Error When Using OfArrayOfContentParts, ToolCall & PDF document

[backkem]

When using the openai-go SDK to create a chat completion with a ChatCompletionAssistantMessageParamContentUnion that includes OfArrayOfContentParts in combination with tool calling & a PDF docucment, the API returns a 400 Bad Request error with the message:
Invalid parameter: messages with role 'tool' must be a response to a preceding message with 'tool_calls'.

This error seems incorrect as there is no such issue in this request. I assume it's masking some other error. In addition, replacing OfArrayOfContentParts with OfString in the same message structure resolves the issue, and the request succeeds. Replacing the PDF document with a text message also doesn't reproduce the issue.

Steps to Reproduce

Use the following code snippet to create a chat completion request:

package main

import (
    "context"
    "fmt"

    "github.com/openai/openai-go"
)

func main() {
	client := openai.NewClient()
	ctx := context.Background()
	params := openai.ChatCompletionNewParams{
		Model: openai.ChatModelGPT4o,
		Messages: []openai.ChatCompletionMessageParamUnion{
			openai.UserMessage("what's the weather in nyc and describe the document contents."),
			{
				OfAssistant: &openai.ChatCompletionAssistantMessageParam{
					Content: openai.ChatCompletionAssistantMessageParamContentUnion{
						OfArrayOfContentParts: []openai.ChatCompletionAssistantMessageParamContentArrayOfContentPartUnion{},
						// OfString: openai.String(""), // Empty string instead works.
					},
					ToolCalls: []openai.ChatCompletionMessageToolCallParam{
						{
							ID:   "call_123",
							Type: "function",
							Function: openai.ChatCompletionMessageToolCallFunctionParam{
								Name:      "get_weather",
								Arguments: `{"location":"NYC","unit":"f"}`,
							},
						},
					},
				},
			},
			{
				OfTool: &openai.ChatCompletionToolMessageParam{
					Content: openai.ChatCompletionToolMessageParamContentUnion{
						OfString: openai.String("40f"),
					},
					ToolCallID: "call_123",
				},
			},
			// openai.UserMessage("Document content: I'm a PDF document."), // No document instead works
			 openai.UserMessage(
			 	[]openai.ChatCompletionContentPartUnionParam{
			 		{
			 			OfFile: &openai.ChatCompletionContentPartFileParam{
			 				File: openai.ChatCompletionContentPartFileFileParam{
			 					Filename: openai.String("test.pdf"),
			 					FileData: openai.String(`data:application/pdf;base64,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`),
			 				},
			 			},
			 		},
			 	},
			 ),
		},
	}

	_, err = client.Chat.Completions.New(ctx, params)
	if err != nil {
		fmt.Printf("Error: %v\n", err)
	} else {
		fmt.Println("Success")
	}
}

output:

Error: POST "https://api.openai.com/v1/chat/completions": 400 Bad Request {
    "message": "Invalid parameter: messages with role 'tool' must be a response to a preceeding message with 'tool_calls'.",
    "type": "invalid_request_error",
    "param": "messages.[2].role",
    "code": null
  }

Expected Behavior

Seems like this should just work.

sysinfo

SDK Version: openai-go v1.5.0
Go Version: go1.24.1 windows/amd64
Model: gpt-4o