fix(usage): preserve final upstream model

backend/internal/service/gateway_record_usage_test.go

@@ -223,6 +223,35 @@ func TestGatewayServiceRecordUsage_PreservesChannelMappedUpstreamModel(t *testin
 	require.Equal(t, "gpt-5.6-terra", *usageRepo.lastLog.UpstreamModel)
 }
 
+func TestGatewayServiceRecordUsage_PreservesLoopedChannelAndAccountUpstreamModel(t *testing.T) {
+	usageRepo := &openAIRecordUsageLogRepoStub{inserted: true}
+	svc := newGatewayRecordUsageServiceForTest(usageRepo, &openAIRecordUsageUserRepoStub{}, &openAIRecordUsageSubRepoStub{})
+
+	err := svc.RecordUsage(context.Background(), &RecordUsageInput{
+		Result: &ForwardResult{
+			RequestID:     "gateway_looped_mapping_models",
+			Usage:         ClaudeUsage{InputTokens: 10, OutputTokens: 6},
+			Model:         "gpt-5.6-terra",
+			UpstreamModel: "gpt-5.6-sol",
+			Duration:      time.Second,
+		},
+		APIKey:  &APIKey{ID: 501, Quota: 100},
+		User:    &User{ID: 601},
+		Account: &Account{ID: 701},
+		ChannelUsageFields: ChannelUsageFields{
+			OriginalModel:      "gpt-5.6-sol",
+			ChannelMappedModel: "gpt-5.6-terra",
+		},
+	})
+
+	require.NoError(t, err)
+	require.NotNil(t, usageRepo.lastLog)
+	require.Equal(t, "gpt-5.6-sol", usageRepo.lastLog.RequestedModel)
+	require.Equal(t, "gpt-5.6-terra", usageRepo.lastLog.Model)
+	require.NotNil(t, usageRepo.lastLog.UpstreamModel)
+	require.Equal(t, "gpt-5.6-sol", *usageRepo.lastLog.UpstreamModel)
+}
+
 func TestGatewayServiceRecordUsage_EmptyImageSizeDefaultsBeforeBillingAndPersistence(t *testing.T) {
 	imagePrice2K := 0.19
 	groupID := int64(901)

backend/internal/service/gateway_usage_billing.go

@@ -986,7 +986,7 @@ func (s *GatewayService) buildRecordUsageLog(
 		RequestID:             requestID,
 		Model:                 result.Model,
 		RequestedModel:        requestedModel,
-		UpstreamModel:         optionalNonEqualStringPtr(result.UpstreamModel, requestedModel),
+		UpstreamModel:         optionalTrimmedStringPtr(result.UpstreamModel),
 		ReasoningEffort:       result.ReasoningEffort,
 		InboundEndpoint:       optionalTrimmedStringPtr(input.InboundEndpoint),
 		UpstreamEndpoint:      optionalTrimmedStringPtr(input.UpstreamEndpoint),

backend/internal/service/openai_gateway_record_usage_test.go

@@ -1369,6 +1369,37 @@ func TestOpenAIGatewayServiceRecordUsage_PreservesChannelMappedUpstreamModel(t *
 	require.Equal(t, "gpt-5.6-terra", *usageRepo.lastLog.UpstreamModel)
 }
 
+func TestOpenAIGatewayServiceRecordUsage_PreservesLoopedChannelAndAccountUpstreamModel(t *testing.T) {
+	usageRepo := &openAIRecordUsageLogRepoStub{inserted: true}
+	userRepo := &openAIRecordUsageUserRepoStub{}
+	subRepo := &openAIRecordUsageSubRepoStub{}
+	svc := newOpenAIRecordUsageServiceForTest(usageRepo, userRepo, subRepo, nil)
+
+	err := svc.RecordUsage(context.Background(), &OpenAIRecordUsageInput{
+		Result: &OpenAIForwardResult{
+			RequestID:     "openai_looped_mapping_models",
+			Model:         "gpt-5.6-terra",
+			UpstreamModel: "gpt-5.6-sol",
+			Usage:         OpenAIUsage{InputTokens: 20, OutputTokens: 10},
+			Duration:      time.Second,
+		},
+		APIKey:  &APIKey{ID: 10},
+		User:    &User{ID: 20},
+		Account: &Account{ID: 30},
+		ChannelUsageFields: ChannelUsageFields{
+			OriginalModel:      "gpt-5.6-sol",
+			ChannelMappedModel: "gpt-5.6-terra",
+		},
+	})
+
+	require.NoError(t, err)
+	require.NotNil(t, usageRepo.lastLog)
+	require.Equal(t, "gpt-5.6-sol", usageRepo.lastLog.RequestedModel)
+	require.Equal(t, "gpt-5.6-terra", usageRepo.lastLog.Model)
+	require.NotNil(t, usageRepo.lastLog.UpstreamModel)
+	require.Equal(t, "gpt-5.6-sol", *usageRepo.lastLog.UpstreamModel)
+}
+
 func TestOpenAIGatewayServiceRecordUsage_BillsMappedRequestsUsingRequestedModel(t *testing.T) {
 	usageRepo := &openAIRecordUsageLogRepoStub{inserted: true}
 	userRepo := &openAIRecordUsageUserRepoStub{}

backend/internal/service/openai_gateway_usage.go

@@ -256,7 +256,7 @@ func (s *OpenAIGatewayService) RecordUsage(ctx context.Context, input *OpenAIRec
 		RequestID:           requestID,
 		Model:               result.Model,
 		RequestedModel:      requestedModel,
-		UpstreamModel:       optionalNonEqualStringPtr(result.UpstreamModel, requestedModel),
+		UpstreamModel:       optionalTrimmedStringPtr(result.UpstreamModel),
 		ServiceTier:         result.ServiceTier,
 		ReasoningEffort:     result.ReasoningEffort,
 		InboundEndpoint:     optionalTrimmedStringPtr(input.InboundEndpoint),

backend/internal/service/usage_log_helpers.go

@@ -10,16 +10,6 @@ func optionalTrimmedStringPtr(raw string) *string {
 	return &trimmed
 }
 
-// optionalNonEqualStringPtr returns a pointer to value if it is non-empty and
-// differs from compare; otherwise nil. Usage logging passes the requested
-// model as compare so a channel mapping still records its effective upstream.
-func optionalNonEqualStringPtr(value, compare string) *string {
-	if value == "" || value == compare {
-		return nil
-	}
-	return &value
-}
-
 func forwardResultBillingModel(requestedModel, upstreamModel string) string {
 	if trimmed := strings.TrimSpace(requestedModel); trimmed != "" {
 		return trimmed