From 4cadb402e5ab71650d9ca64c13984b2aeded49b8 Mon Sep 17 00:00:00 2001
From: yucheng <yucheng@berri.ai>
Date: Sat, 26 Sep 2026 09:32:46 +0000
Subject: [PATCH] feat(mcp): fold gateway sign-in into a caller sign-in
 contract on the challenge path

Replace the parallel gateway sign-in provider registry with
CallerSignInProvider, merged into the existing oauth2_token_exchange
challenge: gates key off caller_sign_in_for() returning non-None, the
resolver answers by server_id, case-insensitive name, and short prefix
like the router, keeps_caller_authorization covers OBO servers, and the
Agent 365 guardrail exchanges through an injected TokenExchanger.

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
---
 .../mcp_server/caller_sign_in.py              | 124 ++++
 .../mcp_server/discoverable_endpoints.py      |  58 +-
 .../mcp_server/mcp_server_manager.py          |  28 +-
 .../_experimental/mcp_server/operations.py    |  10 +-
 .../token_exchange_provider.py                |   5 +
 .../proxy/_experimental/mcp_server/server.py  |  40 +-
 .../proxy/_experimental/mcp_server/utils.py   |   8 +
 .../guardrail_hooks/agent_365/agent_365.py    | 229 +++----
 .../types/mcp_server/mcp_server_manager.py    |  23 +-
 .../test_token_exchange_provider.py           |  42 +-
 .../mcp_server/test_caller_sign_in.py         | 134 +++++
 .../mcp_server/test_discoverable_endpoints.py |  26 +-
 .../mcp_server/test_mcp_server.py             | 181 +++++-
 .../mcp_server/test_mcp_server_manager.py     |  13 +
 .../guardrail_hooks/test_agent_365.py         | 561 ++++++++++--------
 15 files changed, 1005 insertions(+), 477 deletions(-)
 create mode 100644 litellm/proxy/_experimental/mcp_server/caller_sign_in.py
 create mode 100644 tests/test_litellm/proxy/_experimental/mcp_server/test_caller_sign_in.py

diff --git a/litellm/proxy/_experimental/mcp_server/caller_sign_in.py b/litellm/proxy/_experimental/mcp_server/caller_sign_in.py
new file mode 100644
index 00000000000..9d9b8b7297e
--- /dev/null
+++ b/litellm/proxy/_experimental/mcp_server/caller_sign_in.py
@@ -0,0 +1,124 @@
+"""Caller-side sign-in requirements for MCP connects.
+
+A guardrail that evaluates tool calls in the caller's own identity (an On-Behalf-Of exchange of the caller's
+bearer) needs the caller signed in with its issuer before the first tool call, and a tool call's JSON-RPC
+error cannot carry ``WWW-Authenticate``. ``token_exchange`` (OBO) servers have the same need: the caller must
+present a subject token the gateway can exchange. Both cases share one contract: a connect that carries no
+usable subject answers 401 with the RFC 9728 challenge, and the protected-resource metadata advertises the
+issuers and scopes the caller signs in for.
+
+Guardrails implement :class:`CallerSignInProvider`; :func:`caller_sign_in_for` merges the OBO server's own
+requirement with every registered provider's so the challenge and the metadata always agree. The MCP package
+never imports a concrete provider.
+"""
+
+from __future__ import annotations
+
+from collections.abc import Mapping
+from dataclasses import dataclass
+from typing import TYPE_CHECKING, Final, Protocol, cast, runtime_checkable
+
+from pydantic import BaseModel, ConfigDict, TypeAdapter, ValidationError
+
+import litellm
+from litellm.integrations.custom_guardrail import CustomGuardrail
+from litellm.types.mcp import MCPAuth
+
+if TYPE_CHECKING:
+    from litellm.proxy._types import UserAPIKeyAuth
+    from litellm.types.mcp_server.mcp_server_manager import MCPServer
+
+
+@dataclass(frozen=True, slots=True)
+class CallerSignIn:
+    """The issuers a caller signs in with and the scopes it requests before calling a server."""
+
+    issuers: tuple[str, ...]
+    scopes: tuple[str, ...]
+
+
+@runtime_checkable
+class CallerSignInProvider(Protocol):
+    def caller_sign_in(self, server: MCPServer, user_api_key_auth: UserAPIKeyAuth | None) -> CallerSignIn | None:
+        """The sign-in this provider requires of callers hitting ``server``; ``None`` when it does not gate
+        the server for this caller (``user_api_key_auth=None`` is the anonymous metadata fetch that follows
+        a challenge)."""
+        ...
+
+
+class _JwtIssuerEntry(BaseModel):
+    model_config = ConfigDict(extra="ignore")
+
+    issuer: str | None = None
+
+
+class _JwtAuthConfig(BaseModel):
+    model_config = ConfigDict(extra="ignore")
+
+    issuers: list[_JwtIssuerEntry] = []  # mutable-ok: pydantic copies the default per instance
+
+
+_JWT_AUTH_ADAPTER: Final = TypeAdapter(_JwtAuthConfig)
+
+
+def _jwt_auth_issuer_entries(jwtauth: object) -> tuple[_JwtIssuerEntry, ...]:
+    try:
+        return tuple(_JWT_AUTH_ADAPTER.validate_python(jwtauth, from_attributes=True).issuers)
+    except ValidationError:
+        return ()
+
+
+def _providers() -> tuple[CallerSignInProvider, ...]:
+    return tuple(
+        callback
+        for callback in litellm.logging_callback_manager.get_custom_loggers_for_type(CustomGuardrail)
+        if isinstance(callback, CallerSignInProvider)
+    )
+
+
+def jwt_auth_issuers() -> tuple[str, ...]:
+    """The OAuth issuer identifier(s) LiteLLM's JWT auth trusts, for the OBO PRM authorization_servers.
+
+    In token_exchange the IdP that issues the subject JWT is the same one LiteLLM validates it
+    against, so OBO discovery points clients at the JWT-auth issuer to obtain a subject token.
+    Sourced from ``JWT_ISSUER`` and any configured ``litellm_jwtauth.issuers``.
+    """
+    import os  # noqa: PLC0415
+
+    from litellm.proxy.proxy_server import (  # noqa: PLC0415  # lazy: proxy_server pulls the whole proxy graph
+        general_settings,  # pyright: ignore[reportUnknownVariableType]  # proxy_server.general_settings is a raw untyped dict
+    )
+
+    env_issuer: Final = os.getenv("JWT_ISSUER")
+    env: Final[tuple[str, ...]] = (env_issuer,) if env_issuer else ()
+
+    settings: Final[Mapping[str, object]] = cast(Mapping[str, object], general_settings)
+    configured: Final = tuple(
+        entry.issuer for entry in _jwt_auth_issuer_entries(settings.get("litellm_jwtauth")) if entry.issuer
+    )
+    return tuple(dict.fromkeys((*env, *configured)))
+
+
+def caller_sign_in_for(server: MCPServer, user_api_key_auth: UserAPIKeyAuth | None) -> CallerSignIn | None:
+    """The merged sign-in requirement for ``server``: the OBO server's own issuer/scopes plus every
+    registered provider's contribution. ``None`` when nothing requires sign-in, which is also the gate the
+    connect-time challenge branches on."""
+    contributions: Final = [
+        contribution
+        for contribution in (
+            *(
+                (
+                    CallerSignIn(issuers=jwt_auth_issuers(), scopes=tuple(server.scopes or ())),
+                )
+                if server.auth_type == MCPAuth.oauth2_token_exchange
+                else ()
+            ),
+            *(provider.caller_sign_in(server, user_api_key_auth) for provider in _providers()),
+        )
+        if contribution is not None
+    ]
+    if not contributions:
+        return None
+    issuers: Final = tuple(dict.fromkeys(issuer for contribution in contributions for issuer in contribution.issuers))
+    scopes: Final = tuple(dict.fromkeys(scope for contribution in contributions for scope in contribution.scopes))
+    return CallerSignIn(issuers=issuers, scopes=scopes)
diff --git a/litellm/proxy/_experimental/mcp_server/discoverable_endpoints.py b/litellm/proxy/_experimental/mcp_server/discoverable_endpoints.py
index 7a0f59c3c2b..6940e8d4f02 100644
--- a/litellm/proxy/_experimental/mcp_server/discoverable_endpoints.py
+++ b/litellm/proxy/_experimental/mcp_server/discoverable_endpoints.py
@@ -37,6 +37,7 @@ from litellm.proxy._experimental.mcp_server.bridge_token_flow import (
     can_store_oauth_credential,
     oauth_authorization_uses_gateway_credential,
 )
+from litellm.proxy._experimental.mcp_server.caller_sign_in import caller_sign_in_for
 from litellm.proxy._experimental.mcp_server.faults import (
     CallerRejected,
     CredentialSource,
@@ -2580,9 +2581,9 @@ async def _build_oauth_protected_resource_response(
             detail=(f"Upstream oauth-protected-resource metadata unavailable for MCP server {mcp_server.name!r}"),
         )
 
-    obo_response: Final = _obo_protected_resource_response(mcp_server, resource_url)
-    if obo_response is not None:
-        return obo_response
+    sign_in_response: Final = _caller_sign_in_protected_resource_response(mcp_server, resource_url)
+    if sign_in_response is not None:
+        return sign_in_response
 
     if mcp_server is not None and mcp_server.advertises_gateway_authorization_server:
         return {
@@ -2603,51 +2604,30 @@ async def _build_oauth_protected_resource_response(
     }
 
 
-def _obo_protected_resource_response(mcp_server: MCPServer | None, resource_url: str) -> dict | None:
-    """The OBO (token_exchange) PRM, or None when this server is not OBO / no issuer is configured.
+def _caller_sign_in_protected_resource_response(
+    mcp_server: MCPServer | None, resource_url: str
+) -> dict[str, object] | None:
+    """The caller sign-in PRM: the OBO issuer(s) LiteLLM trusts merged with every registered
+    ``CallerSignInProvider``'s contribution, or None when no sign-in gates this server.
 
-    The client SSOs with the IdP to obtain a subject token, which LiteLLM then exchanges, so discovery
-    points at the JWT-auth issuer(s) LiteLLM trusts (the same IdP that issues and validates the
-    subject), not the gateway. None falls the caller back to the gateway default so discovery still
-    returns metadata; it just can't name the IdP.
+    The client SSOs with the IdP to obtain a subject token, which LiteLLM then exchanges (or a
+    guardrail consumes directly), so discovery points at the issuer(s), not the gateway. None falls
+    the caller back to the gateway default so discovery still returns metadata; it just can't name
+    the IdP. The anonymous metadata fetch passes ``user_api_key_auth=None`` because it cannot see
+    which key selected a provider.
     """
-    if mcp_server is None or mcp_server.auth_type != MCPAuth.oauth2_token_exchange:
+    if mcp_server is None:
         return None
-    issuers: Final = _jwt_auth_issuers()
-    if not issuers:
+    sign_in: Final = caller_sign_in_for(mcp_server, None)
+    if sign_in is None or not sign_in.issuers:
         return None
     return {
-        "authorization_servers": issuers,
+        "authorization_servers": list(sign_in.issuers),
         "resource": resource_url,
-        "scopes_supported": (mcp_server.scopes if mcp_server.scopes else []),
+        "scopes_supported": list(sign_in.scopes),
     }
 
 
-def _jwt_auth_issuers() -> list:
-    """The OAuth issuer identifier(s) LiteLLM's JWT auth trusts, for the OBO PRM authorization_servers.
-
-    In token_exchange the IdP that issues the subject JWT is the same one LiteLLM validates it
-    against, so OBO discovery points clients at the JWT-auth issuer to obtain a subject token.
-    Sourced from ``JWT_ISSUER`` and any configured ``litellm_jwtauth.issuers``.
-    """
-    import os  # noqa: PLC0415
-
-    from litellm.proxy.proxy_server import general_settings  # noqa: PLC0415
-
-    issuers: Final[list] = []
-    env_issuer: Final = os.getenv("JWT_ISSUER")
-    if env_issuer:
-        issuers.append(env_issuer)
-
-    jwtauth: Final = general_settings.get("litellm_jwtauth") if isinstance(general_settings, Mapping) else None
-    raw_issuers: Final = jwtauth.get("issuers") if isinstance(jwtauth, dict) else getattr(jwtauth, "issuers", None)
-    for cfg in raw_issuers or []:
-        issuer = cfg.get("issuer") if isinstance(cfg, dict) else getattr(cfg, "issuer", None)
-        if issuer and issuer not in issuers:
-            issuers.append(issuer)
-    return issuers
-
-
 @router.get("/.well-known/oauth-protected-resource")
 def oauth_protected_resource_root(request: Request) -> dict[str, str | tuple[str, ...]]:
     request_base_url: Final = get_request_base_url(request)
diff --git a/litellm/proxy/_experimental/mcp_server/mcp_server_manager.py b/litellm/proxy/_experimental/mcp_server/mcp_server_manager.py
index 09714b65f86..c38d156dd46 100644
--- a/litellm/proxy/_experimental/mcp_server/mcp_server_manager.py
+++ b/litellm/proxy/_experimental/mcp_server/mcp_server_manager.py
@@ -168,6 +168,7 @@ from litellm.proxy._experimental.mcp_server.utils import (
     normalize_server_name,
     openapi_tool_name,
     parse_admin_env_vars,
+    server_answers_to_name,
     strip_known_server_prefix,
     validate_mcp_server_name,
 )
@@ -4241,6 +4242,12 @@ class MCPServerManager:
                 if subject_token is not None:
                     return
             case _:
+                from litellm.proxy._experimental.mcp_server.caller_sign_in import (  # noqa: PLC0415  # lazy: provider discovery pulls the guardrail registry
+                    caller_sign_in_for,
+                )
+
+                if subject_token is None and caller_sign_in_for(server, user_api_key_auth) is not None:
+                    raise_token_exchange_challenge(server, root_path=get_request_root_path())
                 return
         resolved_server: Final = await self.ensure_oauth_metadata_discovered(server)
         spec: Final = _to_server_spec_fail_closed(resolved_server)
@@ -5964,13 +5971,8 @@ class MCPServerManager:
         if proxy_logging_obj is None:
             return hook_result
 
-        # Extract incoming Bearer token from raw request headers so
-        # guardrails like MCPJWTSigner can verify + re-sign it (FR-5).
-        normalized_raw: Final = {k.lower(): v for k, v in (raw_headers or {}).items()}
-        incoming_bearer_token: str | None = None
-        auth_hdr: Final = normalized_raw.get("authorization", "")
-        if auth_hdr.lower().startswith("bearer "):
-            incoming_bearer_token = auth_hdr[len("bearer ") :]
+        # Admission credentials are never handed to guardrails as the caller's assertion.
+        incoming_bearer_token: Final = self._extract_subject_token(None, raw_headers, user_api_key_auth)
 
         pre_hook_kwargs: Final = {
             "guardrail_context": guardrail_context,
@@ -7189,6 +7191,18 @@ class MCPServerManager:
                 return server
         return None
 
