Hide experimental types from external source generators using internal property pattern

ModelContextProtocol.slnx

@@ -78,5 +78,6 @@
     <Project Path="tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj" />
     <Project Path="tests/ModelContextProtocol.TestServer/ModelContextProtocol.TestServer.csproj" />
     <Project Path="tests/ModelContextProtocol.TestSseServer/ModelContextProtocol.TestSseServer.csproj" />
+    <Project Path="tests/ModelContextProtocol.ExperimentalApiRegressionTest/ModelContextProtocol.ExperimentalApiRegressionTest.csproj" />
   </Folder>
 </Solution>

src/ModelContextProtocol.Core/Protocol/CallToolRequestParams.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json;
 using System.Text.Json.Serialization;
 
@@ -33,6 +34,16 @@ public sealed class CallToolRequestParams : RequestParams
     /// When present, indicates that the requestor wants this operation executed as a task.
     /// The receiver must support task augmentation for this specific request type.
     /// </remarks>
+    [Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)]
+    [JsonIgnore]
+    public McpTaskMetadata? Task
+    {
+        get => TaskCore;
+        set => TaskCore = value;
+    }
+
+    // See ExperimentalInternalPropertyTests.cs before modifying this property.
+    [JsonInclude]
     [JsonPropertyName("task")]
-    public McpTaskMetadata? Task { get; set; }
+    internal McpTaskMetadata? TaskCore { get; set; }
 }

src/ModelContextProtocol.Core/Protocol/CallToolResult.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json.Nodes;
 using System.Text.Json.Serialization;
 
@@ -65,6 +66,16 @@ public sealed class CallToolResult : Result
     /// (<see cref="Content"/>, <see cref="StructuredContent"/>, <see cref="IsError"/>) may not be populated.
     /// The actual tool result can be retrieved later via <c>tasks/result</c>.
     /// </remarks>
+    [Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)]
+    [JsonIgnore]
+    public McpTask? Task
+    {
+        get => TaskCore;
+        set => TaskCore = value;
+    }
+
+    // See ExperimentalInternalPropertyTests.cs before modifying this property.
+    [JsonInclude]
     [JsonPropertyName("task")]
-    public McpTask? Task { get; set; }
+    internal McpTask? TaskCore { get; set; }
 }

src/ModelContextProtocol.Core/Protocol/ClientCapabilities.cs

@@ -1,4 +1,5 @@
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json.Serialization;
 using ModelContextProtocol.Client;
 using ModelContextProtocol.Server;
@@ -80,6 +81,16 @@ public sealed class ClientCapabilities
     /// See <see cref="McpTasksCapability"/> for details on configuring which operations support tasks.
     /// </para>
     /// </remarks>
+    [Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)]
+    [JsonIgnore]
+    public McpTasksCapability? Tasks
+    {
+        get => TasksCore;
+        set => TasksCore = value;
+    }
+
+    // See ExperimentalInternalPropertyTests.cs before modifying this property.
+    [JsonInclude]
     [JsonPropertyName("tasks")]
-    public McpTasksCapability? Tasks { get; set; }
+    internal McpTasksCapability? TasksCore { get; set; }
 }

src/ModelContextProtocol.Core/Protocol/CreateMessageRequestParams.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json;
 using System.Text.Json.Serialization;
 
@@ -128,6 +129,16 @@ public sealed class CreateMessageRequestParams : RequestParams
     /// When present, indicates that the requestor wants this operation executed as a task.
     /// The receiver must support task augmentation for this specific request type.
     /// </remarks>
+    [Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)]
+    [JsonIgnore]
+    public McpTaskMetadata? Task
+    {
+        get => TaskCore;
+        set => TaskCore = value;
+    }
+
+    // See ExperimentalInternalPropertyTests.cs before modifying this property.
+    [JsonInclude]
     [JsonPropertyName("task")]
-    public McpTaskMetadata? Task { get; set; }
+    internal McpTaskMetadata? TaskCore { get; set; }
 }

