Merge pull request #49167 from BerriAI/litellm_backport_rc_1_108_0_eddb61

fix(mcp)!: backport #48939 to rc/1.108.0, restore upstream-token admission for legacy delegate servers

litellm/proxy/_experimental/mcp_server/AGENTS.md

@@ -68,8 +68,11 @@ module materially harder to understand.
   behind a single generic branch unless tests prove every mode still behaves
   correctly.
 - Be especially careful with legacy `delegate_auth_to_upstream: true`. `auth_type: oauth2`
-  with `delegate_auth_to_upstream: true` is deprecated: LiteLLM admission is required
-  for matching MCP routes. Use `auth_type: oauth_delegate` for client-forwarded OAuth.
+  with `delegate_auth_to_upstream: true` is deprecated but still supported. A bare
+  `Authorization` that is a LiteLLM credential is validated and never forwarded. Any other
+  bearer is the upstream token: it is forwarded, and the caller is admitted as the JWT or
+  custom-auth user it also proves when that user is granted the server, else anonymously.
+  New servers should use `auth_type: true_passthrough` or `auth_type: oauth_delegate`.
   OAuth discovery endpoints stay public so clients can start the RFC 9728 flow.
 - Keep database-backed fields in sync across migrations, typed models under
   `litellm/types/mcp.py` or `litellm/types/mcp_server/`, config loading, this

litellm/proxy/_experimental/mcp_server/auth/user_api_key_auth_mcp.py

@@ -1,13 +1,14 @@
 import functools
 import re
 import secrets
-from collections.abc import Awaitable, Callable, Mapping, Sequence
+from collections.abc import Awaitable, Callable, Iterator, Mapping, Sequence
 from collections.abc import Set as AbstractSet
 from dataclasses import dataclass
 from datetime import datetime, timezone
 from types import MappingProxyType
-from typing import TYPE_CHECKING, Final, Literal, cast
+from typing import TYPE_CHECKING, Final, Literal, TypeAlias, cast
 
+import jwt
 from fastapi import HTTPException
 from pydantic import SecretStr, TypeAdapter
 from starlette.datastructures import Headers
@@ -64,9 +65,11 @@ from litellm.proxy.agent_endpoints.auth.agent_access_groups import (
 from litellm.proxy.agent_endpoints.auth.agent_caller import agent_caller_auth
 from litellm.proxy.agent_endpoints.auth.managed_authorization import managed_agent_policy
 from litellm.proxy.auth.ip_address_utils import IPAddressUtils
+from litellm.proxy.auth.oauth2_check import OAuth2TokenValidationError
 from litellm.proxy.auth.user_api_key_auth import (  # noqa: F401  # legacy module exports
     _get_bearer_token_or_received_api_key,  # pyright: ignore[reportPrivateUsage]  # shared x-litellm-api-key parser lives with user_api_key_auth
     _run_centralized_common_checks,  # pyright: ignore[reportPrivateUsage,reportUnusedImport]  # backwards-compatible package export
+    oauth2_introspection_applies,
     run_centralized_common_checks,
     user_api_key_auth,
 )
@@ -93,6 +96,9 @@ if TYPE_CHECKING:
 
 _EMPTY_TOOLSET_GRANTS: Final[Mapping[str, Sequence[str]]] = MappingProxyType({})
 OPTIONAL_STRING_ADAPTER: Final[TypeAdapter[str | None]] = TypeAdapter(str | None)
+_PROXY_SETTINGS_ADAPTER: Final = TypeAdapter(Mapping[str, object])
+_CAUSE_CHAIN_MAX_DEPTH: Final = 16
+_IdentityCheck: TypeAlias = Literal["oauth2", "oauth2_proxy", "jwt"]
 
 
 def _normalize_caller_admission_credential(value: str) -> str:
@@ -225,6 +231,136 @@ def _is_legacy_delegate_cold_start(mcp_servers: list[str] | None, client_ip: str
     return True
 
 
+def _cause_chain(exc: BaseException) -> Iterator[BaseException]:
+    link: BaseException | None = exc  # rebind-ok: advances one link of the cause chain per iteration
+    for _ in range(_CAUSE_CHAIN_MAX_DEPTH):
+        if link is None:
+            return
+        yield link
+        link = link.__cause__ or link.__context__
+
+
+def _caused_by(exc: BaseException, error_type: type[BaseException]) -> bool:
+    return any(isinstance(link, error_type) for link in _cause_chain(exc))
+
+
+def _signed_with(token: str, secret: str) -> bool:
+    """True when ``token`` is a JWT whose HS256 signature verifies with ``secret``, whatever its claims say."""
+    try:
+        jwt.decode(
+            token,
+            secret,
+            algorithms=["HS256"],
+            options={"verify_exp": False, "verify_nbf": False, "verify_iat": False, "verify_aud": False},
+        )
+    except jwt.PyJWTError:
+        return False
+    return True
+
+
+async def _bearer_is_litellm_credential(token: str) -> bool:
+    """True when ``token`` is, or may be, a credential LiteLLM issued: the master key or a JWT it signed
+    (dashboard sessions, BYOK sessions), a sealed gateway bearer, or a virtual key the key table holds (a
+    lookup fault counts as held). Only ``sk-`` tokens are looked up, because no other token can be a
+    virtual key."""
+    from litellm.proxy._experimental.mcp_server.bridge_token_flow import (  # noqa: PLC0415  # bridge flow imports this module
+        is_sealed_gateway_bearer,
+        virtual_key_may_exist,
+    )
+    from litellm.proxy._experimental.mcp_server.mcp_server_manager import (  # noqa: PLC0415  # manager imports this module
+        LITELLM_VIRTUAL_KEY_PREFIX,
+    )
+    from litellm.proxy.proxy_server import master_key  # noqa: PLC0415  # startup owns the master key
+
+    if master_key and (secrets.compare_digest(token.encode(), master_key.encode()) or _signed_with(token, master_key)):
+        return True
+    if is_sealed_gateway_bearer(token):
+        return True
+    return token.startswith(LITELLM_VIRTUAL_KEY_PREFIX) and await virtual_key_may_exist(token)
+
+
+def _identity_check(token: str, route: str, settings: Mapping[str, object]) -> _IdentityCheck | None:
+    """The identity check LiteLLM's own auth would run on ``token`` for ``route``, in its order: OAuth2
+    introspection, then trusted reverse-proxy identity headers, then a JWT whose issuer may be LiteLLM's."""
+    from litellm.proxy._experimental.mcp_server.bridge_token_flow import (  # noqa: PLC0415  # bridge flow imports this module
+        jwt_issuer_may_be_litellm,
+    )
+    from litellm.proxy.auth.handle_jwt import JWTHandler  # noqa: PLC0415  # proxy import cycle
+    from litellm.proxy.proxy_server import (  # noqa: PLC0415  # startup owns the active auth configuration
+        jwt_handler,
+        premium_user,
+    )
+
+    enable_jwt_auth: Final = settings.get("enable_jwt_auth") is True
+    if oauth2_introspection_applies(
+        token,
+        route,
+        enable_oauth2_auth=settings.get("enable_oauth2_auth") is True,
+        enable_jwt_auth=enable_jwt_auth,
+        jwt_handler=jwt_handler,
+    ):
+        return "oauth2" if premium_user is True else None
+    if settings.get("enable_oauth2_proxy_auth") is True:
+        return "oauth2_proxy"
+    if enable_jwt_auth and premium_user is True and JWTHandler.is_jwt(token=token) and jwt_issuer_may_be_litellm(token):
+        return "jwt"
+    return None
+
+
+async def _verify_identity(check: Literal["oauth2_proxy", "jwt"], token: str, request: Request) -> None:
+    from litellm.proxy.auth.oauth2_proxy_hook import (  # noqa: PLC0415  # proxy import cycle
+        handle_oauth2_proxy_request,
+    )
+    from litellm.proxy.proxy_server import jwt_handler  # noqa: PLC0415  # startup owns the active auth configuration
+
+    match check:
+        case "oauth2_proxy":
+            await handle_oauth2_proxy_request(request=request)
+        case "jwt":
+            await jwt_handler.auth_jwt(token=token)  # pyright: ignore[reportUnknownMemberType]  # returns a bare dict; only whether it raises matters
+        case _:  # pragma: no cover - exhaustiveness guard, unreachable while the Literal holds
+            assert_never(check)  # pragma: no cover
+
+
+async def _upstream_bearer_identity(token: str, litellm_api_key: str, request: Request) -> UserAPIKeyAuth | None:
+    """The LiteLLM user an upstream bearer also proves, or ``None`` when it proves none.
+
+    Custom auth decides alone, so any rejection is ``None``. Otherwise the identity check LiteLLM would run
+    must vouch for the caller. Reverse-proxy headers and a JWT are verified first, and OAuth2 introspection
+    runs once inside the full check, where a provider that does not vouch is ``None``. Past that, only a
+    user LiteLLM does not know is ``None``; every other refusal (budget, blocked team, disallowed route)
+    propagates."""
+    from litellm.proxy._experimental.mcp_server.bridge_token_flow import (  # noqa: PLC0415  # bridge flow imports this module
+        custom_auth_configured,
+    )
+    from litellm.proxy.auth.auth_utils import get_request_route  # noqa: PLC0415  # proxy import cycle
+    from litellm.proxy.proxy_server import (  # noqa: PLC0415  # startup owns the active auth configuration
+        general_settings,  # pyright: ignore[reportUnknownVariableType]  # legacy global is declared as a bare dict
+    )
+
+    if custom_auth_configured():
+        try:
+            return await user_api_key_auth(api_key=litellm_api_key, request=request)
+        except (HTTPException, ProxyException):
+            return None
+    check: Final = _identity_check(
+        token, get_request_route(request), _PROXY_SETTINGS_ADAPTER.validate_python(general_settings)
+    )
+    if check is None:
+        return None
+    if check != "oauth2":
+        try:
+            await _verify_identity(check, token, request)
+        except Exception:  # noqa: BLE001  # any verification failure means LiteLLM does not vouch for this bearer
+            return None
+    try:
+        return await user_api_key_auth(api_key=litellm_api_key, request=request)
+    except (HTTPException, ProxyException, OAuth2TokenValidationError) as exc:
+        if _caused_by(exc, UserNotFoundError) or _caused_by(exc, OAuth2TokenValidationError):
+            return None
+        raise
+
+
 def _is_litellm_auth_admission_error(exc: Exception) -> bool:
     if isinstance(exc, HTTPException):
         return exc.status_code == 401
@@ -675,6 +811,19 @@ class MCPRequestHandler:
                     route=request_route,
                     mcp_servers=mcp_servers,
                 )
+            elif oauth2_headers and (
+                legacy_delegate_targets := MCPRequestHandler._legacy_delegate_targets(
+                    path=request_route,
+                    mcp_servers=mcp_servers,
+                    client_ip=IPAddressUtils.get_mcp_client_ip(request),
+                )
+            ):
+                validated_user_api_key_auth = await MCPRequestHandler._admit_legacy_delegate_bearer(  # rebind-ok: one assignment per admission arm, like its siblings
+                    authorization_value=oauth2_headers["Authorization"],
+                    litellm_api_key=litellm_api_key,
+                    targets=legacy_delegate_targets,
+                    request=request,
+                )
             elif oauth2_headers:
                 # Authorization on a non-delegated server: the bearer must be a real
                 # LiteLLM credential, so a failed validation is a genuine 401/403 and
@@ -712,10 +861,14 @@ class MCPRequestHandler:
             custom_key_header_name: Final = OPTIONAL_STRING_ADAPTER.validate_python(
                 general_settings.get("litellm_key_header_name")
             )
-            admitted_credential: Final = MCPRequestHandler.caller_admission_credential(
-                headers,
-                validated_user_api_key_auth,
-                custom_key_header_name=custom_key_header_name,
+            admitted_credential: Final = (
+                None
+                if validated_user_api_key_auth.mcp_forwards_caller_authorization
+                else MCPRequestHandler.caller_admission_credential(
+                    headers,
+                    validated_user_api_key_auth,
+                    custom_key_header_name=custom_key_header_name,
+                )
             )
             (
                 oauth2_headers,
@@ -905,6 +1058,53 @@ class MCPRequestHandler:
                 return False
         return True
 
