Add elicitation capability

samples/EverythingServer/Program.cs

@@ -46,7 +46,7 @@
         {
             subscriptions.Add(uri);
 
-            await ctx.Server.RequestSamplingAsync([
+            await ctx.Server.SampleAsync([
                 new ChatMessage(ChatRole.System, "You are a helpful test server"),
                 new ChatMessage(ChatRole.User, $"Resource {uri}, context: A new subscription was started"),
             ],

samples/EverythingServer/Tools/SampleLlmTool.cs

@@ -15,7 +15,7 @@ public static async Task<string> SampleLLM(
         CancellationToken cancellationToken)
     {
         var samplingParams = CreateRequestSamplingParams(prompt ?? string.Empty, "sampleLLM", maxTokens);
-        var sampleResult = await server.RequestSamplingAsync(samplingParams, cancellationToken);
+        var sampleResult = await server.SampleAsync(samplingParams, cancellationToken);
 
         return $"LLM sampling result: {sampleResult.Content.Text}";
     }

samples/TestServerWithHosting/Tools/SampleLlmTool.cs

@@ -18,7 +18,7 @@ public static async Task<string> SampleLLM(
         CancellationToken cancellationToken)
     {
         var samplingParams = CreateRequestSamplingParams(prompt ?? string.Empty, "sampleLLM", maxTokens);
-        var sampleResult = await thisServer.RequestSamplingAsync(samplingParams, cancellationToken);
+        var sampleResult = await thisServer.SampleAsync(samplingParams, cancellationToken);
 
         return $"LLM sampling result: {sampleResult.Content.Text}";
     }

src/ModelContextProtocol/Client/McpClient.cs

@@ -76,6 +76,20 @@ public McpClient(IClientTransport clientTransport, McpClientOptions? options, IL
                     McpJsonUtilities.JsonContext.Default.ListRootsRequestParams,
                     McpJsonUtilities.JsonContext.Default.ListRootsResult);
             }
+
+            if (capabilities.Elicitation is { } elicitationCapability)
+            {
+                if (elicitationCapability.ElicitationHandler is not { } elicitationHandler)
+                {
+                    throw new InvalidOperationException("Elicitation capability was set but it did not provide a handler.");
+                }
+
+                RequestHandlers.Set(
+                    RequestMethods.ElicitationCreate,
+                    (request, _, cancellationToken) => elicitationHandler(request, cancellationToken),
+                    McpJsonUtilities.JsonContext.Default.ElicitRequestParams,
+                    McpJsonUtilities.JsonContext.Default.ElicitResult);
+            }
         }
     }
 

src/ModelContextProtocol/McpJsonUtilities.cs

@@ -96,6 +96,8 @@ internal static bool IsValidMcpToolSchema(JsonElement element)
     [JsonSerializable(typeof(CompleteResult))]
     [JsonSerializable(typeof(CreateMessageRequestParams))]
     [JsonSerializable(typeof(CreateMessageResult))]
+    [JsonSerializable(typeof(ElicitRequestParams))]
+    [JsonSerializable(typeof(ElicitResult))]
     [JsonSerializable(typeof(EmptyResult))]
     [JsonSerializable(typeof(GetPromptRequestParams))]
     [JsonSerializable(typeof(GetPromptResult))]

src/ModelContextProtocol/Protocol/ClientCapabilities.cs

@@ -58,6 +58,13 @@ public class ClientCapabilities
     [JsonPropertyName("sampling")]
     public SamplingCapability? Sampling { get; set; }
 
+    /// <summary>
+    /// Gets or sets the client's elicitation capability, which indicates whether the client 
+    /// supports elicitation of additional information from the user on behalf of the server.
+    /// </summary>
+    [JsonPropertyName("elicitation")]
+    public ElicitationCapability? Elicitation { get; set; }
+
     /// <summary>Gets or sets notification handlers to register with the client.</summary>
     /// <remarks>
     /// <para>

