From f9dec13660fb7052cf65bff4e72d85945d38615c Mon Sep 17 00:00:00 2001
From: Stephen Toub <stoub@microsoft.com>
Date: Thu, 29 Jan 2026 23:04:51 -0500
Subject: [PATCH] Fix race condition in SSE GET request initialization (#1212)

---
 .../StreamableHttpHandler.cs                                | 5 -----
 .../Server/StreamableHttpServerTransport.cs                 | 6 ++++++
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs b/src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs
index 6e11b9b..c50e513 100644
--- a/src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs
+++ b/src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs
@@ -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)
diff --git a/src/ModelContextProtocol.Core/Server/StreamableHttpServerTransport.cs b/src/ModelContextProtocol.Core/Server/StreamableHttpServerTransport.cs
index 6307726..55b6650 100644
--- a/src/ModelContextProtocol.Core/Server/StreamableHttpServerTransport.cs
+++ b/src/ModelContextProtocol.Core/Server/StreamableHttpServerTransport.cs
@@ -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.