Auto-populate completion handlers from AllowedValuesAttribute on prompt/resource parameters

docs/concepts/completions/completions.md

@@ -81,6 +81,42 @@ builder.Services.AddMcpServer()
     });
 ```
 
+### Automatic completions with AllowedValuesAttribute
+
+For parameters with a known set of valid values, you can use `System.ComponentModel.DataAnnotations.AllowedValuesAttribute` on `string` parameters of prompts or resource templates. The server will automatically surface those values as completions without needing a custom completion handler.
+
+#### Prompt parameters
+
+```csharp
+[McpServerPromptType]
+public class MyPrompts
+{
+    [McpServerPrompt, Description("Generates a code review prompt")]
+    public static ChatMessage CodeReview(
+        [Description("The programming language")]
+        [AllowedValues("csharp", "python", "javascript", "typescript", "go", "rust")]
+        string language,
+        [Description("The code to review")] string code)
+        => new(ChatRole.User, $"Please review the following {language} code:\n\n```{language}\n{code}\n```");
+}
+```
+
+#### Resource template parameters
+
+```csharp
+[McpServerResourceType]
+public class MyResources
+{
+    [McpServerResource("config://settings/{section}"), Description("Reads a configuration section")]
+    public static string ReadConfig(
+        [AllowedValues("general", "network", "security", "logging")]
+        string section)
+        => GetConfig(section);
+}
+```
+
+With these attributes in place, when a client sends a `completion/complete` request for the `language` or `section` argument, the server will automatically filter and return matching values based on what the user has typed so far. This approach can be combined with a custom completion handler registered via `WithCompleteHandler`; the handler's results are returned first, followed by any matching `AllowedValues`.
+
 ### Requesting completions on the client
 
 Clients request completions using <xref:ModelContextProtocol.Client.McpClient.CompleteAsync*>. Provide a reference to the prompt or resource template, the argument name, and the current partial value:

src/ModelContextProtocol.Core/Server/McpServerImpl.cs

@@ -245,12 +245,62 @@ private void ConfigureCompletion(McpServerOptions options)
         var completeHandler = options.Handlers.CompleteHandler;
         var completionsCapability = options.Capabilities?.Completions;
 
-        if (completeHandler is null && completionsCapability is null)
+        // Build completion value lookups from prompt/resource collections' [AllowedValues]-attributed parameters.
+        Dictionary<string, Dictionary<string, string[]>>? promptCompletions = BuildAllowedValueCompletions(options.PromptCollection);
+        Dictionary<string, Dictionary<string, string[]>>? resourceCompletions = BuildAllowedValueCompletions(options.ResourceCollection);
+        bool hasCollectionCompletions = promptCompletions is not null || resourceCompletions is not null;
+
+        if (completeHandler is null && completionsCapability is null && !hasCollectionCompletions)
         {
             return;
         }
 
         completeHandler ??= (static async (_, __) => new CompleteResult());
+
+        // Augment the completion handler with allowed values from prompt/resource collections.
+        if (hasCollectionCompletions)
+        {
+            var originalCompleteHandler = completeHandler;
+            completeHandler = async (request, cancellationToken) =>
+            {
+                CompleteResult result = await originalCompleteHandler(request, cancellationToken).ConfigureAwait(false);
+
+                string[]? allowedValues = null;
+                switch (request.Params?.Ref)
+                {
+                    case PromptReference pr when promptCompletions is not null:
+                        if (promptCompletions.TryGetValue(pr.Name, out var promptParams))
+                        {
+                            promptParams.TryGetValue(request.Params.Argument.Name, out allowedValues);
+                        }
+                        break;
+
+                    case ResourceTemplateReference rtr when resourceCompletions is not null:
+                        if (rtr.Uri is not null && resourceCompletions.TryGetValue(rtr.Uri, out var resourceParams))
+                        {
+                            resourceParams.TryGetValue(request.Params.Argument.Name, out allowedValues);
+                        }
+                        break;
+                }
+
+                if (allowedValues is not null)
+                {
+                    string partialValue = request.Params!.Argument.Value;
+                    foreach (var v in allowedValues)
+                    {
+                        if (v.StartsWith(partialValue, StringComparison.OrdinalIgnoreCase))
+                        {
+                            result.Completion.Values.Add(v);
+                        }
+                    }
+
+                    result.Completion.Total = result.Completion.Values.Count;
+                }
+
+                return result;
+            };
+        }
+
         completeHandler = BuildFilterPipeline(completeHandler, options.Filters.Request.CompleteFilters);
 
         ServerCapabilities.Completions = new();
@@ -262,6 +312,76 @@ private void ConfigureCompletion(McpServerOptions options)
             McpJsonUtilities.JsonContext.Default.CompleteResult);
     }
 
+    /// <summary>
+    /// Builds a lookup of primitive name/URI → (parameter name → allowed values) from the enum values
+    /// in the JSON schemas of AIFunction-based prompts or resources.
