> ## Documentation Index > Fetch the complete documentation index at: https://fastmcp-docs-v3-beta2-feature-tracking.mintlify.site/llms.txt > Use this file to discover all available pages before exploring further. # jwt_issuer # `fastmcp.server.auth.jwt_issuer` JWT token issuance and verification for FastMCP OAuth Proxy. This module implements the token factory pattern for OAuth proxies, where the proxy issues its own JWT tokens to clients instead of forwarding upstream provider tokens. This maintains proper OAuth 2.0 token audience boundaries. ## Functions ### `derive_jwt_key` ```python theme={null} derive_jwt_key() -> bytes ``` Derive JWT signing key from a high-entropy or low-entropy key material and server salt. ## Classes ### `JWTIssuer` Issues and validates FastMCP-signed JWT tokens using HS256. This issuer creates JWT tokens for MCP clients with proper audience claims, maintaining OAuth 2.0 token boundaries. Tokens are signed with HS256 using a key derived from the upstream client secret. **Methods:** #### `issue_access_token` ```python theme={null} issue_access_token(self, client_id: str, scopes: list[str], jti: str, expires_in: int = 3600, upstream_claims: dict[str, Any] | None = None) -> str ``` Issue a minimal FastMCP access token. FastMCP tokens are reference tokens containing only the minimal claims needed for validation and lookup. The JTI maps to the upstream token which contains actual user identity and authorization data. **Args:** * `client_id`: MCP client ID * `scopes`: Token scopes * `jti`: Unique token identifier (maps to upstream token) * `expires_in`: Token lifetime in seconds * `upstream_claims`: Optional claims from upstream IdP token to include **Returns:** * Signed JWT token #### `issue_refresh_token` ```python theme={null} issue_refresh_token(self, client_id: str, scopes: list[str], jti: str, expires_in: int, upstream_claims: dict[str, Any] | None = None) -> str ``` Issue a minimal FastMCP refresh token. FastMCP refresh tokens are reference tokens containing only the minimal claims needed for validation and lookup. The JTI maps to the upstream token which contains actual user identity and authorization data. **Args:** * `client_id`: MCP client ID * `scopes`: Token scopes * `jti`: Unique token identifier (maps to upstream token) * `expires_in`: Token lifetime in seconds (should match upstream refresh expiry) * `upstream_claims`: Optional claims from upstream IdP token to include **Returns:** * Signed JWT token #### `verify_token` ```python theme={null} verify_token(self, token: str) -> dict[str, Any] ``` Verify and decode a FastMCP token. Validates JWT signature, expiration, issuer, and audience. **Args:** * `token`: JWT token to verify **Returns:** * Decoded token payload **Raises:** * `JoseError`: If token is invalid, expired, or has wrong claims