Implement SEP-1330: Standards-compliant enum schemas for elicitation (#969)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>

docs/concepts/elicitation/elicitation.md

@@ -16,9 +16,16 @@ The C# SDK registers an instance of <xref:ModelContextProtocol.Server.McpServer>
 so tools can simply add a parameter of type <xref:ModelContextProtocol.Server.McpServer> to their method signature to access it.
 
 The MCP Server must specify the schema of each input value it is requesting from the user.
-Only primitive types (string, number, boolean) are supported for elicitation requests.
+Primitive types (string, number, boolean) and enum types are supported for elicitation requests.
 The schema may include a description to help the user understand what is being requested.
 
+For enum types, the SDK supports several schema formats:
+- **UntitledSingleSelectEnumSchema**: A single-select enum where the enum values serve as both the value and display text
+- **TitledSingleSelectEnumSchema**: A single-select enum with separate display titles for each option (using JSON Schema `oneOf` with `const` and `title`)
+- **UntitledMultiSelectEnumSchema**: A multi-select enum allowing multiple values to be selected
+- **TitledMultiSelectEnumSchema**: A multi-select enum with display titles for each option
+- **LegacyTitledEnumSchema** (deprecated): The legacy enum schema using `enumNames` for backward compatibility
+
 The server can request a single input or multiple inputs at once.
 To help distinguish multiple inputs, each input has a unique name.
 

docs/concepts/elicitation/samples/server/Tools/InteractiveTools.cs

@@ -123,4 +123,152 @@ public sealed class InteractiveTools
             }
         }
     }
+
+    // <snippet_EnumExamples>
+    [McpServerTool, Description("Example tool demonstrating various enum schema types")]
+    public async Task<string> EnumExamples(
+        McpServer server,
+        CancellationToken token
+    )
+    {
+        // Example 1: UntitledSingleSelectEnumSchema - Simple enum without display titles
+        var prioritySchema = new RequestSchema
+        {
+            Properties =
+            {
+                ["Priority"] = new UntitledSingleSelectEnumSchema
+                {
+                    Title = "Priority Level",
+                    Description = "Select the priority level",
+                    Enum = ["low", "medium", "high", "critical"],
+                    Default = "medium"
+                }
+            }
+        };
+
+        var priorityResponse = await server.ElicitAsync(new ElicitRequestParams
+        {
+            Message = "Select a priority level:",
+            RequestedSchema = prioritySchema
+        }, token);
+
+        if (priorityResponse.Action != "accept")
+        {
+            return "Operation cancelled";
+        }
+
+        string? priority = priorityResponse.Content?["Priority"].GetString();
+
+        // Example 2: TitledSingleSelectEnumSchema - Enum with custom display titles
+        var severitySchema = new RequestSchema
+        {
+            Properties =
+            {
+                ["Severity"] = new TitledSingleSelectEnumSchema
+                {
+                    Title = "Issue Severity",
+                    Description = "Select the issue severity level",
+                    OneOf =
+                    [
+                        new EnumSchemaOption { Const = "p0", Title = "P0 - Critical (Immediate attention required)" },
+                        new EnumSchemaOption { Const = "p1", Title = "P1 - High (Urgent, within 24 hours)" },
+                        new EnumSchemaOption { Const = "p2", Title = "P2 - Medium (Within a week)" },
+                        new EnumSchemaOption { Const = "p3", Title = "P3 - Low (As time permits)" }
+                    ],
+                    Default = "p2"
+                }
+            }
+        };
+
+        var severityResponse = await server.ElicitAsync(new ElicitRequestParams
+        {
+            Message = "Select the issue severity:",
+            RequestedSchema = severitySchema
+        }, token);
+
+        if (severityResponse.Action != "accept")
+        {
+            return "Operation cancelled";
+        }
+
+        string? severity = severityResponse.Content?["Severity"].GetString();
+
+        // Example 3: UntitledMultiSelectEnumSchema - Select multiple values
+        var tagsSchema = new RequestSchema
+        {
+            Properties =
+            {
+                ["Tags"] = new UntitledMultiSelectEnumSchema
+                {
+                    Title = "Tags",
+                    Description = "Select one or more tags",
+                    MinItems = 1,
+                    MaxItems = 3,
+                    Items = new UntitledEnumItemsSchema
+                    {
+                        Type = "string",
+                        Enum = ["bug", "feature", "documentation", "enhancement", "question"]
+                    },
+                    Default = ["bug"]
+                }
+            }
+        };
+
+        var tagsResponse = await server.ElicitAsync(new ElicitRequestParams
+        {
+            Message = "Select up to 3 tags:",
+            RequestedSchema = tagsSchema
+        }, token);
+
+        if (tagsResponse.Action != "accept")
+        {
+            return "Operation cancelled";
+        }
+
+        // For multi-select, the value is an array
+        var tags = tagsResponse.Content?["Tags"].EnumerateArray()
+            .Select(e => e.GetString())
+            .ToArray();
+
+        // Example 4: TitledMultiSelectEnumSchema - Multi-select with custom titles
+        var featuresSchema = new RequestSchema
+        {
+            Properties =
+            {
+                ["Features"] = new TitledMultiSelectEnumSchema
+                {
+                    Title = "Features",
+                    Description = "Select desired features",
+                    Items = new TitledEnumItemsSchema
+                    {
+                        AnyOf =
+                        [
+                            new EnumSchemaOption { Const = "auth", Title = "Authentication & Authorization" },
+                            new EnumSchemaOption { Const = "api", Title = "RESTful API" },
+                            new EnumSchemaOption { Const = "ui", Title = "Modern UI Components" },
+                            new EnumSchemaOption { Const = "db", Title = "Database Integration" }
+                        ]
+                    }
+                }
+            }
+        };
+
+        var featuresResponse = await server.ElicitAsync(new ElicitRequestParams
+        {
+            Message = "Select desired features:",
+            RequestedSchema = featuresSchema
+        }, token);
+
+        if (featuresResponse.Action != "accept")
+        {
+            return "Operation cancelled";
+        }
+
+        var features = featuresResponse.Content?["Features"].EnumerateArray()
+            .Select(e => e.GetString())
+            .ToArray();
+
+        return $"Selected: Priority={priority}, Severity={severity}, Tags=[{string.Join(", ", tags ?? [])}], Features=[{string.Join(", ", features ?? [])}]";
+    }
+    // </snippet_EnumExamples>
 }