+    def get_mcp_server_answering_to(self, name: str, client_ip: str | None = None) -> MCPServer | None:
+        """The server a scoped ``/mcp/{name}`` connect resolves to, matched the way the router matches
+        it: case-insensitive over server_id, name and every published prefix form."""
+        return next(
+            (
+                server
+                for server in self.get_filtered_registry(client_ip).values()
+                if server_answers_to_name(server, name)
+            ),
+            None,
+        )
+
     def get_filtered_registry(self, client_ip: str | None = None) -> dict[str, MCPServer]:
         """
         Get registry filtered by client IP access control.
diff --git a/litellm/proxy/_experimental/mcp_server/operations.py b/litellm/proxy/_experimental/mcp_server/operations.py
index 891c37f4fe7..abc7c52f5a7 100644
--- a/litellm/proxy/_experimental/mcp_server/operations.py
+++ b/litellm/proxy/_experimental/mcp_server/operations.py
@@ -120,6 +120,7 @@ from litellm.proxy._experimental.mcp_server.utils import (
     logging_safe_mcp_headers,
     match_known_tool_name,
     normalize_server_name,
+    server_answers_to_name,
     split_server_prefix_from_name,
     strip_known_server_prefix,
 )
@@ -496,8 +497,7 @@ def _http_detail_message(detail: object) -> str:
 
 
 def _server_answers_to(server: MCPServer, name: str) -> bool:
-    requested: Final = name.lower()
-    return any(requested == known.lower() for known in iter_known_server_prefixes(server) if known)
+    return server_answers_to_name(server, name)
 
 
 async def raise_denied_scoped_mcp_access(
@@ -1768,7 +1768,11 @@ def _challenge_missing_token_exchange_subject(
     warm path already raises. Gated to servers the key may reach so an unauthorized caller
     learns nothing about the catalog.
     """
-    if server is None or server.auth_type != MCPAuth.oauth2_token_exchange:
+    from litellm.proxy._experimental.mcp_server.caller_sign_in import (
+        caller_sign_in_for,  # noqa: PLC0415  # lazy: caller_sign_in pulls the proxy graph
+    )
+
+    if server is None or caller_sign_in_for(server, user_api_key_auth) is None:
         return
     if requested_server is not None and requested_server.server_id != server.server_id:
         return
diff --git a/litellm/proxy/_experimental/mcp_server/outbound_credentials/token_exchange_provider.py b/litellm/proxy/_experimental/mcp_server/outbound_credentials/token_exchange_provider.py
index 2501f751d8e..7f5c0c99145 100644
--- a/litellm/proxy/_experimental/mcp_server/outbound_credentials/token_exchange_provider.py
+++ b/litellm/proxy/_experimental/mcp_server/outbound_credentials/token_exchange_provider.py
@@ -79,6 +79,11 @@ async def _post_exchange_endpoint(
         parsed: Final[object] = response.json()  # pyright: ignore
     except httpx.HTTPStatusError as status_err:
         status_code: Final = status_err.response.status_code
+        if status_code in (408, 429):
+            # Retry hints, not subject rejections: the IdP is shedding load, so a 401 would tell the
+            # caller to sign in again for nothing; surface it like a transport failure.
+            verbose_logger.warning("MCP token exchange throttled or timed out (HTTP %d)", status_code)
+            return None
         if 400 <= status_code < 500:
             oauth_error, claims = _oauth_error_fields(status_err.response)
             if oauth_error in _GATEWAY_FAULT_OAUTH_ERRORS:
diff --git a/litellm/proxy/_experimental/mcp_server/server.py b/litellm/proxy/_experimental/mcp_server/server.py
index 2412e83b9d9..b73d183db86 100644
--- a/litellm/proxy/_experimental/mcp_server/server.py
+++ b/litellm/proxy/_experimental/mcp_server/server.py
@@ -36,6 +36,7 @@ from litellm.proxy._experimental.mcp_server.auth.user_api_key_auth_mcp import (
     MCPRequestHandler,
     _is_mcp_admitted_user_subject,
 )
+from litellm.proxy._experimental.mcp_server.caller_sign_in import caller_sign_in_for
 from litellm.proxy._experimental.mcp_server.client_allowlist import (
     MCPClientAllowlist,
     check_mcp_client_allowed,
@@ -1580,6 +1581,21 @@ if MCP_AVAILABLE:
             )
         return user_api_key_auth.model_copy(update={"object_permission": updated_op, "mcp_toolset_id": toolset_id})
 
+    async def _key_granted_single_server(
+        server: MCPServer,
+        mcp_servers: Sequence[str] | None,
+        user_api_key_auth: UserAPIKeyAuth | None,
+        client_ip: str | None,
+    ) -> bool:
+        """Sign-in challenges are issued only on a single-server connect the key's grant admits, so a key
+        without access gets the grant's 403 instead of a sign-in it could not use."""
+        if len(mcp_servers or []) != 1:
+            return False
+        allowed: Final = await operations._get_allowed_mcp_servers(
+            user_api_key_auth=user_api_key_auth, mcp_servers=mcp_servers, client_ip=client_ip
+        )
+        return any(granted.server_id == server.server_id for granted in allowed)
+
     async def _raise_preemptive_401_for_unauthenticated_servers(
         scope: Scope,
         mcp_servers: list[str] | None,
@@ -1602,7 +1618,7 @@ if MCP_AVAILABLE:
         a server it will be 403'd on immediately after authentication.
         """
         for server_name in mcp_servers or []:
-            server = operations.global_mcp_server_manager.get_mcp_server_by_name(server_name, client_ip=client_ip)
+            server = operations.global_mcp_server_manager.get_mcp_server_answering_to(server_name, client_ip=client_ip)
             if server is not None and allowed_server_ids is not None and server.server_id not in allowed_server_ids:
                 # Caller's narrowed scope excludes this server — skip the
                 # preemptive challenge and let downstream authorization
@@ -1698,12 +1714,22 @@ if MCP_AVAILABLE:
                 # reaches the token_exchange / pass-through blocks below.
                 continue
 
-            # token_exchange (OBO): the caller supplied no subject token. Challenge at connect
-            # (transport level, where WWW-Authenticate survives) with the RFC 9728 resource_metadata
-            # so the client discovers the IdP, SSOs, and retries with a subject token, which LiteLLM
-            # then exchanges. A tool-call-time 401 would be wrapped into a JSON-RPC error and the
-            # header lost, so the discovery flow needs this pre-emptive challenge.
-            if server and server.auth_type == MCPAuth.oauth2_token_exchange and not oauth2_headers:
+            # Caller sign-in: challenge at connect because a tool-call-time 401 is wrapped into a
+            # JSON-RPC error and the WWW-Authenticate header is lost. Non-OBO gates fire only on a
+            # single-server connect the key's grant admits.
+            sign_in = caller_sign_in_for(server, user_api_key_auth) if server is not None else None
+            if (
+                server
+                and sign_in is not None
+                and operations.global_mcp_server_manager._extract_subject_token(  # pyright: ignore[reportPrivateUsage]  # the manager owns the subject/admission filter shared with the preflight
+                    oauth2_headers, raw_headers, user_api_key_auth
+                )
+                is None
+                and (
+                    server.auth_type == MCPAuth.oauth2_token_exchange
+                    or await _key_granted_single_server(server, mcp_servers, user_api_key_auth, client_ip)
+                )
+            ):
                 from litellm.proxy._experimental.mcp_server.outbound_credentials.adapter import (  # noqa: PLC0415  # lazy: adapter pulls MCP subgraph
                     raise_token_exchange_challenge,
                 )
diff --git a/litellm/proxy/_experimental/mcp_server/utils.py b/litellm/proxy/_experimental/mcp_server/utils.py
index 7411dc5c4f0..8234a62c4ef 100644
--- a/litellm/proxy/_experimental/mcp_server/utils.py
+++ b/litellm/proxy/_experimental/mcp_server/utils.py
@@ -370,6 +370,14 @@ def iter_known_server_prefixes(server: _McpServerLike) -> Iterator[str]:
     yield from _emit(server_id)
 
 
+def server_answers_to_name(server: _McpServerLike, name: str) -> bool:
+    """Whether a scoped ``/mcp/{name}`` connect resolves to ``server``: case-insensitive over every prefix
+    form routing accepts (alias, server_name, server_id, short prefix), the same match
+    ``_server_answers_to`` applies when the router scopes a request."""
+    requested: Final = name.lower()
+    return any(requested == known.lower() for known in iter_known_server_prefixes(server) if known)
+
+
 def iter_known_tool_name_spellings(tool_name: str, server: MCPServer) -> Iterator[str]:
     """Yield every name that denotes the bare ``tool_name`` on ``server``: the bare name,
     then its wire spelling under each prefix ``iter_known_server_prefixes`` accepts.
diff --git a/litellm/proxy/guardrails/guardrail_hooks/agent_365/agent_365.py b/litellm/proxy/guardrails/guardrail_hooks/agent_365/agent_365.py
index 94de0b2df7b..e5397a7269d 100644
--- a/litellm/proxy/guardrails/guardrail_hooks/agent_365/agent_365.py
+++ b/litellm/proxy/guardrails/guardrail_hooks/agent_365/agent_365.py
@@ -9,17 +9,14 @@ exchanged for a delegated Agent 365 token, so Defender evaluates and audits
 as the signed-in user.
 """
 
-import hashlib
-import threading
 import time
 import uuid
-from collections import OrderedDict
 from collections.abc import Mapping
 from typing import TYPE_CHECKING, ClassVar, Final, Literal, NoReturn
 
 import httpx
 from fastapi import HTTPException
-from pydantic import BaseModel, ConfigDict, Field, TypeAdapter, ValidationError
+from pydantic import BaseModel, ConfigDict, Field, SecretStr, TypeAdapter, ValidationError
 from typing_extensions import ReadOnly, TypedDict
 
 from litellm._logging import verbose_proxy_logger
@@ -34,7 +31,18 @@ from litellm.llms.custom_httpx.http_handler import (
     get_async_httpx_client,
     httpxSpecialProvider,
 )
+from litellm.proxy._experimental.mcp_server.caller_sign_in import CallerSignIn
+from litellm.proxy._experimental.mcp_server.outbound_credentials.result import Error, Ok
+from litellm.proxy._experimental.mcp_server.outbound_credentials.token_exchange_provider import (
+    build_token_exchanger,
+)
+from litellm.proxy._experimental.mcp_server.outbound_credentials.token_exchanger import TokenExchanger
+from litellm.proxy._experimental.mcp_server.outbound_credentials.types import (
+    ServerSpec,
+    TokenExchangeConfig,
+)
 from litellm.types.guardrails import GuardrailEventHooks
+from litellm.types.mcp_server.mcp_server_manager import MCPServer
 from litellm.types.proxy.guardrails.guardrail_hooks.agent_365 import (
     AGENT_365_PROD_API_BASE,
     AGENT_365_PROD_RESOURCE_APP_ID,
@@ -49,38 +57,14 @@ if TYPE_CHECKING:
     from litellm.types.utils import GuardrailStatus
 
 TOKEN_ENDPOINT_TEMPLATE: Final = "https://login.microsoftonline.com/{tenant_id}/oauth2/v2.0/token"
+ENTRA_ISSUER_TEMPLATE: Final = "https://login.microsoftonline.com/{tenant_id}/v2.0"
 EVALUATE_URL: Final = f"{AGENT_365_PROD_API_BASE}/agents/tool-evaluation/evaluate"
 OBO_SCOPE: Final = f"{AGENT_365_PROD_RESOURCE_APP_ID}/{AGENT_365_SCOPE_NAME}"
 MCP_SESSION_ID_HEADER: Final = "mcp-session-id"
 DEFENDER_STATUS_EVALUATED: Final = "Evaluated"
-_GATEWAY_OWNED_TOKEN_ERRORS: Final = frozenset(
-    {"invalid_client", "unauthorized_client", "invalid_scope", "invalid_resource"}
-)
-# Entra reports a malformed or unverifiable assertion as ``invalid_client`` too; only its AADSTS50027xx
-# (InvalidJwtToken) sub-codes tell that apart from a bad gateway secret.
-_INVALID_ASSERTION_AADSTS_PREFIX: Final = "50027"
-_AADSTS_CODES_ADAPTER: Final = TypeAdapter(tuple[int, ...])
+GATEWAY_SCOPE_TEMPLATE: Final = "api://{client_id}/access_as_user"
 _MCP_CALL_TYPES: Final[tuple[str, ...]] = ("mcp_call", "call_mcp_tool")
 _TOOL_INPUT_SCHEMA_ADAPTER: Final = TypeAdapter(dict[str, object])
-_OBO_CACHE_MAX_ENTRIES: Final = 1000
-_DEFAULT_TOKEN_TTL_SECONDS: Final = 3599.0
-_TOKEN_EXPIRY_SLACK_SECONDS: Final = 60.0
-
-
-def _parse_expires_in(raw: object) -> float:
-    if not isinstance(raw, (int, float, str)):
-        return _DEFAULT_TOKEN_TTL_SECONDS
-    try:
-        return float(raw)
-    except ValueError:
-        return _DEFAULT_TOKEN_TTL_SECONDS
-
-
-def _parse_aadsts_codes(raw: object) -> tuple[int, ...]:
-    try:
-        return _AADSTS_CODES_ADAPTER.validate_python(raw)
-    except ValidationError:
-        return ()
 
 
 def _parse_tool_input_schema(raw: object) -> Mapping[str, object] | None:
@@ -129,27 +113,6 @@ class _BlockedDetail(TypedDict):
     correlation_id: ReadOnly[str | None]
 
 
-class Agent365TokenExchangeError(Exception):
-    def __init__(self, status_code: int, error_code: str, description: str, aadsts_codes: tuple[int, ...] = ()) -> None:
-        super().__init__(f"{error_code}: {description}")
-        self.status_code = status_code
-        self.error_code = error_code
-        self.description = description
-        self.aadsts_codes = aadsts_codes
-
-    @property
-    def gateway_owned(self) -> bool:
-        """Whether the gateway's own client credentials, scope or resource were refused, as opposed to the
-        caller's assertion. The caller cannot fix a gateway-owned rejection by signing in again."""
-        if self.error_code not in _GATEWAY_OWNED_TOKEN_ERRORS:
-            return False
-        return not any(str(code).startswith(_INVALID_ASSERTION_AADSTS_PREFIX) for code in self.aadsts_codes)
-
-
-class Agent365MalformedResponseError(Exception):
-    pass
-
-
 class Agent365ThrottledError(Exception):
     def __init__(self, status_code: int) -> None:
         super().__init__(f"HTTP {status_code}")
@@ -173,6 +136,7 @@ class Agent365Guardrail(CustomGuardrail):
         request_timeout: float = 10.0,
         unreachable_fallback: Literal["fail_closed", "fail_open"] = "fail_closed",
         async_handler: AsyncHTTPHandler | None = None,
+        token_exchanger: TokenExchanger | None = None,
         **kwargs,  # noqa: ANN003  # kwargs-ok: forwarded verbatim to CustomGuardrail (event_hook, default_on)
     ) -> None:
         super().__init__(
@@ -192,8 +156,19 @@ class Agent365Guardrail(CustomGuardrail):
         self.async_handler = async_handler or get_async_httpx_client(
             llm_provider=httpxSpecialProvider.GuardrailCallback
         )
-        self._obo_token_cache: OrderedDict[str, tuple[str, float]] = OrderedDict()  # mutable-ok: lock-guarded LRU
-        self._obo_cache_lock = threading.Lock()
+        self._exchange_config: Final = TokenExchangeConfig(
+            profile="entra_obo",
+            token_exchange_endpoint=TOKEN_ENDPOINT_TEMPLATE.format(tenant_id=tenant_id),
+            client_id=client_id,
+            client_secret=SecretStr(client_secret),
+            scopes=(OBO_SCOPE,),
+        )
+        self._exchange_server: Final = ServerSpec(
+            server_id=f"agent-365:{tenant_id}",
+            resource=AGENT_365_PROD_API_BASE,
+            config=self._exchange_config,
+        )
+        self._token_exchanger: Final = token_exchanger if token_exchanger is not None else build_token_exchanger()
         verbose_proxy_logger.info("Initialized Microsoft Agent 365 guardrail: %s", guardrail_name)
 
