diff --git a/internal/control/group_settings.go b/internal/control/group_settings.go
index 3efddccb..81b55293 100644
--- a/internal/control/group_settings.go
+++ b/internal/control/group_settings.go
@@ -246,13 +246,12 @@ func (s *Service) UpdateGroupSettings(
 			updates["weight_manual"] = normalized.weightManual
 		}
 		if normalized.overridesSet {
-			if _, injectUsageOptionsSet := normalized.overrides[state.SettingInjectUsageOptions]; injectUsageOptionsSet &&
-				!groupSupportsOpenAICompletions(group.Protocols) {
-				return app_errors.ErrValidation
-			}
 			group.Config = normalized.encodedOverrides
 			updates["config"] = group.Config
 		}
+		if err := validateGroupInjectUsageOptionsConstraint(group); err != nil {
+			return err
+		}
 		if err := validateGroupRowCandidate(ctx, tx, group); err != nil {
 			return app_errors.ErrValidation
 		}
@@ -280,6 +279,20 @@ func (s *Service) UpdateGroupSettings(
 	return result, nil
 }
 
+func validateGroupInjectUsageOptionsConstraint(group models.Group) error {
+	settings := make(config.Settings)
+	if len(group.Config) > 0 {
+		if err := decodeGroupDiscoveryJSON(group.Config, &settings); err != nil {
+			return app_errors.ErrValidation
+		}
+	}
+	if _, injectUsageOptionsSet := settings[state.SettingInjectUsageOptions]; injectUsageOptionsSet &&
+		!groupSupportsOpenAICompletions(group.Protocols) {
+		return app_errors.ErrValidation
+	}
+	return nil
+}
+
 func groupSupportsOpenAICompletions(raw models.JSON) bool {
 	var protocols []protocol.Protocol
 	if err := decodeGroupDiscoveryJSON(raw, &protocols); err != nil {
diff --git a/internal/control/group_settings_test.go b/internal/control/group_settings_test.go
index b6885287..8f5e3cb3 100644
--- a/internal/control/group_settings_test.go
+++ b/internal/control/group_settings_test.go
@@ -160,6 +160,45 @@ func TestUpdateGroupSettingsValidatesWeightInjectUsageAndUpstreamConfirmation(t
 	}
 }
 
+func TestUpdateGroupSettingsRejectsProtocolChangeThatLeavesInjectUsageOptionsOnNonOpenAIGroup(t *testing.T) {
+	fixture := newServiceFixture(t)
+	created, err := fixture.service.CreateGroup(t.Context(), GroupCreateRequest{
+		UpstreamURL: "https://settings-protocol-constraint.example.com/v1",
+		Protocols:   []protocol.Protocol{protocol.OpenAICompletions},
+		Config: config.Settings{
+			state.SettingInjectUsageOptions: true,
+		},
+		Keys: "sk-settings-protocol-constraint",
+	})
+	if err != nil {
+		t.Fatalf("CreateGroup() error = %v", err)
+	}
+	var before models.Group
+	if err := fixture.db.First(&before, created.GroupID).Error; err != nil {
+		t.Fatal(err)
+	}
+	beforeRevision := fixture.manager.Current().Revision
+
+	_, err = fixture.service.UpdateGroupSettings(t.Context(), created.GroupID, GroupSettingsUpdateRequest{
+		Protocols: optionalField[[]protocol.Protocol]{
+			Set: true, Value: []protocol.Protocol{protocol.Anthropic},
+		},
+	})
+	if !errors.Is(err, app_errors.ErrValidation) {
+		t.Fatalf("UpdateGroupSettings() error = %v, want validation", err)
+	}
+	var after models.Group
+	if err := fixture.db.First(&after, created.GroupID).Error; err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(after, before) {
+		t.Fatalf("protocol constraint changed persisted group: got=%#v want=%#v", after, before)
+	}
+	if got := fixture.manager.Current().Revision; got != beforeRevision {
+		t.Fatalf("protocol constraint published revision %d, want %d", got, beforeRevision)
+	}
+}
+
 func TestGroupSettingsHTTPRejectsStrictJSONAndUnauthorizedWithoutMutation(t *testing.T) {
 	initControlI18n(t)
 	fixture := newServiceFixture(t)