fix(service): preserve anthropic usage fields across compat endpoints

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/internal/service/gateway_forward_as_responses.go

@@ -17,6 +17,7 @@ import (
 	"github.com/Wei-Shaw/sub2api/internal/pkg/logger"
 	"github.com/Wei-Shaw/sub2api/internal/util/responseheaders"
 	"github.com/gin-gonic/gin"
+	"github.com/tidwall/gjson"
 	"go.uber.org/zap"
 )
 
@@ -56,6 +57,7 @@ func (s *GatewayService) ForwardAsResponses(
 
 	// 4. Model mapping
 	mappedModel := originalModel
+	reasoningEffort := ExtractResponsesReasoningEffortFromBody(body)
 	if account.Type == AccountTypeAPIKey {
 		mappedModel = account.GetMappedModel(originalModel)
 	}
@@ -172,14 +174,46 @@ func (s *GatewayService) ForwardAsResponses(
 	var result *ForwardResult
 	var handleErr error
 	if clientStream {
-		result, handleErr = s.handleResponsesStreamingResponse(resp, c, originalModel, mappedModel, startTime)
+		result, handleErr = s.handleResponsesStreamingResponse(resp, c, originalModel, mappedModel, reasoningEffort, startTime)
 	} else {
-		result, handleErr = s.handleResponsesBufferedStreamingResponse(resp, c, originalModel, mappedModel, startTime)
+		result, handleErr = s.handleResponsesBufferedStreamingResponse(resp, c, originalModel, mappedModel, reasoningEffort, startTime)
 	}
 
 	return result, handleErr
 }
 
+// ExtractResponsesReasoningEffortFromBody reads Responses API reasoning.effort
+// and normalizes it for usage logging.
+func ExtractResponsesReasoningEffortFromBody(body []byte) *string {
+	raw := strings.TrimSpace(gjson.GetBytes(body, "reasoning.effort").String())
+	if raw == "" {
+		return nil
+	}
+	normalized := normalizeOpenAIReasoningEffort(raw)
+	if normalized == "" {
+		return nil
+	}
+	return &normalized
+}
+
+func mergeAnthropicUsage(dst *ClaudeUsage, src apicompat.AnthropicUsage) {
+	if dst == nil {
+		return
+	}
+	if src.InputTokens > 0 {
+		dst.InputTokens = src.InputTokens
+	}
+	if src.OutputTokens > 0 {
+		dst.OutputTokens = src.OutputTokens
+	}
+	if src.CacheReadInputTokens > 0 {
+		dst.CacheReadInputTokens = src.CacheReadInputTokens
+	}
+	if src.CacheCreationInputTokens > 0 {
+		dst.CacheCreationInputTokens = src.CacheCreationInputTokens
+	}
+}
+
 // handleResponsesBufferedStreamingResponse reads all Anthropic SSE events from
 // the upstream streaming response, assembles them into a complete Anthropic
 // response, converts to Responses API JSON format, and writes it to the client.
@@ -188,6 +222,7 @@ func (s *GatewayService) handleResponsesBufferedStreamingResponse(
 	c *gin.Context,
 	originalModel string,
 	mappedModel string,
+	reasoningEffort *string,
 	startTime time.Time,
 ) (*ForwardResult, error) {
 	requestID := resp.Header.Get("x-request-id")
@@ -233,21 +268,13 @@ func (s *GatewayService) handleResponsesBufferedStreamingResponse(
 		// message_start carries the initial response structure
 		if event.Type == "message_start" && event.Message != nil {
 			finalResp = event.Message
+			mergeAnthropicUsage(&usage, event.Message.Usage)
 		}
 
 		// message_delta carries final usage and stop_reason
 		if event.Type == "message_delta" {
 			if event.Usage != nil {
-				usage = ClaudeUsage{
-					InputTokens:  event.Usage.InputTokens,
-					OutputTokens: event.Usage.OutputTokens,
-				}
-				if event.Usage.CacheReadInputTokens > 0 {
-					usage.CacheReadInputTokens = event.Usage.CacheReadInputTokens
-				}
-				if event.Usage.CacheCreationInputTokens > 0 {
-					usage.CacheCreationInputTokens = event.Usage.CacheCreationInputTokens
-				}
+				mergeAnthropicUsage(&usage, *event.Usage)
 			}
 			if event.Delta != nil && event.Delta.StopReason != "" && finalResp != nil {
 				finalResp.StopReason = event.Delta.StopReason
@@ -307,12 +334,13 @@ func (s *GatewayService) handleResponsesBufferedStreamingResponse(
 	c.JSON(http.StatusOK, responsesResp)
 
 	return &ForwardResult{
-		RequestID:     requestID,
-		Usage:         usage,
-		Model:         originalModel,
-		UpstreamModel: mappedModel,
-		Stream:        false,
-		Duration:      time.Since(startTime),
+		RequestID:       requestID,
+		Usage:           usage,
+		Model:           originalModel,
+		UpstreamModel:   mappedModel,
+		ReasoningEffort: reasoningEffort,
+		Stream:          false,
+		Duration:        time.Since(startTime),
 	}, nil
 }
 
@@ -323,6 +351,7 @@ func (s *GatewayService) handleResponsesStreamingResponse(
 	c *gin.Context,
 	originalModel string,
 	mappedModel string,
+	reasoningEffort *string,
 	startTime time.Time,
 ) (*ForwardResult, error) {
 	requestID := resp.Header.Get("x-request-id")
@@ -351,13 +380,14 @@ func (s *GatewayService) handleResponsesStreamingResponse(
 
 	resultWithUsage := func() *ForwardResult {
 		return &ForwardResult{
-			RequestID:     requestID,
-			Usage:         usage,
-			Model:         originalModel,
-			UpstreamModel: mappedModel,
-			Stream:        true,
-			Duration:      time.Since(startTime),
-			FirstTokenMs:  firstTokenMs,
+			RequestID:       requestID,
+			Usage:           usage,
+			Model:           originalModel,
+			UpstreamModel:   mappedModel,
+			ReasoningEffort: reasoningEffort,
+			Stream:          true,
+			Duration:        time.Since(startTime),
+			FirstTokenMs:    firstTokenMs,
 		}
 	}
 
@@ -371,22 +401,11 @@ func (s *GatewayService) handleResponsesStreamingResponse(
 
 		// Extract usage from message_delta
 		if event.Type == "message_delta" && event.Usage != nil {
-			usage = ClaudeUsage{
-				InputTokens:  event.Usage.InputTokens,
-				OutputTokens: event.Usage.OutputTokens,
-			}
-			if event.Usage.CacheReadInputTokens > 0 {
-				usage.CacheReadInputTokens = event.Usage.CacheReadInputTokens
-			}
-			if event.Usage.CacheCreationInputTokens > 0 {
-				usage.CacheCreationInputTokens = event.Usage.CacheCreationInputTokens
-			}
+			mergeAnthropicUsage(&usage, *event.Usage)
 		}
 		// Also capture usage from message_start
 		if event.Type == "message_start" && event.Message != nil {
-			if event.Message.Usage.InputTokens > 0 {
-				usage.InputTokens = event.Message.Usage.InputTokens
-			}
+			mergeAnthropicUsage(&usage, event.Message.Usage)
 		}
 
 		// Convert to Responses events