How to return a file within a tool

[rdstettler]

I have an openai runner agent running in the client, something like

agent = Agent(
        name=agent_name,
        instructions=instructions,
        model=model,
        mcp_servers=mcp_servers,
    )

Runner.run_streamed(
                            max_turns=40,
                            starting_agent=agent,
                            input=collapse_history(history)
                        )

So far, for text with works like a charm. However, I have one tool I'd like to return a pdf. Github Copilot always tries to convert the content of the pdf into base64 and return as string, this however very quickly overflows the context window of the AI. OpenAI for example allows to post files, like such
https://platform.openai.com/docs/actions/sending-files

I was looking into something like

from fastmcp.tools.tool import ToolResult

@mcp.tool
def advanced_tool() -> ToolResult:
    """Tool with full control over output."""
    return ToolResult(
        content=[TextContent(text="Human-readable summary")],
        structured_content={"data": "value", "count": 42}
    )

but so far in vain, I don't really know how to structure the answer.

What is the correct way for MCP to return a pdf/image?