Add a callback for receiving stderr output

src/ModelContextProtocol.Core/Client/StdioClientTransport.cs

@@ -146,6 +146,8 @@ public async Task<ITransport> ConnectAsync(CancellationToken cancellationToken =
                         stderrRollingLog.Enqueue(data);
                     }
 
+                    _options.StandardErrorLines?.Invoke(data);
+
                     LogReadStderr(logger, endpointName, data);
                 }
             };

src/ModelContextProtocol.Core/Client/StdioClientTransportOptions.cs

@@ -69,4 +69,9 @@ public required string Command
     /// </para>
     /// </remarks>
     public TimeSpan ShutdownTimeout { get; set; } = TimeSpan.FromSeconds(5);
+
+    /// <summary>
+    /// Gets or sets a callback that is invoked for each line of stderr received from the server process.
+    /// </summary>
+    public Action<string>? StandardErrorLines { get; set; }
 }

tests/ModelContextProtocol.Tests/Transport/StdioClientTransportTests.cs

@@ -1,6 +1,7 @@
 using ModelContextProtocol.Client;
 using ModelContextProtocol.Tests.Utils;
 using System.Runtime.InteropServices;
+using System.Text;
 
 namespace ModelContextProtocol.Tests.Transport;
 
@@ -18,4 +19,31 @@ public async Task CreateAsync_ValidProcessInvalidServer_Throws()
         IOException e = await Assert.ThrowsAsync<IOException>(() => McpClientFactory.CreateAsync(transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken));
         Assert.Contains(id, e.ToString());
     }
+
+    [Fact]
+    public async Task CreateAsync_ValidProcessInvalidServer_StdErrCallbackInvoked()
+    {
+        string id = Guid.NewGuid().ToString("N");
+
+        int count = 0;
+        StringBuilder sb = new();
+        Action<string> stdErrCallback = line =>
+        {
+            Assert.NotNull(line);
+            lock (sb)
+            {
+                sb.AppendLine(line);
+                count++;
+            }
+        };
+
+        StdioClientTransport transport = RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ?
+            new(new() { Command = "cmd", Arguments = ["/C", $"echo \"{id}\" >&2"], StandardErrorLines = stdErrCallback }, LoggerFactory) :
+            new(new() { Command = "ls", Arguments = [id], StandardErrorLines = stdErrCallback }, LoggerFactory);
+
+        await Assert.ThrowsAsync<IOException>(() => McpClientFactory.CreateAsync(transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken));
+
+        Assert.InRange(count, 1, int.MaxValue);
+        Assert.Contains(id, sb.ToString());
+    }
 }