Feature Request: Support Custom Headers for MCP Server Calls

[Khalidzo]

Hi there 👋

I'm currently working with the Anthropic SDK + FastMCP setup, and I ran into a small limitation. Right now, it seems like the only way to pass headers to an MCP server is by using the authorization_token field — which always gets sent as an Authorization: Bearer ... header.

That works for authentication, but what if I just want to pass some extra context to the server? Things like:

• X-Client: abc123
• X-Session-ID: abc123

Right now I have to "abuse" the authorization token just to sneak in custom values, and it just a messy way to get things work.

It’d be amazing if we could pass actual custom headers through the MCP server config, directly in the BetaRequestMCPServerURLDefinitionParam object like this:

class BetaRequestMCPServerURLDefinitionParam(TypedDict, total=False):
    name: Required[str]
    type: Required[Literal["url"]]
    url: Required[str]
    authorization_token: Optional[str]
    tool_configuration: Optional[BetaRequestMCPServerToolConfigurationParam]
    extra_headers: Optional[dict[str, str]]  # <--- new addition

Thanks a lot in advance.
Khalid