src/ModelContextProtocol.Core/Protocol/ElicitRequestParams.cs

@@ -99,8 +99,18 @@ public string Mode
     /// When present, indicates that the requestor wants this operation executed as a task.
     /// The receiver must support task augmentation for this specific request type.
     /// </remarks>
+    [Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)]
+    [JsonIgnore]
+    public McpTaskMetadata? Task
+    {
+        get => TaskCore;
+        set => TaskCore = value;
+    }
+
+    // See ExperimentalInternalPropertyTests.cs before modifying this property.
+    [JsonInclude]
     [JsonPropertyName("task")]
-    public McpTaskMetadata? Task { get; set; }
+    internal McpTaskMetadata? TaskCore { get; set; }
 
     /// <summary>Represents a request schema used in a form mode elicitation request.</summary>
     public sealed class RequestSchema

src/ModelContextProtocol.Core/Protocol/ServerCapabilities.cs

@@ -1,4 +1,5 @@
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json.Serialization;
 using ModelContextProtocol.Server;
 
@@ -79,6 +80,16 @@ public sealed class ServerCapabilities
     /// See <see cref="McpTasksCapability"/> for details on configuring which operations support tasks.
     /// </para>
     /// </remarks>
+    [Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)]
+    [JsonIgnore]
+    public McpTasksCapability? Tasks
+    {
+        get => TasksCore;
+        set => TasksCore = value;
+    }
+
+    // See ExperimentalInternalPropertyTests.cs before modifying this property.
+    [JsonInclude]
     [JsonPropertyName("tasks")]
-    public McpTasksCapability? Tasks { get; set; }
+    internal McpTasksCapability? TasksCore { get; set; }
 }

src/ModelContextProtocol.Core/Protocol/Tool.cs

@@ -120,8 +120,17 @@ public JsonElement? OutputSchema
     /// regarding task augmentation support. See <see cref="ToolExecution"/> for details.
     /// </remarks>
     [Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)]
+    [JsonIgnore]
+    public ToolExecution? Execution
+    {
+        get => ExecutionCore;
+        set => ExecutionCore = value;
+    }
+
+    // See ExperimentalInternalPropertyTests.cs before modifying this property.
+    [JsonInclude]
     [JsonPropertyName("execution")]
-    public ToolExecution? Execution { get; set; }
+    internal ToolExecution? ExecutionCore { get; set; }
 
     /// <summary>
     /// Gets or sets an optional list of icons for this tool.

tests/ModelContextProtocol.ExperimentalApiRegressionTest/ExperimentalPropertyRegressionContext.cs

@@ -0,0 +1,17 @@
+using System.Text.Json.Serialization;
+using ModelContextProtocol.Protocol;
+
+namespace ModelContextProtocol.ExperimentalApiRegressionTest;
+
+/// <summary>
+/// This file validates that the System.Text.Json source generator does not produce
+/// MCPEXP001 diagnostics for MCP protocol types with experimental properties.
+/// </summary>
+[JsonSerializable(typeof(Tool))]
+[JsonSerializable(typeof(ServerCapabilities))]
+[JsonSerializable(typeof(ClientCapabilities))]
+[JsonSerializable(typeof(CallToolResult))]
+[JsonSerializable(typeof(CallToolRequestParams))]
+[JsonSerializable(typeof(CreateMessageRequestParams))]
+[JsonSerializable(typeof(ElicitRequestParams))]
+internal partial class ExperimentalPropertyRegressionContext : JsonSerializerContext;

tests/ModelContextProtocol.ExperimentalApiRegressionTest/ModelContextProtocol.ExperimentalApiRegressionTest.csproj

