Add context to complete requests (#488)

Author: stephentoub

src/ModelContextProtocol.Core/Protocol/CompleteContext.cs

@@ -0,0 +1,19 @@
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// Represents additional context information for completion requests.
+/// </summary>
+/// <remarks>
+/// This context provides information that helps the server generate more relevant 
+/// completion suggestions, such as previously resolved variables in a template.
+/// </remarks>
+public class CompleteContext
+{
+    /// <summary>
+    /// Gets or sets previously-resolved variables in a URI template or prompt.
+    /// </summary>
+    [JsonPropertyName("arguments")]
+    public IDictionary<string, string>? Arguments { get; init; }
+}

src/ModelContextProtocol.Core/Protocol/CompleteRequestParams.cs

@@ -30,5 +30,11 @@ public class CompleteRequestParams : RequestParams
     /// and the current partial input.
     /// </summary>
     [JsonPropertyName("argument")]
-    public required Argument Argument { get; init; }    
+    public required Argument Argument { get; init; }
+
+    /// <summary>
+    /// Gets or sets additional, optional context for completions.
+    /// </summary>
+    [JsonPropertyName("context")]
+    public CompleteContext? Context { get; init; }
 }