+    @staticmethod
+    def _legacy_delegate_targets(
+        path: str, mcp_servers: Sequence[str] | None, client_ip: str | None
+    ) -> tuple[MCPServer, ...]:
+        """Every server the request targets when ALL of them are legacy ``delegate_auth_to_upstream``
+        servers, else ``()``. Names resolve the way downstream routing resolves them, so a permissive
+        ``x-mcp-servers`` header cannot open the gate for a stricter path target."""
+        from litellm.proxy._experimental.mcp_server.mcp_server_manager import (  # noqa: PLC0415  # manager imports this module
+            global_mcp_server_manager,
+            is_legacy_upstream_delegate,
+        )
+
+        servers: Final = tuple(
+            global_mcp_server_manager.get_mcp_server_by_name(name, client_ip=client_ip)
+            for name in MCPRequestHandler._resolve_target_server_names(path=path, mcp_servers_header=mcp_servers)
+        )
+        delegates: Final = tuple(
+            server for server in servers if server is not None and is_legacy_upstream_delegate(server)
+        )
+        return delegates if servers and len(delegates) == len(servers) else ()
+
+    @staticmethod
+    async def _admit_legacy_delegate_bearer(
+        authorization_value: str,
+        litellm_api_key: str,
+        targets: tuple[MCPServer, ...],
+        request: Request,
+    ) -> UserAPIKeyAuth:
+        """Admit a bare ``Authorization`` bearer sent to legacy ``delegate_auth_to_upstream`` servers.
+
+        A LiteLLM credential runs normal admission, and its errors propagate. Any other bearer is the
+        upstream token and is marked for forwarding: a user it also proves who is granted every target is
+        admitted as that user, and every other caller is admitted anonymously."""
+        from litellm.proxy._experimental.mcp_server.mcp_server_manager import (  # noqa: PLC0415  # manager imports this module
+            global_mcp_server_manager,
+        )
+
+        token: Final = _normalize_caller_admission_credential(authorization_value)
+        if await _bearer_is_litellm_credential(token):
+            return await user_api_key_auth(api_key=litellm_api_key, request=request)
+        identity: Final = await _upstream_bearer_identity(token, litellm_api_key, request)
+        if identity is not None:
+            allowed: Final = frozenset(await global_mcp_server_manager.get_allowed_mcp_servers(identity))
+            if all(server.server_id in allowed for server in targets):
+                return identity.model_copy(update={"mcp_forwards_caller_authorization": True})
+        return UserAPIKeyAuth().model_copy(update={"mcp_forwards_caller_authorization": True})
+
     @staticmethod
     def _single_dcr_bridge_delegate_target(
         path: str, mcp_servers: list[str] | None, client_ip: str | None
@@ -1519,7 +1719,7 @@ class MCPRequestHandler:
         return expiry >= datetime.now(timezone.utc)
 
     @staticmethod
-    def _resolve_target_server_names(path: str, mcp_servers_header: list[str] | None) -> list[str]:
+    def _resolve_target_server_names(path: str, mcp_servers_header: Sequence[str] | None) -> Sequence[str]:
         """
         Resolve the target MCP server names exactly as downstream routing
         does (``server.py::extract_mcp_auth_context``).

litellm/proxy/_experimental/mcp_server/bridge_token_flow.py

@@ -61,7 +61,6 @@ async def oauth_authorization_uses_gateway_credential(request: Request) -> bool:
     from litellm.proxy.proxy_server import (  # noqa: PLC0415  # startup owns the active auth configuration
         jwt_handler,
         master_key,
-        user_custom_auth,
     )
 
     if "x-litellm-api-key" in request.headers:
@@ -71,10 +70,26 @@ async def oauth_authorization_uses_gateway_credential(request: Request) -> bool:
         return "authorization" in request.headers
     if token.startswith("sk-") or (master_key and secrets.compare_digest(token.encode(), master_key.encode())):
         return True
-    if user_custom_auth is not None or jwt_handler.litellm_jwtauth.oidc_userinfo_enabled:
+    if custom_auth_configured() or jwt_handler.litellm_jwtauth.oidc_userinfo_enabled:
         return True
     if not JWTHandler.is_jwt(token):
-        return await _opaque_bearer_is_gateway_credential(token)
+        return is_sealed_gateway_bearer(token) or await virtual_key_may_exist(token)
+    return jwt_issuer_may_be_litellm(token)
+
+
+def custom_auth_configured() -> bool:
+    from litellm.proxy.proxy_server import (  # noqa: PLC0415  # startup owns the active auth configuration
+        user_custom_auth,
+    )
+
+    return user_custom_auth is not None
+
+
+def jwt_issuer_may_be_litellm(token: str) -> bool:
+    """False only when the JWT names an issuer LiteLLM's JWT auth is not configured to accept."""
+    from litellm.proxy.auth.handle_jwt import JWTHandler  # noqa: PLC0415  # proxy import cycle
+    from litellm.proxy.proxy_server import jwt_handler  # noqa: PLC0415  # startup owns the active auth configuration
+
     claims: Final = JWTHandler.get_unverified_claims(token)
     issuer: Final = claims.get("iss") if claims is not None else None
     global_issuer: Final = os.getenv("JWT_ISSUER")
@@ -86,13 +101,27 @@ async def oauth_authorization_uses_gateway_credential(request: Request) -> bool:
     )
 
 
-async def _opaque_bearer_is_gateway_credential(token: str) -> bool:
+def is_sealed_gateway_bearer(token: str) -> bool:
+    """True for a bearer only the gateway issues: a bridge or refresh envelope, a v2 GCM blob, or a
+    dashboard UI token. A UI token that cannot be read still counts, so a lookup fault never reads as foreign."""
     from litellm.proxy._experimental.mcp_server.outbound_credentials.envelope import (  # noqa: PLC0415  # envelope imports bridge types
         is_envelope,
         is_refresh_envelope,
     )
-    from litellm.proxy._types import hash_token  # noqa: PLC0415  # proxy import cycle
     from litellm.proxy.auth.auth_checks import ExperimentalUIJWTToken  # noqa: PLC0415  # proxy import cycle
+
+    if is_envelope(token) or is_refresh_envelope(token) or token.startswith(V2_GCM_PREFIX):
+        return True
+    try:
+        return ExperimentalUIJWTToken.get_key_object_from_ui_hash_key(token) is not None
+    except Exception as exc:  # noqa: BLE001  # an identity lookup fault must not permit cookie fallback
+        verbose_logger.debug("OAuth bearer ownership could not be checked (%s)", type(exc).__name__)
+        return True
+
+
+async def virtual_key_may_exist(token: str) -> bool:
+    """False only when the key table answers that ``token`` is not a virtual key; a lookup fault counts as held."""
+    from litellm.proxy._types import hash_token  # noqa: PLC0415  # proxy import cycle
     from litellm.proxy.auth.resolvers.exceptions import KeyNotFoundError  # noqa: PLC0415  # proxy import cycle
     from litellm.proxy.auth.resolvers.store import IdentityStore  # noqa: PLC0415  # proxy import cycle
     from litellm.proxy.proxy_server import (  # noqa: PLC0415  # startup owns the identity store dependencies
@@ -100,11 +129,7 @@ async def _opaque_bearer_is_gateway_credential(token: str) -> bool:
         user_api_key_cache,
     )
 
-    if is_envelope(token) or is_refresh_envelope(token) or token.startswith(V2_GCM_PREFIX):
-        return True
     try:
-        if ExperimentalUIJWTToken.get_key_object_from_ui_hash_key(token) is not None:
-            return True
         await IdentityStore(prisma_client, user_api_key_cache).resolve(hashed_token=hash_token(token))
     except KeyNotFoundError:
         return False

litellm/proxy/_experimental/mcp_server/mcp_server_manager.py

@@ -1189,14 +1189,23 @@ def should_strip_caller_authorization(
       pass-through cold-start case (RFC 9728) the bearer in
       ``Authorization`` is the upstream OAuth token and must be
       forwarded, so we keep it.
-    - **Delegated OAuth servers**: admission always runs and there is no
-      anonymous path, so the caller's separate ``Authorization`` is
+    - **Legacy delegate servers**: forward when MCP admission marked the
+      caller's ``Authorization`` as the upstream token
+      (``mcp_forwards_caller_authorization``); otherwise follow the
+      delegated OAuth rule below.
+    - **Delegated OAuth servers**: the caller's separate ``Authorization`` is
       forwarded only when a distinct ``x-litellm-api-key`` carried
       admission. Without that header the ``Authorization`` *was* the
       admission credential — a virtual key, an IdP JWT, or an SSO / OIDC /
       session token whose ``api_key`` is ``None`` — and must never reach
       the upstream, so it is stripped regardless of the ``api_key`` value.
     """
+    if (
+        user_api_key_auth is not None
+        and user_api_key_auth.mcp_forwards_caller_authorization
+        and is_legacy_upstream_delegate(mcp_server)
+    ):
+        return False
     if _authorization_is_admission_jwt(raw_headers, user_api_key_auth):
         return True
     if mcp_server.auth_type == MCPAuth.oauth2_token_exchange:
@@ -1511,6 +1520,18 @@ async def _materialize_auth_headers(auth: httpx2.Auth | None) -> dict[str, str]
     return {header_name: header_value} if header_value else None
 