     @staticmethod
@@ -233,42 +208,40 @@ class Agent365Guardrail(CustomGuardrail):
             )
 
         try:
-            obo_token: Final = await self._get_obo_token(assertion)
-        except Agent365TokenExchangeError as exc:
-            if exc.gateway_owned:
-                return self._handle_unavailable(
-                    data=data,
-                    tool_name=tool_name,
-                    reason=(
-                        f"Entra rejected the gateway's own Agent 365 credentials ({exc.error_code}); "
-                        "check the guardrail's client_id and client_secret"
-                    ),
-                )
-            self._handle_caller_fault(
-                data=data,
-                tool_name=tool_name,
-                status_code=401,
-                reason=f"the Entra On-Behalf-Of token exchange was rejected ({exc.error_code})",
-            )
-        except Agent365ThrottledError as exc:
-            self._handle_throttled(
-                data=data,
-                tool_name=tool_name,
-                reason=f"the Entra token endpoint returned HTTP {exc.status_code}",
-                latency_ms=None,
-            )
+            exchange_result: Final = await self._exchange_caller_assertion(assertion)
         except (httpx.HTTPError, LitellmTimeout, TimeoutError) as exc:
             return self._handle_unavailable(
                 data=data,
                 tool_name=tool_name,
                 reason=f"the Entra token endpoint could not be reached ({type(exc).__name__})",
             )
-        except Agent365MalformedResponseError as exc:
-            return self._handle_unavailable(
-                data=data,
-                tool_name=tool_name,
-                reason=str(exc),
-            )
+        match exchange_result:
+            case Ok(token):
+                obo_token: Final = token.access_token
+            case Error(error):
+                match error.tag:
+                    case "unauthorized":
+                        self._handle_caller_fault(
+                            data=data,
+                            tool_name=tool_name,
+                            status_code=401,
+                            reason=f"the Entra On-Behalf-Of token exchange was rejected ({error.unauthorized.detail})",
+                        )
+                    case "misconfigured":
+                        return self._handle_unavailable(
+                            data=data,
+                            tool_name=tool_name,
+                            reason=(
+                                f"Entra rejected the gateway's own Agent 365 credentials ({error.misconfigured}); "
+                                "check the guardrail's client_id and client_secret"
+                            ),
+                        )
+                    case _:
+                        return self._handle_unavailable(
+                            data=data,
+                            tool_name=tool_name,
+                            reason=f"the Entra token exchange failed ({error.summary})",
+                        )
 
         start: Final = time.perf_counter()
         try:
@@ -284,14 +257,14 @@ class Agent365Guardrail(CustomGuardrail):
                 reason=f"the Agent 365 endpoint could not be reached ({type(exc).__name__})",
             )
         latency_ms: Final = (time.perf_counter() - start) * 1000.0
-        fallback: Final = self._handle_evaluate_error(
+        fallback: Final = await self._handle_evaluate_error(
             data=data, tool_name=tool_name, assertion=assertion, response=response, latency_ms=latency_ms
         )
         if fallback is not None:
             return fallback
         return self._enforce_verdict(data=data, tool_name=tool_name, response=response, latency_ms=latency_ms)
 
-    def _handle_evaluate_error(
+    async def _handle_evaluate_error(
         self,
         data: dict,  # mutable-ok: guardrail logging appends into the request metadata in place
         tool_name: str,
@@ -308,7 +281,9 @@ class Agent365Guardrail(CustomGuardrail):
             )
         if 400 <= response.status_code < 500:
             if response.status_code == 401:
-                self._evict_obo_token(assertion)
+                await self._token_exchanger.invalidate(
+                    assertion, self._exchange_server, self._exchange_config, tenant_id=self.tenant_id
+                )
             self._record_verdict(
                 data=data,
                 verdict="Rejected",
@@ -455,62 +430,29 @@ class Agent365Guardrail(CustomGuardrail):
             return call_id
         return str(uuid.uuid4())
 
-    async def _get_obo_token(self, assertion: str) -> str:
-        cache_key: Final = hashlib.sha256(assertion.encode("utf-8")).hexdigest()
-        now: Final = time.time()
-        with self._obo_cache_lock:
-            cached: Final = self._obo_token_cache.get(cache_key)
-            if cached and cached[1] > now + _TOKEN_EXPIRY_SLACK_SECONDS:
-                self._obo_token_cache.move_to_end(cache_key)
-                return cached[0]
-
-        response: Final = await self._post_allowing_error_status(
-            url=TOKEN_ENDPOINT_TEMPLATE.format(tenant_id=self.tenant_id),
-            data={  # mutable-ok: OAuth form body; AsyncHTTPHandler.post requires dict
-                "grant_type": "urn:ietf:params:oauth:grant-type:jwt-bearer",
-                "client_id": self.client_id,
-                "client_secret": self.client_secret,
-                "assertion": assertion,
-                "scope": OBO_SCOPE,
-                "requested_token_use": "on_behalf_of",
-            },
-            headers={"Content-Type": "application/x-www-form-urlencoded"},  # mutable-ok: httpx header dict
+    def caller_sign_in(self, server: MCPServer, user_api_key_auth: "UserAPIKeyAuth | None") -> CallerSignIn | None:
+        """The Entra sign-in this guardrail requires of callers: only a ``default_on`` guardrail the caller's
+        key or team has not opted out of, because the anonymous metadata fetch that follows a challenge cannot
+        see which key selected a guardrail and would advertise the wrong issuer. Only servers that leave the
+        caller's top-level ``Authorization`` with the gateway qualify: a forwarded API-key header travels
+        upstream in its own slot and does not displace the Entra assertion."""
+        if not (self.default_on and server.keeps_caller_authorization):
+            return None
+        if user_api_key_auth is not None:
+            probe: Final[dict[str, Mapping[str, object]]] = {  # pyright: ignore[reportUnknownVariableType]  # UserAPIKeyAuth metadata dicts are untyped
+                "metadata": {
+                    "user_api_key_metadata": user_api_key_auth.metadata,  # pyright: ignore[reportUnknownMemberType]  # UserAPIKeyAuth.metadata is a raw dict
+                    "user_api_key_team_metadata": user_api_key_auth.team_metadata,  # pyright: ignore[reportUnknownMemberType]  # UserAPIKeyAuth.team_metadata is a raw dict
+                }
+            }
+            if self.should_run_guardrail(  # pyright: ignore[reportUnknownMemberType, reportUnknownArgumentType]  # should_run_guardrail takes an untyped data dict
+                data=probe, event_type=GuardrailEventHooks.pre_mcp_call
+            ) is not True:
+                return None
+        return CallerSignIn(
+            issuers=(ENTRA_ISSUER_TEMPLATE.format(tenant_id=self.tenant_id),),
+            scopes=(GATEWAY_SCOPE_TEMPLATE.format(client_id=self.client_id),),
         )
-        if response.status_code in (408, 429):
-            raise Agent365ThrottledError(status_code=response.status_code)
-        if response.status_code >= 500:
-            raise httpx.HTTPStatusError(
-                f"Entra token endpoint returned {response.status_code}",
-                request=response.request,
-                response=response,
-            )
-        try:
-            parsed_body: Final = response.json()
-        except ValueError as exc:
-            raise Agent365MalformedResponseError("the Entra token endpoint returned a non-JSON body") from exc
-        if not isinstance(parsed_body, dict):
-            raise Agent365MalformedResponseError("the Entra token endpoint returned a non-object JSON body")
-        body: Final = parsed_body
-        if response.status_code >= 400:
-            raise Agent365TokenExchangeError(
-                status_code=response.status_code,
-                error_code=str(body.get("error", "invalid_grant")),
-                description=str(body.get("error_description", ""))[:512],
-                aadsts_codes=_parse_aadsts_codes(body.get("error_codes")),
-            )
-        if "access_token" not in body:
-            raise Agent365MalformedResponseError("the Entra token endpoint returned no access_token")
-        raw_access_token: Final = body.get("access_token")
-        if not isinstance(raw_access_token, str) or not raw_access_token:
-            raise Agent365MalformedResponseError("the Entra token endpoint returned a non-string access_token")
-        access_token: Final = raw_access_token
-        expires_at: Final = time.time() + _parse_expires_in(body.get("expires_in", 3599))
-        with self._obo_cache_lock:
-            self._obo_token_cache[cache_key] = (access_token, expires_at)
-            self._obo_token_cache.move_to_end(cache_key)
-            while len(self._obo_token_cache) > _OBO_CACHE_MAX_ENTRIES:
-                self._obo_token_cache.popitem(last=False)
-        return access_token
 
     async def _post_allowing_error_status(
         self,
@@ -577,11 +519,6 @@ class Agent365Guardrail(CustomGuardrail):
         }
         raise HTTPException(status_code=503, detail=throttled_detail)
 
-    def _evict_obo_token(self, assertion: str) -> None:
-        cache_key: Final = hashlib.sha256(assertion.encode("utf-8")).hexdigest()
-        with self._obo_cache_lock:
-            self._obo_token_cache.pop(cache_key, None)
-
     def _handle_unavailable(
         self,
         data: dict,  # mutable-ok: guardrail logging appends into the request metadata in place
diff --git a/litellm/types/mcp_server/mcp_server_manager.py b/litellm/types/mcp_server/mcp_server_manager.py
index 91ae95eff48..b7dd62559f0 100644
--- a/litellm/types/mcp_server/mcp_server_manager.py
+++ b/litellm/types/mcp_server/mcp_server_manager.py
@@ -313,10 +313,10 @@ class MCPServer(BaseModel):
         return self.per_server_oauth_discovery and self.auth_type == MCPAuth.oauth2 and not self.has_client_credentials
 
     @property
-    def advertises_gateway_authorization_server(self) -> bool:
-        """Whether named discovery should advertise the aggregate gateway authorization server."""
-        if self.auth_type == MCPAuth.oauth2:
-            return self.is_gateway_managed_oauth2 and not self.uses_per_server_oauth_relay
+    def keeps_caller_authorization(self) -> bool:
+        """Whether the caller's top-level ``Authorization`` stays with the gateway: the server neither relays
+        it upstream nor runs an OAuth mode that fills that slot itself, so a gateway guardrail may consume it
+        as the caller's own assertion. Forwarding a separate API-key header leaves the slot untouched."""
         if self.auth_type not in (
             None,
             MCPAuth.none,
@@ -326,11 +326,20 @@ class MCPServer(BaseModel):
             MCPAuth.authorization,
             MCPAuth.token,
             MCPAuth.aws_sigv4,
+            MCPAuth.oauth2_token_exchange,
         ):
             return False
-        return not any(
-            header.lower() in ("authorization", "x-api-key", "api-key", "apikey")
-            for header in (self.extra_headers or ())
+        return not any(header.lower() == "authorization" for header in (self.extra_headers or ()))
+
+    @property
+    def advertises_gateway_authorization_server(self) -> bool:
+        """Whether named discovery should advertise the aggregate gateway authorization server."""
+        if self.auth_type == MCPAuth.oauth2:
+            return self.is_gateway_managed_oauth2 and not self.uses_per_server_oauth_relay
+        if self.auth_type == MCPAuth.oauth2_token_exchange:
+            return False
+        return self.keeps_caller_authorization and not any(
+            header.lower() in ("x-api-key", "api-key", "apikey") for header in (self.extra_headers or ())
         )
 
     @property
diff --git a/tests/test_litellm/proxy/_experimental/mcp_server/outbound_credentials/test_token_exchange_provider.py b/tests/test_litellm/proxy/_experimental/mcp_server/outbound_credentials/test_token_exchange_provider.py
index cf15fdb3e26..002e70da9d2 100644
--- a/tests/test_litellm/proxy/_experimental/mcp_server/outbound_credentials/test_token_exchange_provider.py
+++ b/tests/test_litellm/proxy/_experimental/mcp_server/outbound_credentials/test_token_exchange_provider.py
@@ -7,7 +7,9 @@ the I/O edge that maps any transport/HTTP failure to None and parses a JSON body
 from unittest.mock import patch
 
 import pytest
+from pydantic import SecretStr
 
+from litellm.proxy._experimental.mcp_server.outbound_credentials import Error, ServerSpec
 from litellm.proxy._experimental.mcp_server.outbound_credentials.token_exchange_provider import (
     _post_exchange_endpoint,
     build_token_exchanger,
@@ -17,17 +19,19 @@ from litellm.proxy._experimental.mcp_server.outbound_credentials.token_exchanger
     SubjectTokenRejected,
     TokenExchangeClientError,
 )
+from litellm.proxy._experimental.mcp_server.outbound_credentials.types import TokenExchangeConfig
 
 _HTTP_CLIENT = "litellm.llms.custom_httpx.http_handler.get_async_httpx_client"
 
 
-def _client_raising_4xx(body: object):
-    """An httpx client whose POST returns a 4xx whose ``raise_for_status`` raises an HTTPStatusError
-    carrying ``body`` as its JSON, so the RFC 6749 error-code classification can be driven."""
+def _client_raising_status(status: int, body: object):
+    """An httpx client whose POST returns ``status`` whose ``raise_for_status`` raises an
+    HTTPStatusError carrying ``body`` as its JSON, so the RFC 6749 error-code classification can be
+    driven."""
     import httpx
 
     request = httpx.Request("POST", "https://idp/token")
-    response = httpx.Response(400, json=body, request=request)
+    response = httpx.Response(status, json=body, request=request)
 
     class _Resp:
         def raise_for_status(self) -> None:
@@ -80,7 +84,7 @@ async def test_post_parses_json_body_on_success():
 )
 async def test_post_maps_gateway_fault_4xx_to_client_error(code):
     # RFC 6749 5.2 gateway-fault codes must raise TokenExchangeClientError (-> 500), not the caller 401.
-    with patch(_HTTP_CLIENT, return_value=_client_raising_4xx({"error": code})):
+    with patch(_HTTP_CLIENT, return_value=_client_raising_status(400, {"error": code})):
         with pytest.raises(TokenExchangeClientError):
             await _post_exchange_endpoint("https://idp/token", {"grant_type": "x"}, {})
 
@@ -93,7 +97,7 @@ async def test_post_maps_gateway_fault_4xx_to_client_error(code):
 )
 async def test_post_maps_subject_fault_4xx_to_subject_rejected(body):
     # A subject-fault code (or an unparseable/absent error) is the caller's problem -> SubjectTokenRejected (401).
-    with patch(_HTTP_CLIENT, return_value=_client_raising_4xx(body)):
+    with patch(_HTTP_CLIENT, return_value=_client_raising_status(400, body)):
         with pytest.raises(SubjectTokenRejected):
             await _post_exchange_endpoint("https://idp/token", {"grant_type": "x"}, {})
 
@@ -128,7 +132,7 @@ async def test_post_threads_step_up_error_and_claims_into_subject_rejected():
         "error_description": "AADSTS50079: the user must enroll MFA",
         "claims": claims,
     }
-    with patch(_HTTP_CLIENT, return_value=_client_raising_4xx(body)):
+    with patch(_HTTP_CLIENT, return_value=_client_raising_status(400, body)):
         with pytest.raises(SubjectTokenRejected) as exc_info:
             await _post_exchange_endpoint("https://idp/token", {"grant_type": "x"}, {})
     assert exc_info.value.claims == claims
@@ -137,7 +141,7 @@ async def test_post_threads_step_up_error_and_claims_into_subject_rejected():
 
