Add X-Accel-Buffering header to SSE responses

[Copilot]

Per MCP spec update (435a718), SSE responses should include X-Accel-Buffering: no to prevent reverse proxy buffering.

Changes

• Added header to StreamableHttpHandler.InitializeSseResponse - the single initialization point for all SSE response paths (Streamable HTTP POST/GET, resumed streams, legacy SSE transport)
• Added test coverage in StreamableHttpServerConformanceTests and updated SseServerIntegrationTests

internal static void InitializeSseResponse(HttpContext context)
{
    context.Response.Headers.ContentType = "text/event-stream";
    context.Response.Headers.CacheControl = "no-cache,no-store";
    context.Response.Headers.ContentEncoding = "identity";
    context.Response.Headers["X-Accel-Buffering"] = "no";  // Added
    context.Features.GetRequiredFeature<IHttpResponseBodyFeature>().DisableBuffering();
}

Original prompt

Context

The MCP specification was updated in modelcontextprotocol/modelcontextprotocol@435a718 to add a new "SSE Stream Configuration" section recommending that servers SHOULD include the X-Accel-Buffering: no header in HTTP responses that return Content-Type: text/event-stream. This header instructs reverse proxies (such as nginx) to disable response buffering, ensuring that SSE events are delivered to clients immediately rather than being held in a buffer. Without this header, proxies may accumulate messages before sending them to the client, introducing unwanted latency and potentially breaking the real-time nature of SSE communication.

Problem

The C# SDK does not currently set the X-Accel-Buffering: no header on any SSE responses. A search for X-Accel-Buffering across the entire repository returns zero results.

Required Change

All SSE responses in the C# SDK flow through a single centralized method: StreamableHttpHandler.InitializeSseResponse in src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs. This method currently sets Content-Type, Cache-Control, Content-Encoding, and disables buffering, but does not set X-Accel-Buffering.

Add the X-Accel-Buffering: no response header in InitializeSseResponse. The current method looks like:

internal static void InitializeSseResponse(HttpContext context)
{
    context.Response.Headers.ContentType = "text/event-stream";
    context.Response.Headers.CacheControl = "no-cache,no-store";

    // Make sure we disable all response buffering for SSE.
    context.Response.Headers.ContentEncoding = "identity";
    context.Features.GetRequiredFeature<IHttpResponseBodyFeature>().DisableBuffering();
}

Add one line to set the header:

context.Response.Headers["X-Accel-Buffering"] = "no";

This single change covers all SSE response paths since InitializeSseResponse is called from:

• HandlePostRequestAsync (Streamable HTTP POST → SSE response)
• HandleGetRequestAsync → HandleUnsolicitedMessageStreamAsync (Streamable HTTP GET → SSE stream)
• HandleResumedStreamAsync (Streamable HTTP GET with Last-Event-ID)
• HandleResumePostResponseStreamAsync (resuming a POST response)
• SseHandler.HandleSseRequestAsync (legacy SSE transport)

This pull request was created from Copilot chat.

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[halter73]

We already set Content-Encoding: identity to prevent similar buffering/compression in IIS. I'm surprised that this pretty NGINX-specific header made its way into the spec, but I don't see the harm in including it by default in the SDK regardless.