fix issue #791

backend/internal/pkg/antigravity/stream_transformer.go

@@ -119,23 +119,33 @@ func (p *StreamingProcessor) ProcessLine(line string) []byte {
 	return result.Bytes()
 }
 
-// Finish 结束处理，返回最终事件和用量
+// Finish 结束处理，返回最终事件和用量。
+// 若整个流未收到任何可解析的上游数据（messageStartSent == false），
+// 则不补发任何结束事件，防止客户端收到没有 message_start 的残缺流。
 func (p *StreamingProcessor) Finish() ([]byte, *ClaudeUsage) {
-	var result bytes.Buffer
-
-	if !p.messageStopSent {
-		_, _ = result.Write(p.emitFinish(""))
-	}
-
 	usage := &ClaudeUsage{
 		InputTokens:          p.inputTokens,
 		OutputTokens:         p.outputTokens,
 		CacheReadInputTokens: p.cacheReadTokens,
 	}
 
+	if !p.messageStartSent {
+		return nil, usage
+	}
+
+	var result bytes.Buffer
+	if !p.messageStopSent {
+		_, _ = result.Write(p.emitFinish(""))
+	}
+
 	return result.Bytes(), usage
 }
 
+// MessageStartSent 报告流中是否已发出过 message_start 事件（即是否收到过有效的上游数据）
+func (p *StreamingProcessor) MessageStartSent() bool {
+	return p.messageStartSent
+}
+
 // emitMessageStart 发送 message_start 事件
 func (p *StreamingProcessor) emitMessageStart(v1Resp *V1InternalResponse) []byte {
 	if p.messageStartSent {

backend/internal/service/antigravity_gateway_service.go

@@ -3696,6 +3696,15 @@ func (s *AntigravityGatewayService) handleClaudeStreamingResponse(c *gin.Context
 				finalEvents, agUsage := processor.Finish()
 				if len(finalEvents) > 0 {
 					cw.Write(finalEvents)
+				} else if !processor.MessageStartSent() && !cw.Disconnected() {
+					// 整个流未收到任何可解析的上游数据（全部 SSE 行均无法被 JSON 解析），
+					// 触发 failover 在同账号重试，避免向客户端发出缺少 message_start 的残缺流
+					logger.LegacyPrintf("service.antigravity_gateway", "[antigravity-Claude-Stream] empty stream response (no valid events parsed), triggering failover")
+					return nil, &UpstreamFailoverError{
+						StatusCode:             http.StatusBadGateway,
+						ResponseBody:           []byte(`{"error":"empty stream response from upstream"}`),
+						RetryableOnSameAccount: true,
+					}
 				}
 				return &antigravityStreamResult{usage: convertUsage(agUsage), firstTokenMs: firstTokenMs, clientDisconnect: cw.Disconnected()}, nil
 			}

backend/internal/service/antigravity_gateway_service_test.go

@@ -998,6 +998,46 @@ func TestHandleClaudeStreamingResponse_ClientDisconnect(t *testing.T) {
 	require.True(t, result.clientDisconnect)
 }
 
+// TestHandleClaudeStreamingResponse_EmptyStream
+// 验证：上游只返回无法解析的 SSE 行时，触发 UpstreamFailoverError 而不是向客户端发出残缺流
+func TestHandleClaudeStreamingResponse_EmptyStream(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	svc := newAntigravityTestService(&config.Config{
+		Gateway: config.GatewayConfig{MaxLineSize: defaultMaxLineSize},
+	})
+
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPost, "/", nil)
+
+	pr, pw := io.Pipe()
+	resp := &http.Response{StatusCode: http.StatusOK, Body: pr, Header: http.Header{}}
+
+	go func() {
+		defer func() { _ = pw.Close() }()
+		// 所有行均为无法 JSON 解析的内容，ProcessLine 全部返回 nil
+		fmt.Fprintln(pw, "data: not-valid-json")
+		fmt.Fprintln(pw, "")
+		fmt.Fprintln(pw, "data: also-invalid")
+		fmt.Fprintln(pw, "")
+	}()
+
+	_, err := svc.handleClaudeStreamingResponse(c, resp, time.Now(), "claude-sonnet-4-5")
+	_ = pr.Close()
+
+	// 应当返回 UpstreamFailoverError 而非 nil，以便上层触发 failover
+	require.Error(t, err)
+	var failoverErr *UpstreamFailoverError
+	require.ErrorAs(t, err, &failoverErr)
+	require.True(t, failoverErr.RetryableOnSameAccount)
+
+	// 客户端不应收到任何 SSE 事件（既无 message_start 也无 message_stop）
+	body := rec.Body.String()
+	require.NotContains(t, body, "event: message_start")
+	require.NotContains(t, body, "event: message_stop")
+	require.NotContains(t, body, "event: message_delta")
+}
+
 // TestHandleClaudeStreamingResponse_ContextCanceled
 // 验证：context 取消时不注入错误事件
 func TestHandleClaudeStreamingResponse_ContextCanceled(t *testing.T) {