@@ -0,0 +1,29 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFrameworks>net10.0;net9.0;net8.0</TargetFrameworks>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+
+    <!--
+      This project validates that MCPEXP001 diagnostics are NOT emitted by the
+      System.Text.Json source generator for types with experimental properties.
+      tests/Directory.Build.props suppresses MCPEXP001 globally for all test projects,
+      but this project must NOT suppress it: its successful compilation IS the test.
+    -->
+    <NoWarn>$(NoWarn.Replace('MCPEXP001',''))</NoWarn>
+
+    <!--
+      [JsonInclude] internal members trigger this warning only because the MCP C# SDK
+      is a project reference here. NuGet consumers won't see it since Roslyn doesn't import
+      internal members from metadata.
+    -->
+    <NoWarn>$(NoWarn);SYSLIB1038</NoWarn>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\ModelContextProtocol.Core\ModelContextProtocol.Core.csproj" />
+  </ItemGroup>
+
+</Project>

tests/ModelContextProtocol.Tests/ExperimentalInternalPropertyTests.cs

@@ -0,0 +1,69 @@
+using System.Reflection;
+using System.Text.Json.Serialization;
+using ModelContextProtocol.Protocol;
+
+namespace ModelContextProtocol.Tests;
+
+/// <summary>
+/// Validates the internal property pattern used for experimental MCP properties.
+/// </summary>
+/// <remarks>
+/// Experimental properties on stable protocol types must use an internal serialization
+/// property to hide the experimental type from external source generators. The public
+/// property is marked [Experimental][JsonIgnore] and delegates to an internal *Core
+/// property marked [JsonInclude][JsonPropertyName].
+/// </remarks>
+public class ExperimentalInternalPropertyTests
+{
+    [Fact]
+    public void ExperimentalProperties_MustBeHiddenFromSourceGenerator()
+    {
+        // [Experimental] properties on stable protocol types must use the internal property
+        // pattern so the STJ source generator does not reference experimental types in
+        // generated code (which would trigger MCPEXP001 for consumers).
+        //
+        // Required pattern:
+        //   1. Mark the public property [Experimental][JsonIgnore]
+        //   2. Add an internal *Core property with [JsonInclude][JsonPropertyName]
+        //      that the public property delegates to
+        //
+        // To stabilize:
+        //   1. Remove [Experimental] and [JsonIgnore] from the public property
+        //   2. Add [JsonPropertyName] to the public property
+        //   3. Convert to auto-property
+        //   4. Remove the internal *Core property
+
+        foreach (var (type, prop) in GetExperimentalPropertiesOnStableTypes())
+        {
+            Assert.True(
+                prop.GetCustomAttribute<JsonIgnoreAttribute>() is not null,
+                $"{type.Name}.{prop.Name} is [Experimental] but missing [JsonIgnore].");
+
+            Assert.True(
+                prop.GetCustomAttribute<JsonPropertyNameAttribute>() is null,
+                $"{type.Name}.{prop.Name} is [Experimental] and must not have [JsonPropertyName].");
+        }
+    }
+
+    private static IEnumerable<(Type Type, PropertyInfo Property)> GetExperimentalPropertiesOnStableTypes()
+    {
+        var protocolTypes = typeof(Tool).Assembly.GetTypes()
+            .Where(t => t.Namespace == "ModelContextProtocol.Protocol" && t.IsClass && !t.IsAbstract);
+
+        foreach (var type in protocolTypes)
+        {
+            if (type.GetCustomAttributes().Any(a => a.GetType().Name == "ExperimentalAttribute"))
+            {
+                continue;
+            }
+
+            foreach (var prop in type.GetProperties(BindingFlags.Public | BindingFlags.Instance))
+            {
+                if (prop.GetCustomAttributes().Any(a => a.GetType().Name == "ExperimentalAttribute"))
+                {
+                    yield return (type, prop);
+                }
+            }
+        }
+    }
+}