 
+def is_legacy_upstream_delegate(server: MCPServer) -> bool:
+    """True for an interactive ``auth_type=oauth2`` server with ``delegate_auth_to_upstream=true``, whose
+    caller ``Authorization`` is the upstream token. The flow is resolved, not read from the column, so an
+    unstamped M2M-shaped row stays excluded: its stored client credentials drive egress, and admitting it
+    anonymously would let any caller act as LiteLLM's service account."""
+    return (
+        server.auth_type == MCPAuth.oauth2
+        and server.delegate_auth_to_upstream is True
+        and MCPServerManager.effective_oauth2_flow(server) != "client_credentials"
+    )
+
+
 def _consumes_caller_authorization(server: MCPServer) -> bool:
     """True when this server's egress forwards the caller's request-wide ``Authorization`` upstream:
     the client-forwarded token modes, legacy OAuth pass-through, and legacy upstream-delegated
@@ -1662,18 +1683,15 @@ def warn_on_shared_identifier_prefixes(servers: Iterable[MCPServer]) -> None:
 
 
 def _warn_legacy_delegate_auth_if_applicable(server: MCPServer, *, source: str) -> None:
-    """Direct legacy delegated OAuth configurations to the admitted replacement."""
-    if server.auth_type != MCPAuth.oauth2:
-        return
-    if getattr(server, "delegate_auth_to_upstream", False) is not True:
-        return
-    if server.has_client_credentials:
+    """Point legacy delegated OAuth configurations at the client-forwarded replacements."""
+    if not is_legacy_upstream_delegate(server):
         return
     label: Final = get_server_prefix(server)
     verbose_logger.warning(
         "MCP server %r (id=%s, source=%s) uses deprecated auth_type=oauth2 with "
-        "delegate_auth_to_upstream=true. LiteLLM admission is now required; migrate to "
-        "auth_type=oauth_delegate for client-forwarded OAuth.",
+        "delegate_auth_to_upstream=true. It still works: a caller without a LiteLLM credential is "
+        "admitted anonymously and its upstream token is forwarded. Migrate to auth_type=true_passthrough "
+        "or auth_type=oauth_delegate.",
         label,
         server.server_id,
         source,
@@ -3723,12 +3741,12 @@ class MCPServerManager:
                 )
             )
             if is_anonymous:
-                passthrough_server_ids: Final = [
+                anonymous_server_ids: Final = [
                     server.server_id
                     for server in self.get_registry().values()
-                    if server.auth_type == MCPAuth.true_passthrough
+                    if server.auth_type == MCPAuth.true_passthrough or is_legacy_upstream_delegate(server)
                 ]
-                combined_servers.update(passthrough_server_ids)
+                combined_servers.update(anonymous_server_ids)
 
             restrict_allow_all: Final = (
                 resolved_general_settings.get("mcp_allow_all_keys_respects_mcp_scope", False)

litellm/proxy/_experimental/mcp_server/server.py

@@ -578,6 +578,7 @@ if MCP_AVAILABLE:
     from litellm.proxy._experimental.mcp_server.mcp_server_manager import (
         MCPServerManager,
         global_mcp_server_manager,
+        is_legacy_upstream_delegate,
     )
 
     ######################################################
@@ -2152,8 +2153,9 @@ if MCP_AVAILABLE:
         oauth2_headers: Mapping[str, str] | None,
         mcp_server_auth_headers: dict[str, dict[str, str]] | None,
         raw_headers: Mapping[str, str] | None,
+        requested_servers: Sequence[str] | None = None,
     ) -> None:
-        """Probe pass-through upstream servers in parallel before the MCP session starts.
+        """Probe pass-through and legacy delegate upstream servers in parallel before the MCP session starts.
 
         Only servers the caller's key is already authorized to reach are probed —
         the list is derived from _get_allowed_mcp_servers so that a user cannot
@@ -2176,6 +2178,18 @@ if MCP_AVAILABLE:
             mcp_servers=mcp_servers,
             client_ip=client_ip,
         )
+        # A legacy delegate is probed only when the single requested name resolves to it the way admission
+        # resolves it, so neither a multi-server connect nor an access group can be failed by one rejected
+        # token; its challenge echoes the requested name, matching the tokenless challenge.
+        requested_scope: Final = requested_servers if requested_servers is not None else mcp_servers
+        single_requested_name: Final = (
+            requested_scope[0] if requested_scope is not None and len(requested_scope) == 1 else None
+        )
+        requested_server: Final = (
+            operations.global_mcp_server_manager.get_mcp_server_by_name(single_requested_name, client_ip=client_ip)
+            if single_requested_name is not None
+            else None
+        )
         prepared_headers: Final = tuple(
             (
                 srv,
@@ -2191,6 +2205,11 @@ if MCP_AVAILABLE:
             )
             for srv in allowed_servers
             if srv.is_oauth_passthrough
+            or (
+                requested_server is not None
+                and srv.server_id == requested_server.server_id
+                and is_legacy_upstream_delegate(srv)
+            )
         )
         probe_headers: Final = tuple(
             (
@@ -2203,7 +2222,7 @@ if MCP_AVAILABLE:
             for srv, (auth, extra) in prepared_headers
         )
         probe_targets: Final[tuple[tuple[MCPServer, str, str], ...]] = tuple(
-            (srv, auth_header, srv.name)
+            (srv, auth_header, srv.name if srv.is_oauth_passthrough else single_requested_name or srv.name)
             for srv, headers in probe_headers
             if (
                 auth_header := next(
@@ -2301,6 +2320,7 @@ if MCP_AVAILABLE:
             if context.mcp_server_auth_headers is not None
             else None,
             raw_headers=context.raw_headers,
+            requested_servers=context.mcp_servers,
         )
         return None
 

litellm/proxy/auth/oauth2_check.py

@@ -12,6 +12,11 @@ from litellm.llms.custom_httpx.http_handler import (
 from litellm.proxy._types import CommonProxyErrors, LitellmUserRoles, UserAPIKeyAuth
 
 
+class OAuth2TokenValidationError(ValueError):
+    """The token-info or introspection endpoint did not vouch for the token: it rejected it, reported it
+    inactive, or could not be reached."""
+
+
 class Oauth2Handler:
     """
     Handles OAuth2 token validation.
@@ -207,7 +212,7 @@ class Oauth2Handler:
             )
         except httpx.HTTPStatusError as e:
             # This will catch any 4xx or 5xx errors
-            raise ValueError(f"Oauth 2.0 Token validation failed: {e}")
+            raise OAuth2TokenValidationError(f"Oauth 2.0 Token validation failed: {e}")
         except Exception as e:
             # This will catch any other errors (like network issues)
-            raise ValueError(f"An error occurred during token validation: {e}")
+            raise OAuth2TokenValidationError(f"An error occurred during token validation: {e}")

litellm/proxy/auth/user_api_key_auth.py

@@ -1098,6 +1098,24 @@ def _matches_routing_override(token_claims: dict, override: "JWTRoutingOverride"
     )
 
 
+def oauth2_introspection_applies(
+    api_key: str, route: str, *, enable_oauth2_auth: bool, enable_jwt_auth: bool, jwt_handler: JWTHandler
+) -> bool:
+    """Whether LiteLLM validates ``api_key`` on ``route`` by OAuth2 token introspection, on LLM API, info and
+    lens routes only: a non-JWT token when OAuth2 auth is on globally, or a JWT matched by a routing override."""
+    is_jwt: Final = enable_jwt_auth and jwt_handler.is_jwt(token=api_key)
+    # Routing uses unverified JWT claims only to choose auth path.
+    # Final authentication is enforced by the selected validator.
+    route_jwt_to_oauth2: Final = is_jwt and _should_route_jwt_to_oauth2_override(token=api_key, jwt_handler=jwt_handler)
+    covered_route: Final = (
+        RouteChecks.is_llm_api_route(route=route)
+        or RouteChecks.is_info_route(route=route)
+        or route == "/lens"
+        or route.startswith("/lens/")
+    )
+    return covered_route and ((enable_oauth2_auth and not is_jwt) or route_jwt_to_oauth2)
+
+
 def _should_route_jwt_to_oauth2_override(token: str, jwt_handler: JWTHandler) -> bool:
     routing_overrides: Final = jwt_handler.litellm_jwtauth.routing_overrides
     if not routing_overrides:
@@ -2194,23 +2212,13 @@ async def user_api_key_auth_builder(
 
         enable_oauth2_auth: Final = general_settings.get("enable_oauth2_auth", False) is True
         enable_jwt_auth: Final = general_settings.get("enable_jwt_auth", False) is True
-        is_jwt = jwt_handler.is_jwt(token=api_key) if enable_jwt_auth else False
-
-        # Routing uses unverified JWT claims only to choose auth path.
-        # Final authentication is enforced by the selected validator.
-        route_jwt_to_oauth2 = is_jwt and _should_route_jwt_to_oauth2_override(token=api_key, jwt_handler=jwt_handler)
-
-        # OAuth2 applies for:
-        # 1) when global OAuth2 auth is enabled on LLM + info routes
-        # 2) JWT tokens that explicitly match routing_overrides on LLM + info routes
-        route_is_lens: Final = route == "/lens" or route.startswith("/lens/")
-        should_apply_override_oauth2: Final = route_jwt_to_oauth2 and (
-            RouteChecks.is_llm_api_route(route=route) or RouteChecks.is_info_route(route=route) or route_is_lens
-        )
-        should_apply_global_oauth2: Final = enable_oauth2_auth and (
-            RouteChecks.is_llm_api_route(route=route) or RouteChecks.is_info_route(route=route) or route_is_lens
-        )
-        if (should_apply_global_oauth2 and not is_jwt) or should_apply_override_oauth2:
+        if oauth2_introspection_applies(
+            api_key,
+            route,
+            enable_oauth2_auth=enable_oauth2_auth,
+            enable_jwt_auth=enable_jwt_auth,
+            jwt_handler=jwt_handler,
+        ):
             from litellm.proxy.proxy_server import premium_user
 
             if premium_user is not True:

litellm/types/mcp_server/mcp_server_manager.py

@@ -204,9 +204,11 @@ class MCPServer(LiteLLMBaseModel):
     access_groups: list[str] | None = None
     allow_all_keys: bool = False
     available_on_public_internet: bool = True
-    # Legacy opt-in to upstream-delegated authentication for ``oauth2``
-    # servers. LiteLLM admission still applies; use ``oauth_delegate`` for the
-    # supported client-forwarded OAuth flow.
+    # Deprecated opt-in to upstream-delegated authentication for ``oauth2``
+    # servers. A bare ``Authorization`` that is not a LiteLLM credential is the
+    # upstream token and is forwarded; see
+    # ``MCPRequestHandler._admit_legacy_delegate_bearer``. New servers should use
+    # ``true_passthrough`` or ``oauth_delegate``.
     #
     # Honored only for ``auth_type == oauth2``; ignored for any other
     # ``auth_type``. OAuth pass-through for non-oauth2 servers

litellm/types/proxy/auth/user_api_key_auth.py

@@ -169,6 +169,10 @@ class UserAPIKeyAuth(LiteLLM_VerificationTokenView):  # the expected response ob
     # above; a forged value could at most narrow, but the stripping keeps the field's provenance
     # single-owner so its meaning stays trustworthy.
     mcp_session_resource_server_ids: tuple[str, ...] | None = Field(default=None, exclude=True)
+    # Set only by MCP admission when a legacy delegate_auth_to_upstream caller's Authorization is the
+    # upstream token rather than a LiteLLM credential, so egress forwards it. Stripped from validated
+    # input like the markers above: a forged value would forward a LiteLLM credential upstream.
+    mcp_forwards_caller_authorization: bool = Field(default=False, exclude=True)
     mcp_toolset_id: str | None = Field(default=None, exclude=True)
     authenticated_by_custom_auth: bool = Field(default=False, exclude=True)
     via_virtual_key: bool = Field(
@@ -230,6 +234,7 @@ class UserAPIKeyAuth(LiteLLM_VerificationTokenView):  # the expected response ob
         values.pop("mcp_explicit_grants_only", None)
         values.pop("mcp_source_team_rpm_limits", None)
         values.pop("mcp_session_resource_server_ids", None)
+        values.pop("mcp_forwards_caller_authorization", None)
         values.pop("mcp_toolset_id", None)
         values.pop("via_virtual_key", None)
         values.pop("authenticated_by_custom_auth", None)

tests/unit/proxy/_experimental/mcp_server/auth/test_user_api_key_auth_mcp.py

@@ -1,7 +1,7 @@
 import contextlib
 import json
 import os
-from collections.abc import Sequence
+from collections.abc import Callable, Iterator, Sequence
 from collections.abc import Set as AbstractSet
 from datetime import datetime, timedelta, timezone
 from types import SimpleNamespace
@@ -1598,9 +1598,7 @@ class TestMCPRequestHandler:
             team_id="test-team-id",
         )
 