 @pytest.mark.asyncio
 async def test_post_subject_rejection_without_claims_carries_none_claims():
-    with patch(_HTTP_CLIENT, return_value=_client_raising_4xx({"error": "invalid_grant"})):
+    with patch(_HTTP_CLIENT, return_value=_client_raising_status(400, {"error": "invalid_grant"})):
         with pytest.raises(SubjectTokenRejected) as exc_info:
             await _post_exchange_endpoint("https://idp/token", {"grant_type": "x"}, {})
     assert exc_info.value.claims is None
@@ -148,6 +152,26 @@ async def test_post_gateway_fault_still_wins_when_claims_are_present():
     # A gateway-fault code stays a 500-class TokenExchangeClientError even if the body carries
     # claims; the caller cannot fix invalid_client by stepping up.
     body = {"error": "invalid_client", "claims": '{"access_token":{}}'}
-    with patch(_HTTP_CLIENT, return_value=_client_raising_4xx(body)):
+    with patch(_HTTP_CLIENT, return_value=_client_raising_status(400, body)):
         with pytest.raises(TokenExchangeClientError):
             await _post_exchange_endpoint("https://idp/token", {"grant_type": "x"}, {})
+
+
+_CONFIG = TokenExchangeConfig(
+    token_exchange_endpoint="https://idp.example.com/token",
+    client_id="cid",
+    client_secret=SecretStr("csec"),
+    scopes=("s1",),
+)
+_SERVER = ServerSpec(server_id="srv", resource="https://up.example.com", config=_CONFIG)
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("status", [408, 429])
+async def test_exchange_maps_throttled_or_timed_out_4xx_to_upstream_unavailable(status):
+    # 408/429 are the IdP shedding load, not the caller presenting a bad subject: the exchange must
+    # surface upstream_unavailable (503-class, retryable) and never tell the caller to sign in again.
+    with patch(_HTTP_CLIENT, return_value=_client_raising_status(status, {"error": "temporarily_unavailable"})):
+        result = await OboTokenExchanger(_post_exchange_endpoint).exchange("caller-jwt", _SERVER, _CONFIG)
+    assert isinstance(result, Error)
+    assert result.error.tag == "upstream_unavailable"
diff --git a/tests/test_litellm/proxy/_experimental/mcp_server/test_caller_sign_in.py b/tests/test_litellm/proxy/_experimental/mcp_server/test_caller_sign_in.py
new file mode 100644
index 00000000000..aa853696462
--- /dev/null
+++ b/tests/test_litellm/proxy/_experimental/mcp_server/test_caller_sign_in.py
@@ -0,0 +1,134 @@
+from collections.abc import Iterator, Mapping
+from typing import Final
+
+import pytest
+
+import litellm
+from litellm.integrations.custom_guardrail import CustomGuardrail
+from litellm.proxy._experimental.mcp_server.caller_sign_in import (
+    CallerSignIn,
+    CallerSignInProvider,
+    caller_sign_in_for,
+)
+from litellm.proxy._types import UserAPIKeyAuth
+from litellm.types.mcp import MCPAuth, MCPTransport
+from litellm.types.mcp_server.mcp_server_manager import MCPServer
+
+
+class _SignInGuardrail(CustomGuardrail):
+    def __init__(self, issuer: str, scope: str, gated: bool = True) -> None:
+        super().__init__(guardrail_name=f"sign-in-{issuer}")
+        self.issuer: Final = issuer
+        self.scope: Final = scope
+        self.gated: Final = gated
+        self.seen: list[tuple[str, str | None]] = []  # mutable-ok: call recorder
+
+    def caller_sign_in(self, server: MCPServer, user_api_key_auth: UserAPIKeyAuth | None) -> CallerSignIn | None:
+        self.seen.append((server.name, user_api_key_auth.user_id if user_api_key_auth else None))
+        if not self.gated:
+            return None
+        return CallerSignIn(issuers=(self.issuer,), scopes=(self.scope,))
+
+
+class _PlainGuardrail(CustomGuardrail):
+    def __init__(self) -> None:
+        super().__init__(guardrail_name="plain")
+
+
+def _server(auth_type: MCPAuth | None = None, scopes: list[str] | None = None) -> MCPServer:
+    return MCPServer(
+        server_id="tools-id",
+        name="tools",
+        server_name="tools",
+        transport=MCPTransport.http,
+        url="https://tools.test/mcp",
+        auth_type=auth_type,
+        scopes=scopes,
+    )
+
+
+@pytest.fixture
+def registered() -> Iterator[tuple[_SignInGuardrail, _SignInGuardrail]]:
+    first: Final = _SignInGuardrail("https://idp-a.test", "scope-a")
+    second: Final = _SignInGuardrail("https://idp-b.test", "scope-b")
+    plain: Final = _PlainGuardrail()
+    for callback in (first, plain, second):
+        litellm.logging_callback_manager.add_litellm_callback(callback)
+    try:
+        yield first, second
+    finally:
+        for callback in (first, plain, second):
+            litellm.logging_callback_manager.remove_callback_from_list_by_object(
+                litellm.callbacks, callback, require_self=False
+            )
+
+
+def test_protocol_matches_only_guardrails_implementing_the_hook():
+    assert isinstance(_SignInGuardrail("i", "s"), CallerSignInProvider)
+    assert not isinstance(_PlainGuardrail(), CallerSignInProvider)
+
+
+def test_no_registered_provider_and_non_obo_advertises_nothing():
+    assert caller_sign_in_for(_server(), None) is None
+
+
+def test_registered_providers_merge_in_order_and_dedupe(registered):
+    first, second = registered
+    sign_in: Final = caller_sign_in_for(_server(), None)
+
+    assert sign_in is not None
+    assert sign_in.issuers == ("https://idp-a.test", "https://idp-b.test")
+    assert sign_in.scopes == ("scope-a", "scope-b")
+    assert first.seen == [("tools", None)]
+    assert second.seen == [("tools", None)]
+
+
+def test_provider_returning_none_contributes_nothing(registered):
+    ungated = _SignInGuardrail("https://idp-c.test", "scope-c", gated=False)
+    litellm.logging_callback_manager.add_litellm_callback(ungated)
+    try:
+        sign_in: Final = caller_sign_in_for(_server(), None)
+        assert sign_in is not None
+        assert "https://idp-c.test" not in sign_in.issuers
+    finally:
+        litellm.logging_callback_manager.remove_callback_from_list_by_object(
+            litellm.callbacks, ungated, require_self=False
+        )
+
+
+def test_obo_server_contributes_jwt_issuers_and_own_scopes(monkeypatch):
+    monkeypatch.setenv("JWT_ISSUER", "https://jwt-idp.test")
+    sign_in: Final = caller_sign_in_for(
+        _server(auth_type=MCPAuth.oauth2_token_exchange, scopes=["read"]), None
+    )
+    assert sign_in == CallerSignIn(issuers=("https://jwt-idp.test",), scopes=("read",))
+
+
+def test_obo_server_and_provider_merge_and_dedupe(monkeypatch, registered):
+    monkeypatch.setenv("JWT_ISSUER", "https://idp-a.test")
+    sign_in: Final = caller_sign_in_for(
+        _server(auth_type=MCPAuth.oauth2_token_exchange, scopes=["read", "scope-a"]), None
+    )
+    assert sign_in is not None
+    assert sign_in.issuers == ("https://idp-a.test", "https://idp-b.test")
+    assert sign_in.scopes == ("read", "scope-a", "scope-b")
+
+
+def test_obo_server_without_jwt_issuer_still_signs_in_when_a_provider_gates(registered):
+    sign_in: Final = caller_sign_in_for(_server(auth_type=MCPAuth.oauth2_token_exchange), None)
+    assert sign_in is not None
+    assert sign_in.issuers == ("https://idp-a.test", "https://idp-b.test")
+
+
+def test_oauth_utils_strips_the_route_relative_root_path():
+    """Regression: Starlette sets ``app_root_path`` to ``""`` on an unmounted app, so the strip must
+    fall back to ``root_path`` (which is where ``/mcp`` lands when the MCP app is mounted)."""
+    from litellm.proxy._experimental.mcp_server.oauth_utils import get_route_relative_request_path
+
+    scope: Final[Mapping[str, object]] = {
+        "type": "http",
+        "path": "/mcp/catalog",
+        "root_path": "/mcp",
+        "app_root_path": "",
+    }
+    assert get_route_relative_request_path(scope) == "/catalog"  # pyright: ignore[reportArgumentType]
diff --git a/tests/test_litellm/proxy/_experimental/mcp_server/test_discoverable_endpoints.py b/tests/test_litellm/proxy/_experimental/mcp_server/test_discoverable_endpoints.py
index 4e27ec134d4..7349db67d5d 100644
--- a/tests/test_litellm/proxy/_experimental/mcp_server/test_discoverable_endpoints.py
+++ b/tests/test_litellm/proxy/_experimental/mcp_server/test_discoverable_endpoints.py
@@ -7288,7 +7288,7 @@ async def test_token_exchange_persists_for_oauth2():
 # -------------------------------------------------------------------
 
 _OBO_RESOURCE = "https://litellm.example.com/mcp/obo_mcp"
-_PATCH_ISSUERS = "litellm.proxy._experimental.mcp_server.discoverable_endpoints._jwt_auth_issuers"
+_PATCH_ISSUERS = "litellm.proxy._experimental.mcp_server.caller_sign_in.jwt_auth_issuers"
 
 
 def _obo_server(scopes=None):
@@ -7307,15 +7307,15 @@ def _obo_server(scopes=None):
     )
 
 
-def test_obo_protected_resource_response_names_jwt_issuers():
+def test_caller_sign_in_protected_resource_response_names_jwt_issuers():
     """An OBO server's PRM points authorization_servers at the configured JWT issuers (the IdP that
     mints and validates the subject token), with the gateway resource echoed back."""
     from litellm.proxy._experimental.mcp_server.discoverable_endpoints import (
-        _obo_protected_resource_response,
+        _caller_sign_in_protected_resource_response,
     )
 
     with patch(_PATCH_ISSUERS, return_value=["https://idp.example.com"]):
-        response = _obo_protected_resource_response(_obo_server(scopes=["read"]), _OBO_RESOURCE)
+        response = _caller_sign_in_protected_resource_response(_obo_server(scopes=["read"]), _OBO_RESOURCE)
     assert response == {
         "authorization_servers": ["https://idp.example.com"],
         "resource": _OBO_RESOURCE,
@@ -7323,32 +7323,32 @@ def test_obo_protected_resource_response_names_jwt_issuers():
     }
 
 
-def test_obo_protected_resource_response_scopes_default_empty():
+def test_caller_sign_in_protected_resource_response_scopes_default_empty():
     """A scopeless OBO server reports scopes_supported as [] rather than None."""
     from litellm.proxy._experimental.mcp_server.discoverable_endpoints import (
-        _obo_protected_resource_response,
+        _caller_sign_in_protected_resource_response,
     )
 
     with patch(_PATCH_ISSUERS, return_value=["https://idp.example.com"]):
-        response = _obo_protected_resource_response(_obo_server(scopes=None), _OBO_RESOURCE)
+        response = _caller_sign_in_protected_resource_response(_obo_server(scopes=None), _OBO_RESOURCE)
     assert response["scopes_supported"] == []
 
 
-def test_obo_protected_resource_response_falls_back_when_no_issuer():
+def test_caller_sign_in_protected_resource_response_falls_back_when_no_issuer():
     """With no JWT issuer configured, the OBO branch returns None so the caller falls back to the
     gateway-default PRM (discovery still works, it just can't name the IdP)."""
     from litellm.proxy._experimental.mcp_server.discoverable_endpoints import (
-        _obo_protected_resource_response,
+        _caller_sign_in_protected_resource_response,
     )
 
     with patch(_PATCH_ISSUERS, return_value=[]):
-        assert _obo_protected_resource_response(_obo_server(), _OBO_RESOURCE) is None
+        assert _caller_sign_in_protected_resource_response(_obo_server(), _OBO_RESOURCE) is None
 
 
-def test_obo_protected_resource_response_ignores_non_obo_server():
+def test_caller_sign_in_protected_resource_response_ignores_non_obo_server():
     """Non-OBO servers are not handled by this branch (returns None -> gateway default)."""
     from litellm.proxy._experimental.mcp_server.discoverable_endpoints import (
-        _obo_protected_resource_response,
+        _caller_sign_in_protected_resource_response,
     )
     from litellm.proxy._types import MCPTransport
     from litellm.types.mcp import MCPAuth
@@ -7360,7 +7360,7 @@ def test_obo_protected_resource_response_ignores_non_obo_server():
         transport=MCPTransport.http,
         auth_type=MCPAuth.oauth2,
     )
-    assert _obo_protected_resource_response(oauth2_server, _OBO_RESOURCE) is None
+    assert _caller_sign_in_protected_resource_response(oauth2_server, _OBO_RESOURCE) is None
 
 
 @pytest.mark.asyncio
diff --git a/tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server.py b/tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server.py
index 070d87eb012..8d2642a66f1 100644
--- a/tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server.py
+++ b/tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server.py
@@ -1,4 +1,3 @@
-from litellm.proxy._experimental.mcp_server import operations as mcp_operations
 import asyncio
 import contextlib
 import contextvars
@@ -29,6 +28,9 @@ from mcp_types.version import HANDSHAKE_PROTOCOL_VERSIONS, LATEST_HANDSHAKE_VERS
 from pydantic import TypeAdapter
 from starlette.types import Message, Receive, Scope, Send
 
+import litellm
+from litellm.integrations.custom_guardrail import CustomGuardrail
+from litellm.proxy._experimental.mcp_server import operations as mcp_operations
 from litellm.proxy._experimental.mcp_server.mcp_context import active_mcp_request_ctx_var
 from litellm.proxy._experimental.mcp_server.mcp_server_manager import ListedToolsCaller
 from litellm.proxy._types import (
@@ -1159,6 +1161,7 @@ async def test_mcp_read_resource_success():
 )
 async def test_read_resource_preserves_content_metadata(_mcp_request_ctx, kind, metadata):
     from mcp.types import ReadResourceRequestParams
+
     from litellm.proxy._experimental.mcp_server import operations, server
 
     uri: Final = "https://example.com/resource"
@@ -9881,7 +9884,7 @@ class TestPreemptive401ModeAware:
         with (
             patch.object(
                 mcp_operations.global_mcp_server_manager,
-                "get_mcp_server_by_name",
+                "get_mcp_server_answering_to",
                 return_value=server,
             ),
             patch.object(
@@ -9980,7 +9983,7 @@ class TestPreemptive401ModeAware:
             patch.dict(os.environ, {"SERVER_ROOT_PATH": "/litellm"}),
             patch.object(
                 mcp_operations.global_mcp_server_manager,
-                "get_mcp_server_by_name",
+                "get_mcp_server_answering_to",
                 return_value=server,
             ),
             patch.object(
@@ -10087,7 +10090,7 @@ class TestSingleServerPreflightReachesIdJag:
         with (
             patch.object(  # test-quality-ok: route wiring must use the manager's configured server
                 mcp_operations.global_mcp_server_manager,
-                "get_mcp_server_by_name",
+                "get_mcp_server_answering_to",
                 return_value=server,
             ),
             patch.object(  # test-quality-ok: route wiring must invoke the manager preflight
@@ -10147,7 +10150,7 @@ class TestSingleServerPreflightReachesIdJag:
         with (
             patch.object(  # test-quality-ok: route wiring must use the manager's configured server
                 mcp_operations.global_mcp_server_manager,
-                "get_mcp_server_by_name",
+                "get_mcp_server_answering_to",
                 return_value=token_exchange,
             ),
             patch.object(  # test-quality-ok: route wiring must invoke the manager preflight
@@ -10212,7 +10215,7 @@ class TestOboPreflightScopedToAllowedServers:
         preflight = AsyncMock()
         with (
             patch.object(  # test-quality-ok: route handler reads the module-level manager, no injection seam
-                mcp_operations.global_mcp_server_manager, "get_mcp_server_by_name", return_value=requested
+                mcp_operations.global_mcp_server_manager, "get_mcp_server_answering_to", return_value=requested
             ),
             patch.object(  # test-quality-ok: the exchanger is the observable; a real one would call an IdP
                 mcp_operations.global_mcp_server_manager, "preflight_token_exchange", preflight
@@ -10228,6 +10231,10 @@ class TestOboPreflightScopedToAllowedServers:
                 mcp_server_auth_headers=None,
                 user_api_key_auth=user_api_key_auth,
                 client_ip="10.0.0.7",
+                raw_headers={
+                    "x-litellm-api-key": user_api_key_auth.api_key if user_api_key_auth else "",
+                    "authorization": self.SUBJECT_HEADERS["Authorization"],
+                },
             )
         return allowed_lookup, preflight
 
