Critical Token Refresh Bugs - Prevents Proactive Refresh

[Norcim133]

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

tldr: Circular logic in mcp/client/auth.py creates two critical issues that prevent proactive token refresh; this would cause frequent re-auth for all users but it creates a third, worse issue for Clients that use TokenStorage (especially noticeable in a multi-user setup)

Background

In mcp/client/auth.py there are only 3 places where OAuthContext.token_expiry_time is set:

1. During init it is set to None
2. During full re-auth after receiving a 401 (in call to _handle_token_response)
3. When a token is deemed invalid in is_token_valid (at start of async_auth_flow)

Issue 1 - Token validity check treats None as Valid

The ultimate problem is that when token_expiry_time is None, a token will pass validity checks, per the below.

def is_token_valid(self) -> bool:
 """Check if current token is valid."""
 return bool(
 self.current_tokens
 and self.current_tokens.access_token
 and (not self.token_expiry_time or time.time() <= self.token_expiry_time)
# ^^^^^^^^^^^^^^^^^^ None makes it perpetually valid so never needs pro-active refresh
 )

This is probably intended behavior to enable None to represent 'perpetual tokens'.

It sets up 2 fatal problems.

Issue 2 - Expired tokens get converted to 'perpetual tokens'

In update_token_expiry an expired token with a token.expires time of 0 is 'Falsey' which sets token_expiry_time to None.

def update_token_expiry(self, token: OAuthToken) -> None:
 """Update token expiry time."""
 if token.expires_in: # <============================ 0 evaluates to False
 self.token_expiry_time = time.time() + token.expires_in
 else:
 self.token_expiry_time = None # <============================ Makes it perpetually valid

Per Issue 1, when token_expiry_time is None the token is always valid (i.e. perpetual).

Undesirable Result - Only full re-auth can refresh a 'perpetual token' that goes stale

Because the token is now perpetual, it never gets proactively refreshed so it always requires a 401 and full re-auth to refresh.

Issue 3 - Clients with storage always start with token_expiry_time set to None

The OAuth init does not check token expiry:

async def _initialize(self) -> None:
 """Load stored tokens and client info."""
 self.context.current_tokens = await self.context.storage.get_tokens()
 self.context.client_info = await self.context.storage.get_client_info()
 self._initialized = True

This is often fine for Clients that persist.

For newly or frequently instantiated Clients that use Token Storage, token_expiry_time is None by default.

Due to Issue 1 that None value cause the token to become perpetual.

Due to Issue 2 the perpetual token is never checked for refresh.

As a result, tokens from storage (which are probably stale) are never checked prior to their first call. So they always trigger a 401 and revert to full re-auth.

Proposed Fixes

Token update should check if token has an expires_in attribute and for an explicit None. This preserves perpetual tokens while treating expired tokens appropriately.

 def update_token_expiry(self, token: OAuthToken) -> None:
 """Update token expiry time."""
 if hasattr(token, 'expires_in') and token.expires_in is not None: # <================ Fix
 self.token_expiry_time = time.time() + token.expires_in
 else:
 self.token_expiry_time = None

Also, the validity check should only work when self.token_expiry_time=None explicitly instead of when 'Falsey':

def is_token_valid(self) -> bool:
 """Check if current token is valid."""
 return bool(
 self.current_tokens
 and self.current_tokens.access_token
 and (self.token_expiry_time is None or time.time() <= self.token_expiry_time) # <============== Fix
 #^^^^^^^^^^^^^^^^^^^^^^^ None can remain signal for perpetual token
 )

Finally, OAuthContext._initialize should call update_token_expiry so stored tokens aren't treated as perpetual by default:

async _initialize(self):
 """Initialize and properly set token expiry from stored tokens."""
 self.context.current_tokens = await self.context.storage.get_tokens()
 self.context.client_info = await self.context.storage.get_client_info()
 
 # Fix: Update token expiry if tokens loaded from storage
 if self.context.current_tokens:
 self.context.update_token_expiry(self.context.current_tokens)
 self._initialized = True

Example Code

Python & MCP Python SDK

I'm on 1.12.4 but the code is still the same for 1.13.1

[Norcim133]

Another bug that ruins token refresh:

tldr: Any person using an MCP Client to connect to an MCP Proxy Server (e.g. that wraps Gmail APIs) can't refresh tokens because oauth_metadata is not set when reviving Client state from token_storage (so it constructs and calls the wrong /token endpoint).

