Automatically normalize "amr" claims in identity tokens to ensure a JSON array is always returned

src/OpenIddict.Server/OpenIddictServerHandlers.Protection.cs

@@ -1384,6 +1384,8 @@ public static partial class OpenIddictServerHandlers
 
                     Claims.Private.Scope when context.TokenType is TokenTypeHints.AccessToken => false,
 
+                    Claims.AuthenticationMethodReference when context.TokenType is TokenTypeHints.IdToken => false,
+
                     _ => true
                 });
 
@@ -1398,10 +1400,27 @@ public static partial class OpenIddictServerHandlers
                     var audiences = context.Principal.GetAudiences();
                     if (audiences.Any())
                     {
-                        claims.Add(Claims.Audience, audiences.Length switch
+                        claims.Add(Claims.Audience, audiences switch
+                        {
+                            [string audience] => audience,
+                                    _         => audiences.ToArray()
+                        });
+                    }
+                }
+
+                // Note: unlike other claims (e.g "aud"), the "amr" claim MUST be represented as a unique
+                // claim representing a JSON array, even if a single authentication method reference is
+                // present in the collection. To ensure an array is always returned, the "amr" claim is
+                // filtered out from the clone principal and manually added as a "string[]" claim value.
+                if (context.TokenType is TokenTypeHints.IdToken)
+                {
+                    var methods = context.Principal.GetClaims(Claims.AuthenticationMethodReference);
+                    if (methods.Any())
+                    {
+                        claims.Add(Claims.AuthenticationMethodReference, methods switch
                         {
-                            1 => audiences.ElementAt(0),
-                            _ => audiences
+                            [string method] => [method],
+                                   _        => methods.ToArray()
                         });
                     }
                 }

src/OpenIddict.Server/OpenIddictServerHandlers.cs

@@ -2216,8 +2216,7 @@ public static partial class OpenIddictServerHandlers
                     => values is [{ ValueType: ClaimValueTypes.String }],
 
                 // The following claims MUST be represented as unique strings or array of strings.
-                Claims.AuthenticationMethodReference or Claims.Private.Audience or
-                Claims.Private.Presenter             or Claims.Private.Resource
+                Claims.Private.Audience or Claims.Private.Presenter or Claims.Private.Resource
                     => values.TrueForAll(static value => value.ValueType is ClaimValueTypes.String) ||
                        // Note: a unique claim using the special JSON_ARRAY claim value type is allowed
                        // if the individual elements of the parsed JSON array are all string values.
@@ -2225,6 +2224,17 @@ public static partial class OpenIddictServerHandlers
                         JsonSerializer.Deserialize<JsonElement>(value) is { ValueKind: JsonValueKind.Array } element &&
                         OpenIddictHelpers.ValidateArrayElements(element, JsonValueKind.String)),
 
+                // Note: unlike other claims (e.g "aud"), the "amr" claim MUST be represented as a unique
+                // claim representing a JSON array, even if a single authentication method reference
+                // is present in the collection. To avoid forcing users to use the special JSON_ARRAY
+                // value type, string values are also allowed here and normalized to JSON arrays
+                // by OpenIddict when generating an identity token based on the specified principal.
+                Claims.AuthenticationMethodReference
+                    => values.TrueForAll(static value => value.ValueType is ClaimValueTypes.String) ||
+                       (values is [{ ValueType: JsonClaimValueTypes.JsonArray, Value: string value }] &&
+                        JsonSerializer.Deserialize<JsonElement>(value) is { ValueKind: JsonValueKind.Array } element &&
+                        OpenIddictHelpers.ValidateArrayElements(element, JsonValueKind.String)),
+
                 // The following claims MUST be represented as unique integers.
                 Claims.Private.AccessTokenLifetime  or Claims.Private.AuthorizationCodeLifetime or
                 Claims.Private.DeviceCodeLifetime   or Claims.Private.IdentityTokenLifetime     or