fix(openai): keep responses stream alive during pre-output failover

backend/internal/service/openai_gateway_service.go

@@ -4008,8 +4008,10 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 	if keepaliveTicker != nil {
 		keepaliveCh = keepaliveTicker.C
 	}
-	// 记录上次收到上游数据的时间，用于控制 keepalive 发送频率
-	lastDataAt := time.Now()
+	// Track downstream writes separately from upstream reads: pre-output failover
+	// can buffer response.created / response.in_progress, so keepalive must be
+	// based on downstream idle time.
+	lastDownstreamWriteAt := time.Now()
 
 	// 仅发送一次错误事件，避免多次写入导致协议混乱。
 	// 注意：OpenAI `/v1/responses` streaming 事件必须符合 OpenAI Responses schema；
@@ -4041,6 +4043,7 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 			return
 		}
 		clientOutputStarted = true
+		lastDownstreamWriteAt = time.Now()
 	}
 
 	needModelReplace := originalModel != mappedModel
@@ -4071,6 +4074,7 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 				logger.LegacyPrintf("service.openai_gateway", "Client disconnected during final flush, returning collected usage")
 			} else if hadBufferedData {
 				clientOutputStarted = true
+				lastDownstreamWriteAt = time.Now()
 			}
 		}
 		return resultWithUsage(), nil
@@ -4114,8 +4118,6 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 		if streamFailoverErr != nil {
 			return
 		}
-		lastDataAt = time.Now()
-
 		// Extract data from SSE line (supports both "data: " and "data:" formats)
 		if data, ok := extractOpenAISSEDataLine(line); ok {
 
@@ -4170,6 +4172,7 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 						logger.LegacyPrintf("service.openai_gateway", "Client disconnected during streaming flush, continuing to drain upstream for billing")
 					} else {
 						clientOutputStarted = true
+						lastDownstreamWriteAt = time.Now()
 					}
 				}
 			}
@@ -4197,6 +4200,7 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 					logger.LegacyPrintf("service.openai_gateway", "Client disconnected during streaming flush, continuing to drain upstream for billing")
 				} else {
 					clientOutputStarted = true
+					lastDownstreamWriteAt = time.Now()
 				}
 			}
 		}
@@ -4283,7 +4287,7 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 			if clientDisconnected {
 				continue
 			}
-			if time.Since(lastDataAt) < keepaliveInterval {
+			if time.Since(lastDownstreamWriteAt) < keepaliveInterval {
 				continue
 			}
 			if _, err := bufferedWriter.WriteString(":\n\n"); err != nil {
@@ -4294,6 +4298,8 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 			if err := flushBuffered(); err != nil {
 				clientDisconnected = true
 				logger.LegacyPrintf("service.openai_gateway", "Client disconnected during keepalive flush, continuing to drain upstream for billing")
+			} else {
+				lastDownstreamWriteAt = time.Now()
 			}
 		}
 	}