diff --git a/pkg/providers/http_provider.go b/pkg/providers/http_provider.go
index 72d5fce..e0dcbbf 100644
--- a/pkg/providers/http_provider.go
+++ b/pkg/providers/http_provider.go
@@ -144,7 +144,45 @@ func toResponsesInputItems(msg Message) []map[string]interface{} {
 	case "system", "developer", "user":
 		return []map[string]interface{}{responsesMessageItem(role, msg.Content, "input_text")}
 	case "assistant":
-		return []map[string]interface{}{responsesMessageItem(role, msg.Content, "output_text")}
+		items := make([]map[string]interface{}, 0, 1+len(msg.ToolCalls))
+		if strings.TrimSpace(msg.Content) != "" || len(msg.ToolCalls) == 0 {
+			items = append(items, responsesMessageItem(role, msg.Content, "output_text"))
+		}
+		for _, tc := range msg.ToolCalls {
+			callID := strings.TrimSpace(tc.ID)
+			if callID == "" {
+				continue
+			}
+			name := strings.TrimSpace(tc.Name)
+			argsRaw := ""
+			if tc.Function != nil {
+				if strings.TrimSpace(tc.Function.Name) != "" {
+					name = strings.TrimSpace(tc.Function.Name)
+				}
+				argsRaw = strings.TrimSpace(tc.Function.Arguments)
+			}
+			if name == "" {
+				continue
+			}
+			if argsRaw == "" {
+				argsJSON, err := json.Marshal(tc.Arguments)
+				if err != nil {
+					argsRaw = "{}"
+				} else {
+					argsRaw = string(argsJSON)
+				}
+			}
+			items = append(items, map[string]interface{}{
+				"type":      "function_call",
+				"call_id":   callID,
+				"name":      name,
+				"arguments": argsRaw,
+			})
+		}
+		if len(items) == 0 {
+			return []map[string]interface{}{responsesMessageItem(role, msg.Content, "output_text")}
+		}
+		return items
 	case "tool":
 		if strings.TrimSpace(msg.ToolCallID) == "" {
 			return []map[string]interface{}{responsesMessageItem("user", msg.Content, "input_text")}
diff --git a/pkg/providers/provider_test.go b/pkg/providers/provider_test.go
index 70bc61b..6a73f49 100644
--- a/pkg/providers/provider_test.go
+++ b/pkg/providers/provider_test.go
@@ -135,6 +135,41 @@ func TestToResponsesInputItems_AssistantUsesOutputText(t *testing.T) {
 	}
 }
 
+func TestToResponsesInputItems_AssistantPreservesToolCalls(t *testing.T) {
+	items := toResponsesInputItems(Message{
+		Role:    "assistant",
+		Content: "",
+		ToolCalls: []ToolCall{
+			{
+				ID:   "call_abc",
+				Name: "exec_command",
+				Arguments: map[string]interface{}{
+					"cmd": "pwd",
+				},
+			},
+		},
+	})
+	if len(items) != 1 {
+		t.Fatalf("expected 1 item, got %d", len(items))
+	}
+	gotType, _ := items[0]["type"].(string)
+	if gotType != "function_call" {
+		t.Fatalf("item type = %q, want function_call", gotType)
+	}
+	gotCallID, _ := items[0]["call_id"].(string)
+	if gotCallID != "call_abc" {
+		t.Fatalf("call_id = %q, want call_abc", gotCallID)
+	}
+	gotName, _ := items[0]["name"].(string)
+	if gotName != "exec_command" {
+		t.Fatalf("name = %q, want exec_command", gotName)
+	}
+	gotArgs, _ := items[0]["arguments"].(string)
+	if !strings.Contains(gotArgs, "\"cmd\":\"pwd\"") {
+		t.Fatalf("arguments = %q, want serialized cmd", gotArgs)
+	}
+}
+
 func containsFunctionCallMarkup(s string) bool {
 	return len(s) > 0 && (strings.Contains(s, "<function_call>") || strings.Contains(s, "</function_call>"))
 }