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Fix race condition in SSE GET request initialization (#1212)

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Stephen Toub 2026-01-29 23:04:51 -05:00 • committed by GitHub
commit f9dec13660
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2 changed files with 6 additions and 5 deletions
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5
src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs
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@ -154,11 +154,6 @@ internal sealed class StreamableHttpHandler(
{
await using var _ = await session.AcquireReferenceAsync(cancellationToken);
InitializeSseResponse(context);
// We should flush headers to indicate a 200 success quickly, because the initialization response
// will be sent in response to a different POST request. It might be a while before we send a message
// over this response body.
await context.Response.Body.FlushAsync(cancellationToken);
await session.Transport.HandleGetRequestAsync(context.Response.Body, cancellationToken);
}
catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)

6
src/ModelContextProtocol.Core/Server/StreamableHttpServerTransport.cs
View file

@ -125,6 +125,12 @@ public sealed class StreamableHttpServerTransport : ITransport
var primingItem = await _storeSseWriter.WriteEventAsync(SseItem.Prime<JsonRpcMessage>(), cancellationToken).ConfigureAwait(false);
await _httpSseWriter.WriteAsync(primingItem, cancellationToken).ConfigureAwait(false);
}
else
{
// If there's no priming write, flush the stream to ensure HTTP response headers are
// sent to the client now that the transport is ready to accept messages via SendMessageAsync.
await sseResponseStream.FlushAsync(cancellationToken).ConfigureAwait(false);
}
}
// Wait for the response to be written before returning from the handler.