MultiAuth breaks EntraOBOToken dependency injection

[npeham]

What happened?

Follow-up to #3035 (closed). Using EntraOBOToken as a tool dependency fails at runtime when the server is configured with MultiAuth, even if MultiAuth wraps an AzureProvider.

MRE:

from fastmcp import FastMCP
from fastmcp.server.auth.providers.azure import AzureProvider, EntraOBOToken
from fastmcp.server.auth import MultiAuth

azure_provider = AzureProvider(...)  # valid AzureProvider config
server = FastMCP("demo", auth=MultiAuth(azure_provider))

@server.tool()
async def call_graph(token: EntraOBOToken) -> str:
    # calling any tool that injects EntraOBOToken raises at startup/request time
    ...

Actual:

RuntimeError: EntraOBOToken requires an AzureProvider as the auth provider. Current provider: MultiAuth

Expected: EntraOBOToken resolves successfully when MultiAuth wraps an AzureProvider.

The failure is in _EntraOBOToken (fastmcp/server/auth/providers/azure.py), which does a hard isinstance(server.auth, AzureProvider) check that doesn't look through MultiAuth — not obvious from the public API.

Version Information

FastMCP version:                                                               3.1.1
MCP version:                                                                  1.26.0
Python version:                                                               3.14.3
Platform:                                          macOS-26.2-arm64-arm-64bit-Mach-O
FastMCP root path: /Users/npeham/.pyenv/versions/3.14.3/lib/python3.14/site-packages