-        with patch(
-            "litellm.proxy._experimental.mcp_server.auth.user_api_key_auth_mcp.user_api_key_auth"
-        ) as mock_user_api_key_auth:
+        with patch("litellm.proxy._experimental.mcp_server.auth.user_api_key_auth_mcp.user_api_key_auth") as mock_user_api_key_auth:
             mock_user_api_key_auth.return_value = mock_auth_result
 
             # Call the method
@@ -1662,10 +1660,10 @@ class TestMCPOAuth2AuthFlow:
     as LiteLLM API keys, causing auth failures and empty tool listings.
     """
 
-    async def test_oauth2_token_in_authorization_header_requires_litellm_admission(self):
+    async def test_oauth2_upstream_token_on_legacy_delegate_is_admitted_anonymously(self) -> None:
         """
-        A bare Authorization token on the legacy delegated mode must establish
-        a LiteLLM principal rather than entering anonymously.
+        A bare upstream OAuth token on the legacy delegated mode is admitted
+        anonymously without LiteLLM key validation and kept for egress.
         """
         from litellm.types.mcp import MCPAuth
 
@@ -1701,10 +1699,11 @@ class TestMCPOAuth2AuthFlow:
                 raw_headers,
             ) = await MCPRequestHandler.process_mcp_request(scope)
 
-            assert isinstance(auth_result, UserAPIKeyAuth)
-            assert auth_result.user_id == "admitted-user"
-            mock_auth.assert_awaited_once()
-            assert oauth2_headers.get("Authorization") == "Bearer atlassian-oauth2-access-token-xyz"
+            assert auth_result.user_id is None
+            assert auth_result.api_key is None
+            assert auth_result.mcp_forwards_caller_authorization is True
+            mock_auth.assert_not_awaited()
+            assert oauth2_headers == {"Authorization": "Bearer atlassian-oauth2-access-token-xyz"}
 
     async def test_explicit_litellm_key_with_oauth2_authorization(self):
         """
@@ -2544,9 +2543,10 @@ class TestMCPDelegateAuthToUpstream:
     """
     Tests for the ``delegate_auth_to_upstream`` per-server flag.
 
-    The legacy flag no longer bypasses LiteLLM admission. OAuth discovery may
-    still use the anonymous cold-start challenge, but a presented bearer must
-    authenticate to LiteLLM unless a separate admission credential is supplied.
+    A bare bearer that is a LiteLLM credential takes normal admission and is
+    never forwarded. Any other bearer is the upstream token: it is forwarded,
+    and the caller is admitted as the JWT or custom-auth user it also proves
+    when that user is granted the server, else anonymously.
     """
 
     @staticmethod
@@ -2651,37 +2651,497 @@ class TestMCPDelegateAuthToUpstream:
             assert isinstance(auth_result, UserAPIKeyAuth)
             mock_auth.assert_awaited_once()
 
-    async def test_delegate_with_only_upstream_token_requires_litellm_auth(self):
-        """
-        An upstream token cannot establish a LiteLLM principal and must not
-        reopen anonymous admission.
-        """
-        from litellm.types.mcp import MCPAuth
-
-        scope = {
+    @staticmethod
+    def _bearer_scope(token: str) -> dict[str, object]:
+        return {
             "type": "http",
             "method": "POST",
             "path": "/mcp/delegated_oauth_server",
-            "headers": [(b"authorization", b"Bearer upstream-pkce-token")],
+            "headers": [(b"authorization", f"Bearer {token}".encode())],
         }
 
+    @staticmethod
+    def _jwt(issuer: str = "https://idp.example") -> str:
+        import jwt
+
+        return jwt.encode({"iss": issuer, "sub": "jwt-user"}, "test-signing-secret", algorithm="HS256")
+
+    @staticmethod
+    @contextlib.contextmanager
+    def _delegate_admission(
+        admission: AsyncMock, allowed_server_ids: Sequence[str] = ("test-server-id",)
+    ) -> Iterator[None]:
+        from litellm.types.mcp import MCPAuth
+
         with (
-            patch(
-                "litellm.proxy._experimental.mcp_server.auth.user_api_key_auth_mcp.user_api_key_auth",
-                new_callable=AsyncMock,
-                side_effect=HTTPException(status_code=401, detail="Invalid API key"),
-            ) as mock_auth,
+            patch("litellm.proxy._experimental.mcp_server.auth.user_api_key_auth_mcp.user_api_key_auth", admission),
             patch("litellm.proxy._experimental.mcp_server.mcp_server_manager.global_mcp_server_manager") as mock_mgr,
         ):
             mock_mgr.get_mcp_server_by_name.return_value = TestMCPDelegateAuthToUpstream._make_server(
                 auth_type=MCPAuth.oauth2,
                 delegate_auth_to_upstream=True,
             )
-            with pytest.raises(HTTPException) as exc_info:
-                await MCPRequestHandler.process_mcp_request(scope)
+            mock_mgr.get_allowed_mcp_servers = AsyncMock(return_value=list(allowed_server_ids))
+            yield
 