src/ModelContextProtocol/Protocol/ElicitRequestParams.cs

@@ -0,0 +1,230 @@
+using System.Diagnostics.CodeAnalysis;
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// Represents a message issued from the server to elicit additional information from the user via the client.
+/// </summary>
+public class ElicitRequestParams
+{
+    /// <summary>
+    /// Gets or sets the message to present to the user.
+    /// </summary>
+    [JsonPropertyName("message")]
+    public string Message { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Gets or sets the requested schema.
+    /// </summary>
+    /// <remarks>
+    /// May be one of <see cref="StringSchema"/>, <see cref="NumberSchema"/>, <see cref="BooleanSchema"/>, or <see cref="EnumSchema"/>.
+    /// </remarks>
+    [JsonPropertyName("requestedSchema")]
+    [field: MaybeNull]
+    public RequestSchema RequestedSchema
+    {
+        get => field ??= new RequestSchema();
+        set => field = value;
+    }
+
+    /// <summary>Represents a request schema used in an elicitation request.</summary>
+    public class RequestSchema
+    {
+        /// <summary>Gets the type of the schema.</summary>
+        /// <remarks>This is always "object".</remarks>
+        [JsonPropertyName("type")]
+        public string Type => "object";
+
+        /// <summary>Gets or sets the properties of the schema.</summary>
+        [JsonPropertyName("properties")]
+        [field: MaybeNull]
+        public IDictionary<string, PrimitiveSchemaDefinition> Properties
+        {
+            get => field ??= new Dictionary<string, PrimitiveSchemaDefinition>();
+            set
+            {
+                Throw.IfNull(value);
+                field = value;
+            }
+        }
+
+        /// <summary>Gets or sets the required properties of the schema.</summary>
+        [JsonPropertyName("required")]
+        public IList<string>? Required { get; set; }
+    }
+
+
+    /// <summary>
+    /// Represents restricted subset of JSON Schema: 
+    /// <see cref="StringSchema"/>, <see cref="NumberSchema"/>, <see cref="BooleanSchema"/>, or <see cref="EnumSchema"/>.
+    /// </summary>
+    [JsonDerivedType(typeof(BooleanSchema))]
+    [JsonDerivedType(typeof(EnumSchema))]
+    [JsonDerivedType(typeof(NumberSchema))]
+    [JsonDerivedType(typeof(StringSchema))]
+    public abstract class PrimitiveSchemaDefinition
+    {
+        protected private PrimitiveSchemaDefinition()
+        {
+        }
+    }
+
+    /// <summary>Represents a schema for a string type.</summary>
+    public sealed class StringSchema : PrimitiveSchemaDefinition
+    {
+        /// <summary>Gets the type of the schema.</summary>
+        /// <remarks>This is always "string".</remarks>
+        [JsonPropertyName("type")]
+        public string Type => "string";
+
+        /// <summary>Gets or sets a title for the string.</summary>
+        [JsonPropertyName("title")]
+        public string? Title { get; set; }
+
+        /// <summary>Gets or sets a description for the string.</summary>
+        [JsonPropertyName("description")]
+        public string? Description { get; set; }
+
+        /// <summary>Gets or sets the minimum length for the string.</summary>
+        [JsonPropertyName("minLength")]
+        public int? MinLength
+        {
+            get => field;
+            set
+            {
+                if (value < 0)
+                {
+                    throw new ArgumentOutOfRangeException(nameof(value), "Minimum length cannot be negative.");
+                }
+
+                field = value;
+            }
+        }
+
+        /// <summary>Gets or sets the maximum length for the string.</summary>
+        [JsonPropertyName("maxLength")]
+        public int? MaxLength
+        {
+            get => field;
+            set
+            {
+                if (value < 0)
+                {
+                    throw new ArgumentOutOfRangeException(nameof(value), "Maximum length cannot be negative.");
+                }
+
+                field = value;
+            }
+        }
+
+        /// <summary>Gets or sets a specific format for the string ("email", "uri", "date", or "date-time").</summary>
+        [JsonPropertyName("format")]
+        public string? Format
+        {
+            get => field;
+            set
+            {
+                if (value is not (null or "email" or "uri" or "date" or "date-time"))
+                {
+                    throw new ArgumentException("Format must be 'email', 'uri', 'date', or 'date-time'.", nameof(value));
+                }
+
+                field = value;
+            }
+        }
+    }
+
+    /// <summary>Represents a schema for a number or integer type.</summary>
+    public sealed class NumberSchema : PrimitiveSchemaDefinition
+    {
+        /// <summary>Gets the type of the schema.</summary>
+        /// <remarks>This should be "number" or "integer".</remarks>
+        [JsonPropertyName("type")]
+        [field: MaybeNull]
+        public string Type
+        {
+            get => field ??= "number";
+            set
+            {
+                if (value is not ("number" or "integer"))
+                {
+                    throw new ArgumentException("Type must be 'number' or 'integer'.", nameof(value));
+                }
+
+                field = value;
+            }
+        }
+
+        /// <summary>Gets or sets a title for the number input.</summary>
+        [JsonPropertyName("title")]
+        public string? Title { get; set; }
+
+        /// <summary>Gets or sets a description for the number input.</summary>
+        [JsonPropertyName("description")]
+        public string? Description { get; set; }
+
+        /// <summary>Gets or sets the minimum allowed value.</summary>
+        [JsonPropertyName("minimum")]
+        public double? Minimum { get; set; }
+
+        /// <summary>Gets or sets the maximum allowed value.</summary>
+        [JsonPropertyName("maximum")]
+        public double? Maximum { get; set; }
+    }
+
+    /// <summary>Represents a schema for a Boolean type.</summary>
+    public sealed class BooleanSchema : PrimitiveSchemaDefinition
+    {
+        /// <summary>Gets the type of the schema.</summary>
+        /// <remarks>This is always "boolean".</remarks>
+        [JsonPropertyName("type")]
+        public string Type => "boolean";
+
+        /// <summary>Gets or sets a title for the Boolean.</summary>
+        [JsonPropertyName("title")]
+        public string? Title { get; set; }
+
+        /// <summary>Gets or sets a description for the Boolean.</summary>
+        [JsonPropertyName("description")]
+        public string? Description { get; set; }
+
+        /// <summary>Gets or sets the default value for the Boolean.</summary>
+        [JsonPropertyName("default")]
+        public bool? Default { get; set; }
+    }
+
+    /// <summary>Represents a schema for an enum type.</summary>
+    public sealed class EnumSchema : PrimitiveSchemaDefinition
+    {
+        /// <summary>Gets the type of the schema.</summary>
+        /// <remarks>This is always "string".</remarks>
+        [JsonPropertyName("type")]
+        public string Type => "string";
+
+        /// <summary>Gets or sets a title for the enum.</summary>
+        [JsonPropertyName("title")]
+        public string? Title { get; set; }
+
+        /// <summary>Gets or sets a description for the enum.</summary>
+        [JsonPropertyName("description")]
+        public string? Description { get; set; }
+
+        /// <summary>Gets or sets the list of allowed string values for the enum.</summary>
+        [JsonPropertyName("enum")]
+        [field: MaybeNull]
+        public IList<string> Enum
+        {
+            get => field ??= [];
+            set
+            {
+                Throw.IfNull(value);
+                field = value;
+            }
+        }
+
+        /// <summary>Gets or sets optional display names corresponding to the enum values.</summary>
+        [JsonPropertyName("enumNames")]
+        public IList<string>? EnumNames { get; set; }
+    }
+}

