diff --git a/dto/openai_response.go b/dto/openai_response.go
index 8d727dab108..5b2172eea20 100644
--- a/dto/openai_response.go
+++ b/dto/openai_response.go
@@ -134,8 +134,37 @@ type FunctionResponse struct {
 	Description string `json:"description,omitempty"`
 	Name        string `json:"name,omitempty"`
 	// call function with arguments in JSON format
-	Parameters any    `json:"parameters,omitempty"` // request
-	Arguments  string `json:"arguments"`            // response
+	Parameters any               `json:"parameters,omitempty"` // request
+	Arguments  ResponseArguments `json:"arguments"`            // response
+}
+
+// ResponseArguments accepts both the canonical JSON string form and the object
+// form occasionally emitted by Responses streaming events.
+type ResponseArguments string
+
+func (a *ResponseArguments) UnmarshalJSON(data []byte) error {
+	if len(data) == 0 || string(data) == "null" {
+		*a = ""
+		return nil
+	}
+	var s string
+	if err := json.Unmarshal(data, &s); err == nil {
+		*a = ResponseArguments(s)
+		return nil
+	}
+	if !json.Valid(data) {
+		return fmt.Errorf("invalid response arguments JSON")
+	}
+	*a = ResponseArguments(string(data))
+	return nil
+}
+
+func (a ResponseArguments) MarshalJSON() ([]byte, error) {
+	return json.Marshal(string(a))
+}
+
+func (a ResponseArguments) String() string {
+	return string(a)
 }
 
 type ChatCompletionsStreamResponse struct {
@@ -346,7 +375,7 @@ type ResponsesOutput struct {
 	Size      string                   `json:"size"`
 	CallId    string                   `json:"call_id,omitempty"`
 	Name      string                   `json:"name,omitempty"`
-	Arguments string                   `json:"arguments,omitempty"`
+	Arguments ResponseArguments        `json:"arguments,omitempty"`
 }
 
 type ResponsesOutputContent struct {
diff --git a/dto/openai_response_test.go b/dto/openai_response_test.go
new file mode 100644
index 00000000000..28dde2b5da6
--- /dev/null
+++ b/dto/openai_response_test.go
@@ -0,0 +1,50 @@
+package dto
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestResponsesOutputArgumentsAcceptsObject(t *testing.T) {
+	var output ResponsesOutput
+	err := json.Unmarshal([]byte(`{"type":"function_call","arguments":{"query":"hello","limit":3}}`), &output)
+	if err != nil {
+		t.Fatalf("Unmarshal ResponsesOutput failed: %v", err)
+	}
+	if got := output.Arguments.String(); got != `{"query":"hello","limit":3}` {
+		t.Fatalf("unexpected arguments: %s", got)
+	}
+}
+
+func TestResponsesOutputArgumentsAcceptsNull(t *testing.T) {
+	var output ResponsesOutput
+	err := json.Unmarshal([]byte(`{"type":"function_call","arguments":null}`), &output)
+	if err != nil {
+		t.Fatalf("Unmarshal ResponsesOutput failed: %v", err)
+	}
+	if got := output.Arguments.String(); got != "" {
+		t.Fatalf("unexpected arguments: %s", got)
+	}
+}
+
+func TestResponsesOutputArgumentsAcceptsArray(t *testing.T) {
+	var output ResponsesOutput
+	err := json.Unmarshal([]byte(`{"type":"function_call","arguments":["hello",3]}`), &output)
+	if err != nil {
+		t.Fatalf("Unmarshal ResponsesOutput failed: %v", err)
+	}
+	if got := output.Arguments.String(); got != `["hello",3]` {
+		t.Fatalf("unexpected arguments: %s", got)
+	}
+}
+
+func TestFunctionResponseArgumentsAcceptsString(t *testing.T) {
+	var fn FunctionResponse
+	err := json.Unmarshal([]byte(`{"name":"x","arguments":"{\"query\":\"hello\"}"}`), &fn)
+	if err != nil {
+		t.Fatalf("Unmarshal FunctionResponse failed: %v", err)
+	}
+	if got := fn.Arguments.String(); got != `{"query":"hello"}` {
+		t.Fatalf("unexpected arguments: %s", got)
+	}
+}
diff --git a/relay/channel/claude/relay-claude.go b/relay/channel/claude/relay-claude.go
index fa8234523c7..661b5f90a4c 100644
--- a/relay/channel/claude/relay-claude.go
+++ b/relay/channel/claude/relay-claude.go
@@ -485,7 +485,7 @@ func StreamResponseClaude2OpenAI(claudeResponse *dto.ClaudeResponse) *dto.ChatCo
 					Type:  "function",
 					Index: common.GetPointer(fcIdx),
 					Function: dto.FunctionResponse{
-						Arguments: *claudeResponse.Delta.PartialJson,
+						Arguments: dto.ResponseArguments(*claudeResponse.Delta.PartialJson),
 					},
 				})
 			case "signature_delta":
@@ -546,7 +546,7 @@ func ResponseClaude2OpenAI(claudeResponse *dto.ClaudeResponse) *dto.OpenAITextRe
 				Type: "function", // compatible with other OpenAI derivative applications
 				Function: dto.FunctionResponse{
 					Name:      message.Name,
-					Arguments: string(args),
+					Arguments: dto.ResponseArguments(string(args)),
 				},
 			})
 		case "thinking":
diff --git a/relay/channel/gemini/relay-gemini.go b/relay/channel/gemini/relay-gemini.go
index 21641e48386..ad522f3d315 100644
--- a/relay/channel/gemini/relay-gemini.go
+++ b/relay/channel/gemini/relay-gemini.go
@@ -1005,7 +1005,7 @@ func getResponseToolCall(item *dto.GeminiPart) *dto.ToolCallResponse {
 		ID:   fmt.Sprintf("call_%s", common.GetUUID()),
 		Type: "function",
 		Function: dto.FunctionResponse{
-			Arguments: string(argsBytes),
+			Arguments: dto.ResponseArguments(string(argsBytes)),
 			Name:      item.FunctionCall.FunctionName,
 		},
 	}
diff --git a/relay/channel/ollama/stream.go b/relay/channel/ollama/stream.go
index 2a264b27e46..8962d00a665 100644
--- a/relay/channel/ollama/stream.go
+++ b/relay/channel/ollama/stream.go
@@ -138,7 +138,7 @@ func ollamaStreamHandler(c *gin.Context, info *relaycommon.RelayInfo, resp *http
 					// arguments -> string
 					argBytes, _ := json.Marshal(tc.Function.Arguments)
 					toolId := fmt.Sprintf("call_%d", toolCallIndex)
-					tr := dto.ToolCallResponse{ID: toolId, Type: "function", Function: dto.FunctionResponse{Name: tc.Function.Name, Arguments: string(argBytes)}}
+					tr := dto.ToolCallResponse{ID: toolId, Type: "function", Function: dto.FunctionResponse{Name: tc.Function.Name, Arguments: dto.ResponseArguments(string(argBytes))}}
 					tr.SetIndex(toolCallIndex)
 					toolCallIndex++
 					delta.Choices[0].Delta.ToolCalls = append(delta.Choices[0].Delta.ToolCalls, tr)
diff --git a/relay/channel/openai/chat_via_responses.go b/relay/channel/openai/chat_via_responses.go
index 5e8ec173de1..4fa6c111a67 100644
--- a/relay/channel/openai/chat_via_responses.go
+++ b/relay/channel/openai/chat_via_responses.go
@@ -258,7 +258,7 @@ func OaiResponsesToChatStreamHandler(c *gin.Context, info *relaycommon.RelayInfo
 			ID:   callID,
 			Type: "function",
 			Function: dto.FunctionResponse{
-				Arguments: argsDelta,
+				Arguments: dto.ResponseArguments(argsDelta),
 			},
 		}
 		tool.SetIndex(idx)
@@ -408,7 +408,7 @@ func OaiResponsesToChatStreamHandler(c *gin.Context, info *relaycommon.RelayInfo
 				toolCallNameByID[callID] = name
 			}
 