-            assert exc_info.value.status_code == 401
-            mock_auth.assert_awaited_once()
+    @staticmethod
+    @contextlib.contextmanager
+    def _jwt_auth_enabled(auth_jwt: AsyncMock) -> Iterator[None]:
+        from litellm.proxy import proxy_server
+        from litellm.proxy._types import LiteLLM_JWTAuth
+
+        with (
+            patch.dict(proxy_server.general_settings, {"enable_jwt_auth": True}),
+            patch.object(proxy_server, "premium_user", True),
+            patch.object(proxy_server, "user_custom_auth", None),
+            patch.object(proxy_server.jwt_handler, "auth_jwt", auth_jwt),
+            patch.object(proxy_server.jwt_handler, "litellm_jwtauth", LiteLLM_JWTAuth(), create=True),
+            patch.dict(os.environ),
+        ):
+            os.environ.pop("JWT_ISSUER", None)
+            yield
+
+    async def test_delegate_forwards_unknown_sk_token_after_key_table_miss(self) -> None:
+        """An sk- token the key table does not hold, such as a provider API key, belongs to the upstream:
+        it is forwarded anonymously rather than rejected for its prefix."""
+        from litellm.proxy.auth.resolvers.exceptions import KeyNotFoundError
+
+        admission: Final = AsyncMock()
+        with (
+            TestMCPDelegateAuthToUpstream._delegate_admission(admission),
+            patch(
+                "litellm.proxy.auth.resolvers.store.IdentityStore.resolve",
+                new_callable=AsyncMock,
+                side_effect=KeyNotFoundError("hashed-token"),
+            ),
+        ):
+            auth_result, _, _, _, oauth2_headers, _ = await MCPRequestHandler.process_mcp_request(
+                TestMCPDelegateAuthToUpstream._bearer_scope("sk-proj-upstream-provider-key")
+            )
+
+        assert auth_result.api_key is None
+        assert auth_result.mcp_forwards_caller_authorization is True
+        assert oauth2_headers == {"Authorization": "Bearer sk-proj-upstream-provider-key"}
+        admission.assert_not_awaited()
+
+    @pytest.mark.parametrize(
+        "lookup_effect",
+        [None, RuntimeError("key table unreachable")],
+        ids=["key-held", "lookup-fault"],
+    )
+    async def test_delegate_litellm_key_refusal_propagates(self, lookup_effect: Exception | None) -> None:
+        """A key the table holds, or one the lookup could not check, takes normal admission, so its
+        refusal reaches the caller instead of becoming an anonymous request that forwards the key."""
+        refusal: Final = ProxyException(
+            message="Budget has been exceeded", type="budget_exceeded", param=None, code=400
+        )
+        with (
+            TestMCPDelegateAuthToUpstream._delegate_admission(AsyncMock(side_effect=refusal)),
+            patch(
+                "litellm.proxy.auth.resolvers.store.IdentityStore.resolve",
+                new_callable=AsyncMock,
+                side_effect=lookup_effect,
+            ),
+        ):
+            with pytest.raises(ProxyException) as exc_info:
+                await MCPRequestHandler.process_mcp_request(TestMCPDelegateAuthToUpstream._bearer_scope("sk-held-key"))
+
+        assert exc_info.value is refusal
+
+    async def test_delegate_master_key_is_admitted_and_never_forwarded(self) -> None:
+        """The master key is not in the key table, so only recognizing it directly stops it from being
+        treated as the upstream's token and sent to the upstream server."""
+        from litellm.proxy import proxy_server
+        from litellm.proxy.auth.resolvers.exceptions import KeyNotFoundError
+
+        admission: Final = AsyncMock(return_value=UserAPIKeyAuth(api_key="sk-master-secret", user_role="proxy_admin"))
+        with (
+            patch.object(proxy_server, "master_key", "sk-master-secret"),
+            TestMCPDelegateAuthToUpstream._delegate_admission(admission),
+            patch(
+                "litellm.proxy.auth.resolvers.store.IdentityStore.resolve",
+                new_callable=AsyncMock,
+                side_effect=KeyNotFoundError("hashed-master-key"),
+            ),
+        ):
+            auth_result, _, _, _, oauth2_headers, _ = await MCPRequestHandler.process_mcp_request(
+                TestMCPDelegateAuthToUpstream._bearer_scope("sk-master-secret")
+            )
+
+        admission.assert_awaited_once()
+        assert auth_result.mcp_forwards_caller_authorization is False
+        assert oauth2_headers is None
+
+    @pytest.mark.parametrize(
+        "signing_secret, expires_in, forwarded",
+        [
+            ("sk-master-secret", timedelta(hours=1), False),
+            ("sk-master-secret", timedelta(hours=-1), False),
+            ("upstream-idp-signing-secret", timedelta(hours=1), True),
+        ],
+        ids=["master-key-signed", "master-key-signed-expired", "signed-elsewhere"],
+    )
+    async def test_delegate_master_key_signed_jwt_is_never_forwarded(
+        self, signing_secret: str, expires_in: timedelta, forwarded: bool
+    ) -> None:
+        """LiteLLM signs dashboard and OAuth tokens with the master key, so such a JWT is LiteLLM's own even
+        once expired: it takes normal admission and never reaches the upstream. A JWT signed by anyone
+        else is the upstream's token."""
+        import jwt
+
+        from litellm.proxy import proxy_server
+
+        token: Final = jwt.encode(
+            {"sub": "dashboard-user", "exp": datetime.now(timezone.utc) + expires_in}, signing_secret, algorithm="HS256"
+        )
+        admission: Final = AsyncMock(return_value=UserAPIKeyAuth(user_id="dashboard-user"))
+        with (
+            patch.object(proxy_server, "master_key", "sk-master-secret"),
+            TestMCPDelegateAuthToUpstream._delegate_admission(admission),
+        ):
+            auth_result, *_ = await MCPRequestHandler.process_mcp_request(
+                TestMCPDelegateAuthToUpstream._bearer_scope(token)
+            )
+
+        assert auth_result.mcp_forwards_caller_authorization is forwarded
+        assert admission.await_count == (0 if forwarded else 1)
+
+    @pytest.mark.parametrize(
+        "token, ui_token_lookup",
+        [
+            ("llm_env_sealed-bridge-envelope", None),
+            ("llm_refresh_sealed-refresh-envelope", None),
+            ("v2:gcm:sealed-credential-blob", None),
+            ("dashboard-login-token", lambda: MagicMock(return_value=UserAPIKeyAuth(user_id="dashboard-user"))),
+            ("unreadable-dashboard-token", lambda: MagicMock(side_effect=ValueError("Invalid hash key"))),
+        ],
+        ids=["bridge-envelope", "refresh-envelope", "gcm-blob", "dashboard-token", "unreadable-dashboard-token"],
+    )
+    async def test_delegate_sealed_litellm_token_takes_normal_admission(
+        self, token: str, ui_token_lookup: Callable[[], MagicMock] | None
+    ) -> None:
+        """A token only LiteLLM issues is LiteLLM's own, even one it fails to read, so it goes through
+        normal admission and is never marked for forwarding upstream."""
+        from litellm.proxy.auth.auth_checks import ExperimentalUIJWTToken
+
+        admission: Final = AsyncMock(return_value=UserAPIKeyAuth(user_id="litellm-user"))
+        ui_lookup: Final = (
+            patch.object(ExperimentalUIJWTToken, "get_key_object_from_ui_hash_key", ui_token_lookup())
+            if ui_token_lookup is not None
+            else contextlib.nullcontext()
+        )
+        with TestMCPDelegateAuthToUpstream._delegate_admission(admission), ui_lookup:
+            auth_result, *_ = await MCPRequestHandler.process_mcp_request(
+                TestMCPDelegateAuthToUpstream._bearer_scope(token)
+            )
+
+        admission.assert_awaited_once()
+        assert auth_result.mcp_forwards_caller_authorization is False
+
+    @staticmethod
+    def _unknown_user_refusal() -> ProxyException:
+        from litellm.types.proxy.auth.auth_checks import UserNotFoundError
+
+        refusal: Final = ProxyException(message="User doesn't exist in db", type="auth_error", param=None, code=401)
+        refusal.__cause__ = UserNotFoundError(user_id="jwt-user")
+        return refusal
+
+    @pytest.mark.parametrize(
+        "auth_jwt_effect, admission, allowed_server_ids, expected_user_id",
+        [
+            (
+                None,
+                lambda: AsyncMock(return_value=UserAPIKeyAuth(user_id="jwt-user", jwt_claims={"sub": "jwt-user"})),
+                ["test-server-id"],
+                "jwt-user",
+            ),
+            (
+                None,
+                lambda: AsyncMock(return_value=UserAPIKeyAuth(user_id="jwt-user", jwt_claims={"sub": "jwt-user"})),
+                [],
+                None,
+            ),
+            (
+                None,
+                lambda: AsyncMock(side_effect=TestMCPDelegateAuthToUpstream._unknown_user_refusal()),
+                ["test-server-id"],
+                None,
+            ),
+            (
+                ValueError("Signature verification failed"),
+                lambda: AsyncMock(return_value=UserAPIKeyAuth(user_id="jwt-user")),
+                ["test-server-id"],
+                None,
+            ),
+        ],
+        ids=["granted-user", "user-not-granted", "user-unknown-to-litellm", "signature-rejected"],
+    )
+    async def test_delegate_jwt_is_forwarded_as_granted_user_or_anonymously(
+        self,
+        auth_jwt_effect: Exception | None,
+        admission: Callable[[], AsyncMock],
+        allowed_server_ids: Sequence[str],
+        expected_user_id: str | None,
+    ) -> None:
+        """A JWT LiteLLM also accepts admits that user when granted the server; a user who is not
+        granted, unknown to LiteLLM, or whose signature fails is admitted anonymously. The JWT is always
+        forwarded, because on this server it is the upstream's token."""
+        token: Final = TestMCPDelegateAuthToUpstream._jwt()
+        with (
+            TestMCPDelegateAuthToUpstream._jwt_auth_enabled(AsyncMock(side_effect=auth_jwt_effect)),
+            TestMCPDelegateAuthToUpstream._delegate_admission(admission(), allowed_server_ids),
+        ):
+            auth_result, _, _, _, oauth2_headers, _ = await MCPRequestHandler.process_mcp_request(
+                TestMCPDelegateAuthToUpstream._bearer_scope(token)
+            )
+
+        assert auth_result.user_id == expected_user_id
+        assert auth_result.mcp_forwards_caller_authorization is True
+        assert oauth2_headers == {"Authorization": f"Bearer {token}"}
+
+    @pytest.mark.parametrize(
+        "refusal",
+        [
+            ProxyException(message="Budget has been exceeded", type="budget_exceeded", param=None, code=400),
+            ProxyException(message="Team=t1 is blocked", type="auth_error", param=None, code=401),
+        ],
+        ids=["over-budget", "blocked-team"],
+    )
+    async def test_delegate_jwt_refusal_after_valid_signature_propagates(self, refusal: ProxyException) -> None:
+        """Once the signature verifies, LiteLLM knows who the caller is, so a budget or an explicit
+        disallowment is enforced and the request fails instead of continuing anonymously."""
+        with (
+            TestMCPDelegateAuthToUpstream._jwt_auth_enabled(AsyncMock(return_value={"sub": "jwt-user"})),
+            TestMCPDelegateAuthToUpstream._delegate_admission(AsyncMock(side_effect=refusal)),
+        ):
+            with pytest.raises(ProxyException) as exc_info:
+                await MCPRequestHandler.process_mcp_request(
+                    TestMCPDelegateAuthToUpstream._bearer_scope(TestMCPDelegateAuthToUpstream._jwt())
+                )
+
+        assert exc_info.value is refusal
+
+    @staticmethod
+    @contextlib.contextmanager
+    def _idp_auth_enabled(
+        mechanism: Literal["oauth2", "oauth2_proxy"], verify: AsyncMock, premium: bool = True
+    ) -> Iterator[None]:
+        from litellm.proxy import proxy_server
+        from litellm.proxy.auth.oauth2_check import Oauth2Handler
+
+        settings: Final = {"enable_oauth2_auth": True} if mechanism == "oauth2" else {"enable_oauth2_proxy_auth": True}
+        verifier: Final = (
+            patch.object(Oauth2Handler, "check_oauth2_token", verify)
+            if mechanism == "oauth2"
+            else patch("litellm.proxy.auth.oauth2_proxy_hook.handle_oauth2_proxy_request", verify)
+        )
+        with (
+            patch.dict(proxy_server.general_settings, settings),
+            patch.object(proxy_server, "premium_user", premium),
+            patch.object(proxy_server, "user_custom_auth", None),
+            verifier,
+        ):
+            yield
+
+    @pytest.mark.parametrize(
+        "verify_effect, allowed_server_ids, expected_user_id",
+        [
+            (None, ["test-server-id"], "idp-user"),
+            (None, [], None),
+            (ValueError("Missing trusted proxy header"), ["test-server-id"], None),
+        ],
+        ids=["granted-user", "user-not-granted", "identity-rejected"],
+    )
+    async def test_delegate_proxy_header_identity_follows_the_jwt_rules(
+        self,
+        verify_effect: Exception | None,
+        allowed_server_ids: Sequence[str],
+        expected_user_id: str | None,
+    ) -> None:
+        """Trusted reverse-proxy headers admit a granted user by name, and a user who is not granted or not
+        vouched for anonymously. The bearer is forwarded either way."""
+        admission: Final = AsyncMock(return_value=UserAPIKeyAuth(user_id="idp-user"))
+        with (
+            TestMCPDelegateAuthToUpstream._idp_auth_enabled("oauth2_proxy", AsyncMock(side_effect=verify_effect)),
+            TestMCPDelegateAuthToUpstream._delegate_admission(admission, allowed_server_ids),
+        ):
+            auth_result, _, _, _, oauth2_headers, _ = await MCPRequestHandler.process_mcp_request(
+                TestMCPDelegateAuthToUpstream._bearer_scope("opaque-idp-token")
+            )
+
+        assert auth_result.user_id == expected_user_id
+        assert auth_result.mcp_forwards_caller_authorization is True
+        assert oauth2_headers == {"Authorization": "Bearer opaque-idp-token"}
+
+    @pytest.mark.parametrize(
+        "provider_status, active, allowed_server_ids, expected_user_id",
+        [
+            (200, True, ["test-server-id"], "idp-user"),
+            (200, True, [], None),
+            (200, False, ["test-server-id"], None),
+            (401, True, ["test-server-id"], None),
+        ],
+        ids=["granted-user", "user-not-granted", "token-inactive", "provider-rejects-token"],
+    )
+    async def test_delegate_oauth2_introspects_once_and_follows_the_jwt_rules(
+        self, provider_status: int, active: bool, allowed_server_ids: Sequence[str], expected_user_id: str | None
+    ) -> None:
+        """The provider is asked about the token once per request. A token it vouches for admits a granted
+        user by name, and one it reports inactive or rejects outright is forwarded anonymously instead of
+        failing the request."""
+        import httpx
+
+        from litellm.proxy import proxy_server
+        from litellm.types.mcp import MCPAuth
+
+        async def introspect(outbound: httpx.Request, **kwargs: object) -> httpx.Response:
+            return httpx.Response(provider_status, request=outbound, json={"active": active, "sub": "idp-user"})
+
+        with (
+            patch.dict(proxy_server.general_settings, {"enable_oauth2_auth": True}),
+            patch.object(proxy_server, "premium_user", True),
+            patch.object(proxy_server, "user_custom_auth", None),
+            patch.object(proxy_server, "master_key", "sk-fixture-master"),
+            patch.object(proxy_server, "prisma_client", None),
+            patch.object(proxy_server, "user_api_key_cache", DualCache()),
+            patch.dict(
+                os.environ,
+                {
+                    "OAUTH_TOKEN_INFO_ENDPOINT": "https://idp.fixture.test/introspect",
+                    "OAUTH_CLIENT_ID": "fixture-client",
+                    "OAUTH_CLIENT_SECRET": "fixture-client-secret",
+                },
+            ),
+            patch("httpx.AsyncClient.send", side_effect=introspect) as network,
+            patch("litellm.proxy._experimental.mcp_server.mcp_server_manager.global_mcp_server_manager") as mock_mgr,
+        ):
+            mock_mgr.get_mcp_server_by_name.return_value = TestMCPDelegateAuthToUpstream._make_server(
+                auth_type=MCPAuth.oauth2, delegate_auth_to_upstream=True
+            )
+            mock_mgr.get_allowed_mcp_servers = AsyncMock(return_value=list(allowed_server_ids))
+            auth_result, _, _, _, oauth2_headers, _ = await MCPRequestHandler.process_mcp_request(
+                TestMCPDelegateAuthToUpstream._bearer_scope("opaque-idp-token")
+            )
+
+        assert network.call_count == 1
+        assert auth_result.user_id == expected_user_id
+        assert auth_result.mcp_forwards_caller_authorization is True
+        assert oauth2_headers == {"Authorization": "Bearer opaque-idp-token"}
+
+    async def test_delegate_oauth2_rejection_admits_anonymously_whether_or_not_auth_wraps_it(self) -> None:
+        """A provider rejection is the provider declining to vouch, so the bearer is forwarded anonymously
+        even when it reaches admission as the bare validation error rather than a wrapped auth error."""
+        from litellm.proxy.auth.oauth2_check import OAuth2TokenValidationError
+
+        with (
+            TestMCPDelegateAuthToUpstream._idp_auth_enabled("oauth2", AsyncMock()),
+            TestMCPDelegateAuthToUpstream._delegate_admission(
+                AsyncMock(side_effect=OAuth2TokenValidationError("Token is not active"))
+            ),
+        ):
+            auth_result, _, _, _, oauth2_headers, _ = await MCPRequestHandler.process_mcp_request(
+                TestMCPDelegateAuthToUpstream._bearer_scope("opaque-idp-token")
+            )
+
+        assert auth_result.user_id is None
+        assert auth_result.mcp_forwards_caller_authorization is True
+        assert oauth2_headers == {"Authorization": "Bearer opaque-idp-token"}
+
+    @pytest.mark.parametrize("mechanism", ["oauth2", "oauth2_proxy"])
+    async def test_delegate_idp_user_refusal_propagates(self, mechanism: Literal["oauth2", "oauth2_proxy"]) -> None:
+        """Once the provider vouches for the caller, LiteLLM's budget and permission refusals apply."""
+        refusal: Final = ProxyException(
+            message="Budget has been exceeded", type="budget_exceeded", param=None, code=400
+        )
+        with (
+            TestMCPDelegateAuthToUpstream._idp_auth_enabled(mechanism, AsyncMock()),
+            TestMCPDelegateAuthToUpstream._delegate_admission(AsyncMock(side_effect=refusal)),
+        ):
+            with pytest.raises(ProxyException) as exc_info:
+                await MCPRequestHandler.process_mcp_request(
+                    TestMCPDelegateAuthToUpstream._bearer_scope("opaque-idp-token")
+                )
+
+        assert exc_info.value is refusal
+
+    async def test_delegate_oauth2_without_premium_never_calls_the_provider(self) -> None:
+        """OAuth2 introspection is enterprise-only, so without a license the bearer is the upstream's alone."""
+        introspect: Final = AsyncMock()
+        with (
+            TestMCPDelegateAuthToUpstream._idp_auth_enabled("oauth2", introspect, premium=False),
+            TestMCPDelegateAuthToUpstream._delegate_admission(
+                AsyncMock(return_value=UserAPIKeyAuth(user_id="idp-user"))
+            ),
+        ):
+            auth_result, *_ = await MCPRequestHandler.process_mcp_request(
+                TestMCPDelegateAuthToUpstream._bearer_scope("opaque-idp-token")
+            )
+
+        assert auth_result.user_id is None
+        assert auth_result.mcp_forwards_caller_authorization is True
+        introspect.assert_not_awaited()
+
+    async def test_delegate_jwt_from_unconfigured_issuer_is_forwarded_without_jwt_auth(self) -> None:
+        """A JWT from an issuer LiteLLM's JWT auth is not configured for never reaches JWT auth."""
+        auth_jwt: Final = AsyncMock(return_value={"sub": "jwt-user"})
+        admission: Final = AsyncMock(return_value=UserAPIKeyAuth(user_id="jwt-user"))
+        with (
+            TestMCPDelegateAuthToUpstream._jwt_auth_enabled(auth_jwt),
+            patch.dict(os.environ, {"JWT_ISSUER": "https://litellm-idp.example"}),
+            TestMCPDelegateAuthToUpstream._delegate_admission(admission),
+        ):
+            auth_result, *_ = await MCPRequestHandler.process_mcp_request(
+                TestMCPDelegateAuthToUpstream._bearer_scope(
+                    TestMCPDelegateAuthToUpstream._jwt(issuer="https://upstream-idp.example")
+                )
+            )
+
+        assert auth_result.user_id is None
+        assert auth_result.mcp_forwards_caller_authorization is True
+        auth_jwt.assert_not_awaited()
+
+    @pytest.mark.parametrize(
+        "admission, expected_user_id",
+        [
+            (
+                lambda: AsyncMock(return_value=UserAPIKeyAuth(user_id="custom-user", api_key="upstream-custom-token")),
+                "custom-user",
+            ),
+            (
+                lambda: AsyncMock(
+                    side_effect=ProxyException(message="custom auth rejected", type="auth_error", param=None, code=401)
+                ),
+                None,
+            ),
+        ],
+        ids=["accepted", "rejected"],
+    )
+    async def test_delegate_custom_auth_decides_alone(
+        self, admission: Callable[[], AsyncMock], expected_user_id: str | None
+    ) -> None:
+        """With custom auth configured, its answer alone decides: accepted admits that user, any failure
+        admits anonymously. The bearer is forwarded either way."""
+        from litellm.proxy import proxy_server
+
+        with (
+            patch.object(proxy_server, "user_custom_auth", AsyncMock()),
+            TestMCPDelegateAuthToUpstream._delegate_admission(admission()),
+        ):
+            auth_result, _, _, _, oauth2_headers, _ = await MCPRequestHandler.process_mcp_request(
+                TestMCPDelegateAuthToUpstream._bearer_scope("upstream-custom-token")
+            )
+
+        assert auth_result.user_id == expected_user_id
+        assert auth_result.mcp_forwards_caller_authorization is True
+        assert oauth2_headers == {"Authorization": "Bearer upstream-custom-token"}
+
+    def test_forwarding_marker_cannot_be_forged_from_input(self) -> None:
+        """Only admission may mark a caller's Authorization for forwarding; caller-shaped input never can."""
+        assert UserAPIKeyAuth(mcp_forwards_caller_authorization=True).mcp_forwards_caller_authorization is False
+        assert (
+            UserAPIKeyAuth.model_validate({"mcp_forwards_caller_authorization": True}).mcp_forwards_caller_authorization
+            is False
+        )
 
