Add client completion notification and details

src/ModelContextProtocol.Core/Client/ClientCompletionDetails.cs

@@ -0,0 +1,21 @@
+namespace ModelContextProtocol.Client;
+
+/// <summary>
+/// Provides details about why an MCP client session completed.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Transport implementations may return derived types with additional strongly-typed
+/// information, such as <see cref="StdioClientCompletionDetails"/>.
+/// </para>
+/// </remarks>
+public class ClientCompletionDetails
+{
+    /// <summary>
+    /// Gets the exception that caused the session to close, if any.
+    /// </summary>
+    /// <remarks>
+    /// This is <see langword="null"/> for graceful closure.
+    /// </remarks>
+    public Exception? Exception { get; set; }
+}

src/ModelContextProtocol.Core/Client/HttpClientCompletionDetails.cs

@@ -0,0 +1,15 @@
+using System.Net;
+
+namespace ModelContextProtocol.Client;
+
+/// <summary>
+/// Provides details about the completion of an HTTP-based MCP client session,
+/// including sessions using the legacy SSE transport or the Streamable HTTP transport.
+/// </summary>
+public sealed class HttpClientCompletionDetails : ClientCompletionDetails
+{
+    /// <summary>
+    /// Gets the HTTP status code that caused the session to close, or <see langword="null"/> if unavailable.
+    /// </summary>
+    public HttpStatusCode? HttpStatusCode { get; set; }
+}

src/ModelContextProtocol.Core/Client/McpClient.cs

@@ -53,4 +53,21 @@ protected McpClient()
     /// </para>
     /// </remarks>
     public abstract string? ServerInstructions { get; }
+
+    /// <summary>
+    /// Gets a <see cref="Task{TResult}"/> that completes when the client session has completed.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// The task always completes successfully. The result provides details about why the session
+    /// completed. Transport implementations may return derived types with additional strongly-typed
+    /// information, such as <see cref="StdioClientCompletionDetails"/>.
+    /// </para>
+    /// <para>
+    /// For graceful closure (e.g., explicit disposal), <see cref="ClientCompletionDetails.Exception"/>
+    /// will be <see langword="null"/>. For unexpected closure (e.g., process crash, network failure),
+    /// it may contain an exception that caused or that represents the failure.
+    /// </para>
+    /// </remarks>
+    public abstract Task<ClientCompletionDetails> Completion { get; }
 }

src/ModelContextProtocol.Core/Client/McpClientImpl.cs

@@ -521,6 +521,9 @@ private void RegisterTaskHandlers(RequestHandlers requestHandlers, IMcpTaskStore
     /// <inheritdoc/>
     public override string? ServerInstructions => _serverInstructions;
 
+    /// <inheritdoc/>
+    public override Task<ClientCompletionDetails> Completion => _sessionHandler.CompletionTask;
+
     /// <summary>
     /// Asynchronously connects to an MCP server, establishes the transport connection, and completes the initialization handshake.
     /// </summary>
@@ -655,6 +658,14 @@ public override async ValueTask DisposeAsync()
         _taskCancellationTokenProvider?.Dispose();
         await _sessionHandler.DisposeAsync().ConfigureAwait(false);
         await _transport.DisposeAsync().ConfigureAwait(false);
+
+        // After disposal, the channel writer is complete but ProcessMessagesCoreAsync
+        // may have been cancelled with unread items still buffered. ChannelReader.Completion
+        // only resolves once all items are consumed, so drain remaining items.
+        while (_transport.MessageReader.TryRead(out var _));
+
+        // Then ensure all work has quiesced.
+        await Completion.ConfigureAwait(false);
     }
 
     [LoggerMessage(Level = LogLevel.Information, Message = "{EndpointName} client received server '{ServerInfo}' capabilities: '{Capabilities}'.")]

