The act claim was reserved in FusionAuth 1.70.0 because it is defined in RFC 8693 (OAuth 2.0 Token Exchange) as a standard claim representing the "actor" in delegated token scenarios. FusionAuth began enforcing this reservation to ensure compliance with the RFC and prevent conflicts with the claim's intended usage in token exchange flows.

Why This Is Classified as Minor

FusionAuth follows semantic versioning, and this change was classified as minor rather than breaking because:

The act claim is a standardized RFC claim with a specific defined purpose Using non-standard values in RFC-defined claims can cause interoperability issues with other OAuth/OIDC systems The change aligns FusionAuth's behavior with industry standards

However, the lack of logging when a reserved claim is silently discarded is a valid concern and makes troubleshooting difficult.

Recommended Solutions 1. Use a Custom Claim Namespace

Instead of using act directly, use a namespaced custom claim that won't conflict with RFC-defined claims:

// In your JWT populate lambda jwt['https://yourapp.com/actor'] = 'your-actor-id';

This approach is standard practice in OAuth/OIDC and prevents future collisions with reserved claims.

2. Use applicationId or Other Non-Reserved Claims

If the actor represents an application or service account, consider using the built-in applicationId claim or another custom claim that describes your specific use case (e.g., actorId, serviceAccountId).

Why Reserved Claims Cannot Be Overridden

Reserved claims like act, sub, iss, aud, etc., are protected because they have specific meanings defined by RFCs and must maintain consistent behavior for:

Interoperability: Other systems expect these claims to have specific formats and meanings Security: Claims like iss and aud are critical for token validation and cannot be arbitrarily changed Standards Compliance: FusionAuth aims to be a standards-compliant identity provider Migration Path

If you're currently using act in production:

Update your JWT populate lambda to use a namespaced custom claim Update your application to read from the new claim name Test thoroughly in a non-production environment before upgrading

If you need to maintain backward compatibility during migration, you could temporarily set both claims in your lambda (the custom namespaced one and attempt to set act, knowing it will be discarded) and update your application to check for both, prioritizing the custom claim.

Related Documentation JWT Populate Lambda - Documentation on customizing JWT claims with lambdas Client Credentials JWT Populate Lambda - Documentation for lambdas in the Client Credentials grant OAuth Token Exchange (RFC 8693) - FusionAuth's implementation of RFC 8693, which defines the act claim FusionAuth 1.70.0 Release Notes - Release notes for version 1.70.0 FusionAuth 1.70 Announcement: Token Exchange - Blog post explaining the new token exchange feature and scoped delegation GitHub Issue #1721: JWT claim overriding, namespacing or the ability to unreserve claims - Open issue discussing the ability to override or unreserve claims Additional Context on Reserved Claims

As of FusionAuth 1.70.0, the following claims are reserved in JWT Populate Lambdas and cannot be modified:

act (added in 1.70.0) amr auth_time authenticationType cnf fa_uid gty iat sub tid tty

Note: The exp claim can be lowered (shortened) but not extended or removed.

For other lambda types (Client Credentials, UserInfo, Token Exchange), different sets of reserved claims apply. Always use namespaced custom claims (e.g., https://example.com/my-claim) to avoid conflicts with current or future reserved claims.