OpenAiChatOptions where toolChoice can only be of the String type

[zmwei666]

Bug description
Error message:

{
    "error": {
        "message": "Invalid value: '{\"t...\"}}'. Supported values are: 'none', 'auto', and 'required'.",
        "type": "invalid_request_error",
        "param": "tool_choice",
        "code": "invalid_value"
    }
}

The toolChoice of OpenAiChatOptions is defined as a String, but in fact, it should be an Object. It can be a String or a Map. As defined in ToolChoiceBuilder, but now I can't set it to map.
Open ai explains the toolChoice field as follows:

Controls which (if any) tool is called by the model. none means the model will not call any tool and instead generates a message. auto means the model can pick between generating a message or calling one or more tools. required means the model must call one or more tools. Specifying a particular tool via {"type": "function", "function": {"name": "my_function"}} forces the model to call that tool.

Reference link: https://platform.openai.com/docs/api-reference/chat/create

Environment
JDK: 21
Spring AI: 1.0.0-M4
OS: macOS 15.0.1

Steps to reproduce

@SpringBootTest
public class ToolChoiceTest {
    @Resource
    private OpenAiChatModel openAiChatModel;

    @Resource
    private ObjectMapper objectMapper;

    @Test
    void test() throws JsonProcessingException {
        Object currentWeather = OpenAiApi.ChatCompletionRequest.ToolChoiceBuilder.FUNCTION("CurrentWeather");
        OpenAiChatOptions chatOptions = OpenAiChatOptions.builder()
                .withFunctionCallbacks(
                        List.of(FunctionCallback.builder()
                                .description("Get the weather in location")
                                .function("CurrentWeather", new MockWeatherService())
                                .inputType(MockWeatherService.Request.class)
                                .build()))

//                .withToolChoice(OpenAiApi.ChatCompletionRequest.ToolChoiceBuilder.AUTO)  // ok
//                .withToolChoice(OpenAiApi.ChatCompletionRequest.ToolChoiceBuilder.NONE)  // ok
//                .withToolChoice(OpenAiApi.ChatCompletionRequest.ToolChoiceBuilder.FUNCTION("CurrentWeather"))  // compilation failed
                .withToolChoice(objectMapper.writeValueAsString(currentWeather)) // NonTransientAiException: 400
                // request body: {"messages":[{"content":"What is the temperature in Shanghai now?","role":"user"}],"model":"gpt-4o","stream":false,"temperature":0.7,"tools":[{"type":"function","function":{"description":"Get the weather in location","name":"CurrentWeather","parameters":{"$schema":"https://json-schema.org/draft/2020-12/schema","type":"object","properties":{"location":{"type":"string"},"unit":{"type":"string","enum":["C","F"]}}}}}],"tool_choice":"{\"type\":\"function\",\"function\":{\"name\":\"CurrentWeather\"}}"}
                // response body: {"error":{"message":"Invalid value: '{\"t...\"}}'. Supported values are: 'none', 'auto', and 'required'.","type":"invalid_request_error","param":"tool_choice","code":"invalid_value"}}

                .build();

        chatOptions.setProxyToolCalls(true);
        ChatResponse chatResponse = openAiChatModel.call(new Prompt("What is the temperature in Shanghai now?", chatOptions));
        System.out.println(chatResponse.getResult().getOutput());

    }


    public static class MockWeatherService implements Function<MockWeatherService.Request, MockWeatherService.Response> {

        public enum Unit { C, F }
        public record Request(String location, Unit unit) {}
        public record Response(double temp, Unit unit) {}

        public Response apply(Request request) {
            return new Response(30.0, Unit.C);
        }
    }
}

From the request body, it can be seen that tool_choice is serialized:

"tool_choice":"{\"type\":\"function\",\"function\":{\"name\":\"CurrentWeather\"}

open ai response:

{
    "error": {
        "message": "Invalid value: '{\"t...\"}}'. Supported values are: 'none', 'auto', and 'required'.",
        "type": "invalid_request_error",
        "param": "tool_choice",
        "code": "invalid_value"
    }
}

Expected behavior
Change the toolChoice of OpenAiChatOptions to Object type

After the change
After I changed the toolChoice of OpenAiChatOptions to type Object, the following code worked:

@Test
void test() throws JsonProcessingException {
    OpenAiChatOptions chatOptions = OpenAiChatOptions.builder()
            .withFunctionCallbacks(
                    List.of(FunctionCallback.builder()
                            .description("Get the weather in location")
                            .function("CurrentWeather", new MockWeatherService())
                            .inputType(MockWeatherService.Request.class)
                            .build()))

            .withToolChoice(OpenAiApi.ChatCompletionRequest.ToolChoiceBuilder.FUNCTION("CurrentWeather"))  // ok
            // request body: {"messages":[{"content":"What is the temperature in Shanghai now?","role":"user"}],"model":"gpt-4o","stream":false,"temperature":0.7,"tools":[{"type":"function","function":{"description":"Get the weather in location","name":"CurrentWeather","parameters":{"$schema":"https://json-schema.org/draft/2020-12/schema","type":"object","properties":{"location":{"type":"string"},"unit":{"type":"string","enum":["C","F"]}}}}}],"tool_choice":{"function":{"name":"CurrentWeather"},"type":"function"}}
            // response body: {"id":"chatcmpl-AcvLRP5ikO8N2ln9DVY8ZbBuhHwHw","object":"chat.completion","created":1733840261,"model":"gpt-4o-2024-08-06","choices":[{"index":0,"message":{"role":"assistant","content":null,"tool_calls":[{"id":"call_VPXdnJqOEbUWmrk1WEGfTPlE","type":"function","function":{"name":"CurrentWeather","arguments":"{\"location\":\"Shanghai\"}"}}],"refusal":null},"logprobs":null,"finish_reason":"stop"}],"usage":{"prompt_tokens":70,"completion_tokens":5,"total_tokens":75,"prompt_tokens_details":{"cached_tokens":0,"audio_tokens":0},"completion_tokens_details":{"reasoning_tokens":0,"audio_tokens":0,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"system_fingerprint":"fp_9d50cd990b"}

            .build();

    chatOptions.setProxyToolCalls(true);
    ChatResponse chatResponse = openAiChatModel.call(new Prompt("What is the temperature in Shanghai now?", chatOptions));
    System.out.println(chatResponse.getResult().getOutput());

}

After changing toolChoice to type Object, I can use ToolChoiceBuilder directly

OpenAiApi.ChatCompletionRequest.ToolChoiceBuilder.FUNCTION("CurrentWeather")

In the final request body, toolChoice is:

"tool_choice":{"function":{"name":"CurrentWeather"},"type":"function"}

[zmwei666]

pr: Fix OpenAI API Tool Choice type