fastmcp/docs/python-sdk/fastmcp-server-auth-oauth_proxy.mdx

---
title: oauth_proxy
sidebarTitle: oauth_proxy
---

# `fastmcp.server.auth.oauth_proxy`


OAuth Proxy Provider for FastMCP.

This provider acts as a transparent proxy to an upstream OAuth Authorization Server,
handling Dynamic Client Registration locally while forwarding all other OAuth flows.
This enables authentication with upstream providers that don't support DCR or have
restricted client registration policies.

Key features:
- Proxies authorization and token endpoints to upstream server
- Implements local Dynamic Client Registration with fixed upstream credentials
- Validates tokens using upstream JWKS
- Maintains minimal local state for bookkeeping
- Enhanced logging with request correlation

This implementation is based on the OAuth 2.1 specification and is designed for
production use with enterprise identity providers.


## Functions

### `create_consent_html` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L236" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
create_consent_html(client_id: str, redirect_uri: str, scopes: list[str], txn_id: str, csrf_token: str, client_name: str | None = None, title: str = 'Application Access Request', server_name: str | None = None, server_icon_url: str | None = None, server_website_url: str | None = None, client_website_url: str | None = None) -> str
```


Create a styled HTML consent page for OAuth authorization requests.


## Classes

### `OAuthTransaction` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L108" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>


OAuth transaction state for consent flow.

Stored server-side to track active authorization flows with client context.
Includes CSRF tokens for consent protection per MCP security best practices.


### `ClientCode` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L129" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>


Client authorization code with PKCE and upstream tokens.

Stored server-side after upstream IdP callback. Contains the upstream
tokens bound to the client's PKCE challenge for secure token exchange.


### `UpstreamTokenSet` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L147" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>


Stored upstream OAuth tokens from identity provider.

These tokens are obtained from the upstream provider (Google, GitHub, etc.)
and stored in plaintext within this model. Encryption is handled transparently
at the storage layer via FernetEncryptionWrapper. Tokens are never exposed to MCP clients.


### `JTIMapping` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L169" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>


Maps FastMCP token JTI to upstream token ID.

This allows stateless JWT validation while still being able to look up
the corresponding upstream token when tools need to access upstream APIs.


### `ProxyDCRClient` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L181" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>


Client for DCR proxy with configurable redirect URI validation.

This special client class is critical for the OAuth proxy to work correctly
with Dynamic Client Registration (DCR). Here's why it exists:

Problem:
--------
When MCP clients use OAuth, they dynamically register with random localhost
ports (e.g., http://localhost:55454/callback). The OAuth proxy needs to:
1. Accept these dynamic redirect URIs from clients based on configured patterns
2. Use its own fixed redirect URI with the upstream provider (Google, GitHub, etc.)
3. Forward the authorization code back to the client's dynamic URI

Solution:
---------
This class validates redirect URIs against configurable patterns,
while the proxy internally uses its own fixed redirect URI with the upstream
provider. This allows the flow to work even when clients reconnect with
different ports or when tokens are cached.

Without proper validation, clients could get "Redirect URI not registered" errors
when trying to authenticate with cached tokens, or security vulnerabilities could
arise from accepting arbitrary redirect URIs.


**Methods:**

#### `validate_redirect_uri` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L210" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
validate_redirect_uri(self, redirect_uri: AnyUrl | None) -> AnyUrl
```

Validate redirect URI against allowed patterns.

Since we're acting as a proxy and clients register dynamically,
we validate their redirect URIs against configurable patterns.
This is essential for cached token scenarios where the client may
reconnect with a different port.


### `TokenHandler` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L383" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>


TokenHandler that returns OAuth 2.1 compliant error responses.

The MCP SDK always returns HTTP 400 for all client authentication issues.
However, OAuth 2.1 Section 5.3 and the MCP specification require that
invalid or expired tokens MUST receive a HTTP 401 response.

This handler extends the base MCP SDK TokenHandler to transform client
authentication failures into OAuth 2.1 compliant responses:
- Changes 'unauthorized_client' to 'invalid_client' error code
- Returns HTTP 401 status code instead of 400 for client auth failures

Per OAuth 2.1 Section 5.3: "The authorization server MAY return an HTTP 401
(Unauthorized) status code to indicate which HTTP authentication schemes
are supported."

