工具调用能力太差，基本稳定复现

Start by reproducing the weather-tool workflow described in the issue and inspect the generated messages payload, especially the empty assistant content and subsequent weather result. Trace where tool results are converted into messages; done means the request no longer returns the missing messages.content 400 error and the workflow completes normally.

调用一个天气查询，执行后将天气查询结果放在text中，然后模型报400，missing messages.content parameter，进而工作流异常结束
{
"model": "doubao-seed-1-8-251228",
"messages": [
{
"role": "system",
"content": "Answer user questions to the best of your knowledge.\n1. use the 'getCurrentTime' tool to query the city...s current time.\n2. use the 'getWeather' tool to query the city...s current weather.\n\n\nYou are an agent. Your internal name is "ark_agent". The description about you is "A helpful assistant for user questions."."
},
{
"role": "user",
"content": [
{
"type": "text",
"text": "............"
}
]
},
{
"role": "assistant",
"content": []
},
{
"role": "user",
"content": [
{
"type": "text",
"text": "{"status":"success","report":"The weather in ...... is sunny with a temperature of 25 degrees Celsius (77 degrees Fahrenheit)."}"
}
]
}
],
"tools": [
{
"type": "function",
"function": {
"name": "getWeather",
"description": "Get the weather for a given city",
"parameters": {
"properties": {
"city": {
"description": "The name of the city for which to retrieve the weather report",
"type": "string"
}
},
"required": [
"city"
],
"type": "object"
}
}
},
{
"type": "function",
"function": {
"name": "getCurrentTime",
"description": "Get the current time for a given city",
"parameters": {
"properties": {
"city": {
"description": "Name of the city to get the time for",
"type": "string"
}
},
"required": [
"city"
],
"type": "object"
}
}
}
]
}