src/ModelContextProtocol.Core/Client/SseClientSessionTransport.cs

@@ -2,6 +2,7 @@
 using Microsoft.Extensions.Logging.Abstractions;
 using ModelContextProtocol.Protocol;
 using System.Diagnostics;
+using System.Net;
 using System.Net.Http.Headers;
 using System.Net.ServerSentEvents;
 using System.Text.Json;
@@ -124,7 +125,7 @@ private async Task CloseAsync()
         }
         finally
         {
-            SetDisconnected();
+            SetDisconnected(new TransportClosedException(new HttpClientCompletionDetails()));
         }
     }
 
@@ -143,6 +144,7 @@ public override async ValueTask DisposeAsync()
 
     private async Task ReceiveMessagesAsync(CancellationToken cancellationToken)
     {
+        HttpStatusCode? failureStatusCode = null;
         try
         {
             using var request = new HttpRequestMessage(HttpMethod.Get, _sseEndpoint);
@@ -151,6 +153,11 @@ private async Task ReceiveMessagesAsync(CancellationToken cancellationToken)
 
             using var response = await _httpClient.SendAsync(request, message: null, cancellationToken).ConfigureAwait(false);
 
+            if (!response.IsSuccessStatusCode)
+            {
+                failureStatusCode = response.StatusCode;
+            }
+
             await response.EnsureSuccessStatusCodeWithResponseBodyAsync(cancellationToken).ConfigureAwait(false);
 
             using var stream = await response.Content.ReadAsStreamAsync(cancellationToken).ConfigureAwait(false);
@@ -179,14 +186,20 @@ private async Task ReceiveMessagesAsync(CancellationToken cancellationToken)
             }
             else
             {
+                SetDisconnected(new TransportClosedException(new HttpClientCompletionDetails
+                {
+                    HttpStatusCode = failureStatusCode,
+                    Exception = ex,
+                }));
+
                 LogTransportReadMessagesFailed(Name, ex);
                 _connectionEstablished.TrySetException(ex);
                 throw;
             }
         }
         finally
         {
-            SetDisconnected();
+            SetDisconnected(new TransportClosedException(new HttpClientCompletionDetails()));
         }
     }
 

src/ModelContextProtocol.Core/Client/StdioClientCompletionDetails.cs

@@ -0,0 +1,22 @@
+namespace ModelContextProtocol.Client;
+
+/// <summary>
+/// Provides details about the completion of a stdio-based MCP client session.
+/// </summary>
+public sealed class StdioClientCompletionDetails : ClientCompletionDetails
+{
+    /// <summary>
+    /// Gets the process ID of the server process, or <see langword="null"/> if unavailable.
+    /// </summary>
+    public int? ProcessId { get; set; }
+
+    /// <summary>
+    /// Gets the exit code of the server process, or <see langword="null"/> if unavailable.
+    /// </summary>
+    public int? ExitCode { get; set; }
+
+    /// <summary>
+    /// Gets the last lines of the server process's standard error output, or <see langword="null"/> if unavailable.
+    /// </summary>
+    public IReadOnlyList<string>? StandardErrorTail { get; set; }
+}

src/ModelContextProtocol.Core/Client/StdioClientSessionTransport.cs

@@ -5,14 +5,22 @@
 namespace ModelContextProtocol.Client;
 
 /// <summary>Provides the client side of a stdio-based session transport.</summary>
-internal sealed class StdioClientSessionTransport(
-    StdioClientTransportOptions options, Process process, string endpointName, Queue<string> stderrRollingLog, ILoggerFactory? loggerFactory) :
-    StreamClientSessionTransport(process.StandardInput.BaseStream, process.StandardOutput.BaseStream, encoding: null, endpointName, loggerFactory)
+internal sealed class StdioClientSessionTransport : StreamClientSessionTransport
 {
-    private readonly StdioClientTransportOptions _options = options;
-    private readonly Process _process = process;
-    private readonly Queue<string> _stderrRollingLog = stderrRollingLog;
+    private readonly StdioClientTransportOptions _options;
+    private readonly Process _process;
+    private readonly Queue<string> _stderrRollingLog;
     private int _cleanedUp = 0;
+    private readonly int? _processId;
+
+    public StdioClientSessionTransport(StdioClientTransportOptions options, Process process, string endpointName, Queue<string> stderrRollingLog, ILoggerFactory? loggerFactory) :
+        base(process.StandardInput.BaseStream, process.StandardOutput.BaseStream, encoding: null, endpointName, loggerFactory)
+    {
+        _options = options;
+        _process = process;
+        _stderrRollingLog = stderrRollingLog;
+        try { _processId = process.Id; } catch { }
+    }
 
