Add ElicitAsync<T> (#630)

src/ModelContextProtocol.Core/Protocol/ElicitRequestParams.cs

@@ -126,7 +126,23 @@ public class Converter : JsonConverter<PrimitiveSchemaDefinition>
                     switch (propertyName)
                     {
                         case "type":
-                            type = reader.GetString();
+                            if (reader.TokenType == JsonTokenType.String)
+                            {
+                                type = reader.GetString();
+                            }
+                            else if (reader.TokenType == JsonTokenType.StartArray)
+                            {
+                                var types = JsonSerializer.Deserialize(ref reader, McpJsonUtilities.JsonContext.Default.StringArray);
+                                if (types is [var leftNullableType, "null"])
+                                {
+                                    type = leftNullableType;
+                                } 
+                                else if (types is ["null", var rightNullableType])
+                                {
+                                    type = rightNullableType;
+                                }
+                            }
+
                             break;
 
                         case "title":

src/ModelContextProtocol.Core/Protocol/ElicitResult.cs

@@ -48,3 +48,22 @@ public sealed class ElicitResult : Result
     [JsonPropertyName("content")]
     public IDictionary<string, JsonElement>? Content { get; set; }
 }
+
+/// <summary>
+/// Represents the client's response to an elicitation request, with typed content payload.
+/// </summary>
+/// <typeparam name="T">The type of the expected content payload.</typeparam>
+public sealed class ElicitResult<T> : Result
+{
+    /// <summary>
+    /// Gets or sets the user action in response to the elicitation.
+    /// </summary>
+    [JsonPropertyName("action")]
+    public string Action { get; set; } = "cancel";
+
+    /// <summary>
+    /// Gets or sets the submitted form data as a typed value.
+    /// </summary>
+    [JsonPropertyName("content")]
+    public T? Content { get; set; }
+}

src/ModelContextProtocol.Core/Server/McpServerExtensions.cs

@@ -4,6 +4,8 @@
 using System.Runtime.CompilerServices;
 using System.Text;
 using System.Text.Json;
+using System.Text.Json.Nodes;
+using System.Text.Json.Serialization.Metadata;
 
 namespace ModelContextProtocol.Server;
 
@@ -234,6 +236,204 @@ public static ValueTask<ElicitResult> ElicitAsync(
             cancellationToken: cancellationToken);
     }
 