Per MCP spec: "Invalid or expired tokens MUST receive a HTTP 401 response."


**Methods:**

#### `response` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L402" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
response(self, obj: TokenSuccessResponse | TokenErrorResponse)
```

Override response method to provide OAuth 2.1 compliant error handling.


### `OAuthProxy` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L432" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>


OAuth provider that presents a DCR-compliant interface while proxying to non-DCR IDPs.

Purpose
-------
MCP clients expect OAuth providers to support Dynamic Client Registration (DCR),
where clients can register themselves dynamically and receive unique credentials.
Most enterprise IDPs (Google, GitHub, Azure AD, etc.) don't support DCR and require
pre-registered OAuth applications with fixed credentials.

This proxy bridges that gap by:
- Presenting a full DCR-compliant OAuth interface to MCP clients
- Translating DCR registration requests to use pre-configured upstream credentials
- Proxying all OAuth flows to the upstream IDP with appropriate translations
- Managing the state and security requirements of both protocols

Architecture Overview
--------------------
The proxy maintains a single OAuth app registration with the upstream provider
while allowing unlimited MCP clients to register and authenticate dynamically.
It implements the complete OAuth 2.1 + DCR specification for clients while
translating to whatever OAuth variant the upstream provider requires.

Key Translation Challenges Solved
---------------------------------
1. Dynamic Client Registration:
   - MCP clients expect to register dynamically and get unique credentials
   - Upstream IDPs require pre-registered apps with fixed credentials
   - Solution: Accept DCR requests, return shared upstream credentials

2. Dynamic Redirect URIs:
   - MCP clients use random localhost ports that change between sessions
   - Upstream IDPs require fixed, pre-registered redirect URIs
   - Solution: Use proxy's fixed callback URL with upstream, forward to client's dynamic URI

3. Authorization Code Mapping:
   - Upstream returns codes for the proxy's redirect URI
   - Clients expect codes for their own redirect URIs
   - Solution: Exchange upstream code server-side, issue new code to client

4. State Parameter Collision:
   - Both client and proxy need to maintain state through the flow
   - Only one state parameter available in OAuth
   - Solution: Use transaction ID as state with upstream, preserve client's state

5. Token Management:
   - Clients may expect different token formats/claims than upstream provides
   - Need to track tokens for revocation and refresh
   - Solution: Store token relationships, forward upstream tokens transparently

OAuth Flow Implementation
------------------------
1. Client Registration (DCR):
   - Accept any client registration request
   - Store ProxyDCRClient that accepts dynamic redirect URIs

2. Authorization:
   - Store transaction mapping client details to proxy flow
   - Redirect to upstream with proxy's fixed redirect URI
   - Use transaction ID as state parameter with upstream

3. Upstream Callback:
   - Exchange upstream authorization code for tokens (server-side)
   - Generate new authorization code bound to client's PKCE challenge
   - Redirect to client's original dynamic redirect URI

4. Token Exchange:
   - Validate client's code and PKCE verifier
   - Return previously obtained upstream tokens
   - Clean up one-time use authorization code

5. Token Refresh:
   - Forward refresh requests to upstream using authlib
   - Handle token rotation if upstream issues new refresh token
   - Update local token mappings

State Management
---------------
The proxy maintains minimal but crucial state:
- _oauth_transactions: Active authorization flows with client context
- _client_codes: Authorization codes with PKCE challenges and upstream tokens
- _access_tokens, _refresh_tokens: Token storage for revocation
- Token relationship mappings for cleanup and rotation

Security Considerations
----------------------
- PKCE enforced end-to-end (client to proxy, proxy to upstream)
- Authorization codes are single-use with short expiry
- Transaction IDs are cryptographically random
- All state is cleaned up after use to prevent replay
- Token validation delegates to upstream provider

Provider Compatibility
---------------------
Works with any OAuth 2.0 provider that supports:
- Authorization code flow
- Fixed redirect URI (configured in provider's app settings)
- Standard token endpoint

Handles provider-specific requirements:
- Google: Ensures minimum scope requirements
- GitHub: Compatible with OAuth Apps and GitHub Apps
- Azure AD: Handles tenant-specific endpoints
- Generic: Works with any spec-compliant provider


**Methods:**

#### `get_client` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L813" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
get_client(self, client_id: str) -> OAuthClientInformationFull | None
```

Get client information by ID. This is generally the random ID
provided to the DCR client during registration, not the upstream client ID.

