Further allow configuring FastMCP through environment variables

[jvdmr]

Enhancement

While many options are already configurable through environment variables, some are still missing.

Particularly 'transport' is unavailable, rendering the other options useless in some use cases: you can set FASTMCP_HOST and FASTMCP_PORT, but as the default transport setting is 'stdio' and there is no FASTMCP_TRANSPORT option, setting the host and port in environment variables is useless.

[marvin-context-protocol]

Found 1 possible duplicate issue:

1. Add Typed MCPConfig for Server Initialization & Dynamic Configuration #1447: Add Typed MCPConfig for Server Initialization & Dynamic Configuration

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[jlowin]

We have already deprecated host and port as init settings, which is the reason they are currently able to be set by environment variable. Rather than add new env vars blindly which need to be respected everywhere (and which would really only affect blind mcp.run() calls invoked by python server.py, which is not the recommended path for this framework), I'd like to better understand why this is helpful to you? fastmcp run <server.py> --transport <transport> --port 1234 is already the encouraged way to run a server, and the new fastmcp.json is intended to be a source of truth.

[brightsparc]

I am finding that I need to configure the transport security settings, and there doesn't appear to be an easy way to pass this in without create a duplicate implementation of the create_streamable_http_app - would it be possible to extend this to include an additional property for security_settings?

    def http_app(
        self,
        path: str | None = None,
        middleware: list[ASGIMiddleware] | None = None,
        json_response: bool | None = None,
        stateless_http: bool | None = None,
        transport: Literal["http", "streamable-http", "sse"] = "http",
        security_settings: TransportSecuritySettings | None = None, # <----- additional field
    ) -> StarletteWithLifespan:



def create_streamable_http_app(
    server: FastMCP[LifespanResultT],
    streamable_http_path: str,
    event_store: EventStore | None = None,
    auth: AuthProvider | None = None,
    json_response: bool = False,
    stateless_http: bool = False,
    security_settings: TransportSecuritySettings | None = None, # <----- additional field
    debug: bool = False,
    routes: list[BaseRoute] | None = None,
    middleware: list[Middleware] | None = None,
) -> StarletteWithLifespan:


    # Create session manager using the provided event store
    session_manager = StreamableHTTPSessionManager(
        app=server._mcp_server,
        event_store=event_store,
        json_response=json_response,
        stateless=stateless_http,
        security_settings=security_settings, # <----- additional field
    )

[jvdmr]

We are running AWS' MCP servers (https://github.com/awslabs/mcp) in an ECS container.

They use FastMCP, and (as far as I can tell) do not out-of-the-box support configuring FastMCP through any config files.

Introducing a FASTMCP_TRANSPORT env variable seems the simplest way forward, whether this variable then gets picked up in the AWS MCP code and passed on to mcp.run(transport=...), or it gets handled in FastMCP itself, but the latter option seems the cleanest to me.

[jlowin]

Ah I see, so the use case is you're using someone else's (AWS) server and they haven't exposed transport options. I understand better - I'm still hesitant to add the env var because it might just be better to add a proper transport layer (which would address @brightsparc concern) but on the other hand I do want to unblock this