synapse/docs/admin_api/client_server_api_extensions.md

Client-Server API Extensions

Server administrators can set special account data to change how the Client-Server API behaves for their clients. Setting the account data, or having it already set, as a non-admin has no effect.

All configuration options can be set through the io.element.synapse.admin_client_config global account data on the admin's user account.

Example:

PUT /_matrix/client/v3/user/{adminUserId}/account_data/io.element.synapse.admin_client_config
{
    "return_soft_failed_events": true
}

See soft failed events

Learn more about soft failure from the spec.

To receive soft failed events in APIs like /sync and /messages, set return_soft_failed_events to true in the admin client config. When false, the normal behaviour of these endpoints is to exclude soft failed events.

Default: false