     async def test_delegate_off_still_requires_litellm_auth(self):
         """
@@ -2752,7 +3212,12 @@ class TestMCPDelegateAuthToUpstream:
                 await MCPRequestHandler.process_mcp_request(scope)
             assert exc_info.value.status_code == 401
 
-    async def test_delegate_mixed_targets_fail_closed(self):
+    @pytest.mark.parametrize(
+        "bearer_headers",
+        [[], [(b"authorization", b"Bearer upstream-pkce-token")]],
+        ids=["no-bearer", "upstream-bearer"],
+    )
+    async def test_delegate_mixed_targets_fail_closed(self, bearer_headers: list[tuple[bytes, bytes]]) -> None:
         """
         x-mcp-servers can list multiple targets. If ANY of them does not opt in
         to delegate_auth_to_upstream, the bypass must NOT fire — otherwise an
@@ -2768,6 +3233,7 @@ class TestMCPDelegateAuthToUpstream:
             "path": "/mcp",
             "headers": [
                 (b"x-mcp-servers", b"delegated_oauth,plain_oauth"),
+                *bearer_headers,
             ],
         }
 
@@ -2862,9 +3328,8 @@ class TestMCPDelegateAuthToUpstream:
 
     async def test_authorization_bearer_on_delegate_server_establishes_litellm_principal(self):
         """
-        A bare Authorization bearer now follows normal LiteLLM admission. A
-        separate x-litellm-api-key is required when Authorization is intended
-        for the upstream server.
+        A bare sk- bearer the key table may hold takes normal LiteLLM admission
+        and is never marked for forwarding upstream.
         """
         from litellm.types.mcp import MCPAuth
 
@@ -2897,6 +3362,7 @@ class TestMCPDelegateAuthToUpstream:
             ) = await MCPRequestHandler.process_mcp_request(scope)
             assert isinstance(auth_result, UserAPIKeyAuth)
             assert auth_result.user_id == "real-user"
+            assert auth_result.mcp_forwards_caller_authorization is False
             assert oauth2_headers.get("Authorization") == "Bearer sk-9876"
             mock_auth.assert_awaited_once()
 
@@ -3078,8 +3544,9 @@ class TestMCPDelegateAuthToUpstream:
             mock_auth.assert_awaited_once()
             assert auth.api_key is None
 
-    async def test_get_allowed_servers_excludes_legacy_delegates(self):
-        """Legacy delegated servers are never added to anonymous access."""
+    async def test_get_allowed_servers_includes_interactive_legacy_delegates_for_anonymous(self) -> None:
+        """Anonymous callers reach interactive legacy delegates, never an M2M server whose stored
+        client credentials would let them act as LiteLLM's service account."""
         from litellm.proxy._experimental.mcp_server.mcp_server_manager import (
             MCPServerManager,
         )
@@ -3117,7 +3584,7 @@ class TestMCPDelegateAuthToUpstream:
         ):
             result = await manager.get_allowed_mcp_servers(None)
 