+    /// <summary>
+    /// Requests additional information from the user via the client, constructing a request schema from the
+    /// public serializable properties of <typeparamref name="T"/> and deserializing the response into <typeparamref name="T"/>.
+    /// </summary>
+    /// <typeparam name="T">The type describing the expected input shape. Only primitive members are supported (string, number, boolean, enum).</typeparam>
+    /// <param name="server">The server initiating the request.</param>
+    /// <param name="message">The message to present to the user.</param>
+    /// <param name="serializerOptions">Serializer options that influence property naming and deserialization.</param>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests.</param>
+    /// <returns>An <see cref="ElicitResult{T}"/> with the user's response, if accepted.</returns>
+    /// <remarks>
+    /// Elicitation uses a constrained subset of JSON Schema and only supports strings, numbers/integers, booleans and string enums.
+    /// Unsupported member types are ignored when constructing the schema.
+    /// </remarks>
+    public static async ValueTask<ElicitResult<T?>> ElicitAsync<T>(
+        this IMcpServer server,
+        string message,
+        JsonSerializerOptions? serializerOptions = null,
+        CancellationToken cancellationToken = default) where T : class
+    {
+        Throw.IfNull(server);
+        ThrowIfElicitationUnsupported(server);
+
+        serializerOptions ??= McpJsonUtilities.DefaultOptions;
+        serializerOptions.MakeReadOnly();
+
+        var schema = BuildRequestSchema<T>(serializerOptions);
+
+        var request = new ElicitRequestParams
+        {
+            Message = message,
+            RequestedSchema = schema,
+        };
+
+        var raw = await server.ElicitAsync(request, cancellationToken).ConfigureAwait(false);
+
+        if (!string.Equals(raw.Action, "accept", StringComparison.OrdinalIgnoreCase) || raw.Content is null)
+        {
+            return new ElicitResult<T?> { Action = raw.Action, Content = default };
+        }
+
+        var obj = new JsonObject();
+        foreach (var kvp in raw.Content)
+        {
+            obj[kvp.Key] = JsonNode.Parse(kvp.Value.GetRawText());
+        }
+
+        T? typed = JsonSerializer.Deserialize(obj, serializerOptions.GetTypeInfo<T>());
+        return new ElicitResult<T?> { Action = raw.Action, Content = typed };
+    }
+
+    private static ElicitRequestParams.RequestSchema BuildRequestSchema<T>(JsonSerializerOptions serializerOptions)
+    {
+        var schema = new ElicitRequestParams.RequestSchema();
+        var props = schema.Properties;
+
+        JsonTypeInfo<T> typeInfo = serializerOptions.GetTypeInfo<T>();
+
+        if (typeInfo.Kind != JsonTypeInfoKind.Object)
+        {
+            throw new McpException($"Type '{typeof(T).FullName}' is not supported for elicitation requests.");
+        }
+
+        foreach (JsonPropertyInfo pi in typeInfo.Properties)
+        {
+            var memberType = pi.PropertyType;
+            var def = CreatePrimitiveSchema(memberType, serializerOptions);
+            if (def is not null)
+            {
+                props[pi.Name] = def;
+            }
+        }
+
+        return schema;
+    }
+
+    private static ElicitRequestParams.PrimitiveSchemaDefinition? CreatePrimitiveSchema(Type type, JsonSerializerOptions serializerOptions)
+    {
+        JsonTypeInfo typeInfo = serializerOptions.GetTypeInfo(type);
+
+        if (typeInfo.Kind != JsonTypeInfoKind.None)
+        {
+            throw new McpException($"Type '{type.FullName}' is not a supported property type for elicitation requests.");
+        }
+
+        var jsonElement = AIJsonUtilities.CreateJsonSchema(type, serializerOptions: serializerOptions);
+
+        if (!TryValidateElicitationPrimitiveSchema(type, jsonElement, out var error))
+        {
+            throw new McpException(error);
+        }
+
+        var primitiveSchemaDefinition =
+            jsonElement.Deserialize(McpJsonUtilities.JsonContext.Default.PrimitiveSchemaDefinition);
+        return primitiveSchemaDefinition;
+    }
+
+    /// <summary>
+    /// Validate the produced schema strictly to the subset we support. We only accept an object schema
+    /// with a supported primitive type keyword and no additional unsupported keywords.Reject things like
+    /// {}, 'true', or schemas that include unrelated keywords(e.g.items, properties, patternProperties, etc.).
+    /// </summary>
+    /// <param name="type">The type of the schema being validated.</param>
+    /// <param name="schema">The schema to validate.</param>
+    /// <param name="error">The error message, if validation fails.</param>
+    /// <returns></returns>
+    private static bool TryValidateElicitationPrimitiveSchema(Type type, JsonElement schema, [NotNullWhen(false)] out string? error)
+    {
+        if (schema.ValueKind is not JsonValueKind.Object)
+        {
+            error = $"Schema generated for type '{type.FullName}' is invalid: expected an object schema.";
+            return false;
+        }
+
+        if (!schema.TryGetProperty("type", out JsonElement typeProperty)
+            || !(typeProperty.ValueKind is JsonValueKind.String or JsonValueKind.Array))
+        {
+            error = $"Schema generated for type '{type.FullName}' is invalid: missing or invalid 'type' keyword.";
+            return false;
+        }
+
+        string? typeKeyword = null;
+        if (typeProperty.ValueKind == JsonValueKind.Array) // bool? will parse as ["boolean", "null"]
+        {
+            var types = JsonSerializer.Deserialize(typeProperty.GetRawText(), McpJsonUtilities.JsonContext.Default.StringArray);
+            if (types is [var leftNullableType, "null"])
+            {
+                typeKeyword = leftNullableType;
+            }
+            else if (types is ["null", var rightNullableType])
+            {
+                typeKeyword = rightNullableType;
+            }
+            else
+            {
+                error = $"Schema generated for type '{type.FullName}' is invalid: unsupported 'type' array.";
+                return false;
+            }
+        }
+        else
+        {
+            typeKeyword = typeProperty.GetString();
+        }
+
+        if (string.IsNullOrEmpty(typeKeyword))
+        {
+            error = $"Schema generated for type '{type.FullName}' is invalid: empty 'type' value.";
+            return false;
+        }
+
+        // Accept number or integer as the numeric primitive (both map to NumberSchema)
+        bool isString = typeKeyword == "string";
+        bool isBoolean = typeKeyword == "boolean";
+        bool isNumber = typeKeyword == "number" || typeKeyword == "integer";
+        if (!isString && !isBoolean && !isNumber)
+        {
+            error = $"Schema generated for type '{type.FullName}' is invalid: unsupported primitive type '{typeKeyword}'.";
+            return false;
+        }
+
+        // Allowed property names per primitive schema we support.
+        HashSet<string> allowed = new(StringComparer.Ordinal)
+        {
+            "type",
+            "title",
+            "description"
+        };
+        if (isString)
+        {
+            allowed.Add("minLength");
+            allowed.Add("maxLength");
+            allowed.Add("format");
+            allowed.Add("enum"); // for string enums
+            allowed.Add("enumNames"); // for string enums
+        }
+        else if (isNumber)
+        {
+            allowed.Add("minimum");
+            allowed.Add("maximum");
+        }
+        else if (isBoolean)
+        {
+            allowed.Add("default");
+        }
+
+        foreach (JsonProperty prop in schema.EnumerateObject())
+        {
+            if (!allowed.Contains(prop.Name))
+            {
+                error = $"The property '{type.FullName}.{prop.Name}' is not supported for elicitation.";
+                return false;
+            }
+        }
+
+        error = string.Empty;
+        return true;
+    }
+
     private static void ThrowIfSamplingUnsupported(IMcpServer server)
     {
         if (server.ClientCapabilities?.Sampling is null)