+    /// </summary>
+    private static Dictionary<string, Dictionary<string, string[]>>? BuildAllowedValueCompletions<T>(
+        McpServerPrimitiveCollection<T>? primitives) where T : class, IMcpServerPrimitive
+    {
+        if (primitives is null)
+        {
+            return null;
+        }
+
+        Dictionary<string, Dictionary<string, string[]>>? result = null;
+        foreach (var primitive in primitives)
+        {
+            JsonElement schema;
+            string id;
+            if (primitive is AIFunctionMcpServerPrompt aiPrompt)
+            {
+                schema = aiPrompt.AIFunction.JsonSchema;
+                id = aiPrompt.ProtocolPrompt.Name;
+            }
+            else if (primitive is AIFunctionMcpServerResource aiResource && aiResource.IsTemplated)
+            {
+                schema = aiResource.AIFunction.JsonSchema;
+                id = aiResource.ProtocolResourceTemplate.UriTemplate;
+            }
+            else
+            {
+                continue;
+            }
+
+            if (schema.TryGetProperty("properties", out JsonElement properties) &&
+                properties.ValueKind is JsonValueKind.Object)
+            {
+                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;
+                }
+            }
+        }
+
+        return result;
+    }
+
     private void ConfigureExperimental(McpServerOptions options)
     {
         ServerCapabilities.Experimental = options.Capabilities?.Experimental;

src/ModelContextProtocol.Core/Server/McpServerPrompt.cs

@@ -116,6 +116,11 @@ namespace ModelContextProtocol.Server;
 /// <para>
 /// Other returned types will result in an <see cref="InvalidOperationException"/> being thrown.
 /// </para>
+/// <para>
+/// Parameters of type <see cref="string"/> that are decorated with <c>AllowedValuesAttribute</c>
+/// will automatically have their allowed values surfaced as completions in response to <c>completion/complete</c> requests from clients,
+/// without requiring a custom <see cref="McpServerHandlers.CompleteHandler"/> to be configured.
+/// </para>
 /// </remarks>
 public abstract class McpServerPrompt : IMcpServerPrimitive
 {

src/ModelContextProtocol.Core/Server/McpServerPromptAttribute.cs

@@ -103,6 +103,11 @@ namespace ModelContextProtocol.Server;
 /// <para>
 /// Other returned types will result in an <see cref="InvalidOperationException"/> being thrown.
 /// </para>
+/// <para>
+/// Parameters of type <see cref="string"/> that are decorated with <c>AllowedValuesAttribute</c>
+/// will automatically have their allowed values surfaced as completions in response to <c>completion/complete</c> requests from clients,
+/// without requiring a custom <see cref="McpServerHandlers.CompleteHandler"/> to be configured.
+/// </para>
 /// </remarks>
 [AttributeUsage(AttributeTargets.Method)]
 public sealed class McpServerPromptAttribute : Attribute

src/ModelContextProtocol.Core/Server/McpServerResource.cs

@@ -121,6 +121,11 @@ namespace ModelContextProtocol.Server;
 /// <para>
 /// Other returned types will result in an <see cref="InvalidOperationException"/> being thrown.
 /// </para>
+/// <para>
+/// Parameters of type <see cref="string"/> that are decorated with <c>AllowedValuesAttribute</c>
+/// will automatically have their allowed values surfaced as completions in response to <c>completion/complete</c> requests from clients,
+/// without requiring a custom <see cref="McpServerHandlers.CompleteHandler"/> to be configured.
+/// </para>
 /// </remarks>
 public abstract class McpServerResource : IMcpServerPrimitive
 {

src/ModelContextProtocol.Core/Server/McpServerResourceAttribute.cs

@@ -105,6 +105,11 @@ namespace ModelContextProtocol.Server;
 /// <para>
 /// Other returned types will result in an <see cref="InvalidOperationException"/> being thrown.
 /// </para>
+/// <para>
+/// Parameters of type <see cref="string"/> that are decorated with <c>AllowedValuesAttribute</c>
+/// will automatically have their allowed values surfaced as completions in response to <c>completion/complete</c> requests from clients,
+/// without requiring a custom <see cref="McpServerHandlers.CompleteHandler"/> to be configured.
+/// </para>
 /// </remarks>
 [AttributeUsage(AttributeTargets.Method)]
 public sealed class McpServerResourceAttribute : Attribute