diff --git a/internal/handler/agent.go b/internal/handler/agent.go
index b338c04f..4af89989 100644
--- a/internal/handler/agent.go
+++ b/internal/handler/agent.go
@@ -23,6 +23,7 @@ import (
 	"cyberstrike-ai/internal/mcp"
 	"cyberstrike-ai/internal/mcp/builtin"
 	"cyberstrike-ai/internal/multiagent"
+	"cyberstrike-ai/internal/openai"
 
 	"github.com/gin-gonic/gin"
 	"github.com/robfig/cron/v3"
@@ -1158,7 +1159,16 @@ func (h *AgentHandler) createProgressCallback(runCtx context.Context, cancelRun
 			return
 		}
 		if eventType == "response_delta" {
-			respPlan.b.WriteString(message)
+			if dataMap, ok := data.(map[string]interface{}); ok {
+				if acc, okAcc := dataMap[openai.SSEAccumulatedKey].(string); okAcc {
+					respPlan.b.Reset()
+					respPlan.b.WriteString(acc)
+				} else {
+					respPlan.b.WriteString(message)
+				}
+			} else {
+				respPlan.b.WriteString(message)
+			}
 			if dataMap, ok := data.(map[string]interface{}); ok && respPlan.meta == nil {
 				respPlan.meta = make(map[string]interface{}, len(dataMap))
 				for k, v := range dataMap {
@@ -1213,8 +1223,12 @@ func (h *AgentHandler) createProgressCallback(runCtx context.Context, cancelRun
 					} else if tb.persistAs == "" {
 						tb.persistAs = persistAs
 					}
-					// delta 片段直接拼接
-					tb.b.WriteString(message)
+					if acc, okAcc := dataMap[openai.SSEAccumulatedKey].(string); okAcc {
+						tb.b.Reset()
+						tb.b.WriteString(acc)
+					} else {
+						tb.b.WriteString(message)
+					}
 					// 有时 delta 先到 start 未到，补充元信息
 					for k, v := range dataMap {
 						tb.meta[k] = v
diff --git a/internal/openai/sse_stream.go b/internal/openai/sse_stream.go
new file mode 100644
index 00000000..a86d6306
--- /dev/null
+++ b/internal/openai/sse_stream.go
@@ -0,0 +1,20 @@
+package openai
+
+// SSEAccumulatedKey 为 SSE progress 事件 data 中的服务端权威流式全文快照字段。
+// 前端应优先用该字段更新 buffer，避免对 delta 二次 normalize 导致叠字。
+const SSEAccumulatedKey = "accumulated"
+
+// WithSSEAccumulated 在 progress data 中附带当前流式累计全文（权威快照）。
+func WithSSEAccumulated(data map[string]interface{}, accumulated string) map[string]interface{} {
+	if data == nil {
+		data = make(map[string]interface{}, 1)
+	}
+	data[SSEAccumulatedKey] = accumulated
+	return data
+}
+
+// NormalizeStreamingDelta 将可能是“累计片段/重发片段”的内容归一化为“纯增量”。
+// 与 unexported normalizeStreamingDelta 相同，供 agent / multiagent 等包在发 SSE 前累计正文。
+func NormalizeStreamingDelta(current, incoming string) (next, delta string) {
+	return normalizeStreamingDelta(current, incoming)
+}