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Add a callback for receiving stderr output #525

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Merged
merged 2 commits into from
Jun 17, 2025
+35 −0
Merged

Add a callback for receiving stderr output #525

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f42e729
Add a callback for receiving stderr output
stephentoub Jun 17, 2025
f9d5ea4
Add asserts to test
stephentoub Jun 17, 2025
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2 changes: 2 additions & 0 deletions src/ModelContextProtocol.Core/Client/StdioClientTransport.cs
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Expand Up @@ -146,6 +146,8 @@ public async Task<ITransport> ConnectAsync(CancellationToken cancellationToken =
stderrRollingLog.Enqueue(data);
}

_options.StandardErrorLines?.Invoke(data);

LogReadStderr(logger, endpointName, data);
}
};
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5 changes: 5 additions & 0 deletions src/ModelContextProtocol.Core/Client/StdioClientTransportOptions.cs
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Expand Up @@ -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; }
}
17 changes: 17 additions & 0 deletions tests/ModelContextProtocol.Tests/Transport/StdioClientTransportTests.cs
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Expand Up @@ -18,4 +18,21 @@ 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;
Action<string> stdErrCallback = line => Interlocked.Increment(ref 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);
}
}
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