Identity Bridge

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

SCOPE = f"api://{CLIENT_ID}/access_as_user"   # FULLY-QUALIFIED — required

mcp = FastMCP(
    "mcp-prm-obo-azure",
    auth=AuthSettings(
        issuer_url=f"https://login.microsoftonline.com/{TENANT_ID}/v2.0",
        resource_server_url=BASE_URL,          # published in PRM (RFC 9728)
        required_scopes=[SCOPE],               # drives PRM + middleware check
    ),
    token_verifier=EntraTokenVerifier(),
)
# Serves /.well-known/oauth-protected-resource and /mcp (Streamable HTTP).

import jwt
from jwt import PyJWKClient

_jwks = PyJWKClient(f"https://login.microsoftonline.com/{TENANT_ID}/discovery/v2.0/keys")
VALID_ISSUERS = {
    f"https://login.microsoftonline.com/{TENANT_ID}/v2.0",
    f"https://sts.windows.net/{TENANT_ID}/",     # v1 issuer — also accepted
}

def verify(token: str) -> dict:
    key = _jwks.get_signing_key_from_jwt(token).key
    claims = jwt.decode(token, key, algorithms=["RS256"],
                        audience=f"api://{CLIENT_ID}")   # no issuer= (checked below)
    if claims["iss"] not in VALID_ISSUERS:
        raise PermissionError("bad issuer")
    if "access_as_user" not in claims.get("scp", "").split():
        raise PermissionError("missing scope")
    return claims

import msal

_app = msal.ConfidentialClientApplication(
    CLIENT_ID, client_credential=CLIENT_SECRET,
    authority=f"https://login.microsoftonline.com/{TENANT_ID}",
)

def arm_token_for_user(user_assertion: str) -> str:
    result = _app.acquire_token_on_behalf_of(
        user_assertion=user_assertion,                      # the caller's token
        scopes=["https://management.azure.com/.default"],   # downstream: ARM
    )
    if "access_token" not in result:
        raise PermissionError(result.get("error_description", "OBO failed"))
    return result["access_token"]
# Use this token for ARM REST calls -> the user's own RBAC applies.

You are a senior Azure identity engineer and an expert on the Model Context Protocol (MCP),
OAuth 2.1, Microsoft Entra ID, and the On-Behalf-Of (OBO) flow.

Build me a production-shaped, REMOTE, AUTHENTICATED MCP server (Python) that lets an AI
client call Azure AS THE SIGNED-IN USER — never with a standing service identity.

REQUIREMENTS

1. Transport & framework
   - MCP over Streamable HTTP using FastMCP. Host on Azure Functions (Flex Consumption,
     Linux, Python) via an ASGI entry point. routePrefix "" so /mcp and /.well-known/*
     are at the root.

2. Standards-based auth (the core of this build)
   - Act as an OAuth 2.1 Resource Server. Publish Protected Resource Metadata (RFC 9728)
     at /.well-known/oauth-protected-resource pointing clients at Microsoft Entra ID.
   - Advertise the FULLY-QUALIFIED scope api:///access_as_user (a bare scope
     name makes Entra resolve against Microsoft Graph and fail).
   - Validate incoming Entra bearer tokens: JWKS signature, issuer, audience, scope.
     Accept BOTH v2 (login.microsoftonline.com/.../v2.0) and v1 (sts.windows.net/...)
     issuers.

3. On-Behalf-Of
   - Use MSAL ConfidentialClientApplication.acquire_token_on_behalf_of to exchange the
     caller's token for an Azure Resource Manager token (scope
     https://management.azure.com/.default), then call ARM REST as the user (their RBAC).

4. Tools
   - whoami(), list_subscriptions(), list_resource_groups(subscription_id),
     list_resources(subscription_id, resource_group?). Read-only. Return JSON.

5. Config & deploy
   - Read ALL config from .env only (never os.environ): TENANT_ID, CLIENT_ID,
     CLIENT_SECRET, REQUIRED_SCOPE, BASE_URL.
   - Provide Bicep infra (storage, plan, Function App, App Insights, RBAC), an azd
     azure.yaml with a post-provision hook that writes .env, and a PowerShell script
     that creates the Entra app registration (sets requestedAccessTokenVersion = 2).
   - Pass allowed_hosts/allowed_origins from BASE_URL to avoid 421 Invalid Host header
     once deployed.

6. Docs
   - README with the discover -> sign in -> call -> validate -> OBO -> act flow, a
     .vscode/mcp.json example, and a troubleshooting table (AADSTS65002, 401
     invalid_token, 421 Invalid Host, 403 BlobUploadFailedException).

Generate the full project and the exact az / azd commands, then tell me how to verify it
from VS Code (PRM discovery + sign-in). Ask me anything you need; otherwise build it.