Authenticating HTTP Servers

Why stdio never needed this

A stdio server (every lesson through 19) is launched directly by whoever's talking to it, there's no network in between, so there's nothing to authenticate. An HTTP server (Lesson 20) is different: it's sitting on a port, and anything that can reach that port can send it requests. add is a harmless demo, but a real server exposing, say, database access absolutely needs to know who's asking.

Verifying a bearer token

The SDK's building block is TokenVerifier, a small protocol with one method.

from mcp.server.auth.provider import AccessToken, TokenVerifier

VALID_TOKEN = "secret-token-123"


class StaticTokenVerifier(TokenVerifier):
    async def verify_token(self, token: str) -> AccessToken | None:
        if token != VALID_TOKEN:
            return None
        return AccessToken(token=token, client_id="demo-client", scopes=["mcp"])

Returning None means "reject this token", returning an AccessToken means "accept it, and here's what it's allowed to do" (scopes). A real server would check the token against a database or call out to an identity provider here, this lesson hardcodes one valid token to keep the demo self-contained.

Wiring it into the server

from mcp.server.auth.settings import AuthSettings
from mcp.server.fastmcp import FastMCP

mcp = FastMCP(
    "auth-demo-server",
    port=8932,
    stateless_http=True,
    token_verifier=StaticTokenVerifier(),
    auth=AuthSettings(
        issuer_url="http://127.0.0.1:8932",
        resource_server_url="http://127.0.0.1:8932",
        required_scopes=["mcp"],
    ),
)

AuthSettings describes this server as an OAuth resource server, issuer_url and resource_server_url are metadata a real deployment would point at an actual identity provider; here they just point back at the demo server itself. MCP recommends OAuth for production HTTP servers precisely so a server doesn't need to invent its own token format, this lesson's StaticTokenVerifier is a minimal stand-in to demonstrate the mechanism, not a production auth scheme.

Sending the token from a client

async with streamablehttp_client(
    "http://127.0.0.1:8932/mcp",
    headers={"Authorization": "Bearer secret-token-123"},
) as (read, write, _):
    ...

streamablehttp_client takes a plain headers dict, same as any HTTP client. Omit the header, or send the wrong token, and the connection fails with a 401 Unauthorized before initialize() ever completes.