Pin built-in MCP clients to the current protocol

Author: NikolayPianikov

src/AI.Tools/DefaultToolSessionFactory.cs

@@ -15,6 +15,8 @@ public sealed class DefaultToolSessionFactory(
     IChatTemporaryDirectory temporaryDirectory,
     IToolEnvironment toolEnvironment) : IMcpServerConnection
 {
+    private const string ProtocolVersion = "2026-07-28";
+
     public Guid ServerId => DefaultMcpServer.Id;
 
     /// <summary>Name of the environment variable through which the built-in server receives its directory grants.</summary>
@@ -30,7 +32,12 @@ public async Task<IToolSession> OpenAsync(IReadOnlyList<ToolDirectoryGrant> dire
         // replaced. Every call has returned by then, so there is nothing left to wait for.
         var transport = toolEnvironment.Launch(new ToolProcessLaunch("Default tools", executable, [], null, environment,
             InheritEnvironment: false, ShutdownTimeout: TimeSpan.FromMilliseconds(500)));
-        var client = await McpClient.CreateAsync(transport, cancellationToken: cancellationToken);
+        // This is our own current server, so do not let a slow discovery response make the SDK
+        // fall back to the legacy initialize handshake on the same connection.
+        var client = await McpClient.CreateAsync(transport, new McpClientOptions
+        {
+            ProtocolVersion = ProtocolVersion
+        }, cancellationToken: cancellationToken);
         try
         {
             var tools = await client.ListToolsAsync(cancellationToken: cancellationToken);

tests/AI.Mcp.BuiltIn.Tests/SandboxedToolTests.cs

@@ -14,6 +14,8 @@ namespace AI.Mcp.BuiltIn.Tests;
 [Trait("Category", "Integration")]
 public sealed class SandboxedToolTests : IAsyncLifetime
 {
+    private const string ProtocolVersion = "2026-07-28";
+
     private static readonly string[] ReadOnly = ["read"];
     private static readonly string[] Everything = ["read", "write", "edit", "delete"];
 
@@ -41,17 +43,20 @@ public async ValueTask InitializeAsync()
             new { root = _writable, recursive = true, capabilities = Everything, purpose = "project" },
             new { root = _temporary, recursive = true, capabilities = Everything, purpose = "chatTemporary" }
         });
-        _client = await McpClient.CreateAsync(new StdioClientTransport(new StdioClientTransportOptions
-        {
-            Name = "Default tools",
-            Command = "dotnet",
-            Arguments = [Path.Combine(AppContext.BaseDirectory, "AI.Mcp.BuiltIn.dll")],
-            EnvironmentVariables = new Dictionary<string, string?>
+        _client = await McpClient.CreateAsync(
+            new StdioClientTransport(new StdioClientTransportOptions
             {
-                ["AI_CLIENT_DIRECTORY_GRANTS"] = grants
-            },
-            ShutdownTimeout = TimeSpan.FromMilliseconds(500)
-        }), cancellationToken: Token);
+                Name = "Default tools",
+                Command = "dotnet",
+                Arguments = [Path.Combine(AppContext.BaseDirectory, "AI.Mcp.BuiltIn.dll")],
+                EnvironmentVariables = new Dictionary<string, string?>
+                {
+                    ["AI_CLIENT_DIRECTORY_GRANTS"] = grants
+                },
+                ShutdownTimeout = TimeSpan.FromMilliseconds(500)
+            }),
+            new McpClientOptions { ProtocolVersion = ProtocolVersion },
+            cancellationToken: Token);
     }
 
     public async ValueTask DisposeAsync()

tests/AI.Mcp.BuiltIn.Tests/ScriptRunToolTests.cs

@@ -14,6 +14,8 @@ namespace AI.Mcp.BuiltIn.Tests;
 [Trait("Category", "Integration")]
 public sealed class ScriptRunToolTests : IAsyncLifetime
 {
+    private const string ProtocolVersion = "2026-07-28";
+
     private static readonly string[] TwoArguments = ["hello", "world"];
     private static readonly string[] XmlImports = ["System.Xml.Linq"];
     private static readonly string[] XmlReferences = ["System.Xml.Linq"];
@@ -27,15 +29,18 @@ public async ValueTask InitializeAsync()
     {
         var assembly = Path.Combine(AppContext.BaseDirectory, "AI.Mcp.BuiltIn.dll");
         File.Exists(assembly).ShouldBeTrue($"The server assembly must be next to the tests: {assembly}");
-        _client = await McpClient.CreateAsync(new StdioClientTransport(new StdioClientTransportOptions
-        {
-            Name = "Default tools",
-            // Started through the muxer rather than the apphost so the test does not depend on
-            // which runtime the test host resolved.
-            Command = "dotnet",
-            Arguments = [assembly],
-            ShutdownTimeout = TimeSpan.FromMilliseconds(500)
-        }), cancellationToken: Token);
+        _client = await McpClient.CreateAsync(
+            new StdioClientTransport(new StdioClientTransportOptions
+            {
+                Name = "Default tools",
+                // Started through the muxer rather than the apphost so the test does not depend on
+                // which runtime the test host resolved.
+                Command = "dotnet",
+                Arguments = [assembly],
+                ShutdownTimeout = TimeSpan.FromMilliseconds(500)
+            }),
+            new McpClientOptions { ProtocolVersion = ProtocolVersion },
+            cancellationToken: Token);
     }
 
     public async ValueTask DisposeAsync() => await _client.DisposeAsync();