The Setup - An MCP Client is connecting to an MCP Proxy Server (i.e. that wraps a 3rd party resource)

1. For MCP Proxy Servers, your well-known endpoint is your url (e.g. auth.palpable.io/.well-known...).
2. From that endpoint, you serve up OauthMetada for your 3rd party's endpoints (e.g. authorization_endpoint: "https://login.microsoft...)
3. This works on initial auth because async_auth_flow checks your endpoint and stores 3rd party endpoints in OauthMetadata

The Problem - When a Client is instantiated from Token Storage, OAuthMetadata is missing - so it tries to construct a /token url from server_url (will be wrong because it is a proxy server)

1. If a client is killed and re-instantiated using tokens, no OAuthMetadata is present (only filled in discovery)
2. The _refresh_token method tries to get the /token endpoint from stored OAuthMetadata (missing)
3. As a fallback, it manually adds /token to the auth_base_url which is derived from the server_url set on init
4. Problem: For an MCP Proxy Server, the auth_base_url will be wrong (e.g. auth.palpable.io/token instead of auth.microsoft...)

async def _refresh_token(self) -> httpx.Request:
 """Build token refresh request."""
 if not self.context.current_tokens or not self.context.current_tokens.refresh_token:
 raise OAuthTokenError("No refresh token available")

 if not self.context.client_info:
 raise OAuthTokenError("No client info available")

 if self.context.oauth_metadata and self.context.oauth_metadata.token_endpoint: <=====Fails here... not oauth_metadata
 token_url = str(self.context.oauth_metadata.token_endpoint)
 else:
 auth_base_url = self.context.get_authorization_base_url(self.context.server_url)
 token_url = urljoin(auth_base_url, "/token") <========================Falls back to MCP Proxy url which fails

5. Upon 404 from the/token endpoint all the tokens are cleared and auth is started from scratch

Potential Fixes -

1. Force discovery on _initialize

async def _initialize(self) -> None:
 self.context.current_tokens = await self.context.storage.get_tokens()
 self.context.client_info = await self.context.storage.get_client_info()
 # MISSING: self.context.update_token_expiry(self.context.current_tokens)
 # ALSO MISSING: <discovery> + set OAuthMetadata
 self._initialized = True

2. Add get and set OAuthMetaData to TokenStorage. Or add to OAuthClientInformationFull and extract it from a call to get_client_info.

3. Create a proxy_server_url attribute in OAuthContext and have the discovery checks check it on init but use server_url for the fallback /token endpoint construction.

[keurcien]

Should the values of token expired_at or created_at be persisted in the token storage layer? Can we make this explicit in the token storage protocol? Currently users need to guess this requirement to successfully implement a token storage with effective token refresh. Or maybe be part of the OAuthToken (but this introduces an unnecessary field for the server)?

[Norcim133]

Should the values of token expired_at or created_at be persisted in the token storage layer? Can we make this explicit in the token storage protocol? Currently users need to guess this requirement to successfully implement a token storage with effective token refresh. Or maybe be part of the OAuthToken (but this introduces an unnecessary field for the server)?

Currently, per logic above, it doesn't matter if it is in the token layer.

The context layer ensures refresh never happens.

[keurcien]

Agreed, but this fix should be enough? Combined with update on expiry logic using expired_at.

async def _initialize(self):
 """Initialize and properly set token expiry from stored tokens."""
 self.context.current_tokens = await self.context.storage.get_tokens()
 self.context.client_info = await self.context.storage.get_client_info()
 
 # Fix: Update token expiry if tokens loaded from storage
 if self.context.current_tokens:
 self.context.update_token_expiry(self.context.current_tokens)
 self._initialized = True

Also +1 for storing OAuthMetadata.

[rosaboyle]

This is sadly a crazy bug that is effecting every other major MCP - including Linear, substack etc.

[DRayX]

Related FastMCP issue: PrefectHQ/fastmcp#2862