diff --git a/dto/openai_request.go b/dto/openai_request.go
index 8c104ddd242d..44cb5c9a57be 100644
--- a/dto/openai_request.go
+++ b/dto/openai_request.go
@@ -1,6 +1,7 @@
 package dto
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
 	"strings"
@@ -947,6 +948,58 @@ func (r *OpenAIResponsesRequest) GetToolsMap() []map[string]any {
 	return toolsMap
 }
 
+// NormalizeInputNullContent rewrites Responses API message input items whose
+// `content` is explicitly null into an empty string. Some OpenAI-compatible
+// chat clients emit empty assistant messages as null, while Responses-compatible
+// upstreams reject null content. Non-message input items such as reasoning and
+// function_call use different schemas and must be preserved.
+func (r *OpenAIResponsesRequest) NormalizeInputNullContent() error {
+	if r == nil || len(r.Input) == 0 {
+		return nil
+	}
+	if common.GetJsonType(r.Input) != "array" {
+		return nil
+	}
+
+	var items []any
+	if err := common.Unmarshal(r.Input, &items); err != nil {
+		return err
+	}
+
+	changed := false
+	for _, rawItem := range items {
+		item, ok := rawItem.(map[string]any)
+		if !ok || item == nil {
+			continue
+		}
+		itemType := strings.TrimSpace(common.Interface2String(item["type"]))
+		role := strings.TrimSpace(common.Interface2String(item["role"]))
+		if itemType != "" && itemType != "message" {
+			continue
+		}
+		if itemType == "" && role == "" {
+			continue
+		}
+		content, exists := item["content"]
+		if !exists || content != nil {
+			continue
+		}
+		item["content"] = ""
+		changed = true
+	}
+
+	if !changed {
+		return nil
+	}
+
+	normalized, err := common.Marshal(items)
+	if err != nil {
+		return err
+	}
+	r.Input = json.RawMessage(bytes.TrimSpace(normalized))
+	return nil
+}
+
 type Reasoning struct {
 	Effort  string `json:"effort,omitempty"`
 	Summary string `json:"summary,omitempty"`
diff --git a/dto/openai_request_zero_value_test.go b/dto/openai_request_zero_value_test.go
index 4b0dbd7c25ee..154f882daaeb 100644
--- a/dto/openai_request_zero_value_test.go
+++ b/dto/openai_request_zero_value_test.go
@@ -71,3 +71,27 @@ func TestOpenAIResponsesRequestPreserveExplicitZeroValues(t *testing.T) {
 	require.True(t, gjson.GetBytes(encoded, "stream").Exists())
 	require.True(t, gjson.GetBytes(encoded, "top_p").Exists())
 }
+
+func TestOpenAIResponsesRequestNormalizeInputNullContent(t *testing.T) {
+	raw := []byte(`{
+		"model":"gpt-5.4",
+		"input":[
+			{"role":"user","content":"hello"},
+			{"role":"assistant","content":null},
+			{"type":"function_call","call_id":"call_1","name":"demo","arguments":"{}"},
+			{"type":"reasoning","summary":[],"content":null,"encrypted_content":"secret"}
+		]
+	}`)
+
+	var req OpenAIResponsesRequest
+	err := common.Unmarshal(raw, &req)
+	require.NoError(t, err)
+
+	err = req.NormalizeInputNullContent()
+	require.NoError(t, err)
+
+	require.Equal(t, "\"\"", gjson.GetBytes(req.Input, "1.content").Raw)
+	require.Equal(t, "", gjson.GetBytes(req.Input, "1.content").String())
+	require.Equal(t, "hello", gjson.GetBytes(req.Input, "0.content").String())
+	require.Equal(t, "null", gjson.GetBytes(req.Input, "3.content").Raw)
+}
diff --git a/relay/responses_handler.go b/relay/responses_handler.go
index 58324aa7cec9..43c3c54ff375 100644
--- a/relay/responses_handler.go
+++ b/relay/responses_handler.go
@@ -59,6 +59,9 @@ func ResponsesHelper(c *gin.Context, info *relaycommon.RelayInfo) (newAPIError *
 	if err != nil {
 		return types.NewError(fmt.Errorf("failed to copy request to GeneralOpenAIRequest: %w", err), types.ErrorCodeInvalidRequest, types.ErrOptionWithSkipRetry())
 	}
+	if err := request.NormalizeInputNullContent(); err != nil {
+		return types.NewError(err, types.ErrorCodeInvalidRequest, types.ErrOptionWithSkipRetry())
+	}
 
 	err = helper.ModelMappedHelper(c, info, request)
 	if err != nil {
diff --git a/service/openaicompat/chat_to_responses.go b/service/openaicompat/chat_to_responses.go
index 16096b88f597..1015f9471ba5 100644
--- a/service/openaicompat/chat_to_responses.go
+++ b/service/openaicompat/chat_to_responses.go
@@ -398,5 +398,9 @@ func ChatCompletionsRequestToResponsesRequest(req *dto.GeneralOpenAIRequest) (*d
 		}
 	}
 
+	if err := out.NormalizeInputNullContent(); err != nil {
+		return nil, err
+	}
+
 	return out, nil
 }