fix: resolve EntraOBOToken dependency injection through MultiAuth (#3609)

Co-authored-by: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>

src/fastmcp/server/auth/providers/azure.py

@@ -14,6 +14,7 @@ import httpx
 from key_value.aio.protocols import AsyncKeyValue
 
 from fastmcp.dependencies import Dependency
+from fastmcp.server.auth.auth import MultiAuth
 from fastmcp.server.auth.oauth_proxy import OAuthProxy
 from fastmcp.server.auth.providers.jwt import JWTVerifier
 from fastmcp.utilities.auth import decode_jwt_payload, parse_scopes
@@ -24,6 +25,8 @@ if TYPE_CHECKING:
     from mcp.server.auth.provider import AuthorizationParams
     from mcp.shared.auth import OAuthClientInformationFull
 
+    from fastmcp.server.auth.auth import AuthProvider
+
 logger = get_logger(__name__)
 
 # Standard OIDC scopes that should never be prefixed with identifier_uri.
@@ -663,6 +666,17 @@ def _require_azure_identity(feature: str) -> None:
         ) from e
 
 
+def _find_azure_provider(auth: AuthProvider | None) -> AzureProvider | None:
+    """Extract an AzureProvider from an auth provider, unwrapping MultiAuth if needed."""
+    if isinstance(auth, AzureProvider):
+        return auth
+
+    if isinstance(auth, MultiAuth) and isinstance(auth.server, AzureProvider):
+        return auth.server
+
+    return None
+
+
 class _EntraOBOToken(Dependency[str]):
     """Dependency that performs OBO token exchange for Microsoft Entra.
 
@@ -687,13 +701,14 @@ class _EntraOBOToken(Dependency[str]):
             )
 
         server = get_server()
-        if not isinstance(server.auth, AzureProvider):
+        azure_provider = _find_azure_provider(server.auth)
+        if azure_provider is None:
             raise RuntimeError(
                 "EntraOBOToken requires an AzureProvider as the auth provider. "
                 f"Current provider: {type(server.auth).__name__}"
             )
 
-        credential = await server.auth.get_obo_credential(
+        credential = await azure_provider.get_obo_credential(
             user_assertion=access_token.token,
         )
 

tests/server/auth/providers/test_azure_scopes.py

@@ -3,12 +3,14 @@
 import pytest
 from key_value.aio.stores.memory import MemoryStore
 
+from fastmcp.server.auth.auth import MultiAuth
 from fastmcp.server.auth.providers.azure import (
     OIDC_SCOPES,
     AzureJWTVerifier,
     AzureProvider,
+    _find_azure_provider,
 )
-from fastmcp.server.auth.providers.jwt import RSAKeyPair
+from fastmcp.server.auth.providers.jwt import RSAKeyPair, StaticTokenVerifier
 
 
 @pytest.fixture
@@ -725,3 +727,42 @@ class TestAzureOBOIntegration:
 
         dep = _EntraOBOToken(["scope"])
         assert isinstance(dep, Dependency)
+
+
+class TestFindAzureProvider:
+    """Tests for _find_azure_provider helper used by EntraOBOToken."""
+
+    def test_returns_azure_provider_directly(self, memory_storage):
+        """When auth is an AzureProvider, return it directly."""
+        provider = AzureProvider(
+            tenant_id="test-tenant",
+            client_id="test-client",
+            client_secret="test-secret",
+            client_storage=memory_storage,
+            base_url="https://example.com",
+            required_scopes=["read"],
+        )
+        assert _find_azure_provider(provider) is provider
+
+    def test_unwraps_multiauth_with_azure_server(self, memory_storage):
+        """When auth is a MultiAuth wrapping an AzureProvider, return the inner provider."""
+        provider = AzureProvider(
+            tenant_id="test-tenant",
+            client_id="test-client",
+            client_secret="test-secret",
+            client_storage=memory_storage,
+            base_url="https://example.com",
+            required_scopes=["read"],
+        )
+        multi = MultiAuth(server=provider)
+        assert _find_azure_provider(multi) is provider
+
+    def test_returns_none_for_no_auth(self):
+        """When auth is None, return None."""
+        assert _find_azure_provider(None) is None
+
+    def test_returns_none_for_multiauth_without_azure_server(self):
+        """When MultiAuth has no server or a non-Azure server, return None."""
+        verifier = StaticTokenVerifier(tokens={"t": {"client_id": "c", "scopes": []}})
+        multi = MultiAuth(verifiers=[verifier])
+        assert _find_azure_provider(multi) is None