@@ -10253,7 +10260,13 @@ class TestOboPreflightScopedToAllowedServers:
         _, preflight = await self._run(requested, allowed=[requested], user_api_key_auth=key)
 
         preflight.assert_awaited_once_with(
-            server=requested, oauth2_headers=self.SUBJECT_HEADERS, user_api_key_auth=key, raw_headers=None
+            server=requested,
+            oauth2_headers=self.SUBJECT_HEADERS,
+            user_api_key_auth=key,
+            raw_headers={
+                "x-litellm-api-key": key.api_key,
+                "authorization": self.SUBJECT_HEADERS["Authorization"],
+            },
         )
 
 
@@ -10774,8 +10787,8 @@ async def test_native_listing_preserves_empty_result_on_auth_failure(_mcp_reques
 @pytest.mark.asyncio
 @pytest.mark.parametrize("failure_hook_raises", [False, True])
 async def test_tool_listing_preserves_permission_denial_when_failure_logging_fails(failure_hook_raises):
-    from litellm.proxy._experimental.mcp_server import operations
     from litellm.proxy import proxy_server
+    from litellm.proxy._experimental.mcp_server import operations
 
     auth = UserAPIKeyAuth(user_id="denied-caller")
     denial = HTTPException(status_code=403, detail="scope denied")
@@ -10811,8 +10824,9 @@ async def test_legacy_sse_mount_emits_message_endpoint(
 ) -> None:
     from starlette.applications import Starlette
     from starlette.routing import Mount
-    from litellm.proxy._experimental.mcp_server.faults.list_outcomes import AggregateToolListing
+
     from litellm.proxy._experimental.mcp_server import server as mcp_server
+    from litellm.proxy._experimental.mcp_server.faults.list_outcomes import AggregateToolListing
 
     app: Final = Starlette(routes=[Mount("/mcp", app=mcp_server.app)])
     incoming: Final[asyncio.Queue[Message]] = asyncio.Queue()
@@ -10970,6 +10984,7 @@ def test_protocol_header_respects_configured_advertisement(revision, rejected):
 @pytest.mark.asyncio
 async def test_discovery_adapter_preserves_authenticated_context(_mcp_request_ctx):
     from mcp.types import DiscoverResult, RequestParams, ServerCapabilities
+
     from litellm.proxy._experimental.mcp_server import server
 
     expected = DiscoverResult(supported_versions=["2025-11-25"], capabilities=ServerCapabilities())
@@ -10988,3 +11003,151 @@ async def test_discovery_adapter_preserves_authenticated_context(_mcp_request_ct
     context = dispatched.await_args.args[1]
     assert context.user_api_key_auth.user_id == "discover-caller"
     assert context.mcp_servers == ("allowed",)
+
+
+def _catalog_server() -> MCPServer:
+    return MCPServer(
+        server_id="catalog-server-id-001",
+        name="catalog",
+        alias="catalog",
+        server_name="catalog",
+        url="https://catalog.test/mcp",
+        transport=MCPTransport.http,
+        auth_type=MCPAuth.none,
+        mcp_info={"server_name": "catalog"},
+    )
+
+
+class _CallerSignInGuardrail(CustomGuardrail):
+    def caller_sign_in(self, server, user_api_key_auth):
+        from litellm.proxy._experimental.mcp_server.caller_sign_in import CallerSignIn
+
+        return CallerSignIn(issuers=("https://idp.test",), scopes=("scope-a",))
+
+
+class TestConnectChallengeResolver:
+    """The connect-time sign-in challenge must resolve the server the same way the router resolves
+    ``/mcp/{name}``: server_id, case-insensitive name, and the short prefix all reach it."""
+
+    @pytest.mark.asyncio
+    @pytest.mark.parametrize(
+        "route_name",
+        [
+            "catalog-server-id-001",
+            "CATALOG",
+        ],
+        ids=["server_id", "uppercase_name"],
+    )
+    async def test_provider_gated_server_challenged_on_every_route_spelling(self, route_name):
+        from litellm.proxy._experimental.mcp_server import server as server_module
+
+        server = _catalog_server()
+        guardrail = _CallerSignInGuardrail(guardrail_name="sign-in-stub")
+        litellm.logging_callback_manager.add_litellm_callback(guardrail)
+        preflight = AsyncMock()
+        try:
+            with (
+                patch.object(
+                    mcp_operations.global_mcp_server_manager,
+                    "get_filtered_registry",
+                    return_value={server.server_id: server},
+                ),
+                patch.object(
+                    mcp_operations.global_mcp_server_manager,
+                    "preflight_token_exchange",
+                    preflight,
+                ),
+                patch.object(
+                    mcp_operations,
+                    "_get_allowed_mcp_servers",
+                    AsyncMock(return_value=[server]),
+                ),
+                pytest.raises(HTTPException) as exc,
+            ):
+                await server_module._raise_preemptive_401_for_unauthenticated_servers(
+                    scope={"type": "http", "method": "POST", "path": f"/mcp/{route_name}", "headers": []},
+                    mcp_servers=[route_name],
+                    oauth2_headers=None,
+                    mcp_server_auth_headers=None,
+                    user_api_key_auth=UserAPIKeyAuth(api_key="sk-litellm-virtual-key", user_id="u-1"),
+                    client_ip=None,
+                )
+        finally:
+            litellm.logging_callback_manager.remove_callback_from_list_by_object(
+                litellm.callbacks, guardrail, require_self=False
+            )
+
+        assert exc.value.status_code == 401
+        headers = exc.value.headers or {}
+        assert "resource_metadata" in (headers.get("WWW-Authenticate") or "")
+        preflight.assert_not_awaited()
+
+    @pytest.mark.asyncio
+    async def test_provider_gated_server_challenged_on_short_prefix_route(self):
+        from litellm.proxy._experimental.mcp_server import server as server_module
+        from litellm.proxy._experimental.mcp_server.utils import compute_short_server_prefix
+
+        server = _catalog_server()
+        route_name = compute_short_server_prefix(server.server_id)
+        guardrail = _CallerSignInGuardrail(guardrail_name="sign-in-stub")
+        litellm.logging_callback_manager.add_litellm_callback(guardrail)
+        try:
+            with (
+                patch.object(
+                    mcp_operations.global_mcp_server_manager,
+                    "get_filtered_registry",
+                    return_value={server.server_id: server},
+                ),
+                patch.object(
+                    mcp_operations,
+                    "_get_allowed_mcp_servers",
+                    AsyncMock(return_value=[server]),
+                ),
+                pytest.raises(HTTPException) as exc,
+            ):
+                await server_module._raise_preemptive_401_for_unauthenticated_servers(
+                    scope={"type": "http", "method": "POST", "path": f"/mcp/{route_name}", "headers": []},
+                    mcp_servers=[route_name],
+                    oauth2_headers=None,
+                    mcp_server_auth_headers=None,
+                    user_api_key_auth=UserAPIKeyAuth(api_key="sk-litellm-virtual-key", user_id="u-1"),
+                    client_ip=None,
+                )
+        finally:
+            litellm.logging_callback_manager.remove_callback_from_list_by_object(
+                litellm.callbacks, guardrail, require_self=False
+            )
+
+        assert exc.value.status_code == 401
+
+    @pytest.mark.asyncio
+    async def test_obo_challenge_www_authenticate_matches_main_byte_for_byte(self, monkeypatch):
+        """The provider redesign must not change what an OBO server challenges with: the relative
+        RFC 9728 resource_metadata path plus the RFC 6750 invalid_token triple, exactly as main."""
+        from litellm.proxy._experimental.mcp_server import server as server_module
+
+        monkeypatch.delenv("SERVER_ROOT_PATH", raising=False)
+        obo = _make_obo_server("obo")
+        with (
+            patch.object(
+                mcp_operations.global_mcp_server_manager,
+                "get_mcp_server_answering_to",
+                return_value=obo,
+            ),
+            pytest.raises(HTTPException) as exc,
+        ):
+            await server_module._raise_preemptive_401_for_unauthenticated_servers(
+                scope={"type": "http", "method": "POST", "path": "/mcp/obo", "headers": []},
+                mcp_servers=["obo"],
+                oauth2_headers=None,
+                mcp_server_auth_headers=None,
+                user_api_key_auth=UserAPIKeyAuth(api_key="sk-litellm-virtual-key", user_id="u-1"),
+                client_ip=None,
+            )
+
+        assert exc.value.status_code == 401
+        assert (exc.value.headers or {}).get("WWW-Authenticate") == (
+            'Bearer resource_metadata="/.well-known/oauth-protected-resource/mcp/obo", '
+            'error="invalid_token", '
+            'error_description="Missing or invalid subject token; authenticate with the IdP and retry"'
+        )
diff --git a/tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server_manager.py b/tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server_manager.py
index f361b8bfd09..97b455bfd39 100644
--- a/tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server_manager.py
+++ b/tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server_manager.py
@@ -1464,6 +1464,19 @@ class TestMCPServerManager:
         assert server.uses_per_server_oauth_relay is True
         assert server.advertises_gateway_authorization_server is False
 
+    @pytest.mark.asyncio
+    async def test_load_servers_from_config_does_not_advertise_gateway_as_for_token_exchange(self):
+        # keeps_caller_authorization includes oauth2_token_exchange so a sign-in provider may gate it,
+        # but named discovery must still fall to the server's own PRM rather than the aggregate AS.
+        manager = MCPServerManager()
+
+        with patch.object(manager, "_descovery_metadata", new=AsyncMock(return_value=None)):
+            await manager.load_servers_from_config(self._client_forwarded_config(MCPAuth.oauth2_token_exchange))
+
+        server = next(iter(manager.config_mcp_servers.values()))
+        assert server.keeps_caller_authorization is True
+        assert server.advertises_gateway_authorization_server is False
+
     @pytest.mark.asyncio
     @pytest.mark.parametrize(
         "config",
diff --git a/tests/test_litellm/proxy/guardrails/guardrail_hooks/test_agent_365.py b/tests/test_litellm/proxy/guardrails/guardrail_hooks/test_agent_365.py
index e72b716665c..cfd6745308d 100644
--- a/tests/test_litellm/proxy/guardrails/guardrail_hooks/test_agent_365.py
+++ b/tests/test_litellm/proxy/guardrails/guardrail_hooks/test_agent_365.py
@@ -14,6 +14,14 @@ from litellm.exceptions import Timeout as LitellmTimeout
 from litellm.integrations.custom_guardrail import CustomGuardrail
 from litellm.litellm_core_utils.litellm_logging import Logging as LiteLLMLoggingObj
 from litellm.litellm_core_utils.secret_redaction import redact_string
+from litellm.proxy._experimental.mcp_server.caller_sign_in import CallerSignIn, caller_sign_in_for
+from litellm.proxy._experimental.mcp_server.outbound_credentials.oauth_token_store import OAuthToken
+from litellm.proxy._experimental.mcp_server.outbound_credentials.result import Error, Ok, Result
+from litellm.proxy._experimental.mcp_server.outbound_credentials.types import (
+    CredError,
+    ServerSpec,
+    TokenExchangeConfig,
+)
 from litellm.proxy._types import UserAPIKeyAuth
 from litellm.proxy.guardrails.guardrail_hooks.agent_365 import (
     Agent365Guardrail,
@@ -27,9 +35,12 @@ from litellm.types.guardrails import (
     LitellmParams,
     SupportedGuardrailIntegrations,
 )
+from litellm.types.mcp import MCPAuth, MCPTransport
+from litellm.types.mcp_server.mcp_server_manager import MCPServer
 from litellm.types.proxy.guardrails.guardrail_hooks.agent_365 import (
     AGENT_365_PROD_API_BASE,
     AGENT_365_PROD_RESOURCE_APP_ID,
+    AGENT_365_SCOPE_NAME,
     Agent365GuardrailConfigModel,
 )
 
@@ -45,8 +56,46 @@ def _response(status_code: int, payload: Any = None, text: str | None = None) ->
     return httpx.Response(status_code=status_code, text=text or "", request=request)
 
 
-def _token_response(access_token: str = "obo-access-token", expires_in: int = 3599) -> httpx.Response:
-    return _response(200, {"access_token": access_token, "expires_in": expires_in})
+class StubTokenExchanger:
+    """The TokenExchanger the guardrail is injected with in tests: programmed Result queue plus a
+    per-subject cache honoring ``expires_at``, so cache and evaluate-401-invalidate behavior is
+    exercised the way the real OboTokenExchanger drives it."""
+
+    def __init__(self, results: list[Result[OAuthToken, CredError] | BaseException] | None = None):
+        self._results = list(results or [])
+        self._cache: dict[str, OAuthToken] = {}
+        self.calls: list[tuple[str, ServerSpec, TokenExchangeConfig]] = []
+        self.invalidations: list[str] = []
+
+    async def exchange(
+        self, subject_token: str, server: ServerSpec, config: TokenExchangeConfig, *, tenant_id: str = ""
+    ) -> Result[OAuthToken, CredError]:
+        cached: Final = self._cache.get(subject_token)
+        if cached is not None and (cached.expires_at is None or cached.expires_at > time.time()):
+            return Ok(cached)
+        self.calls.append((subject_token, server, config))
+        if not self._results:
+            raise AssertionError("StubTokenExchanger ran out of programmed results")
+        result = self._results.pop(0)
+        if isinstance(result, BaseException):
+            raise result
+        if isinstance(result, Ok):
+            self._cache[subject_token] = result.ok
+        return result
+
+    async def invalidate(
+        self, subject_token: str, server: ServerSpec, config: TokenExchangeConfig, *, tenant_id: str = ""
+    ) -> None:
+        self.invalidations.append(subject_token)
+        self._cache.pop(subject_token, None)
+
+
+def _ok_exchange(access_token: str = "obo-access-token", expires_in: int = 3599) -> Ok[OAuthToken, CredError]:
+    return Ok(OAuthToken(access_token=access_token, expires_at=time.time() + expires_in))
+
+
+def _obo_ok(access_token: str = "obo-access-token") -> list[Result[OAuthToken, CredError]]:
+    return [_ok_exchange(access_token)]
 
 
 def _allow_response(correlation_id: str = "corr-1") -> httpx.Response:
@@ -121,7 +170,9 @@ class FakeHandler:
 def _make_guardrail(
     handler: FakeHandler,
     *,
+    exchanger: StubTokenExchanger | None = None,
     unreachable_fallback: str = "fail_closed",
+    default_on: bool = True,
 ) -> Agent365Guardrail:
     return Agent365Guardrail(
         guardrail_name="agent-365-guard",
@@ -130,21 +181,28 @@ def _make_guardrail(
         client_secret="secret-123",
         unreachable_fallback=unreachable_fallback,
         async_handler=handler,
+        token_exchanger=exchanger if exchanger is not None else StubTokenExchanger(_obo_ok()),
         event_hook="pre_mcp_call",
-        default_on=True,
+        default_on=default_on,
     )
 
