PrefectHQ/fastmcp · error · ValueError

Invalid transport: {transport}

Error message

Invalid transport: {transport}

What it means

The run_server_in_process test helper only supports the 'sse' transport when spawning a server subprocess; any other value hits the else branch and raises ValueError. It exists because Starlette apps are constructed (not pickled) inside the child process based on the transport string.

Source

Thrown at fastmcp_slim/fastmcp/utilities/tests.py:73

    try:
        # apply the new settings
        for attr, value in kwargs.items():
            settings.set_setting(attr, value)
        yield

    finally:
        # restore the old settings
        for attr in kwargs:
            settings.set_setting(attr, old_settings.get_setting(attr))


def _run_server(mcp_server: FastMCP, transport: Literal["sse"], port: int) -> None:
    # Some Starlette apps are not pickleable, so we need to create them here based on the indicated transport
    if transport == "sse":
        app = mcp_server.http_app(transport="sse")
    else:
        raise ValueError(f"Invalid transport: {transport}")
    uvicorn_server = uvicorn.Server(
        config=uvicorn.Config(
            app=app,
            host="127.0.0.1",
            port=port,
            log_level="error",
            ws="websockets-sansio",
        )
    )
    uvicorn_server.run()


@contextmanager
def run_server_in_process(
    server_fn: Callable[..., None],
    *args: Any,
    provide_host_and_port: bool = True,
    host: str = "127.0.0.1",

View on GitHub (pinned to 1f02114297)

Solutions

1. Use transport="sse" (the only value this helper accepts)
2. Use a different server-start helper (e.g. run_server_async or http_app directly) for other transports
3. Fix typos — the literal string must be exactly "sse"

Example fix

// before
with run_server_in_process(mcp_server, transport="http") as url: ...

// after
with run_server_in_process(mcp_server, transport="sse") as url: ...

if transport != "sse":
    raise ValueError(f"run_server_in_process only supports 'sse', got {transport!r}")

def is_supported_transport(t: str) -> bool:
    return t == "sse"

try:
    with run_server_in_process(mcp, transport=transport) as url:
        yield url
except ValueError as e:
    pytest.skip(f"unsupported transport for helper: {e}")

When it happens

Trigger: Passing transport="http"/"streamable-http"/"stdio" (or a typo like "https") to run_server_in_process / mcp_server_url instead of "sse".

Common situations: Copying test helper code between FastMCP versions where more transports are supported elsewhere; assuming the test utility accepts every transport the client does; simple typos.

Related errors

• SSE transport does not support stateless mode
• Unknown transport: {transport}
• Both 'httpx_client_factory' and 'verify' were provided. The
• Could not create the CLI state directory
• OAuth provider has no server URL. Either pass mcp_url to OAu

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/30098a61ef0449b5. Report an issue: GitHub.