diff --git a/backend/internal/platform/api_call_payload.go b/backend/internal/platform/api_call_payload.go
index b34918216..9927bb858 100644
--- a/backend/internal/platform/api_call_payload.go
+++ b/backend/internal/platform/api_call_payload.go
@@ -11,6 +11,7 @@ import (
 	"net/http"
 	"net/url"
 	"strings"
+	"unicode/utf8"
 )
 
 const maxAPICallPayloadBytes = 128 << 10
@@ -151,5 +152,14 @@ func truncateAPICallPayload(value string) string {
 	if len(value) <= maxAPICallPayloadBytes {
 		return value
 	}
-	return value[:maxAPICallPayloadBytes] + fmt.Sprintf("\n[报文已截断，原始长度 %d 字节]", len(value))
+	cut := maxAPICallPayloadBytes
+	for cut > 0 && !utf8.RuneStart(value[cut]) {
+		cut--
+	}
+	// 上游报文不保证是合法 UTF-8，直接入库会被 PostgreSQL 以 invalid byte sequence 拒收。
+	truncated := value[:cut]
+	if !utf8.ValidString(truncated) {
+		truncated = strings.ToValidUTF8(truncated, string(utf8.RuneError))
+	}
+	return truncated + fmt.Sprintf("\n[报文已截断，原始长度 %d 字节]", len(value))
 }
diff --git a/backend/internal/platform/api_call_payload_test.go b/backend/internal/platform/api_call_payload_test.go
new file mode 100644
index 000000000..6016c5e95
--- /dev/null
+++ b/backend/internal/platform/api_call_payload_test.go
@@ -0,0 +1,61 @@
+package platform
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+	"unicode/utf8"
+)
+
+// 回归测试：截断必须落在 UTF-8 字符边界上，且产出永远是合法 UTF-8，
+// 否则调用日志写 PostgreSQL 会报 invalid byte sequence，订单被误标 uncertain。
+func TestTruncateAPICallPayloadAlwaysValidUTF8(t *testing.T) {
+	cases := []struct {
+		name  string
+		value string
+	}{
+		{"短文本原样返回", "你好，世界"},
+		{"纯 ASCII 长文", strings.Repeat("a", maxAPICallPayloadBytes+500)},
+		{"中文长文（3 字节字符）", strings.Repeat("测", (maxAPICallPayloadBytes+500)/3)},
+		{"emoji 长文（4 字节字符）", strings.Repeat("🎬", (maxAPICallPayloadBytes+500)/4)},
+		{"中日韩混排", strings.Repeat("测试テスト", (maxAPICallPayloadBytes+500)/18)},
+		{"源报文本身含非法 UTF-8", strings.Repeat("ok", 70000) + "\xff\xfe" + strings.Repeat("尾", 100)},
+	}
+	for _, tc := range cases {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			got := truncateAPICallPayload(tc.value)
+			if !utf8.ValidString(got) {
+				t.Fatalf("截断结果含非法 UTF-8")
+			}
+			if len(tc.value) <= maxAPICallPayloadBytes {
+				if got != tc.value {
+					t.Fatalf("短文本应原样返回")
+				}
+				return
+			}
+			if !strings.HasSuffix(got, fmt.Sprintf("[报文已截断，原始长度 %d 字节]", len(tc.value))) {
+				t.Fatalf("缺少截断标记: %q", got[len(got)-60:])
+			}
+			if len(got) > maxAPICallPayloadBytes+64 {
+				t.Fatalf("截断后长度越界: got %d", len(got))
+			}
+		})
+	}
+}
+
+func TestSanitizeAPICallPayloadTruncatedJSONIsValidUTF8(t *testing.T) {
+	// 中文 prompt + 大报文，走 json.MarshalIndent 格式化后仍超限的路径。
+	big := make([]string, 0, 20000)
+	for i := 0; i < 20000; i++ {
+		big = append(big, `{"prompt":"用参考图画一段夜晚赛博朋克城市的天际线航拍镜头","index":`+fmt.Sprint(i)+`}`)
+	}
+	data := []byte("[" + strings.Join(big, ",") + "]")
+	got := SanitizeAPICallPayload(data, "application/json")
+	if !utf8.ValidString(got) {
+		t.Fatalf("JSON 载体截断结果含非法 UTF-8")
+	}
+	if len(data) <= maxAPICallPayloadBytes {
+		t.Fatalf("用例应超过截断阈值")
+	}
+}