tests/ModelContextProtocol.Tests/ExperimentalPropertySerializationTests.cs

@@ -0,0 +1,118 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using ModelContextProtocol.Protocol;
+
+namespace ModelContextProtocol.Tests;
+
+/// <summary>
+/// Validates that the internal property pattern used for experimental properties
+/// produces the expected serialization behavior for SDK consumers using source generators.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Experimental properties (e.g. <see cref="Tool.Execution"/>, <see cref="ServerCapabilities.Tasks"/>)
+/// use an internal <c>*Core</c> property for serialization. A consumer's source-generated
+/// <see cref="JsonSerializerContext"/> cannot see internal members, so experimental data is
+/// silently dropped unless the consumer chains the SDK's resolver into their options.
+/// </para>
+/// <para>
+/// These tests depend on <see cref="Tool.Execution"/> and <see cref="ServerCapabilities.Tasks"/>
+/// being experimental. When those APIs stabilize, update these tests to reference whatever
+/// experimental properties exist at that time, or remove them entirely if no experimental
+/// APIs remain.
+/// </para>
+/// </remarks>
+public class ExperimentalPropertySerializationTests
+{
+    [Fact]
+    public void ExperimentalProperties_Dropped_WithConsumerContextOnly()
+    {
+        var options = new JsonSerializerOptions
+        {
+            TypeInfoResolverChain = { ConsumerJsonContext.Default }
+        };
+
+        var tool = new Tool
+        {
+            Name = "test-tool",
+            Execution = new ToolExecution { TaskSupport = ToolTaskSupport.Optional }
+        };
+
+        string json = JsonSerializer.Serialize(tool, options);
+        Assert.DoesNotContain("\"execution\"", json);
+        Assert.Contains("\"name\"", json);
+    }
+
+    [Fact]
+    public void ExperimentalProperties_IgnoredOnDeserialize_WithConsumerContextOnly()
+    {
+        string json = JsonSerializer.Serialize(
+            new Tool
+            {
+                Name = "test-tool",
+                Execution = new ToolExecution { TaskSupport = ToolTaskSupport.Optional }
+            },
+            McpJsonUtilities.DefaultOptions);
+        Assert.Contains("\"execution\"", json);
+
+        var options = new JsonSerializerOptions
+        {
+            TypeInfoResolverChain = { ConsumerJsonContext.Default }
+        };
+        var deserialized = JsonSerializer.Deserialize<Tool>(json, options)!;
+        Assert.Equal("test-tool", deserialized.Name);
+        Assert.Null(deserialized.Execution);
+    }
+
+    [Fact]
+    public void ExperimentalProperties_RoundTrip_WhenSdkResolverIsChained()
+    {
+        var options = new JsonSerializerOptions
+        {
+            TypeInfoResolverChain =
+            {
+                McpJsonUtilities.DefaultOptions.TypeInfoResolver!,
+                ConsumerJsonContext.Default,
+            }
+        };
+
+        var tool = new Tool
+        {
+            Name = "test-tool",
+            Execution = new ToolExecution { TaskSupport = ToolTaskSupport.Optional }
+        };
+
+        string json = JsonSerializer.Serialize(tool, options);
+        Assert.Contains("\"execution\"", json);
+        Assert.Contains("\"name\"", json);
+
+        var deserialized = JsonSerializer.Deserialize<Tool>(json, options)!;
+        Assert.Equal("test-tool", deserialized.Name);
+        Assert.NotNull(deserialized.Execution);
+        Assert.Equal(ToolTaskSupport.Optional, deserialized.Execution.TaskSupport);
+    }
+
+    [Fact]
+    public void ExperimentalProperties_RoundTrip_WithDefaultOptions()
+    {
+        var capabilities = new ServerCapabilities
+        {
+            Tasks = new McpTasksCapability()
+        };
+
+        string json = JsonSerializer.Serialize(capabilities, McpJsonUtilities.DefaultOptions);
+        Assert.Contains("\"tasks\"", json);
+
+        var deserialized = JsonSerializer.Deserialize<ServerCapabilities>(json, McpJsonUtilities.DefaultOptions)!;
+        Assert.NotNull(deserialized.Tasks);
+    }
+}
+
+[JsonSerializable(typeof(Tool))]
+[JsonSerializable(typeof(ServerCapabilities))]
+[JsonSerializable(typeof(ClientCapabilities))]
+[JsonSerializable(typeof(CallToolResult))]
+[JsonSerializable(typeof(CallToolRequestParams))]
+[JsonSerializable(typeof(CreateMessageRequestParams))]
+[JsonSerializable(typeof(ElicitRequestParams))]
+internal partial class ConsumerJsonContext : JsonSerializerContext;