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When a user attempts to authenticate with an SP, the SP can provide a list of supported IdPs to the user. If the SP has a preferred IdP, it can suggest that IdP to the user by including an IDP IdP hint in the authentication request. The IDP IdP hint can be in the form of a URL or an identifier, such as the IdP's entity ID.
If the user has previously authenticated with the suggested IdP, they may be automatically redirected to that IdP to complete the authentication process. This can save the user time and effort by eliminating the need to select an IdP from a list or search for the appropriate IdP.
Overall, IDP IdP hinting can improve the user experience by providing a more seamless authentication flow, while also allowing SPs to promote their preferred IdPs to users.