Invalid value for 'tool_choice'

[Muleskinner]

Overview

After upgrading from version 8.5.4 to 8.8.1, I now encounter an error when calling GetCompletionAsync without using tools.

DeserializeAsync Failed! HTTP status code: BadRequest | Response body: {
  "error": {
    "message": "Invalid value for 'tool_choice': 'tool_choice' is only allowed when 'tools' are specified.",
    "type": "invalid_request_error",
    "param": "tool_choice",
    "code": null
  }
}

Im creating the ChatRequest like this:
new ChatRequest(messages, null, model: model, temperature: temperature, jsonSchema: jsonScheme); ie: tools = null and im not providing tool_choice.

That is, tools = null and I'm not explicitly providing a tool_choice. I've also tested the following variations — all result in the same error:

new ChatRequest(messages, null, toolChoice: "none", model: model, temperature: temperature, jsonSchema: jsonScheme); 

and this

new ChatRequest(messages, null, toolChoice: null, model: model, temperature: temperature, jsonSchema: jsonScheme); 

all with same error.

To Reproduce

Steps to reproduce the behavior:

var chatRequest = new ChatRequest(messages, null, model: model, temperature: temperature);
var result = await Client.ChatEndpoint.GetCompletionAsync(chatRequest);

Result:

DeserializeAsync Failed! HTTP status code: BadRequest | Response body: {
  "error": {
    "message": "Invalid value for 'tool_choice': 'tool_choice' is only allowed when 'tools' are specified.",
    "type": "invalid_request_error",
    "param": "tool_choice",
    "code": null
  }
}

Expected behavior

When no tools are specified, tool_choice should not be included in the request payload — or at least not default to "none".

Additional context

This worked correctly in v8.5.4, even with:

new ChatRequest(messages, null, toolChoice: "auto", model: model, temperature: temperature, jsonSchema: jsonScheme); 

[Muleskinner]

Update / Workaround found

After digging into this further, I discovered that the issue only occurs when using the ChatRequest constructor that includes the tools parameter, even if tools is explicitly set to null.

The following causes the error:
// This triggers tool_choice = "none", which causes the API to reject the request
new ChatRequest(messages, tools: null, model: "gpt-4o");'

But using the overload that doesn't accept a tools parameter at all works perfectly:
// This avoids the bug entirely
new ChatRequest(messages, model: "gpt-4o");

So until the internal logic is fixed to avoid setting tool_choice when tools == null, the safest workaround is to avoid passing the tools argument entirely when you're not using tools.

Thanks again — and hope this helps others running into the same error!

[StephenHodgson]

This is the intended behavior, some models require the choice be set to none even when no tools are passed.