-        assert result == []
+        assert result == ["pkce-server"]
 
     async def test_true_passthrough_skips_litellm_auth_anonymously(self):
         """auth_type=true_passthrough performs no admission auth: the caller's Authorization is an
@@ -7643,9 +8110,7 @@ class TestMCPDcrBridgeDualCredential:
                     new_callable=AsyncMock,
                     return_value=UserAPIKeyAuth(user_id=presented_user, api_key="any-hash"),
                 ),
-                patch(
-                    "litellm.proxy._experimental.mcp_server.mcp_server_manager.global_mcp_server_manager"
-                ) as mock_mgr,
+                patch("litellm.proxy._experimental.mcp_server.mcp_server_manager.global_mcp_server_manager") as mock_mgr,
                 patch("litellm.proxy.proxy_server.master_key", self._MASTER_KEY),
             ):
                 mock_mgr.get_mcp_server_by_name.return_value = self._DELEGATE._bridge_delegate_server()
@@ -7882,9 +8347,7 @@ class TestAggregateGatewayDcrChallenge:
         ):
             with (
                 patch(self._AUTH_PATCH_TARGET, side_effect=self._auth_401()),
-                patch(
-                    "litellm.proxy._experimental.mcp_server.mcp_server_manager.global_mcp_server_manager"
-                ) as mock_mgr,
+                patch("litellm.proxy._experimental.mcp_server.mcp_server_manager.global_mcp_server_manager") as mock_mgr,
             ):
                 mock_mgr.get_mcp_server_by_name.return_value = server
                 with pytest.raises(HTTPException) as exc_info:
@@ -7930,9 +8393,7 @@ class TestAggregateGatewayDcrChallenge:
             with (
                 patch.dict(os.environ, {"SERVER_ROOT_PATH": "/litellm"}),
                 patch(self._AUTH_PATCH_TARGET, side_effect=self._auth_401()),
-                patch(
-                    "litellm.proxy._experimental.mcp_server.mcp_server_manager.global_mcp_server_manager"
-                ) as mock_mgr,
+                patch("litellm.proxy._experimental.mcp_server.mcp_server_manager.global_mcp_server_manager") as mock_mgr,
             ):
                 mock_mgr.get_mcp_server_by_name.return_value = server
                 with pytest.raises(HTTPException) as exc_info:
@@ -7963,9 +8424,7 @@ class TestAggregateGatewayDcrChallenge:
         ):
             with (
                 patch(self._AUTH_PATCH_TARGET, side_effect=self._auth_401()),
-                patch(
-                    "litellm.proxy._experimental.mcp_server.mcp_server_manager.global_mcp_server_manager"
-                ) as mock_mgr,
+                patch("litellm.proxy._experimental.mcp_server.mcp_server_manager.global_mcp_server_manager") as mock_mgr,
             ):
                 mock_mgr.get_mcp_server_by_name.return_value = resolved
                 mock_mgr.get_mcp_server_by_identifier.return_value = resolved
@@ -8010,9 +8469,7 @@ class TestAggregateGatewayDcrChallenge:
             (None, None),
         ]
         for resolved, expected in cases:
-            with patch(
-                "litellm.proxy._experimental.mcp_server.mcp_server_manager.global_mcp_server_manager"
-            ) as mock_mgr:
+            with patch("litellm.proxy._experimental.mcp_server.mcp_server_manager.global_mcp_server_manager") as mock_mgr:
                 mock_mgr.get_mcp_server_by_identifier.return_value = resolved
                 assert _gateway_dcr_challenge_target("/mcp/srv", None, None) == expected, resolved
         assert _gateway_dcr_challenge_target("/mcp/a,b", None, None) is None

tests/unit/proxy/_experimental/mcp_server/test_mcp_server_tool_calls_and_headers.py

@@ -4,6 +4,7 @@ import contextlib
 import contextvars
 import json
 import os
+from collections.abc import Coroutine, Iterator, Sequence
 from datetime import datetime, timedelta
 from types import SimpleNamespace
 from typing import Final, Literal
@@ -664,6 +665,75 @@ def test_prepare_mcp_server_headers_legacy_delegate_strips_repeated_admission_ke
     assert extra_headers is None
 
 
+@pytest.mark.parametrize(
+    "caller_fields",
+    [{}, {"user_id": "jwt-user", "api_key": "upstream-jwt", "jwt_claims": {"sub": "jwt-user"}}],
+    ids=["anonymous", "admitted-jwt-user"],
+)
+def test_prepare_mcp_server_headers_legacy_delegate_forwards_marked_upstream_authorization(
+    caller_fields: dict[str, object],
+) -> None:
+    """When admission marked the caller's Authorization as the upstream token, a legacy delegate forwards
+    it, including a JWT that also admitted the caller and would otherwise be stripped as an admission JWT."""
+    from litellm.proxy._types import UserAPIKeyAuth
+    from litellm.proxy._experimental.mcp_server.server import (
+        _prepare_mcp_server_headers,
+    )
+
+    server = MCPServer(
+        server_id="legacy-delegate-marked",
+        name="legacy-delegate",
+        transport=MCPTransport.http,
+        auth_type=MCPAuth.oauth2,
+        delegate_auth_to_upstream=True,
+    )
+    caller: Final = UserAPIKeyAuth.model_validate(caller_fields).model_copy(
+        update={"mcp_forwards_caller_authorization": True}
+    )
+
+    server_auth_header, extra_headers = _prepare_mcp_server_headers(
+        server=server,
+        mcp_server_auth_headers=None,
+        mcp_auth_header=None,
+        oauth2_headers={"Authorization": "Bearer upstream-jwt"},
+        raw_headers={"authorization": "Bearer upstream-jwt"},
+        user_api_key_auth=caller,
+    )
+
+    assert server_auth_header is None
+    assert extra_headers == {"Authorization": "Bearer upstream-jwt"}
+
+
+def test_prepare_mcp_server_headers_forwarding_marker_is_ignored_off_legacy_delegates() -> None:
+    """The forwarding marker only applies to legacy delegates, so it can never carry a LiteLLM key to an
+    oauth_delegate upstream."""
+    from litellm.proxy._types import UserAPIKeyAuth
+    from litellm.proxy._experimental.mcp_server.server import (
+        _prepare_mcp_server_headers,
+    )
+
+    server = MCPServer(
+        server_id="oauth-delegate-marked",
+        name="oauth-delegate",
+        transport=MCPTransport.http,
+        auth_type=MCPAuth.oauth_delegate,
+    )
+    caller: Final = UserAPIKeyAuth(api_key="sk-litellm-key").model_copy(
+        update={"mcp_forwards_caller_authorization": True}
+    )
+
+    _, extra_headers = _prepare_mcp_server_headers(
+        server=server,
+        mcp_server_auth_headers=None,
+        mcp_auth_header=None,
+        oauth2_headers={"Authorization": "Bearer sk-litellm-key"},
+        raw_headers={"authorization": "Bearer sk-litellm-key"},
+        user_api_key_auth=caller,
+    )
+
+    assert extra_headers is None
+
+
 def test_prepare_mcp_server_headers_m2m_skips_authorization_from_raw_extra_headers():
     """M2M must not merge caller Authorization from raw_headers when extra_headers lists it."""
     try:
@@ -7173,15 +7243,19 @@ async def test_legacy_delegate_bare_token_is_not_probed_upstream():  # test-qual
 
 
 @pytest.mark.asyncio
-async def test_legacy_delegate_dual_credentials_are_not_probed_upstream():  # test-quality-ok: this removed security-sensitive egress has no return value; non-invocation is the contract
-    """A separate upstream bearer never triggers the removed legacy probe."""
+@pytest.mark.usefixtures("httpx_transport")
+async def test_legacy_delegate_dual_credentials_probe_only_the_upstream_token() -> None:
+    """With a separate x-litellm-api-key, the preflight probes the upstream with its own token and never
+    with the LiteLLM key."""
+    import respx
+
+    from litellm.proxy._types import UserAPIKeyAuth
     from litellm.proxy._experimental.mcp_server.server import (
         _check_passthrough_upstream_auth,
     )
-    from litellm.proxy._types import UserAPIKeyAuth
 
-    server = _delegate_auth_mcp_server()
-    scope = _delegate_scope(
+    server: Final = _delegate_auth_mcp_server()
+    scope: Final = _delegate_scope(
         [
             (b"x-litellm-api-key", b"sk-litellm-proxy-key"),
             (b"authorization", b"Bearer upstream-token"),
@@ -7189,15 +7263,14 @@ async def test_legacy_delegate_dual_credentials_are_not_probed_upstream():  # te
     )
 
     with (
+        _patch_delegate_resolver(server, "delegate_test"),
         patch(  # test-quality-ok: isolate authorized-server resolution so this test targets the preflight boundary
             "litellm.proxy._experimental.mcp_server.operations._get_allowed_mcp_servers",
             new=AsyncMock(return_value=[server]),
         ),
-        patch(  # test-quality-ok: the removed probe call is the security regression under test
-            "litellm.proxy._experimental.mcp_server.server._probe_upstream_auth",
-            new=AsyncMock(),
-        ) as probe,
+        respx.mock as upstream,
     ):
+        probe: Final = upstream.post(server.url).respond(200)
         await _check_passthrough_upstream_auth(
             scope=scope,
             user_api_key_auth=UserAPIKeyAuth(user_id="admitted-user"),
@@ -7208,7 +7281,160 @@ async def test_legacy_delegate_dual_credentials_are_not_probed_upstream():  # te
             raw_headers={"x-litellm-api-key": "sk-litellm-proxy-key", "Authorization": "Bearer upstream-token"},
         )
 