     /// <inheritdoc/>
     public override async Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
@@ -47,17 +55,26 @@ protected override async ValueTask CleanupAsync(Exception? error = null, Cancell
         // so create an exception with details about that.
         error ??= await GetUnexpectedExitExceptionAsync(cancellationToken).ConfigureAwait(false);
 
-        // Now terminate the server process.
+        // Terminate the server process (or confirm it already exited), then build
+        // and publish strongly-typed completion details while the process handle
+        // is still valid so we can read the exit code.
         try
         {
-            StdioClientTransport.DisposeProcess(_process, processRunning: true, shutdownTimeout: _options.ShutdownTimeout);
+            StdioClientTransport.DisposeProcess(
+                _process, 
+                processRunning: true,
+                _options.ShutdownTimeout,
+                beforeDispose: () => SetDisconnected(new TransportClosedException(BuildCompletionDetails(error))));
         }
         catch (Exception ex)
         {
             LogTransportShutdownFailed(Name, ex);
+            SetDisconnected(new TransportClosedException(BuildCompletionDetails(error)));
         }
 
-        // And handle cleanup in the base type.
+        // And handle cleanup in the base type. SetDisconnected has already been
+        // called above, so the base call is a no-op for disconnect state but
+        // still performs other cleanup (cancelling the read task, etc.).
         await base.CleanupAsync(error, cancellationToken).ConfigureAwait(false);
     }
 
@@ -104,4 +121,32 @@ protected override async ValueTask CleanupAsync(Exception? error = null, Cancell
 
         return new IOException(errorMessage);
     }
+
+    private StdioClientCompletionDetails BuildCompletionDetails(Exception? error)
+    {
+        StdioClientCompletionDetails details = new()
+        {
+            Exception = error,
+            ProcessId = _processId,
+        };
+
+        try
+        {
+            if (StdioClientTransport.HasExited(_process))
+            {
+                details.ExitCode = _process.ExitCode;
+            }
+        }
+        catch { }
+
+        lock (_stderrRollingLog)
+        {
+            if (_stderrRollingLog.Count > 0)
+            {
+                details.StandardErrorTail = _stderrRollingLog.ToArray();
+            }
+        }
+
+        return details;
+    }
 }

src/ModelContextProtocol.Core/Client/StdioClientTransport.cs

@@ -213,7 +213,7 @@ public async Task<ITransport> ConnectAsync(CancellationToken cancellationToken =
     }
 
     internal static void DisposeProcess(
-        Process? process, bool processRunning, TimeSpan shutdownTimeout)
+        Process? process, bool processRunning, TimeSpan shutdownTimeout, Action? beforeDispose = null)
     {
         if (process is not null)
         {
@@ -239,6 +239,10 @@ internal static void DisposeProcess(
                 {
                     process.WaitForExit();
                 }
+
+                // Invoke the callback while the process handle is still valid,
+                // e.g. to read ExitCode before Dispose() invalidates it.
+                beforeDispose?.Invoke();
             }
             finally
             {

src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs

@@ -20,11 +20,12 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
 
     private readonly McpHttpClient _httpClient;
     private readonly HttpClientTransportOptions _options;
-    private readonly CancellationTokenSource _connectionCts;
+    private readonly CancellationTokenSource _connectionCts = new();
     private readonly ILogger _logger;
 
     private string? _negotiatedProtocolVersion;
     private Task? _getReceiveTask;
+    private volatile TransportClosedException? _disconnectError;
 
     private readonly SemaphoreSlim _disposeLock = new(1, 1);
     private bool _disposed;
@@ -42,7 +43,6 @@ public StreamableHttpClientSessionTransport(
 
