Add X-Accel-Buffering header to SSE responses (#1294)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs

@@ -339,6 +339,7 @@ internal sealed class StreamableHttpHandler(
 
         // Make sure we disable all response buffering for SSE.
         context.Response.Headers.ContentEncoding = "identity";
+        context.Response.Headers["X-Accel-Buffering"] = "no";
         context.Features.GetRequiredFeature<IHttpResponseBodyFeature>().DisableBuffering();
     }
 

tests/ModelContextProtocol.AspNetCore.Tests/SseServerIntegrationTests.cs

@@ -26,6 +26,7 @@ public class SseServerIntegrationTests(SseServerIntegrationTestFixture fixture,
         Assert.NotNull(sseResponse.Headers.CacheControl);
         Assert.True(sseResponse.Headers.CacheControl.NoStore);
         Assert.True(sseResponse.Headers.CacheControl.NoCache);
+        Assert.Equal("no", Assert.Single(sseResponse.Headers.GetValues("X-Accel-Buffering")));
     }
 
     [Fact]

tests/ModelContextProtocol.AspNetCore.Tests/StreamableHttpServerConformanceTests.cs

@@ -92,6 +92,17 @@ public class StreamableHttpServerConformanceTests(ITestOutputHelper outputHelper
         Assert.Equal("text/event-stream", Assert.Single(response.Content.Headers.GetValues("content-type")));
     }
 
+    [Fact]
+    public async Task SseResponse_Includes_XAccelBufferingHeader()
+    {
+        await StartAsync();
+
+        using var response = await HttpClient.PostAsync("", JsonContent(InitializeRequest), TestContext.Current.CancellationToken);
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        Assert.Equal("text/event-stream", response.Content.Headers.ContentType?.MediaType);
+        Assert.Equal("no", Assert.Single(response.Headers.GetValues("X-Accel-Buffering")));
+    }
+
     [Fact]
     public async Task PostRequest_IsUnsupportedMediaType_WithoutJsonContentType()
     {