-			newArgs := streamResp.Item.Arguments
+			newArgs := streamResp.Item.Arguments.String()
 			prevArgs := toolCallArgsByID[callID]
 			argsDelta := ""
 			if newArgs != "" {
diff --git a/relay/channel/openai/helper.go b/relay/channel/openai/helper.go
index 08811a77205..6e01b026745 100644
--- a/relay/channel/openai/helper.go
+++ b/relay/channel/openai/helper.go
@@ -85,7 +85,7 @@ func ProcessStreamResponse(streamResponse dto.ChatCompletionsStreamResponse, res
 			}
 			for _, tool := range choice.Delta.ToolCalls {
 				responseTextBuilder.WriteString(tool.Function.Name)
-				responseTextBuilder.WriteString(tool.Function.Arguments)
+				responseTextBuilder.WriteString(tool.Function.Arguments.String())
 			}
 		}
 	}
@@ -132,7 +132,7 @@ func processChatCompletions(streamResp string, streamItems []string, responseTex
 				}
 				for _, tool := range choice.Delta.ToolCalls {
 					responseTextBuilder.WriteString(tool.Function.Name)
-					responseTextBuilder.WriteString(tool.Function.Arguments)
+					responseTextBuilder.WriteString(tool.Function.Arguments.String())
 				}
 			}
 		}
diff --git a/service/convert.go b/service/convert.go
index 95acf835ee4..11c4f9f922f 100644
--- a/service/convert.go
+++ b/service/convert.go
@@ -343,13 +343,14 @@ func StreamResponseOpenAI2Claude(openAIResponse *dto.ChatCompletionsStreamRespon
 			claudeResponses = append(claudeResponses, resp)
 			// 首块包含工具 delta，则追加 input_json_delta
 			if toolCall.Function.Arguments != "" {
+				partialJSON := toolCall.Function.Arguments.String()
 				idx := 0
 				claudeResponses = append(claudeResponses, &dto.ClaudeResponse{
 					Index: &idx,
 					Type:  "content_block_delta",
 					Delta: &dto.ClaudeMediaMessage{
 						Type:        "input_json_delta",
-						PartialJson: &toolCall.Function.Arguments,
+						PartialJson: &partialJSON,
 					},
 				})
 			}
@@ -515,12 +516,13 @@ func StreamResponseOpenAI2Claude(openAIResponse *dto.ChatCompletionsStreamRespon
 				}
 
 				if len(toolCall.Function.Arguments) > 0 {
+					partialJSON := toolCall.Function.Arguments.String()
 					claudeResponses = append(claudeResponses, &dto.ClaudeResponse{
 						Index: &idx,
 						Type:  "content_block_delta",
 						Delta: &dto.ClaudeMediaMessage{
 							Type:        "input_json_delta",
-							PartialJson: &toolCall.Function.Arguments,
+							PartialJson: &partialJSON,
 						},
 					})
 				}