-    probe.assert_not_awaited()
+    assert probe.call_count == 1
+    sent_headers: Final = probe.calls[0].request.headers
+    assert sent_headers["Authorization"] == "Bearer upstream-token"
+    assert all("sk-litellm-proxy-key" not in value for value in sent_headers.values())
+
+
+def _expired_delegate_token_preflight(mcp_servers: Sequence[str]) -> Coroutine[object, object, None]:
+    from litellm.proxy._types import UserAPIKeyAuth
+    from litellm.proxy._experimental.mcp_server.server import (
+        _check_passthrough_upstream_auth,
+    )
+
+    return _check_passthrough_upstream_auth(
+        scope=_delegate_scope([(b"authorization", b"Bearer expired-upstream-token")]),
+        user_api_key_auth=UserAPIKeyAuth().model_copy(update={"mcp_forwards_caller_authorization": True}),
+        mcp_servers=list(mcp_servers),
+        client_ip=None,
+        oauth2_headers={"Authorization": "Bearer expired-upstream-token"},
+        mcp_server_auth_headers=None,
+        raw_headers={"authorization": "Bearer expired-upstream-token"},
+    )
+
+
+@contextlib.contextmanager
+def _rejecting_delegate_upstream() -> Iterator[None]:
+    server: Final = _delegate_auth_mcp_server()
+    with (
+        _patch_delegate_resolver(server, "delegate_test"),
+        patch(  # test-quality-ok: isolate authorized-server resolution so this test targets the preflight boundary
+            "litellm.proxy._experimental.mcp_server.operations._get_allowed_mcp_servers",
+            new=AsyncMock(return_value=[server]),
+        ),
+        patch(  # test-quality-ok: stands in for the upstream rejecting an expired token
+            "litellm.proxy._experimental.mcp_server.server._probe_upstream_auth",
+            new=AsyncMock(return_value=(401, 'Bearer error="invalid_token"')),
+        ),
+    ):
+        yield
+
+
+@pytest.mark.asyncio
+async def test_legacy_delegate_expired_upstream_token_gets_reauthorization_challenge() -> None:
+    """An expired upstream token on a single-server route gets the RFC 9728 re-authorization challenge
+    for the requested name, so the client signs in again instead of seeing an empty tool list."""
+    with _rejecting_delegate_upstream(), pytest.raises(HTTPException) as exc_info:
+        await _expired_delegate_token_preflight(["delegate_test"])
+
+    assert exc_info.value.status_code == 401
+    challenge: Final = exc_info.value.headers["www-authenticate"]
+    assert 'error="invalid_token"' in challenge
+    assert (
+        'resource_metadata="http://localhost:4000/.well-known/oauth-protected-resource/mcp/delegate_test"' in challenge
+    )
+
+
+@pytest.mark.asyncio
+async def test_legacy_delegate_multi_server_scope_is_not_failed_by_one_upstream_token() -> None:
+    """A multi-server scope is not probed, so one rejected upstream token cannot fail the whole connect."""
+    with _rejecting_delegate_upstream():
+        assert await _expired_delegate_token_preflight(["delegate_test", "other_server"]) is None
+
+
+@pytest.mark.asyncio
+@pytest.mark.usefixtures("httpx_transport")
+async def test_legacy_delegate_probe_checks_the_callers_token_not_a_static_authorization() -> None:
+    """The probe is what checks a delegate caller's own token, so a static Authorization the server sends on
+    tool calls cannot stand in for it: a junk token still gets the re-authorization challenge."""
+    import respx
+
+    server: Final = _delegate_auth_mcp_server().model_copy(
+        update={"static_headers": {"Authorization": "Bearer operator-token"}}
+    )
+    with (
+        _patch_delegate_resolver(server, "delegate_test"),
+        patch(  # test-quality-ok: isolate authorized-server resolution so this test targets the preflight boundary
+            "litellm.proxy._experimental.mcp_server.operations._get_allowed_mcp_servers",
+            new=AsyncMock(return_value=[server]),
+        ),
+        respx.mock as upstream,
+    ):
+        upstream.post(server.url).mock(
+            side_effect=lambda request: httpx.Response(
+                200 if request.headers["Authorization"] == "Bearer operator-token" else 401
+            )
+        )
+        outcome: Final = await asyncio.gather(
+            _expired_delegate_token_preflight(["delegate_test"]), return_exceptions=True
+        )
+
+    assert getattr(outcome[0], "status_code", None) == 401
+
+
+@pytest.mark.asyncio
+@pytest.mark.usefixtures("httpx_transport")
+@pytest.mark.parametrize(
+    "requested, expected_status",
+    [(("delegate_test",), 401), (("delegate_test", "other_server"), None)],
+    ids=["single-server", "two-requested-one-reachable"],
+)
+async def test_modern_interaction_probes_legacy_delegate_only_for_a_single_requested_server(
+    requested: tuple[str, ...], expected_status: int | None
+) -> None:
+    """The single-server rule follows what the caller asked for, not what it can reach, so a request
+    naming two servers is not failed by one expired delegate token."""
+    import respx
+
+    from litellm.proxy._types import UserAPIKeyAuth
+    from litellm.proxy._experimental.mcp_server import server
+    from litellm.proxy._experimental.mcp_server.contracts import OperationContext
+
+    target: Final = _delegate_auth_mcp_server()
+    scope: Final = _delegate_scope(
+        [
+            (b"authorization", b"Bearer expired-upstream-token"),
+            (b"mcp-protocol-version", b"2026-07-28"),
+            (b"mcp-method", b"tools/call"),
+            (b"mcp-name", b"delegate-add"),
+        ]
+    )
+    context: Final = OperationContext(
+        _caller=UserAPIKeyAuth().model_copy(update={"mcp_forwards_caller_authorization": True}),
+        mcp_servers=requested,
+        oauth2_headers={"Authorization": "Bearer expired-upstream-token"},
+        raw_headers={"authorization": "Bearer expired-upstream-token"},
+    )
+    body: Final = json.dumps(
+        {
+            "jsonrpc": "2.0",
+            "id": 1,
+            "method": "tools/call",
+            "params": {
+                "name": "delegate-add",
+                "arguments": {"a": 2, "b": 3},
+                "_meta": {
+                    "io.modelcontextprotocol/protocolVersion": "2026-07-28",
+                    "io.modelcontextprotocol/clientCapabilities": {},
+                },
+            },
+        }
+    ).encode()
+    with (
+        _patch_delegate_resolver(target, "delegate_test"),
+        patch(  # test-quality-ok: isolate authorized-server resolution so this test targets the preflight boundary
+            "litellm.proxy._experimental.mcp_server.operations._get_allowed_mcp_servers",
+            new=AsyncMock(return_value=[target]),
+        ),
+        respx.mock as upstream,
+    ):
+        upstream.post(target.url).respond(401, headers={"www-authenticate": 'Bearer error="invalid_token"'})
+        outcome: Final = await asyncio.gather(
+            server._preflight_modern_interaction(scope, body, context), return_exceptions=True
+        )
+
+    assert getattr(outcome[0], "status_code", None) == expected_status
 
 
 @pytest.mark.parametrize(
@@ -7445,39 +7671,47 @@ async def test_delegate_not_probed_when_named_only_via_server_id():
 
 
 @pytest.mark.asyncio
-async def test_delegate_probe_not_fanned_out_to_access_group_members():
-    """A single access-group name passes the one-target route gate but must not fan
-    the delegate probe out to group-expanded member servers; the group name resolves
-    to no server under admission's resolver, so no probe fires."""
+@pytest.mark.usefixtures("httpx_transport")
+async def test_delegate_probe_not_fanned_out_to_access_group_members() -> None:
+    """A single access-group name resolves to no server under admission's resolver, so the delegate probe is
+    not fanned out to the group's members and one member rejecting the token cannot fail the whole group."""
+    import respx
+
+    from litellm.proxy._types import UserAPIKeyAuth
     from litellm.proxy._experimental.mcp_server.server import (
         _check_passthrough_upstream_auth,
     )
-    from litellm.proxy._types import UserAPIKeyAuth
-
-    group_member = _delegate_auth_mcp_server()
 
+    rejecting: Final = _delegate_auth_mcp_server()
+    accepting: Final = _delegate_auth_mcp_server("delegate-2").model_copy(
+        update={"name": "delegate_two", "url": "http://upstream-two:9402/mcp"}
+    )
     with (
-        _patch_delegate_resolver(group_member, "delegate_test"),
-        patch(
+        _patch_delegate_resolver(rejecting, "delegate_test"),
+        patch(  # test-quality-ok: isolate authorized-server resolution so this test targets the preflight boundary
             "litellm.proxy._experimental.mcp_server.operations._get_allowed_mcp_servers",
-            new=AsyncMock(return_value=[group_member]),
+            new=AsyncMock(return_value=[rejecting, accepting]),
         ),
-        patch(
-            "litellm.proxy._experimental.mcp_server.server._probe_upstream_auth",
-            new=AsyncMock(return_value=(401, None)),
-        ) as probe,
+        respx.mock as upstream,
     ):
-        await _check_passthrough_upstream_auth(
-            scope=_delegate_scope([(b"authorization", b"Bearer bogus-token")]),
-            user_api_key_auth=UserAPIKeyAuth(),
-            mcp_servers=["prod_tools_group"],
-            client_ip=None,
-            oauth2_headers=None,
-            mcp_server_auth_headers=None,
-            raw_headers=None,
+        upstream.post(rejecting.url).respond(401, headers={"www-authenticate": 'Bearer error="invalid_token"'})
+        upstream.post(accepting.url).respond(200)
+        assert (
+            await _check_passthrough_upstream_auth(
+                scope=_delegate_scope(
+                    [(b"x-litellm-api-key", b"sk-litellm-proxy-key"), (b"authorization", b"Bearer upstream-token")]
+                ),
+                user_api_key_auth=UserAPIKeyAuth(user_id="admitted-user"),
+                mcp_servers=["prod_tools_group"],
+                client_ip=None,
+                oauth2_headers={"Authorization": "Bearer upstream-token"},
+                mcp_server_auth_headers=None,
+                raw_headers={"x-litellm-api-key": "sk-litellm-proxy-key", "Authorization": "Bearer upstream-token"},
+            )
+            is None
         )
 
-    probe.assert_not_awaited()
+    assert upstream.calls.call_count == 0
 
 
 @pytest.mark.asyncio
@@ -8753,8 +8987,7 @@ async def test_get_allowed_mcp_servers_from_mcp_server_names_unknown_name_fails_
     ]
 
     with patch(
-        "litellm.proxy._experimental.mcp_server.auth.user_api_key_auth_mcp."
-        "MCPRequestHandler.get_mcp_servers_from_access_groups",
+        "litellm.proxy._experimental.mcp_server.auth.user_api_key_auth_mcp.MCPRequestHandler.get_mcp_servers_from_access_groups",
         new_callable=AsyncMock,
         return_value=[],
     ):
@@ -8812,8 +9045,7 @@ async def test_get_allowed_mcp_servers_from_mcp_server_names_known_alias_returns
     ]
 
     with patch(
-        "litellm.proxy._experimental.mcp_server.auth.user_api_key_auth_mcp."
-        "MCPRequestHandler.get_mcp_servers_from_access_groups",
+        "litellm.proxy._experimental.mcp_server.auth.user_api_key_auth_mcp.MCPRequestHandler.get_mcp_servers_from_access_groups",
         new_callable=AsyncMock,
         return_value=[],
     ):
@@ -8845,8 +9077,7 @@ async def test_get_allowed_mcp_servers_from_mcp_server_names_mixed_known_and_unk
     ]
 
     with patch(
-        "litellm.proxy._experimental.mcp_server.auth.user_api_key_auth_mcp."
-        "MCPRequestHandler.get_mcp_servers_from_access_groups",
+        "litellm.proxy._experimental.mcp_server.auth.user_api_key_auth_mcp.MCPRequestHandler.get_mcp_servers_from_access_groups",
         new_callable=AsyncMock,
         return_value=[],
     ):
@@ -8878,8 +9109,7 @@ async def test_get_allowed_mcp_servers_from_mcp_server_names_access_group_resolv
     ]
 
     with patch(
-        "litellm.proxy._experimental.mcp_server.auth.user_api_key_auth_mcp."
-        "MCPRequestHandler.get_mcp_servers_from_access_groups",
+        "litellm.proxy._experimental.mcp_server.auth.user_api_key_auth_mcp.MCPRequestHandler.get_mcp_servers_from_access_groups",
         new_callable=AsyncMock,
         return_value=["id-b"],
     ):
@@ -9207,8 +9437,7 @@ async def test_get_allowed_mcp_servers_includes_active_servers_submitted_by_user
             return_value={"submitted-1": submitted_server},
         ),
         patch(
-            "litellm.proxy._experimental.mcp_server.auth.user_api_key_auth_mcp."
-            "MCPRequestHandler.get_allowed_mcp_servers",
+            "litellm.proxy._experimental.mcp_server.auth.user_api_key_auth_mcp.MCPRequestHandler.get_allowed_mcp_servers",
             AsyncMock(return_value=[]),
         ),
         patch("litellm.proxy.proxy_server.prisma_client", MagicMock()),

uv.lock

@@ -1657,60 +1657,70 @@ wheels = [
 
 [[package]]
 name = "ddtrace"
-version = "4.11.0"
+version = "4.15.6"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
     { name = "bytecode" },
+    { name = "ddtrace-internal" },
     { name = "envier" },
     { name = "opentelemetry-api" },
     { name = "wrapt" },
 ]
-sdist = { url = "https://files.pythonhosted.org/packages/81/51/a628f0177274bab5b67c93a1558fd222babb15286a427e4e8c1d65a265b2/ddtrace-4.11.0.tar.gz", hash = "sha256:260c5b46e80565f4fd08cec2650f707627fb57cd6f8951a2d8c9e6a02b490074", size = 2422158, upload-time = "2026-07-10T08:57:53.396Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/f6/94/f0f005257dc49a7062c6fed98a6dbcca7691e9d1db5c64e8a2d14f204cac/ddtrace-4.15.6.tar.gz", hash = "sha256:2914f646d4ae338c644b814c4ad80c963e7aadfb39d1e67b0367a15d5c4bc0db", size = 2655968, upload-time = "2026-10-08T09:46:58.392Z" }
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