         _options = transportOptions;
         _httpClient = httpClient;
-        _connectionCts = new CancellationTokenSource();
         _logger = (ILogger?)loggerFactory?.CreateLogger<HttpClientTransport>() ?? NullLogger.Instance;
 
         // We connect with the initialization request with the MCP transport. This means that any errors won't be observed
@@ -96,6 +96,13 @@ internal async Task<HttpResponseMessage> SendHttpRequestAsync(JsonRpcMessage mes
         // We'll let the caller decide whether to throw or fall back given an unsuccessful response.
         if (!response.IsSuccessStatusCode)
         {
+            // Per the MCP spec, a 404 response to a request containing an Mcp-Session-Id
+            // indicates the session has ended. Signal completion so McpClient.Completion resolves.
+            if (response.StatusCode == HttpStatusCode.NotFound && SessionId is not null)
+            {
+                SetSessionExpired(response.StatusCode);
+            }
+
             return response;
         }
 
@@ -184,18 +191,14 @@ public override async ValueTask DisposeAsync()
             {
                 LogTransportShutdownFailed(Name, ex);
             }
-            finally
-            {
-                _connectionCts.Dispose();
-            }
         }
         finally
         {
             // If we're auto-detecting the transport and failed to connect, leave the message Channel open for the SSE transport.
             // This class isn't directly exposed to public callers, so we don't have to worry about changing the _state in this case.
             if (_options.TransportMode is not HttpTransportMode.AutoDetect || _getReceiveTask is not null)
             {
-                SetDisconnected();
+                SetDisconnected(_disconnectError ?? new TransportClosedException(new HttpClientCompletionDetails()));
             }
         }
     }
@@ -204,8 +207,8 @@ private async Task ReceiveUnsolicitedMessagesAsync()
     {
         var state = new SseStreamState();
 
-        // Continuously receive unsolicited messages until canceled
-        while (!_connectionCts.Token.IsCancellationRequested)
+        // Continuously receive unsolicited messages until canceled or disconnected
+        while (!_connectionCts.Token.IsCancellationRequested && IsConnected)
         {
             await SendGetSseRequestWithRetriesAsync(
                 relatedRpcRequest: null,
@@ -285,6 +288,13 @@ await SendGetSseRequestWithRetriesAsync(
 
                 if (!response.IsSuccessStatusCode)
                 {
+                    // Per the MCP spec, a 404 response to a request containing an Mcp-Session-Id
+                    // indicates the session has ended. Signal completion so McpClient.Completion resolves.
+                    if (response.StatusCode == HttpStatusCode.NotFound && SessionId is not null)
+                    {
+                        SetSessionExpired(response.StatusCode);
+                    }
+
                     // If the server could be reached but returned a non-success status code,
                     // retrying likely won't change that.
                     return null;
@@ -474,4 +484,23 @@ private static TimeSpan ElapsedSince(long stopwatchTimestamp)
         return TimeSpan.FromSeconds((double)(Stopwatch.GetTimestamp() - stopwatchTimestamp) / Stopwatch.Frequency);
 #endif
     }
+
+    private void SetSessionExpired(HttpStatusCode statusCode)
+    {
+        // Store the error before canceling so DisposeAsync can use it if it races us, especially
+        // after the call to Cancel below, to invoke SetDisconnected.
+        _disconnectError = new TransportClosedException(new HttpClientCompletionDetails
+        {
+            HttpStatusCode = statusCode,
+            Exception = new McpException(
+                "The server returned HTTP 404 for a request with an Mcp-Session-Id, indicating the session has expired. " +
+                "To continue, create a new client session or call ResumeSessionAsync with a new connection."),
+        });
+
+        // Cancel to unblock any in-flight operations (e.g., SSE stream reads in
+        // SendGetSseRequestWithRetriesAsync) that are waiting on _connectionCts.Token.
+        _connectionCts.Cancel();
+
+        SetDisconnected(_disconnectError);
+    }
 }