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fastmcp.server.auth.oauth_proxy.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.

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Classes

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OAuthProxy

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

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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

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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.

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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

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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

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State Management

The proxy maintains minimal but crucial state via pluggable storage (client_storage):
  • _oauth_transactions: Active authorization flows with client context
  • _client_codes: Authorization codes with PKCE challenges and upstream tokens
  • _jti_mapping_store: Maps FastMCP token JTIs to upstream token IDs
  • _refresh_token_store: Refresh token metadata (keyed by token hash)
All state is stored in the configured client_storage backend (Redis, disk, etc.) enabling horizontal scaling across multiple instances.

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Security Considerations

  • Refresh tokens stored by hash only (defense in depth if storage compromised)
  • 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

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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:

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set_mcp_path 

set_mcp_path(self, mcp_path: str | None) -> None
Set the MCP endpoint path and create JWTIssuer with correct audience. This method is called by get_routes() to configure the resource URL and create the JWTIssuer. The JWT audience is set to the full resource URL (e.g., http://localhost:8000/mcp) to ensure tokens are bound to this specific MCP endpoint. Args:
  • mcp_path: The path where the MCP endpoint is mounted (e.g., “/mcp”)

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jwt_issuer 

jwt_issuer(self) -> JWTIssuer
Get the JWT issuer, ensuring it has been initialized. The JWT issuer is created when set_mcp_path() is called (via get_routes()). This property ensures a clear error if used before initialization.

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get_client 

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).

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register_client 

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.

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authorize 

authorize(self, client: OAuthClientInformationFull, params: AuthorizationParams) -> str
Start OAuth transaction and route through consent interstitial. Flow:
  1. Validate client’s resource matches server’s resource URL (security check)
  2. Store transaction with client details and PKCE (if forwarding)
  3. Return local /consent URL; browser visits consent first
  4. 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.

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load_authorization_code 

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.

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exchange_authorization_code 

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.

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load_refresh_token 

load_refresh_token(self, client: OAuthClientInformationFull, refresh_token: str) -> RefreshToken | None
Load refresh token metadata from distributed storage. Looks up by token hash and reconstructs the RefreshToken object. Validates that the token belongs to the requesting client.

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exchange_refresh_token 

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)

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load_access_token 

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.

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revoke_token 

revoke_token(self, token: AccessToken | RefreshToken) -> None
Revoke token locally and with upstream server if supported. For refresh tokens, removes from local storage by hash. For all tokens, attempts upstream revocation if endpoint is configured. Access token JTI mappings expire via TTL.

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get_routes 

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.