Disable response buffering for SSE responses

src/ModelContextProtocol.AspNetCore/McpEndpointRouteBuilderExtensions.cs

@@ -1,4 +1,5 @@
 using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Http.Features;
 using Microsoft.AspNetCore.Routing;
 using Microsoft.AspNetCore.WebUtilities;
 using Microsoft.Extensions.DependencyInjection;
@@ -39,7 +40,7 @@ public static IEndpointConventionBuilder MapMcp(this IEndpointRouteBuilder endpo
             var requestAborted = context.RequestAborted;
 
             response.Headers.ContentType = "text/event-stream";
-            response.Headers.CacheControl = "no-store";
+            response.Headers.CacheControl = "no-cache,no-store";
 
             var sessionId = MakeNewSessionId();
             await using var transport = new SseResponseStreamTransport(response.Body, $"/message?sessionId={sessionId}");
@@ -50,6 +51,10 @@ public static IEndpointConventionBuilder MapMcp(this IEndpointRouteBuilder endpo
 
             try
             {
+                // Make sure we disable all response buffering for SSE
+                context.Response.Headers.ContentEncoding = "identity";
+                context.Features.GetRequiredFeature<IHttpResponseBodyFeature>().DisableBuffering();
+
                 var transportTask = transport.RunAsync(cancellationToken: requestAborted);
                 await using var server = McpServerFactory.Create(transport, mcpServerOptions.Value, loggerFactory, endpoints.ServiceProvider);
 

tests/ModelContextProtocol.Tests/SseServerIntegrationTests.cs

@@ -280,6 +280,21 @@ public async Task CallTool_Sse_EchoServer_Concurrently()
         }
     }
 
+    [Fact]
+    public async Task EventSourceResponse_Includes_ExpectedHeaders()
+    {
+        using var httpClient = GetHttpClient();
+        using var sseResponse = await httpClient.GetAsync("", HttpCompletionOption.ResponseHeadersRead, TestContext.Current.CancellationToken);
+
+        sseResponse.EnsureSuccessStatusCode();
+
+        Assert.Equal("text/event-stream", sseResponse.Content.Headers.ContentType?.MediaType);
+        Assert.Equal("identity", sseResponse.Content.Headers.ContentEncoding.ToString());
+        Assert.NotNull(sseResponse.Headers.CacheControl);
+        Assert.True(sseResponse.Headers.CacheControl.NoStore);
+        Assert.True(sseResponse.Headers.CacheControl.NoCache);
+    }
+
     [Fact]
     public async Task EventSourceStream_Includes_MessageEventType()
     {