For unregistered clients, returns None (which will raise an error in the SDK).


#### `register_client` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L829" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
register_client(self, client_info: OAuthClientInformationFull) -> None
```

Register a client locally

When a client registers, we create a ProxyDCRClient that is more
forgiving about validating redirect URIs, since the DCR client's
redirect URI will likely be localhost or unknown to the proxied IDP. The
proxied IDP only knows about this server's fixed redirect URI.


#### `authorize` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L876" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
authorize(self, client: OAuthClientInformationFull, params: AuthorizationParams) -> str
```

Start OAuth transaction and route through consent interstitial.

Flow:
1. Store transaction with client details and PKCE (if forwarding)
2. Return local /consent URL; browser visits consent first
3. Consent handler redirects to upstream IdP if approved/already approved

If consent is disabled (require_authorization_consent=False), skip the consent screen
and redirect directly to the upstream IdP.


#### `load_authorization_code` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L951" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
load_authorization_code(self, client: OAuthClientInformationFull, authorization_code: str) -> AuthorizationCode | None
```

Load authorization code for validation.

Look up our client code and return authorization code object
with PKCE challenge for validation.


#### `exchange_authorization_code` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L994" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
exchange_authorization_code(self, client: OAuthClientInformationFull, authorization_code: AuthorizationCode) -> OAuthToken
```

Exchange authorization code for FastMCP-issued tokens.

Implements the token factory pattern:
1. Retrieves upstream tokens from stored authorization code
2. Extracts user identity from upstream token
3. Encrypts and stores upstream tokens
4. Issues FastMCP-signed JWT tokens
5. Returns FastMCP tokens (NOT upstream tokens)

PKCE validation is handled by the MCP framework before this method is called.


#### `load_refresh_token` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L1159" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
load_refresh_token(self, client: OAuthClientInformationFull, refresh_token: str) -> RefreshToken | None
```

Load refresh token from local storage.


#### `exchange_refresh_token` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L1167" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
exchange_refresh_token(self, client: OAuthClientInformationFull, refresh_token: RefreshToken, scopes: list[str]) -> OAuthToken
```

Exchange FastMCP refresh token for new FastMCP access token.

Implements two-tier refresh:
1. Verify FastMCP refresh token
2. Look up upstream token via JTI mapping
3. Refresh upstream token with upstream provider
4. Update stored upstream token
5. Issue new FastMCP access token
6. Keep same FastMCP refresh token (unless upstream rotates)


#### `load_access_token` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L1368" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
load_access_token(self, token: str) -> AccessToken | None
```

Validate FastMCP JWT by swapping for upstream token.

This implements the token swap pattern:
1. Verify FastMCP JWT signature (proves it's our token)
2. Look up upstream token via JTI mapping
3. Decrypt upstream token
4. Validate upstream token with provider (GitHub API, JWT validation, etc.)
5. Return upstream validation result

The FastMCP JWT is a reference token - all authorization data comes
from validating the upstream token via the TokenVerifier.


#### `revoke_token` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L1424" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
revoke_token(self, token: AccessToken | RefreshToken) -> None
```

Revoke token locally and with upstream server if supported.

Removes tokens from local storage and attempts to revoke them with
the upstream server if a revocation endpoint is configured.


#### `get_routes` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L1468" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
get_routes(self, mcp_path: str | None = None) -> list[Route]
```

Get OAuth routes with custom handlers for better error UX.

This method creates standard OAuth routes and replaces:
- /authorize endpoint: Enhanced error responses for unregistered clients
- /token endpoint: OAuth 2.1 compliant error codes

**Args:**
- `mcp_path`: The path where the MCP endpoint is mounted (e.g., "/mcp")
This is used to advertise the resource URL in metadata.