diff --git a/server/internal/proxy/convert/json_responses.go b/server/internal/proxy/convert/json_responses.go
index 2c12c87..abaf567 100644
--- a/server/internal/proxy/convert/json_responses.go
+++ b/server/internal/proxy/convert/json_responses.go
@@ -240,11 +240,9 @@ func chatToResponsesReq(body []byte) ([]byte, error) {
 	if len(system) > 0 {
 		out["instructions"] = strings.Join(system, "\n")
 	}
-	if len(input) == 1 {
-		out["input"] = input[0] // 单条消息项
-	} else {
-		out["input"] = input
-	}
+	// input 必须是数组：部分上游（如火山方舟）只接受 []*InputItem，
+	// 单对象会被拒（400 Mismatch type）。
+	out["input"] = input
 
 	if len(req.Tools) > 0 {
 		tools := make([]any, 0, len(req.Tools))