diff --git a/internal/adapter/openai/responses_handler.go b/internal/adapter/openai/responses_handler.go
index ff324b4..e04fb5f 100644
--- a/internal/adapter/openai/responses_handler.go
+++ b/internal/adapter/openai/responses_handler.go
@@ -159,7 +159,6 @@ func (h *Handler) handleResponsesStream(w http.ResponseWriter, r *http.Request,
 		if bufferToolContent {
 			for _, evt := range flushToolSieve(&sieve, toolNames) {
 				if evt.Content != "" {
-					finalText += evt.Content
 					sendEvent("response.output_text.delta", util.BuildOpenAIResponsesTextDeltaPayload(responseID, evt.Content))
 				}
 				if len(evt.ToolCalls) > 0 {
diff --git a/internal/adapter/openai/responses_stream_test.go b/internal/adapter/openai/responses_stream_test.go
new file mode 100644
index 0000000..4633388
--- /dev/null
+++ b/internal/adapter/openai/responses_stream_test.go
@@ -0,0 +1,70 @@
+package openai
+
+import (
+	"bufio"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func TestHandleResponsesStreamNoDuplicateTailInCompletedOutputText(t *testing.T) {
+	h := &Handler{}
+	req := httptest.NewRequest(http.MethodPost, "/v1/responses", nil)
+	rec := httptest.NewRecorder()
+
+	sseLine := func(v string) string {
+		b, _ := json.Marshal(map[string]any{
+			"p": "response/content",
+			"v": v,
+		})
+		return "data: " + string(b) + "\n"
+	}
+
+	tail := `{"tool_calls":[{"name":"read_file","input":`
+	streamBody := sseLine("Before ") + sseLine(tail) + "data: [DONE]\n"
+	resp := &http.Response{
+		StatusCode: http.StatusOK,
+		Body:       io.NopCloser(strings.NewReader(streamBody)),
+	}
+
+	h.handleResponsesStream(rec, req, resp, "owner-a", "resp_test", "deepseek-chat", "prompt", false, false, []string{"read_file"})
+
+	completed, ok := extractSSEEventPayload(rec.Body.String(), "response.completed")
+	if !ok {
+		t.Fatalf("expected response.completed event, body=%s", rec.Body.String())
+	}
+	responseObj, _ := completed["response"].(map[string]any)
+	outputText, _ := responseObj["output_text"].(string)
+	if strings.Count(outputText, tail) != 1 {
+		t.Fatalf("expected tail to appear once in output_text, got output_text=%q", outputText)
+	}
+}
+
+func extractSSEEventPayload(body, targetEvent string) (map[string]any, bool) {
+	scanner := bufio.NewScanner(strings.NewReader(body))
+	matched := false
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if strings.HasPrefix(line, "event: ") {
+			evt := strings.TrimSpace(strings.TrimPrefix(line, "event: "))
+			matched = evt == targetEvent
+			continue
+		}
+		if !matched || !strings.HasPrefix(line, "data: ") {
+			continue
+		}
+		raw := strings.TrimSpace(strings.TrimPrefix(line, "data: "))
+		if raw == "" || raw == "[DONE]" {
+			continue
+		}
+		var payload map[string]any
+		if err := json.Unmarshal([]byte(raw), &payload); err != nil {
+			return nil, false
+		}
+		return payload, true
+	}
+	return nil, false
+}