Incorrect type for mcp_call Response output item

[Floriferous]

Confirm this is a Node library issue and not an underlying OpenAI API issue

• This is an issue with the Node library

Describe the bug

There's a type mistake in the mcp_call response output item. The type of item.error says it should be string | null, but I received an object with multiple properties. Not sure if the issue is the type definition, or if the error wasn't turned into a string somewhere else.

Here's the type definition of McpCall:

  export interface McpCall {
    /**
     * The unique ID of the tool call.
     */
    id: string;

    /**
     * A JSON string of the arguments passed to the tool.
     */
    arguments: string;

    /**
     * The name of the tool that was run.
     */
    name: string;

    /**
     * The label of the MCP server running the tool.
     */
    server_label: string;

    /**
     * The type of the item. Always `mcp_call`.
     */
    type: 'mcp_call';

    /**
     * The error from the tool call, if any.
     */
    error?: string | null;

    /**
     * The output from the tool call.
     */
    output?: string | null;
  }

And here's an mcp_call item I just got from the responses API:

const item = {
  id: 'mcp_68653a694bec8198a308e96e37ba509b09483f21bc1f2ea7',
  type: 'mcp_call',
  approval_request_id: null,
  arguments: '{"query":"latest tech gadgets"}',
  error: {
    type: 'mcp_protocol_error',
    code: 32600,
    message: 'Session terminated'
  },
  name: 'search',
  output: null,
  server_label: 'my-mcp-server'
}

To Reproduce

Use the responses API like this:

  const response = await openaiClient.responses.create({
    input: "Hello world, please call a tool",
    model: 'gpt-4o',
    tools: [
      {
        require_approval: 'never',
        server_label: 'my-mcp-server',
        server_url: mcpServerUrl,
        type: 'mcp',
      },
    ],
  });

Have a failing MCP server (not sure what the actual underlying Session terminated error is) that generated this kind of error.

Code snippets

OS

macOS

Node version

v20

Library version

openai v5.8.2