestApplyUpdate:
         current = TTSSettings(voice="alice", language="en")
         delta = TTSSettings(voice="bob")
         changed = current.apply_update(delta)
-        assert changed == {"voice"}
+        assert changed.keys() == {"voice"}
+        assert changed["voice"] == "alice"  # old value
         assert current.voice == "bob"
         assert current.language == "en"
 
@@ -108,14 +109,14 @@ class TestApplyUpdate:
         current = TTSSettings(voice="alice", language="en")
         delta = TTSSettings(voice="alice")
         changed = current.apply_update(delta)
-        assert changed == set()
+        assert changed == {}
         assert current.voice == "alice"
 
     def test_apply_update_not_given_skipped(self):
         current = TTSSettings(voice="alice", language="en")
         delta = TTSSettings()  # all NOT_GIVEN
         changed = current.apply_update(delta)
-        assert changed == set()
+        assert changed == {}
         assert current.voice == "alice"
         assert current.language == "en"
 
@@ -123,7 +124,9 @@ class TestApplyUpdate:
         current = LLMSettings(temperature=0.7, max_tokens=100)
         delta = LLMSettings(temperature=0.9, max_tokens=200, top_p=0.95)
         changed = current.apply_update(delta)
-        assert changed == {"temperature", "max_tokens", "top_p"}
+        assert changed.keys() == {"temperature", "max_tokens", "top_p"}
+        assert changed["temperature"] == 0.7
+        assert changed["max_tokens"] == 100
         assert current.temperature == 0.9
         assert current.max_tokens == 200
         assert current.top_p == 0.95
@@ -135,6 +138,7 @@ class TestApplyUpdate:
         delta.extra = {"speed": 1.2}
         changed = current.apply_update(delta)
         assert "speed" in changed
+        assert changed["speed"] == 1.0  # old value
         assert current.extra == {"speed": 1.2, "stability": 0.5}
 
     def test_apply_update_extra_no_change(self):
@@ -143,13 +147,14 @@ class TestApplyUpdate:
         delta = TTSSettings()
         delta.extra = {"speed": 1.0}
         changed = current.apply_update(delta)
-        assert changed == set()
+        assert changed == {}
 
     def test_apply_update_model_field(self):
         current = ServiceSettings(model="old-model")
         delta = ServiceSettings(model="new-model")
         changed = current.apply_update(delta)
-        assert changed == {"model"}
+        assert changed.keys() == {"model"}
+        assert changed["model"] == "old-model"
         assert current.model == "new-model"
 
     def test_apply_update_none_is_a_valid_value(self):
@@ -165,6 +170,7 @@ class TestApplyUpdate:
         delta = TTSSettings(language="en")
         changed = current.apply_update(delta)
         assert "language" in changed
+        assert changed["language"] is None  # old value was None
         assert current.language == "en"
 
 
@@ -293,7 +299,9 @@ class TestRoundtrip:
         delta = TTSSettings.from_mapping(raw)
 
         changed = current.apply_update(delta)
-        assert changed == {"voice", "speed"}
+        assert changed.keys() == {"voice", "speed"}
+        assert changed["voice"] == "alice"
+        assert changed["speed"] == 1.0
         assert current.voice == "bob"
         assert current.language == "en"
         assert current.extra["speed"] == 1.2
@@ -303,6 +311,7 @@ class TestRoundtrip:
         current = LLMSettings(model="gpt-4o", temperature=0.7)
         delta = LLMSettings.from_mapping({"model": "gpt-4o-mini", "temperature": 0.9})
         changed = current.apply_update(delta)
-        assert changed == {"model", "temperature"}
+        assert changed.keys() == {"model", "temperature"}
+        assert changed["model"] == "gpt-4o"
         assert current.model == "gpt-4o-mini"
         assert current.temperature == 0.9