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fastmcp.server.auth.identity_assertion

Server-side identity assertion (ID-JAG) support for FastMCP (SEP-990). .. warning:: Beta Feature: Identity assertion support is currently in beta. The API may change in future releases. Please report any issues you encounter. SEP-990 defines an enterprise “on-behalf-of” flow. A corporate identity provider (Okta, Entra, etc.) issues an ID-JAG (Identity Assertion JWT Authorization Grant) that asserts an employee’s identity to a specific MCP authorization server. The client presents that ID-JAG at the token endpoint using the RFC 7523 urn:ietf:params:oauth:grant-type:jwt-bearer grant (the RFC 8693 token-exchange profile). This module validates the assertion and lets the authorization server mint a short-lived access token carrying the asserted subject, with no refresh token — the client re-exchanges a fresh ID-JAG instead, and revocation lives at the IdP. This module provides:
  • IdentityAssertion: a small pydantic config model attached to OAuthProxy via the identity_assertion parameter.
  • IdentityAssertionValidator: validates an ID-JAG per RFC 7523 §3 and the SEP-990 processing rules, reusing FastMCP’s :class:JWTVerifier for signature, issuer, audience, and expiry checks, and enforcing typ, sub presence, and jti replay protection on top.

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Functions

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normalize_resource_url

Normalize a resource URL by removing query parameters and trailing slashes. RFC 8707 allows clients to include query parameters in resource URLs, but the server’s configured resource URL typically doesn’t include them. This normalizes both sides for comparison by stripping query and fragment.

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server_url_has_query

Check if a URL has query parameters.

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Classes

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IdentityAssertion

Configuration for server-side identity assertion (ID-JAG) support. When attached to an :class:~fastmcp.server.auth.oauth_proxy.OAuthProxy via the identity_assertion parameter, the proxy’s token endpoint accepts the RFC 7523 jwt-bearer grant carrying an ID-JAG issued by one of the trusted_issuers, and mints a short-lived FastMCP access token for the asserted subject.

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IdentityAssertionError

Raised when an ID-JAG fails validation. The message is for server-side logging only; the token endpoint maps this to a generic OAuth error response and does not leak the detail to the client.

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IdentityAssertionValidator

Validates ID-JAG assertions for the SEP-990 jwt-bearer grant. Reuses :class:JWTVerifier for signature, issuer, audience, and expiry checks (with JWKS fetching and caching), and layers on the SEP-990 processing rules that the generic verifier does not cover: the typ JOSE header, a mandatory sub, and jti replay rejection. JTI replay protection mirrors :class:CIMDAssertionValidator: seen jti values are cached until the assertion would expire anyway, with periodic cleanup and an emergency size cap. Like CIMD, the cache is per-process, so replay protection is not shared across horizontally-scaled workers or replicas; see the identity-assertion docs for the deployment caveat. Methods:

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validate

Validate an ID-JAG and return its claims. Args:
  • assertion: The compact-serialized ID-JAG JWT.
  • client_id: The authenticated client presenting the assertion. Must match the assertion’s signed client_id claim — checked before the jti is recorded as consumed, so an assertion presented by the wrong client is rejected without burning it for the right one.
  • resource_url: This server’s resource URL, if configured. Must match the assertion’s signed resource claim, for the same reason.
Returns:
  • The verified claims (including sub, iss, and any resource/scope).
Raises:
  • IdentityAssertionError: If the assertion is invalid for any reason.