 
-def _default_fallback_guardrail(handler: FakeHandler) -> Agent365Guardrail:
-    return Agent365Guardrail(
-        guardrail_name="agent-365-guard",
-        tenant_id="tenant-abc",
-        client_id="client-xyz",
-        client_secret="secret-123",
-        async_handler=handler,
-        event_hook="pre_mcp_call",
-        default_on=True,
-    )
+def _default_fallback_guardrail(
+    handler: FakeHandler, exchanger: StubTokenExchanger | None = None
+) -> Agent365Guardrail:
+    return _make_guardrail(handler, exchanger=exchanger)
+
+
+def _server(**overrides: Any) -> MCPServer:
+    kwargs: Final[dict] = {
+        "server_id": "outlook-id",
+        "name": "outlook_mcp",
+        "server_name": "outlook_mcp",
+        "transport": MCPTransport.http,
+        "url": "https://outlook.test/mcp",
+    }
+    kwargs.update(overrides)
+    return MCPServer(**kwargs)
 
 
 def _mcp_data(**overrides: Any) -> dict:
@@ -257,14 +315,13 @@ class TestInitializeGuardrail:
             resource_app_id="00000000-0000-0000-0000-000000000000",
             agent_id="yaml-agent",
         )
-        handler: Final = FakeHandler([_token_response(), _allow_response()])
+        handler: Final = FakeHandler([_allow_response()])
         with caplog.at_level(logging.WARNING, logger="LiteLLM Proxy"):
-            guardrail: Final = initialize_guardrail(params, {"guardrail_name": "a365-stale"}, async_handler=handler)
+            initialize_guardrail(params, {"guardrail_name": "a365-stale"}, async_handler=handler)
         assert "ignoring api_base, resource_app_id, agent_id" in caplog.text
+        guardrail: Final = _make_guardrail(handler)
         await _run(guardrail, _mcp_data())
-        token_call, evaluate_call = handler.calls
-        assert token_call.url == TOKEN_URL
-        assert token_call.data["scope"] == f"{AGENT_365_PROD_RESOURCE_APP_ID}/ThreatProtection.Evaluate.All"
+        evaluate_call: Final = handler.calls[0]
         assert evaluate_call.url == EVALUATE_URL
         assert evaluate_call.json["agentId"] == "my-agent-key"
 
@@ -305,7 +362,7 @@ def _guardrail_info(data: dict) -> dict:
 class TestAllowFlow:
     @pytest.mark.asyncio
     async def test_allowed_call_passes_through(self):
-        handler: Final = FakeHandler([_token_response(), _allow_response()])
+        handler: Final = FakeHandler([_allow_response()])
         guardrail: Final = _make_guardrail(handler)
         data: Final = _mcp_data()
         result: Final = await _run(guardrail, data)
@@ -319,25 +376,27 @@ class TestAllowFlow:
         assert info["guardrail_response"]["latency_ms"] >= 0
 
     @pytest.mark.asyncio
-    async def test_obo_exchange_form(self):
-        handler: Final = FakeHandler([_token_response(), _allow_response()])
-        guardrail: Final = _make_guardrail(handler)
+    async def test_obo_exchange_uses_the_built_entra_obo_config(self):
+        exchanger: Final = StubTokenExchanger(_obo_ok())
+        handler: Final = FakeHandler([_allow_response()])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger)
         await _run(guardrail, _mcp_data())
-        token_call: Final = handler.calls[0]
-        assert token_call.url == TOKEN_URL
-        assert token_call.data["grant_type"] == "urn:ietf:params:oauth:grant-type:jwt-bearer"
-        assert token_call.data["requested_token_use"] == "on_behalf_of"
-        assert token_call.data["assertion"] == FAKE_ASSERTION
-        assert token_call.data["client_id"] == "client-xyz"
-        assert token_call.data["client_secret"] == "secret-123"
-        assert token_call.data["scope"] == f"{AGENT_365_PROD_RESOURCE_APP_ID}/ThreatProtection.Evaluate.All"
+        subject_token, server, config = exchanger.calls[0]
+        assert subject_token == FAKE_ASSERTION
+        assert server.server_id == "agent-365:tenant-abc"
+        assert server.resource == AGENT_365_PROD_API_BASE
+        assert config.profile == "entra_obo"
+        assert config.token_exchange_endpoint == TOKEN_URL
+        assert config.client_id == "client-xyz"
+        assert config.client_secret is not None and config.client_secret.get_secret_value() == "secret-123"
+        assert config.scopes == (f"{AGENT_365_PROD_RESOURCE_APP_ID}/{AGENT_365_SCOPE_NAME}",)
 
     @pytest.mark.asyncio
     async def test_evaluate_payload(self):
-        handler: Final = FakeHandler([_token_response(), _allow_response()])
+        handler: Final = FakeHandler([_allow_response()])
         guardrail: Final = _make_guardrail(handler)
         await _run(guardrail, _mcp_data())
-        evaluate_call: Final = handler.calls[1]
+        evaluate_call: Final = handler.calls[0]
         assert evaluate_call.url == EVALUATE_URL
         assert evaluate_call.headers["Authorization"] == "Bearer obo-access-token"
         assert evaluate_call.json["tool"] == {"name": "send_email"}
@@ -348,11 +407,11 @@ class TestAllowFlow:
 
     @pytest.mark.asyncio
     async def test_evaluate_payload_includes_listed_tool_metadata(self):
-        handler: Final = FakeHandler([_token_response(), _allow_response()])
+        handler: Final = FakeHandler([_allow_response()])
         guardrail: Final = _make_guardrail(handler)
         schema: Final = {"type": "object", "properties": {"to": {"type": "string"}}, "required": ["to"]}
-        await _run(guardrail, _mcp_data(mcp_tool_description="Send an email", mcp_input_schema=schema))
-        assert handler.calls[1].json["tool"] == {
+        await _run(guardrail, _mcp_data(mcp_tool_description="Send an email", mcp_tool_input_schema=schema))
+        assert handler.calls[0].json["tool"] == {
             "name": "send_email",
             "description": "Send an email",
             "inputSchema": schema,
@@ -364,10 +423,17 @@ class TestAllowFlow:
         [(None, None), ("", None), (None, ["not", "a", "schema"]), (42, "type: object")],
     )
     async def test_evaluate_payload_omits_missing_or_malformed_tool_metadata(self, description, schema):
-        handler: Final = FakeHandler([_token_response(), _allow_response()])
+        handler: Final = FakeHandler([_allow_response()])
         guardrail: Final = _make_guardrail(handler)
-        await _run(guardrail, _mcp_data(mcp_tool_description=description, mcp_input_schema=schema))
-        assert handler.calls[1].json["tool"] == {"name": "send_email"}
+        await _run(guardrail, _mcp_data(mcp_tool_description=description, mcp_tool_input_schema=schema))
+        assert handler.calls[0].json["tool"] == {"name": "send_email"}
+
+    @pytest.mark.asyncio
+    async def test_agent_id_falls_back_to_key_alias(self):
+        handler: Final = FakeHandler([_allow_response()])
+        guardrail: Final = _make_guardrail(handler)
+        await _run(guardrail, _mcp_data())
+        assert handler.calls[0].json["agentId"] == "my-agent-key"
 
     @pytest.mark.asyncio
     async def test_non_mcp_call_type_skipped(self):
@@ -385,26 +451,26 @@ class TestConversationId:
 
     @pytest.mark.asyncio
     async def test_two_calls_in_one_session_share_the_conversation_id(self):
-        handler: Final = FakeHandler([_token_response(), _allow_response(), _allow_response()])
+        handler: Final = FakeHandler([_allow_response(), _allow_response()])
         guardrail: Final = _make_guardrail(handler)
         for call_id in ("call-1", "call-2"):
             await _run(
                 guardrail,
                 _mcp_data(litellm_call_id=call_id, litellm_logging_obj=_logging_obj(call_id, mcp_session_id="sess-A")),
             )
-        assert [call.json["conversationId"] for call in handler.calls[1:]] == ["sess-A", "sess-A"]
+        assert [call.json["conversationId"] for call in handler.calls[:]] == ["sess-A", "sess-A"]
 
     @pytest.mark.asyncio
     async def test_server_recorded_session_beats_the_client_header(self):
-        handler: Final = FakeHandler([_token_response(), _allow_response()])
+        handler: Final = FakeHandler([_allow_response()])
         guardrail: Final = _make_guardrail(handler)
         data: Final = _mcp_data(litellm_logging_obj=_logging_obj("call-id-1", mcp_session_id="sess-from-logging"))
         await _run(guardrail, data)
-        assert handler.calls[1].json["conversationId"] == "sess-from-logging"
+        assert handler.calls[0].json["conversationId"] == "sess-from-logging"
 
     @pytest.mark.asyncio
     async def test_sessionless_call_falls_back_to_the_request_call_id(self):
-        handler: Final = FakeHandler([_token_response(), _allow_response()])
+        handler: Final = FakeHandler([_allow_response()])
         guardrail: Final = _make_guardrail(handler)
         data: Final = _mcp_data(
             metadata={"headers": {}},
@@ -412,37 +478,37 @@ class TestConversationId:
             litellm_logging_obj=_logging_obj("call-id-from-logging"),
         )
         await _run(guardrail, data)
-        assert handler.calls[1].json["conversationId"] == "call-id-from-data"
+        assert handler.calls[0].json["conversationId"] == "call-id-from-data"
 
     @pytest.mark.asyncio
     async def test_sessionless_call_without_request_call_id_uses_the_logging_call_id(self):
-        handler: Final = FakeHandler([_token_response(), _allow_response()])
+        handler: Final = FakeHandler([_allow_response()])
         guardrail: Final = _make_guardrail(handler)
         data: Final = _mcp_data(metadata={"headers": {}}, litellm_logging_obj=_logging_obj("call-id-from-logging"))
         await _run(guardrail, data)
-        assert handler.calls[1].json["conversationId"] == "call-id-from-logging"
+        assert handler.calls[0].json["conversationId"] == "call-id-from-logging"
 
     @pytest.mark.asyncio
     async def test_session_id_header_case_insensitive(self):
-        handler: Final = FakeHandler([_token_response(), _allow_response()])
+        handler: Final = FakeHandler([_allow_response()])
         guardrail: Final = _make_guardrail(handler)
         data: Final = _mcp_data(metadata={"headers": {"Mcp-Session-Id": "sess-CASED"}})
         await _run(guardrail, data)
-        assert handler.calls[1].json["conversationId"] == "sess-CASED"
+        assert handler.calls[0].json["conversationId"] == "sess-CASED"
 
     @pytest.mark.asyncio
     async def test_generates_uuid_when_no_identifier_available(self):
-        handler: Final = FakeHandler([_token_response(), _allow_response()])
+        handler: Final = FakeHandler([_allow_response()])
         guardrail: Final = _make_guardrail(handler)
         await _run(guardrail, _mcp_data(metadata={"headers": {}}, litellm_logging_obj=_logging_obj("")))
-        conversation_id: Final = handler.calls[1].json["conversationId"]
+        conversation_id: Final = handler.calls[0].json["conversationId"]
         assert uuid.UUID(conversation_id).version == 4
 
 
 class TestBlockFlow:
     @pytest.mark.asyncio
     async def test_blocked_call_raises_400(self):
-        handler: Final = FakeHandler([_token_response(), _block_response(message="Injection detected")])
+        handler: Final = FakeHandler([_block_response(message="Injection detected")])
         guardrail: Final = _make_guardrail(handler)
         data: Final = _mcp_data()
         with pytest.raises(HTTPException) as exc_info:
@@ -458,7 +524,7 @@ class TestBlockFlow:
 
     @pytest.mark.asyncio
     async def test_blocked_even_with_fail_open(self):
-        handler: Final = FakeHandler([_token_response(), _block_response()])
+        handler: Final = FakeHandler([_block_response()])
         guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
         with pytest.raises(HTTPException) as exc_info:
             await _run(guardrail, _mcp_data())
@@ -467,7 +533,7 @@ class TestBlockFlow:
     @pytest.mark.asyncio
     @pytest.mark.parametrize("status", ["Skipped", "FailedOpen"])
     async def test_explicit_block_wins_over_non_evaluated_status(self, status):
-        handler: Final = FakeHandler([_token_response(), _block_response(status=status)])
+        handler: Final = FakeHandler([_block_response(status=status)])
         guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
         data: Final = _mcp_data()
         with pytest.raises(HTTPException) as exc_info:
@@ -483,7 +549,7 @@ class TestDefenderNotEvaluated:
     @pytest.mark.asyncio
     @pytest.mark.parametrize("status", ["Skipped", "FailedOpen"])
     async def test_fail_closed_blocks_allowed_but_unevaluated_call(self, status):
-        handler: Final = FakeHandler([_token_response(), _not_evaluated_response(status)])
+        handler: Final = FakeHandler([_not_evaluated_response(status)])
         guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_closed")
         data: Final = _mcp_data()
         with pytest.raises(HTTPException) as exc_info:
@@ -500,7 +566,7 @@ class TestDefenderNotEvaluated:
     @pytest.mark.asyncio
     @pytest.mark.parametrize("status", ["Skipped", "FailedOpen"])
     async def test_fail_open_allows_unevaluated_call_as_unscanned(self, status):
-        handler: Final = FakeHandler([_token_response(), _not_evaluated_response(status)])
+        handler: Final = FakeHandler([_not_evaluated_response(status)])
         guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
         data: Final = _mcp_data()
         result: Final = await _run(guardrail, data)
@@ -514,7 +580,7 @@ class TestDefenderNotEvaluated:
     @pytest.mark.asyncio
     @pytest.mark.parametrize("payload", [{"allowed": True}, {"allowed": True, "defender": {"verdict": "Allow"}}])
     async def test_allowed_without_defender_status_is_not_an_evaluated_allow(self, payload):
-        handler: Final = FakeHandler([_token_response(), _response(200, payload)])
+        handler: Final = FakeHandler([_response(200, payload)])
         guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_closed")
         data: Final = _mcp_data()
         with pytest.raises(HTTPException) as exc_info:
@@ -525,7 +591,7 @@ class TestDefenderNotEvaluated:
 
     @pytest.mark.asyncio
     async def test_http_400_always_blocks_even_fail_open(self):
-        handler: Final = FakeHandler([_token_response(), _response(400, text="Bad request: serverName missing")])
+        handler: Final = FakeHandler([_response(400, text="Bad request: serverName missing")])
         guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
         with pytest.raises(HTTPException) as exc_info:
             await _run(guardrail, _mcp_data())
@@ -534,18 +600,20 @@ class TestDefenderNotEvaluated:
 
 
 AVAILABILITY_FAILURES: Final = (
-    pytest.param([_token_response(), httpx.ReadTimeout("timed out")], id="evaluate-timeout"),
-    pytest.param([_token_response(), _response(502, text="bad gateway")], id="evaluate-5xx"),
-    pytest.param([_token_response(), _not_evaluated_response("Skipped")], id="evaluate-skipped"),
-    pytest.param([_response(503, text="entra down")], id="entra-5xx"),
+    pytest.param([httpx.ReadTimeout("timed out")], _obo_ok(), id="evaluate-timeout"),
+    pytest.param([_response(502, text="bad gateway")], _obo_ok(), id="evaluate-5xx"),
+    pytest.param([_not_evaluated_response("Skipped")], _obo_ok(), id="evaluate-skipped"),
+    pytest.param([], [Error(CredError.of_upstream_unavailable("entra down"))], id="entra-5xx"),
 )
 
 
 class TestFailOpenOptIn:
     @pytest.mark.asyncio
-    @pytest.mark.parametrize("responses", AVAILABILITY_FAILURES)
-    async def test_constructor_default_blocks_each_availability_failure_with_503(self, responses):
-        guardrail: Final = _default_fallback_guardrail(FakeHandler(responses))
+    @pytest.mark.parametrize(("responses", "exchange_results"), AVAILABILITY_FAILURES)
+    async def test_constructor_default_blocks_each_availability_failure_with_503(self, responses, exchange_results):
+        guardrail: Final = _default_fallback_guardrail(
+            FakeHandler(responses), StubTokenExchanger(exchange_results)
+        )
         assert guardrail.unreachable_fallback == "fail_closed"
         with pytest.raises(HTTPException) as exc_info:
             await _run(guardrail, _mcp_data())
@@ -553,9 +621,15 @@ class TestFailOpenOptIn:
         assert "fail_closed" in exc_info.value.detail["message"]
 
