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Fix stdin message splitting for back-to-back newline-delimited messages #1758

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Fix stdin message splitting for back-to-back newline-delimited messages #1758

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f72b360
Initial plan
Copilot Jul 13, 2026
d60dbea
Fix stdin message splitting to handle back-to-back messages
Copilot Jul 13, 2026
c228328
Use UTF-8 Decoder to handle multi-byte characters across read boundaries
Copilot Jul 13, 2026
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118 changes: 79 additions & 39 deletions DevProxy/Stdio/ProxySession.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -197,6 +197,11 @@ private async Task ForwardStdinAsync()
{
var buffer = new byte[4096];
using var stdin = Console.OpenStandardInput();
// Buffer for accumulating partial messages that don't end with a newline
var pendingData = new StringBuilder();
// Decoder maintains state across calls to handle multi-byte UTF-8
// characters that may be split across buffer boundaries
var decoder = Encoding.UTF8.GetDecoder();

try
{
Expand Down Expand Up @@ -231,57 +236,92 @@ private async Task ForwardStdinAsync()
break;
}

var data = buffer[..bytesRead];
Log("STDIN >>>", buffer, bytesRead);

// Log intercepted stdin (similar to HTTP InterceptedRequest)
await LogInterceptedStdioAsync(data, StdioMessageDirection.Stdin);

// Execute plugins and check if message was consumed
var responseState = new ResponseState();
var requestArgs = new StdioRequestArgs
{
Body = data,
ResponseState = responseState,
Session = _stdioSession!,
SessionData = _sessionData,
GlobalData = _globalData
};
// Split the buffer into individual newline-delimited messages.
// A single read may contain multiple messages when they arrive
// back-to-back (e.g., a notification followed by a request).
var charCount = decoder.GetCharCount(buffer, 0, bytesRead, flush: false);
var chars = new char[charCount];
decoder.GetChars(buffer, 0, bytesRead, chars, 0, flush: false);
pendingData.Append(chars);

await ExecuteBeforeStdinPluginsAsync(requestArgs);
var pending = pendingData.ToString();
var lines = pending.Split('\n');

// Only forward to child if not consumed
if (!responseState.HasBeenSet)
// If the chunk doesn't end with a newline, the last element is
// a partial message that needs to be carried over to the next read.
if (!pending.EndsWith('\n'))
{
await _writeSemaphore.WaitAsync(_cts.Token);
try
{
await _childStdin!.WriteAsync(buffer.AsMemory(0, bytesRead), _cts.Token);
await _childStdin.FlushAsync(_cts.Token);
}
finally
{
_ = _writeSemaphore.Release();
}
pendingData.Clear();
pendingData.Append(lines[^1]);
lines = lines[..^1];
}
else
{
// Send mock response if set by a plugin
await SendMockResponseAsync(requestArgs);
pendingData.Clear();
}

// Create request log entry for this stdin message
var requestLog = new StdioRequestLog
foreach (var line in lines)
{
Command = _stdioSession!.Command,
StdinBody = data,
StdinTimestamp = DateTimeOffset.UtcNow,
MessageType = responseState.HasBeenSet ? MessageType.Mocked : MessageType.PassedThrough
};
_requestLogs.Add(requestLog);
var message = line.TrimEnd('\r');
if (message.Length == 0)
{
continue;
}

var messageBytes = Encoding.UTF8.GetBytes(message);

// Log intercepted stdin (similar to HTTP InterceptedRequest)
await LogInterceptedStdioAsync(messageBytes, StdioMessageDirection.Stdin);

// Execute plugins and check if message was consumed
var responseState = new ResponseState();
var requestArgs = new StdioRequestArgs
{
Body = messageBytes,
ResponseState = responseState,
Session = _stdioSession!,
SessionData = _sessionData,
GlobalData = _globalData
};

// Notify plugins of the request log
await NotifyStdioRequestLogAsync(requestLog);
await ExecuteBeforeStdinPluginsAsync(requestArgs);

// Only forward to child if not consumed
if (!responseState.HasBeenSet)
{
var forwardBytes = Encoding.UTF8.GetBytes(message + "\n");
await _writeSemaphore.WaitAsync(_cts.Token);
Comment on lines +292 to +295
try
{
await _childStdin!.WriteAsync(forwardBytes.AsMemory(), _cts.Token);
await _childStdin.FlushAsync(_cts.Token);
}
finally
{
_ = _writeSemaphore.Release();
}
}
else
{
// Send mock response if set by a plugin
await SendMockResponseAsync(requestArgs);
}

// Create request log entry for this stdin message
var requestLog = new StdioRequestLog
{
Command = _stdioSession!.Command,
StdinBody = messageBytes,
StdinTimestamp = DateTimeOffset.UtcNow,
MessageType = responseState.HasBeenSet ? MessageType.Mocked : MessageType.PassedThrough
};
_requestLogs.Add(requestLog);

// Notify plugins of the request log
await NotifyStdioRequestLogAsync(requestLog);
}
}
}
catch (OperationCanceledException)
Expand Down
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