src/ModelContextProtocol/Protocol/ElicitResult.cs

@@ -0,0 +1,41 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// Represents the client's response to an elicitation request.
+/// </summary>
+public class ElicitResult
+{
+    /// <summary>
+    /// Gets or sets the user action in response to the elicitation.
+    /// </summary>
+    /// <remarks>
+    /// <list type="bullet">
+    ///   <item>
+    ///     <term>"accept"</term>
+    ///     <description>User submitted the form/confirmed the action</description>
+    ///   </item>
+    ///   <item>
+    ///     <term>"decline"</term>
+    ///     <description>User explicitly declined the action</description>
+    ///   </item>
+    ///   <item>
+    ///     <term>"cancel"</term>
+    ///     <description>User dismissed without making an explicit choice</description>
+    ///   </item>
+    /// </list>
+    /// </remarks>
+    [JsonPropertyName("action")]
+    public string Action { get; set; } = "cancel";
+
+    /// <summary>
+    /// Gets or sets the submitted form data.
+    /// </summary>
+    /// <remarks>
+    /// This is typically omitted if the action is "cancel" or "decline".
+    /// </remarks>
+    [JsonPropertyName("content")]
+    public JsonElement? Content { get; set; }
+}

src/ModelContextProtocol/Protocol/ElicitationCapability.cs

@@ -0,0 +1,36 @@
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// Represents the capability for a client to provide server-requested additional information during interactions.
+/// </summary>
+/// <remarks>
+/// <para>
+/// This capability enables the MCP client to respond to elicitation requests from an MCP server.
+/// </para>
+/// <para>
+/// When this capability is enabled, an MCP server can request the client to provide additional information
+/// during interactions. The client must set a <see cref="ElicitationHandler"/> to process these requests.
+/// </para>
+/// </remarks>
+public class ElicitationCapability
+{
+    // Currently empty in the spec, but may be extended in the future.
+
+    /// <summary>
+    /// Gets or sets the handler for processing <see cref="RequestMethods.ElicitationCreate"/> requests.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This handler function is called when an MCP server requests the client to provide additional
+    /// information during interactions. The client must set this property for the elicitation capability to work.
+    /// </para>
+    /// <para>
+    /// The handler receives message parameters and a cancellation token.
+    /// It should return a <see cref="ElicitResult"/> containing the response to the elicitation request.
+    /// </para>
+    /// </remarks>
+    [JsonIgnore]
+    public Func<ElicitRequestParams?, CancellationToken, ValueTask<ElicitResult>>? ElicitationHandler { get; set; }
+}

src/ModelContextProtocol/Protocol/RequestMethods.cs

@@ -93,6 +93,15 @@ public static class RequestMethods
     /// </remarks>
     public const string SamplingCreateMessage = "sampling/createMessage";
 
+    /// <summary>
+    /// The name of the request method sent from the client to the server to elicit additional information from the user via the client.
+    /// </summary>
+    /// <remarks>
+    /// This request is used when the server needs more information from the client to proceed with a task or interaction.
+    /// Servers can request structured data from users, with optional JSON schemas to validate responses.
+    /// </remarks>
+    public const string ElicitationCreate = "elicitation/create";
+
     /// <summary>
     /// The name of the request method sent from the client to the server when it first connects, asking it initialize.
     /// </summary>