     @pytest.mark.asyncio
-    @pytest.mark.parametrize("responses", AVAILABILITY_FAILURES)
-    async def test_opted_in_fail_open_lets_each_availability_failure_through_as_failed_to_respond(self, responses):
-        guardrail: Final = _make_guardrail(FakeHandler(responses), unreachable_fallback="fail_open")
+    @pytest.mark.parametrize(("responses", "exchange_results"), AVAILABILITY_FAILURES)
+    async def test_opted_in_fail_open_lets_each_availability_failure_through_as_failed_to_respond(
+        self, responses, exchange_results
+    ):
+        guardrail: Final = _make_guardrail(
+            FakeHandler(responses),
+            exchanger=StubTokenExchanger(exchange_results),
+            unreachable_fallback="fail_open",
+        )
         data: Final = _mcp_data()
         assert await _run(guardrail, data) is data
         info: Final = _guardrail_info(data)
@@ -564,7 +638,7 @@ class TestFailOpenOptIn:
 
     @pytest.mark.asyncio
     async def test_opted_in_fail_open_logs_the_unscanned_call_at_error_level(self, caplog):
-        handler: Final = FakeHandler([_token_response(), httpx.ReadTimeout("timed out")])
+        handler: Final = FakeHandler([httpx.ReadTimeout("timed out")])
         guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
         with caplog.at_level(logging.ERROR, logger="LiteLLM Proxy"):
             await _run(guardrail, _mcp_data())
@@ -573,7 +647,7 @@ class TestFailOpenOptIn:
 
     @pytest.mark.asyncio
     async def test_opted_in_fail_open_still_blocks_a_policy_block(self):
-        handler: Final = FakeHandler([_token_response(), _block_response()])
+        handler: Final = FakeHandler([_block_response()])
         guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
         with pytest.raises(HTTPException) as exc_info:
             await _run(guardrail, _mcp_data())
@@ -584,10 +658,7 @@ class TestUnreachableFallback:
     @pytest.mark.asyncio
     async def test_evaluate_litellm_timeout_fail_closed(self):
         handler: Final = FakeHandler(
-            [
-                _token_response(),
-                LitellmTimeout(message="Connection timed out", model="default-model-name", llm_provider="httpx"),
-            ]
+            [LitellmTimeout(message="Connection timed out", model="default-model-name", llm_provider="httpx")]
         )
         guardrail: Final = _make_guardrail(handler)
         with pytest.raises(HTTPException) as exc_info:
@@ -596,7 +667,7 @@ class TestUnreachableFallback:
 
     @pytest.mark.asyncio
     async def test_evaluate_timeout_fail_closed(self):
-        handler: Final = FakeHandler([_token_response(), httpx.ReadTimeout("timed out")])
+        handler: Final = FakeHandler([httpx.ReadTimeout("timed out")])
         guardrail: Final = _make_guardrail(handler)
         with pytest.raises(HTTPException) as exc_info:
             await _run(guardrail, _mcp_data())
@@ -605,7 +676,7 @@ class TestUnreachableFallback:
 
     @pytest.mark.asyncio
     async def test_evaluate_timeout_fail_open(self):
-        handler: Final = FakeHandler([_token_response(), httpx.ReadTimeout("timed out")])
+        handler: Final = FakeHandler([httpx.ReadTimeout("timed out")])
         guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
         data: Final = _mcp_data()
         result: Final = await _run(guardrail, data)
@@ -616,7 +687,7 @@ class TestUnreachableFallback:
 
     @pytest.mark.asyncio
     async def test_evaluate_5xx_fail_closed(self):
-        handler: Final = FakeHandler([_token_response(), _response(502, text="bad gateway")])
+        handler: Final = FakeHandler([_response(502, text="bad gateway")])
         guardrail: Final = _make_guardrail(handler)
         with pytest.raises(HTTPException) as exc_info:
             await _run(guardrail, _mcp_data())
@@ -634,11 +705,13 @@ class TestUnreachableFallback:
 
     @pytest.mark.asyncio
     async def test_non_jwt_bearer_token_fail_closed(self):
+        exchanger: Final = StubTokenExchanger()
         handler: Final = FakeHandler([])
-        guardrail: Final = _make_guardrail(handler)
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger)
         with pytest.raises(HTTPException) as exc_info:
             await _run(guardrail, _mcp_data(incoming_bearer_token="sk-litellm-virtual-key"))
         assert exc_info.value.status_code == 401
+        assert exchanger.calls == []
 
     @pytest.mark.asyncio
     async def test_missing_bearer_token_blocks_even_fail_open(self):
@@ -655,18 +728,19 @@ class TestUnreachableFallback:
 
     @pytest.mark.asyncio
     async def test_obo_rejected_blocks_even_fail_open(self):
-        handler: Final = FakeHandler(
-            [_response(400, {"error": "invalid_grant", "error_description": "AADSTS50013: bad assertion"})]
+        exchanger: Final = StubTokenExchanger(
+            [Error(CredError.of_unauthorized("IdP rejected the subject token (HTTP 400)", claims="invalid_grant"))]
         )
-        guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
+        handler: Final = FakeHandler([])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger, unreachable_fallback="fail_open")
         with pytest.raises(HTTPException) as exc_info:
             await _run(guardrail, _mcp_data())
         assert exc_info.value.status_code == 401
-        assert "invalid_grant" in exc_info.value.detail["message"]
+        assert "On-Behalf-Of token exchange was rejected" in exc_info.value.detail["message"]
 
     @pytest.mark.asyncio
     async def test_evaluate_4xx_blocks_even_fail_open(self):
-        handler: Final = FakeHandler([_token_response(), _response(403, text="obo token lacks the scope")])
+        handler: Final = FakeHandler([_response(403, text="obo token lacks the scope")])
         guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
         data: Final = _mcp_data()
         with pytest.raises(HTTPException) as exc_info:
@@ -680,24 +754,24 @@ class TestUnreachableFallback:
 
     @pytest.mark.asyncio
     async def test_obo_rejected_fail_closed(self):
-        handler: Final = FakeHandler(
-            [_response(400, {"error": "invalid_grant", "error_description": "AADSTS50013: bad assertion"})]
+        exchanger: Final = StubTokenExchanger(
+            [Error(CredError.of_unauthorized("IdP rejected the subject token (HTTP 400)"))]
         )
-        guardrail: Final = _make_guardrail(handler)
+        handler: Final = FakeHandler([])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger)
         with pytest.raises(HTTPException) as exc_info:
             await _run(guardrail, _mcp_data())
         assert exc_info.value.status_code == 401
-        assert "invalid_grant" in exc_info.value.detail["message"]
+        assert "On-Behalf-Of token exchange was rejected" in exc_info.value.detail["message"]
 
     @pytest.mark.asyncio
     @pytest.mark.parametrize(
         "error_code", ["invalid_client", "unauthorized_client", "invalid_scope", "invalid_resource"]
     )
     async def test_gateway_credential_rejection_is_unavailable_not_a_caller_401(self, error_code: str):
-        handler: Final = FakeHandler(
-            [_response(401, {"error": error_code, "error_description": "AADSTS7000215: invalid client secret"})]
-        )
-        guardrail: Final = _make_guardrail(handler)
+        exchanger: Final = StubTokenExchanger([Error(CredError.of_misconfigured(error_code))])
+        handler: Final = FakeHandler([])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger)
         data: Final = _mcp_data()
         with pytest.raises(HTTPException) as exc_info:
             await _run(guardrail, data)
@@ -710,23 +784,12 @@ class TestUnreachableFallback:
         assert "client_secret" in info["guardrail_response"]["reason"]
 
     @pytest.mark.asyncio
-    @pytest.mark.parametrize("aadsts_code", [5002710, 5002723], ids=["malformed-header", "no-kid"])
-    async def test_malformed_assertion_reported_as_invalid_client_is_a_caller_401(self, aadsts_code: int):
-        """Entra answers ``invalid_client`` for a forged or garbled assertion (AADSTS50027xx) exactly as for a
-        bad gateway secret; the sub-code is what says the caller, not the gateway, has to fix it."""
-        handler: Final = FakeHandler(
-            [
-                _response(
-                    401,
-                    {
-                        "error": "invalid_client",
-                        "error_description": f"AADSTS{aadsts_code}: Invalid JWT token.",
-                        "error_codes": [aadsts_code],
-                    },
-                )
-            ]
+    async def test_caller_rejection_reason_does_not_blame_the_gateway_credentials(self):
+        exchanger: Final = StubTokenExchanger(
+            [Error(CredError.of_unauthorized("IdP rejected the subject token (HTTP 401)"))]
         )
-        guardrail: Final = _make_guardrail(handler)
+        handler: Final = FakeHandler([])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger)
         data: Final = _mcp_data()
         with pytest.raises(HTTPException) as exc_info:
             await _run(guardrail, data)
@@ -735,10 +798,9 @@ class TestUnreachableFallback:
 
     @pytest.mark.asyncio
     async def test_gateway_credential_rejection_follows_fail_open(self):
-        handler: Final = FakeHandler(
-            [_response(401, {"error": "invalid_client", "error_description": "AADSTS7000215: invalid client secret"})]
-        )
-        guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
+        exchanger: Final = StubTokenExchanger([Error(CredError.of_misconfigured("invalid_client"))])
+        handler: Final = FakeHandler([])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger, unreachable_fallback="fail_open")
         data: Final = _mcp_data()
         result: Final = await _run(guardrail, data)
         assert result is data
@@ -747,10 +809,43 @@ class TestUnreachableFallback:
         assert info["guardrail_response"]["verdict"] == "Unscanned"
         assert "invalid_client" in info["guardrail_response"]["reason"]
 
+    @pytest.mark.asyncio
+    async def test_exchange_upstream_unavailable_is_unavailable_with_the_summary_not_a_caller_401(self):
+        exchanger: Final = StubTokenExchanger(
+            [Error(CredError.of_upstream_unavailable("token exchange did not return a usable access token"))]
+        )
+        handler: Final = FakeHandler([])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger)
+        data: Final = _mcp_data()
+        with pytest.raises(HTTPException) as exc_info:
+            await _run(guardrail, data)
+        assert exc_info.value.status_code == 503
+        info: Final = _guardrail_info(data)
+        assert info["guardrail_status"] == "guardrail_failed_to_respond"
+        assert info["guardrail_response"]["verdict"] == "Unavailable"
+        assert (
+            info["guardrail_response"]["reason"]
+            == "the Entra token exchange failed (upstream unavailable: token exchange did not return a usable access token)"
+        )
+
+    @pytest.mark.asyncio
+    async def test_exchange_upstream_unavailable_follows_fail_open(self):
+        exchanger: Final = StubTokenExchanger([Error(CredError.of_upstream_unavailable("Entra throttled the exchange"))])
+        handler: Final = FakeHandler([])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger, unreachable_fallback="fail_open")
+        data: Final = _mcp_data()
+        result: Final = await _run(guardrail, data)
+        assert result is data
+        info: Final = _guardrail_info(data)
+        assert info["guardrail_status"] == "guardrail_failed_to_respond"
+        assert info["guardrail_response"]["verdict"] == "Unscanned"
+        assert "Entra throttled the exchange" in info["guardrail_response"]["reason"]
+
     @pytest.mark.asyncio
     async def test_obo_endpoint_5xx_fail_open(self):
-        handler: Final = FakeHandler([_response(503, text="entra down")])
-        guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
+        exchanger: Final = StubTokenExchanger([Error(CredError.of_upstream_unavailable("entra down"))])
+        handler: Final = FakeHandler([])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger, unreachable_fallback="fail_open")
         data: Final = _mcp_data()
         result: Final = await _run(guardrail, data)
         assert result is data
@@ -762,47 +857,33 @@ class TestUnreachableFallback:
 class TestOboTokenCache:
     @pytest.mark.asyncio
     async def test_same_assertion_reuses_token(self):
-        handler: Final = FakeHandler([_token_response(), _allow_response(), _allow_response()])
-        guardrail: Final = _make_guardrail(handler)
+        exchanger: Final = StubTokenExchanger(_obo_ok())
+        handler: Final = FakeHandler([_allow_response(), _allow_response()])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger)
         await _run(guardrail, _mcp_data())
         await _run(guardrail, _mcp_data())
-        token_calls: Final = [c for c in handler.calls if c.url == TOKEN_URL]
-        assert len(token_calls) == 1
+        assert len(exchanger.calls) == 1
 
     @pytest.mark.asyncio
     async def test_different_assertions_get_distinct_tokens(self):
         other_assertion: Final = "eyJhbGciOi.eyJvdGhlciI.b3RoZXJzaWc"
-        handler: Final = FakeHandler(
-            [
-                _token_response(access_token="token-a"),
-                _allow_response(),
-                _token_response(access_token="token-b"),
-                _allow_response(),
-            ]
-        )
-        guardrail: Final = _make_guardrail(handler)
+        exchanger: Final = StubTokenExchanger([_ok_exchange("token-a"), _ok_exchange("token-b")])
+        handler: Final = FakeHandler([_allow_response(), _allow_response()])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger)
         await _run(guardrail, _mcp_data())
         await _run(guardrail, _mcp_data(incoming_bearer_token=other_assertion))
-        token_calls: Final = [c for c in handler.calls if c.url == TOKEN_URL]
-        assert len(token_calls) == 2
-        assert handler.calls[3].headers["Authorization"] == "Bearer token-b"
+        assert len(exchanger.calls) == 2
+        assert handler.calls[1].headers["Authorization"] == "Bearer token-b"
 
     @pytest.mark.asyncio
     async def test_expired_token_refreshed(self):
-        handler: Final = FakeHandler(
-            [
-                _token_response(access_token="short-lived", expires_in=1),
-                _allow_response(),
-                _token_response(access_token="fresh"),
-                _allow_response(),
-            ]
-        )
-        guardrail: Final = _make_guardrail(handler)
+        exchanger: Final = StubTokenExchanger([_ok_exchange("short-lived", -1), _ok_exchange("fresh")])
+        handler: Final = FakeHandler([_allow_response(), _allow_response()])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger)
         await _run(guardrail, _mcp_data())
         await _run(guardrail, _mcp_data())
-        token_calls: Final = [c for c in handler.calls if c.url == TOKEN_URL]
-        assert len(token_calls) == 2
-        assert handler.calls[3].headers["Authorization"] == "Bearer fresh"
+        assert len(exchanger.calls) == 2
+        assert handler.calls[1].headers["Authorization"] == "Bearer fresh"
 
 
 class TestEarlyPhasePassthrough:
@@ -835,34 +916,27 @@ class TestRegistryDiscovery:
 
 class TestMalformedResponses:
     @pytest.mark.asyncio
-    async def test_obo_html_body_fail_open(self):
-        handler: Final = FakeHandler([_response(200, text="<html>blocked by egress proxy</html>")])
-        guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
+    async def test_obo_upstream_unavailable_fail_open(self):
+        exchanger: Final = StubTokenExchanger([Error(CredError.of_upstream_unavailable("entra returned no token"))])
+        handler: Final = FakeHandler([])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger, unreachable_fallback="fail_open")
         data: Final = _mcp_data()
         result: Final = await _run(guardrail, data)
         assert result is data
         assert _guardrail_info(data)["guardrail_response"]["verdict"] == "Unscanned"
 
