Pass session id's to MCP endpoints. (#466)

* Pass session id's to MCP endpoints.

* Update src/ModelContextProtocol/Client/McpClient.cs

Co-authored-by: Stephen Halter <halter73@gmail.com>

* Update src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs

Co-authored-by: Stephen Halter <halter73@gmail.com>

* Make init callback asynchronous.

* Address feedback.

---------

Co-authored-by: Stephen Halter <halter73@gmail.com>

tests/Common/Utils/TestServerTransport.cs

@@ -16,6 +16,8 @@ public class TestServerTransport : ITransport
 
     public Action<JsonRpcMessage>? OnMessageSent { get; set; }
 
+    public string? SessionId => null;
+
     public TestServerTransport()
     {
         _messageChannel = Channel.CreateUnbounded<JsonRpcMessage>(new UnboundedChannelOptions

tests/ModelContextProtocol.AspNetCore.Tests/HttpServerIntegrationTests.cs

@@ -52,6 +52,15 @@ public abstract class HttpServerIntegrationTests : LoggedTest, IClassFixture<Sse
         // Assert
         Assert.NotNull(client.ServerCapabilities);
         Assert.NotNull(client.ServerInfo);
+
+        if (ClientTransportOptions.Endpoint.AbsolutePath.EndsWith("/sse"))
+        {
+            Assert.Null(client.SessionId);
+        }
+        else
+        {
+            Assert.NotNull(client.SessionId);
+        }
     }
 
     [Fact]
@@ -90,6 +99,35 @@ public abstract class HttpServerIntegrationTests : LoggedTest, IClassFixture<Sse
         Assert.Equal("Echo: Hello MCP!", textContent.Text);
     }
 
+    [Fact]
+    public async Task CallTool_EchoSessionId_ReturnsTheSameSessionId()
+    {
+        // arrange
+
+        // act
+        await using var client = await GetClientAsync();
+        var result1 = await client.CallToolAsync("echoSessionId", cancellationToken: TestContext.Current.CancellationToken);
+        var result2 = await client.CallToolAsync("echoSessionId", cancellationToken: TestContext.Current.CancellationToken);
+        var result3 = await client.CallToolAsync("echoSessionId", cancellationToken: TestContext.Current.CancellationToken);
+
+        // assert
+        Assert.NotNull(result1);
+        Assert.NotNull(result2);
+        Assert.NotNull(result3);
+
+        Assert.False(result1.IsError);
+        Assert.False(result2.IsError);
+        Assert.False(result3.IsError);
+        
+        var textContent1 = Assert.Single(result1.Content);
+        var textContent2 = Assert.Single(result2.Content);
+        var textContent3 = Assert.Single(result3.Content);
+
+        Assert.NotNull(textContent1.Text);
+        Assert.Equal(textContent1.Text, textContent2.Text);
+        Assert.Equal(textContent1.Text, textContent3.Text);
+    }
+
     [Fact]
     public async Task ListResources_Sse_TestServer()
     {

tests/ModelContextProtocol.TestServer/Program.cs

@@ -133,6 +133,16 @@ internal static class Program
                                 """),
                         },
                         new Tool()
+                        {
+                            Name = "echoSessionId",
+                            Description = "Echoes the session id back to the client.",
+                            InputSchema = JsonSerializer.Deserialize<JsonElement>("""
+                                {
+                                    "type": "object"
+                                }
+                                """, McpJsonUtilities.DefaultOptions),
+                        },
+                        new Tool()
                         {
                             Name = "sampleLLM",
                             Description = "Samples from an LLM using MCP's sampling feature.",
@@ -170,6 +180,13 @@ internal static class Program
                         Content = [new Content() { Text = "Echo: " + message.ToString(), Type = "text" }]
                     };
                 }
+                else if (request.Params?.Name == "echoSessionId")
+                {
+                    return new CallToolResponse()
+                    {
+                        Content = [new Content() { Text = request.Server.SessionId, Type = "text" }]
+                    };
+                }
                 else if (request.Params?.Name == "sampleLLM")
                 {
                     if (request.Params?.Arguments is null ||

tests/ModelContextProtocol.TestSseServer/Program.cs

@@ -128,6 +128,16 @@ public class Program
                                     """, McpJsonUtilities.DefaultOptions),
                             },
                             new Tool()
+                            {
+                                Name = "echoSessionId",
+                                Description = "Echoes the session id back to the client.",
+                                InputSchema = JsonSerializer.Deserialize<JsonElement>("""
+                                    {
+                                        "type": "object"
+                                    }
+                                    """, McpJsonUtilities.DefaultOptions),
+                            },
+                            new Tool()
                             {
                                 Name = "sampleLLM",
                                 Description = "Samples from an LLM using MCP's sampling feature.",
@@ -168,6 +178,13 @@ public class Program
                             Content = [new Content() { Text = "Echo: " + message.ToString(), Type = "text" }]
                         };
                     }
+                    else if (request.Params.Name == "echoSessionId")
+                    {
+                        return new CallToolResponse()
+                        {
+                            Content = [new Content() { Text = request.Server.SessionId, Type = "text" }]
+                        };
+                    }
                     else if (request.Params.Name == "sampleLLM")
                     {
                         if (request.Params.Arguments is null ||

tests/ModelContextProtocol.Tests/Client/McpClientFactoryTests.cs

@@ -109,6 +109,7 @@ public class McpClientFactoryTests
         private readonly Channel<JsonRpcMessage> _channel = Channel.CreateUnbounded<JsonRpcMessage>();
 
         public bool IsConnected => true;
+        public string? SessionId => null;
 
         public ChannelReader<JsonRpcMessage> MessageReader => _channel.Reader;
 

tests/ModelContextProtocol.Tests/ClientIntegrationTests.cs

@@ -54,6 +54,8 @@ public partial class ClientIntegrationTests : LoggedTest, IClassFixture<ClientIn
         Assert.NotNull(client.ServerInfo);
         if (clientId != "everything")   // Note: Comment the below assertion back when the everything server is updated to provide instructions
             Assert.NotNull(client.ServerInstructions);
+
+        Assert.Null(client.SessionId);
     }
 
     [Theory]
@@ -94,6 +96,22 @@ public partial class ClientIntegrationTests : LoggedTest, IClassFixture<ClientIn
         Assert.Equal("Echo: Hello MCP!", textContent.Text);
     }
 
+    [Fact]
+    public async Task CallTool_Stdio_EchoSessionId_ReturnsNull()
+    {
+        // arrange
+
+        // act
+        await using var client = await _fixture.CreateClientAsync("test_server");
+        var result = await client.CallToolAsync("echoSessionId", cancellationToken: TestContext.Current.CancellationToken);
+
+        // assert
+        Assert.NotNull(result);
+        Assert.False(result.IsError);
+        var textContent = Assert.Single(result.Content, c => c.Type == "text");
+        Assert.Null(textContent.Text);
+    }
+
     [Theory]
     [MemberData(nameof(GetClients))]
     public async Task CallTool_Stdio_ViaAIFunction_EchoServer(string clientId)

tests/ModelContextProtocol.Tests/Server/McpServerTests.cs

@@ -650,6 +650,7 @@ public class McpServerTests : LoggedTest
 
         public ValueTask DisposeAsync() => default;
 
+        public string? SessionId => throw new NotImplementedException();
         public Implementation? ClientInfo => throw new NotImplementedException();
         public IServiceProvider? Services => throw new NotImplementedException();
         public LoggingLevel? LoggingLevel => throw new NotImplementedException();