Avoid rebuilding registered primitive collections per stateless request

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerPrompt.cs

@@ -177,6 +177,13 @@ private static McpServerPromptCreateOptions DeriveOptions(MethodInfo method, Mcp
     /// <summary>Gets the <see cref="AIFunction"/> wrapped by this prompt.</summary>
     internal AIFunction AIFunction { get; }
 
+    /// <summary>Backing cache for <see cref="AllowedValueCompletions"/>.</summary>
+    private object? _allowedValueCompletions;
+
+    /// <summary>Gets the values that each restricted parameter of the prompt allows, or <see langword="null"/> if none are restricted.</summary>
+    internal Dictionary<string, string[]>? AllowedValueCompletions =>
+        Server.AllowedValueCompletions.GetOrCreate(ref _allowedValueCompletions, AIFunction.JsonSchema);
+
     /// <summary>Initializes a new instance of the <see cref="McpServerPrompt"/> class.</summary>
     private AIFunctionMcpServerPrompt(AIFunction function, Prompt prompt, IReadOnlyList<object> metadata)
     {

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerResource.cs

@@ -289,6 +289,13 @@ private static string DeriveUriTemplate(string name, AIFunction function)
     /// <summary>Gets the <see cref="AIFunction"/> wrapped by this resource.</summary>
     internal AIFunction AIFunction { get; }
 
+    /// <summary>Backing cache for <see cref="AllowedValueCompletions"/>.</summary>
+    private object? _allowedValueCompletions;
+
+    /// <summary>Gets the values that each restricted parameter of the resource allows, or <see langword="null"/> if none are restricted.</summary>
+    internal Dictionary<string, string[]>? AllowedValueCompletions =>
+        Server.AllowedValueCompletions.GetOrCreate(ref _allowedValueCompletions, AIFunction.JsonSchema);
+
     /// <summary>Initializes a new instance of the <see cref="McpServerResource"/> class.</summary>
     private AIFunctionMcpServerResource(AIFunction function, ResourceTemplate resourceTemplate, IReadOnlyList<object> metadata)
     {

src/ModelContextProtocol.Core/Server/AllowedValueCompletions.cs

@@ -0,0 +1,68 @@
+using System.Text.Json;
+
+namespace ModelContextProtocol.Server;
+
+/// <summary>
+/// Extracts the values that parameters of an AIFunction-based prompt or resource are restricted to, from the enum values
+/// in the function's JSON schema, for use as completions.
+/// </summary>
+internal static class AllowedValueCompletions
+{
+    /// <summary>Marks a cache whose function was found to have no restricted parameters.</summary>
+    private static readonly object s_none = new();
+
+    /// <summary>
+    /// Gets the parameter name → allowed values lookup for <paramref name="schema"/>, computing and caching it in
+    /// <paramref name="cache"/> on first use. The returned dictionary is shared and must not be modified.
+    /// </summary>
+    /// <remarks>
+    /// The schema of an AIFunction is immutable, so the result can be computed once per primitive rather than once per
+    /// session. Concurrent first calls may each compute it; the results are equivalent.
+    /// </remarks>
+    public static Dictionary<string, string[]>? GetOrCreate(ref object? cache, JsonElement schema)
+    {
+        object? cached = Volatile.Read(ref cache);
+        if (cached is null)
+        {
+            cached = (object?)Create(schema) ?? s_none;
+            Volatile.Write(ref cache, cached);
+        }
+
+        return cached as Dictionary<string, string[]>;
+    }
+
+    private static Dictionary<string, string[]>? Create(JsonElement schema)
+    {
+        if (!schema.TryGetProperty("properties", out JsonElement properties) ||
+            properties.ValueKind is not JsonValueKind.Object)
+        {
+            return null;
+        }
+
+        Dictionary<string, string[]>? paramValues = null;
+        foreach (var param in properties.EnumerateObject())
+        {
+            if (param.Value.TryGetProperty("enum", out JsonElement enumValues) &&
+                enumValues.ValueKind is JsonValueKind.Array)
+            {
+                List<string>? values = null;
+                foreach (var item in enumValues.EnumerateArray())
+                {
+                    if (item.ValueKind is JsonValueKind.String && item.GetString() is { } str)
+                    {
+                        values ??= [];
+                        values.Add(str);
+                    }
+                }
+
+                if (values is not null)
+                {
+                    paramValues ??= new(StringComparer.Ordinal);
+                    paramValues[param.Name] = [.. values];
+                }
+            }
+        }
+
+        return paramValues;
+    }
+}

src/ModelContextProtocol.Core/Server/McpServerImpl.cs

@@ -1119,55 +1119,27 @@ private void ConfigureCompletion(McpServerOptions options)
         Dictionary<string, Dictionary<string, string[]>>? result = null;
         foreach (var primitive in primitives)
         {
-            JsonElement schema;
+            Dictionary<string, string[]>? paramValues;
             string id;
             if (primitive is AIFunctionMcpServerPrompt aiPrompt)
             {
-                schema = aiPrompt.AIFunction.JsonSchema;
+                paramValues = aiPrompt.AllowedValueCompletions;
                 id = aiPrompt.ProtocolPrompt.Name;
             }
             else if (primitive is AIFunctionMcpServerResource aiResource && aiResource.IsTemplated)
             {
-                schema = aiResource.AIFunction.JsonSchema;
+                paramValues = aiResource.AllowedValueCompletions;
                 id = aiResource.ProtocolResourceTemplate.UriTemplate;
             }
             else
             {
                 continue;
             }
 
-            if (schema.TryGetProperty("properties", out JsonElement properties) &&
-                properties.ValueKind is JsonValueKind.Object)
+            if (paramValues is not null)
             {
-                Dictionary<string, string[]>? paramValues = null;
-                foreach (var param in properties.EnumerateObject())
-                {
-                    if (param.Value.TryGetProperty("enum", out JsonElement enumValues) &&
-                        enumValues.ValueKind is JsonValueKind.Array)
-                    {
-                        List<string>? values = null;
-                        foreach (var item in enumValues.EnumerateArray())
-                        {
-                            if (item.ValueKind is JsonValueKind.String && item.GetString() is { } str)
-                            {
-                                values ??= [];
-                                values.Add(str);
-                            }
-                        }
-
-                        if (values is not null)
-                        {
-                            paramValues ??= new(StringComparer.Ordinal);
-                            paramValues[param.Name] = [.. values];
-                        }
-                    }
-                }
-
-                if (paramValues is not null)
-                {
-                    result ??= new(StringComparer.Ordinal);
-                    result[id] = paramValues;
-                }
+                result ??= new(StringComparer.Ordinal);
+                result[id] = paramValues;
             }
         }