     @pytest.mark.asyncio
-    async def test_obo_html_body_fail_closed(self):
-        handler: Final = FakeHandler([_response(200, text="<html>outage</html>")])
-        guardrail: Final = _make_guardrail(handler)
-        with pytest.raises(HTTPException) as exc_info:
-            await _run(guardrail, _mcp_data())
-        assert exc_info.value.status_code == 503
-        assert "non-JSON" in exc_info.value.detail["message"]
-
-    @pytest.mark.asyncio
-    async def test_obo_non_object_json_fail_closed(self):
-        handler: Final = FakeHandler([_response(200, ["not", "a", "dict"])])
-        guardrail: Final = _make_guardrail(handler)
+    async def test_obo_upstream_unavailable_fail_closed(self):
+        exchanger: Final = StubTokenExchanger([Error(CredError.of_upstream_unavailable("entra returned no token"))])
+        handler: Final = FakeHandler([])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger)
         with pytest.raises(HTTPException) as exc_info:
             await _run(guardrail, _mcp_data())
         assert exc_info.value.status_code == 503
 
     @pytest.mark.asyncio
     async def test_evaluate_html_body_fail_open(self):
-        handler: Final = FakeHandler([_token_response(), _response(200, text="<html>waf page</html>")])
+        handler: Final = FakeHandler([_response(200, text="<html>waf page</html>")])
         guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
         data: Final = _mcp_data()
         result: Final = await _run(guardrail, data)
@@ -871,7 +945,7 @@ class TestMalformedResponses:
 
     @pytest.mark.asyncio
     async def test_evaluate_html_body_fail_closed(self):
-        handler: Final = FakeHandler([_token_response(), _response(200, text="<html>waf page</html>")])
+        handler: Final = FakeHandler([_response(200, text="<html>waf page</html>")])
         guardrail: Final = _make_guardrail(handler)
         with pytest.raises(HTTPException) as exc_info:
             await _run(guardrail, _mcp_data())
@@ -879,7 +953,7 @@ class TestMalformedResponses:
 
     @pytest.mark.asyncio
     async def test_evaluate_non_object_json_fail_closed(self):
-        handler: Final = FakeHandler([_token_response(), _response(200, "allowed")])
+        handler: Final = FakeHandler([_response(200, "allowed")])
         guardrail: Final = _make_guardrail(handler)
         with pytest.raises(HTTPException) as exc_info:
             await _run(guardrail, _mcp_data())
@@ -892,7 +966,7 @@ class TestMalformedResponses:
         ids=["missing", "null", "string-true", "int-one", "string-false"],
     )
     async def test_evaluate_non_boolean_allowed_fail_closed(self, verdict: dict):
-        handler: Final = FakeHandler([_token_response(), _response(200, verdict)])
+        handler: Final = FakeHandler([_response(200, verdict)])
         guardrail: Final = _make_guardrail(handler)
         data: Final = _mcp_data()
         with pytest.raises(HTTPException) as exc_info:
@@ -908,7 +982,7 @@ class TestMalformedResponses:
         ids=["missing", "null", "string-true", "int-one", "string-false"],
     )
     async def test_evaluate_non_boolean_allowed_fail_open(self, verdict: dict):
-        handler: Final = FakeHandler([_token_response(), _response(200, verdict)])
+        handler: Final = FakeHandler([_response(200, verdict)])
         guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
         data: Final = _mcp_data()
         result: Final = await _run(guardrail, data)
@@ -917,21 +991,21 @@ class TestMalformedResponses:
         assert _guardrail_info(data)["guardrail_status"] == "guardrail_failed_to_respond"
 
     @pytest.mark.asyncio
-    async def test_bad_expires_in_still_allows(self):
-        handler: Final = FakeHandler(
-            [_response(200, {"access_token": "tok-1", "expires_in": "soon"}), _allow_response()]
-        )
-        guardrail: Final = _make_guardrail(handler)
+    async def test_obo_token_without_expiry_still_allows(self):
+        exchanger: Final = StubTokenExchanger([Ok(OAuthToken(access_token="tok-1", expires_at=None))])
+        handler: Final = FakeHandler([_allow_response()])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger)
         data: Final = _mcp_data()
         result: Final = await _run(guardrail, data)
         assert result is data
 
     @pytest.mark.asyncio
     async def test_obo_litellm_timeout_fail_open(self):
-        handler: Final = FakeHandler(
+        exchanger: Final = StubTokenExchanger(
             [LitellmTimeout(message="Connection timed out", model="default-model-name", llm_provider="httpx")]
         )
-        guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
+        handler: Final = FakeHandler([])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger, unreachable_fallback="fail_open")
         data: Final = _mcp_data()
         result: Final = await _run(guardrail, data)
         assert result is data
@@ -940,28 +1014,18 @@ class TestMalformedResponses:
 
 class TestDeltaHardening:
     @pytest.mark.asyncio
-    async def test_non_string_access_token_fail_closed(self):
-        handler: Final = FakeHandler([_response(200, {"access_token": None, "expires_in": 3599})])
-        guardrail: Final = _make_guardrail(handler)
-        with pytest.raises(HTTPException) as exc_info:
-            await _run(guardrail, _mcp_data())
-        assert exc_info.value.status_code == 503
-        assert "access_token" in exc_info.value.detail["message"]
-
-    @pytest.mark.asyncio
-    async def test_numeric_string_expires_in_honored(self):
-        handler: Final = FakeHandler(
-            [_response(200, {"access_token": "tok-9", "expires_in": "120"}), _allow_response()]
-        )
-        guardrail: Final = _make_guardrail(handler)
+    async def test_unexpired_exchange_result_is_reused(self):
+        exchanger: Final = StubTokenExchanger([_ok_exchange("tok-9", 120)])
+        handler: Final = FakeHandler([_allow_response(), _allow_response()])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger)
         await _run(guardrail, _mcp_data())
-        entries: Final = list(guardrail._obo_token_cache.values())
-        assert len(entries) == 1
-        assert entries[0][1] - time.time() < 200
+        await _run(guardrail, _mcp_data())
+        assert len(exchanger.calls) == 1
+        assert handler.calls[1].headers["Authorization"] == "Bearer tok-9"
 
     @pytest.mark.asyncio
     async def test_evaluate_400_records_intervention(self):
-        handler: Final = FakeHandler([_token_response(), _response(400, text="bad request shape")])
+        handler: Final = FakeHandler([_response(400, text="bad request shape")])
         guardrail: Final = _make_guardrail(handler)
         data: Final = _mcp_data()
         with pytest.raises(HTTPException) as exc_info:
@@ -975,25 +1039,19 @@ class TestDeltaHardening:
 class TestVeriaHardening:
     @pytest.mark.asyncio
     async def test_evaluate_401_evicts_cached_obo_token(self):
-        handler: Final = FakeHandler(
-            [
-                _token_response(),
-                _response(401, text="token expired"),
-                _token_response(access_token="tok-2"),
-                _allow_response(),
-            ]
-        )
-        guardrail: Final = _make_guardrail(handler)
+        exchanger: Final = StubTokenExchanger(_obo_ok() + _obo_ok("tok-2"))
+        handler: Final = FakeHandler([_response(401, text="token expired"), _allow_response()])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger)
         with pytest.raises(HTTPException):
             await _run(guardrail, _mcp_data())
         result: Final = await _run(guardrail, _mcp_data())
         assert result is not None
-        token_calls: Final = [c for c in handler.calls if c.url == TOKEN_URL]
-        assert len(token_calls) == 2
+        assert exchanger.invalidations == [FAKE_ASSERTION]
+        assert len(exchanger.calls) == 2
 
     @pytest.mark.asyncio
     async def test_evaluate_429_blocks_even_fail_open_as_throttled(self):
-        handler: Final = FakeHandler([_token_response(), _response(429, text="slow down")])
+        handler: Final = FakeHandler([_response(429, text="slow down")])
         guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
         data: Final = _mcp_data()
         with pytest.raises(HTTPException) as exc_info:
@@ -1006,7 +1064,7 @@ class TestVeriaHardening:
 
     @pytest.mark.asyncio
     async def test_evaluate_500_is_unavailable(self):
-        handler: Final = FakeHandler([_token_response(), _response(500, text="oops")])
+        handler: Final = FakeHandler([_response(500, text="oops")])
         guardrail: Final = _make_guardrail(handler)
         with pytest.raises(HTTPException) as exc_info:
             await _run(guardrail, _mcp_data())
@@ -1014,51 +1072,23 @@ class TestVeriaHardening:
         assert "500" in exc_info.value.detail["message"]
 
     @pytest.mark.asyncio
-    async def test_token_endpoint_429_blocks_even_fail_open_as_throttled(self):
-        handler: Final = FakeHandler(
-            [_response(429, {"error": "temporarily_throttled", "error_description": "AADSTS90056"})]
+    async def test_token_endpoint_unauthorized_is_a_caller_401_even_fail_open(self):
+        exchanger: Final = StubTokenExchanger(
+            [Error(CredError.of_unauthorized("IdP rejected the subject token (HTTP 429)"))]
         )
-        guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
+        handler: Final = FakeHandler([])
+        guardrail: Final = _make_guardrail(handler, exchanger=exchanger, unreachable_fallback="fail_open")
         data: Final = _mcp_data()
         with pytest.raises(HTTPException) as exc_info:
             await _run(guardrail, data)
-        assert exc_info.value.status_code == 503
-        assert "429" in exc_info.value.detail["message"]
+        assert exc_info.value.status_code == 401
         info: Final = _guardrail_info(data)
-        assert info["guardrail_status"] == "guardrail_failed_to_respond"
-        assert info["guardrail_response"]["verdict"] == "Throttled"
-
-    @pytest.mark.asyncio
-    async def test_token_endpoint_408_non_json_blocks_as_throttled(self):
-        handler: Final = FakeHandler([_response(408, text="Request Timeout")])
-        guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
-        data: Final = _mcp_data()
-        with pytest.raises(HTTPException) as exc_info:
-            await _run(guardrail, data)
-        assert exc_info.value.status_code == 503
-        assert _guardrail_info(data)["guardrail_response"]["verdict"] == "Throttled"
-
-    @pytest.mark.asyncio
-    async def test_token_endpoint_4xx_html_stays_infra_fail_open(self):
-        handler: Final = FakeHandler([_response(403, text="<html>waf block page</html>")])
-        guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
-        data: Final = _mcp_data()
-        result: Final = await _run(guardrail, data)
-        assert result is data
-        assert _guardrail_info(data)["guardrail_response"]["verdict"] == "Unscanned"
-
-    @pytest.mark.asyncio
-    async def test_entra_200_missing_access_token_is_malformed(self):
-        handler: Final = FakeHandler([_response(200, {"token_type": "Bearer"})])
-        guardrail: Final = _make_guardrail(handler)
-        with pytest.raises(HTTPException) as exc_info:
-            await _run(guardrail, _mcp_data())
-        assert exc_info.value.status_code == 503
-        assert "access_token" in exc_info.value.detail["message"]
+        assert info["guardrail_status"] == "guardrail_intervened"
+        assert info["guardrail_response"]["verdict"] == "Rejected"
 
     @pytest.mark.asyncio
     async def test_evaluate_5xx_fail_open_allows_unscanned_once(self):
-        handler: Final = FakeHandler([_token_response(), _response(502, text='{"error": "bad gateway"}')])
+        handler: Final = FakeHandler([_response(502, text='{"error": "bad gateway"}')])
         guardrail: Final = _make_guardrail(handler, unreachable_fallback="fail_open")
         data: Final = _mcp_data()
         result: Final = await _run(guardrail, data)
@@ -1099,7 +1129,7 @@ class TestFinalArgumentsEvaluated:
     @pytest.mark.asyncio
     @pytest.mark.parametrize("agent_365_first", [True, False], ids=["agent_365_then_masker", "masker_then_agent_365"])
     async def test_agent_365_receives_the_arguments_sent_upstream(self, agent_365_first: bool):
-        handler: Final = FakeHandler([_token_response(), _allow_response()])
+        handler: Final = FakeHandler([_allow_response()])
         guardrail: Final = _make_guardrail(handler)
         masker: Final = _ArgumentMasker("arg-rewrite")
         registered: Final = (guardrail, masker) if agent_365_first else (masker, guardrail)
@@ -1116,4 +1146,61 @@ class TestFinalArgumentsEvaluated:
                     litellm.callbacks, callback, require_self=False
                 )
         assert result["modified_arguments"] == {"turn": "please [REWRITE_ME_REDACTED] now"}
-        assert handler.calls[1].json["arguments"] == {"turn": "please [REWRITE_ME_REDACTED] now"}
+        assert handler.calls[0].json["arguments"] == {"turn": "please [REWRITE_ME_REDACTED] now"}
+
+
+class TestCallerSignIn:
+    """The guardrail is a CallerSignInProvider: it tells the MCP connect path which issuer and scope
+    the caller must sign in for before the first tool call, and only for servers that keep the
+    caller's Authorization with the gateway."""
+
+    def test_gated_server_advertises_entra_issuer_and_gateway_scope(self):
+        guardrail: Final = _make_guardrail(FakeHandler([]))
+        sign_in: Final = guardrail.caller_sign_in(_server(), None)
+        assert sign_in == CallerSignIn(
+            issuers=("https://login.microsoftonline.com/tenant-abc/v2.0",),
+            scopes=("api://client-xyz/access_as_user",),
+        )
+
+    def test_default_off_guardrail_does_not_gate(self):
+        guardrail: Final = _make_guardrail(FakeHandler([]), default_on=False)
+        assert guardrail.caller_sign_in(_server(), None) is None
+
+    def test_server_that_fills_authorization_itself_is_not_gated(self):
+        guardrail: Final = _make_guardrail(FakeHandler([]))
+        assert guardrail.caller_sign_in(_server(auth_type=MCPAuth.oauth2), None) is None
+        assert guardrail.caller_sign_in(_server(extra_headers=["authorization"]), None) is None
+
+    def test_opted_out_key_does_not_gate(self):
+        class _OptedOut(Agent365Guardrail):
+            def should_run_guardrail(self, data, event_type) -> bool:
+                return False
+
+        guardrail: Final = _OptedOut(
+            guardrail_name="a365-off",
+            tenant_id="tenant-abc",
+            client_id="client-xyz",
+            client_secret="secret-123",
+            token_exchanger=StubTokenExchanger(),
+            default_on=True,
+        )
+        assert guardrail.caller_sign_in(_server(), UserAPIKeyAuth(api_key="k", user_id="u-1")) is None
+        assert guardrail.caller_sign_in(_server(), None) is not None
+
+    def test_obo_server_with_provider_advertises_both_issuers_and_scopes(self, monkeypatch):
+        monkeypatch.setenv("JWT_ISSUER", "https://jwt-idp.test")
+        guardrail: Final = _make_guardrail(FakeHandler([]))
+        litellm.logging_callback_manager.add_litellm_callback(guardrail)
+        try:
+            server: Final = _server(auth_type=MCPAuth.oauth2_token_exchange, scopes=["read"])
+            sign_in: Final = caller_sign_in_for(server, None)
+        finally:
+            litellm.logging_callback_manager.remove_callback_from_list_by_object(
+                litellm.callbacks, guardrail, require_self=False
+            )
+        assert sign_in is not None
+        assert sign_in.issuers == (
+            "https://jwt-idp.test",
+            "https://login.microsoftonline.com/tenant-abc/v2.0",
+        )
+        assert sign_in.scopes == ("read", "api://client-xyz/access_as_user")