Standardize SSE response headers

- Extract SSE response preparation into HttpServletResponseUtils
- Add X-Accel-Buffering: no header to SSE responses
- Add no-transform to the SSE Cache-Control header
- Drop Connection: keep-alive from SSE responses (prohibited in HTTP/2)

Fixes #293

Signed-off-by: Daniel Garnier-Moiroux <git@garnier.wf>

Author: Debasishhh

mcp-core/src/main/java/io/modelcontextprotocol/modern/server/transport/HttpServletMcpTransport.java

@@ -345,7 +345,7 @@ private String eventFrame(JSONRPCMessage message) {
 	private static PrintWriter startEventStream(HttpServletResponse response) throws IOException {
 		response.setContentType(TEXT_EVENT_STREAM);
 		response.setCharacterEncoding(UTF_8);
-		response.setHeader("Cache-Control", "no-cache");
+		response.setHeader("Cache-Control", "no-cache, no-transform");
 		response.setHeader("X-Accel-Buffering", "no");
 		response.setStatus(HttpServletResponse.SC_OK);
 		response.flushBuffer();

mcp-core/src/main/java/io/modelcontextprotocol/server/transport/HttpServletResponseUtils.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2026-2026 the original author or authors.
+ */
+
+package io.modelcontextprotocol.server.transport;
+
+import jakarta.servlet.http.HttpServletResponse;
+
+/**
+ * Utility methods for working with {@link HttpServletResponse}. For internal use only.
+ *
+ * @author Daniel Garnier-Moiroux
+ */
+final class HttpServletResponseUtils {
+
+	private static final String TEXT_EVENT_STREAM = "text/event-stream";
+
+	private static final String UTF_8 = "UTF-8";
+
+	private HttpServletResponseUtils() {
+	}
+
+	/**
+	 * Prepares the response for streaming Server-Sent Events: sets the content type,
+	 * character encoding and the headers that keep intermediaries from caching,
+	 * transforming or buffering the stream.
+	 * @param response The HTTP servlet response
+	 */
+	static void prepareSseResponse(HttpServletResponse response) {
+		response.setContentType(TEXT_EVENT_STREAM);
+		response.setCharacterEncoding(UTF_8);
+		response.setHeader("Cache-Control", "no-cache, no-transform");
+		response.setHeader("X-Accel-Buffering", "no");
+	}
+
+}

mcp-core/src/main/java/io/modelcontextprotocol/server/transport/HttpServletSseServerTransportProvider.java

@@ -300,10 +300,7 @@ protected void doGet(HttpServletRequest request, HttpServletResponse response)
 			return;
 		}
 
-		response.setContentType("text/event-stream");
-		response.setCharacterEncoding(UTF_8);
-		response.setHeader("Cache-Control", "no-cache");
-		response.setHeader("Connection", "keep-alive");
+		HttpServletResponseUtils.prepareSseResponse(response);
 
 		String sessionId = UUID.randomUUID().toString();
 		AsyncContext asyncContext = request.startAsync();

mcp-core/src/main/java/io/modelcontextprotocol/server/transport/HttpServletStreamableServerTransportProvider.java

@@ -426,10 +426,7 @@ protected void doGet(HttpServletRequest request, HttpServletResponse response)
 		McpTransportContext transportContext = this.contextExtractor.extract(request);
 
 		try {
-			response.setContentType(TEXT_EVENT_STREAM);
-			response.setCharacterEncoding(UTF_8);
-			response.setHeader("Cache-Control", "no-cache");
-			response.setHeader("Connection", "keep-alive");
+			HttpServletResponseUtils.prepareSseResponse(response);
 
 			AsyncContext asyncContext = request.startAsync();
 			asyncContext.setTimeout(0);
@@ -601,10 +598,7 @@ else if (message instanceof McpSchema.JSONRPCNotification jsonrpcNotification) {
 			}
 			else if (message instanceof McpSchema.JSONRPCRequest jsonrpcRequest) {
 				// For streaming responses, we need to return SSE
-				response.setContentType(TEXT_EVENT_STREAM);
-				response.setCharacterEncoding(UTF_8);
-				response.setHeader("Cache-Control", "no-cache");
-				response.setHeader("Connection", "keep-alive");
+				HttpServletResponseUtils.prepareSseResponse(response);
 
 				AsyncContext asyncContext = request.startAsync();
 				asyncContext.setTimeout(0);

mcp-test/src/test/java/io/modelcontextprotocol/modern/server/transport/HttpServletMcpTransportIntegrationTests.java

@@ -211,6 +211,8 @@ void streamingToolCallReturnsSse() throws Exception {
 		assertThat(response.statusCode()).isEqualTo(200);
 		assertThat(response.headers().firstValue("Content-Type"))
 			.hasValueSatisfying(v -> assertThat(v).contains("text/event-stream"));
+		assertThat(response.headers().firstValue("Cache-Control")).contains("no-cache, no-transform");
+		assertThat(response.headers().firstValue("X-Accel-Buffering")).contains("no");
 		assertThat(response.body()).contains("notifications/progress");
 		assertThat(response.body()).contains("\"result\"");
 	}

mcp-test/src/test/java/io/modelcontextprotocol/server/HttpServletSseIntegrationTests.java

@@ -182,6 +182,29 @@ void testRejectsWhenContentLengthHeaderExceedsLimit(String clientType) {
 		}
 	}
 
+	@Test
+	void sseResponseIncludesXAccelBufferingHeader() throws Exception {
+		// https://github.com/modelcontextprotocol/java-sdk/issues/293 - without this
+		// header, proxies like Nginx buffer the SSE response, breaking real-time
+		// streaming.
+		prepareAsyncServerBuilder().build();
+
+		var httpClient = HttpClient.newHttpClient();
+		var sseRequest = HttpRequest.newBuilder()
+			.uri(URI.create("http://localhost:" + PORT + CUSTOM_SSE_ENDPOINT))
+			.header("Accept", "text/event-stream")
+			.GET()
+			.build();
+
+		var response = httpClient.send(sseRequest, HttpResponse.BodyHandlers.ofInputStream());
+		try {
+			assertThat(response.headers().firstValue("X-Accel-Buffering")).contains("no");
+		}
+		finally {
+			response.body().close();
+		}
+	}
+
 	@Test
 	void rejectsWhenBodyBytesExceedLimitWithoutContentLengthHeader() throws Exception {
 		var httpClient = HttpClient.newHttpClient();

mcp-test/src/test/java/io/modelcontextprotocol/server/HttpServletStreamableIntegrationTests.java

@@ -14,6 +14,7 @@
 import java.net.http.HttpResponse;
 import java.nio.ByteBuffer;
 import java.time.Duration;
+import java.util.List;
 import java.util.Map;
 import java.util.Queue;
 import java.util.concurrent.CompletableFuture;
@@ -212,6 +213,69 @@ void testMissingHandlerReturnsMethodNotFoundError() {
 		}
 	}
 
+	/**
+	 * https://github.com/modelcontextprotocol/java-sdk/issues/293 - without this header,
+	 * proxies like Nginx buffer the SSE response, breaking real-time streaming.
+	 */
+	@Test
+	void listeningStreamIncludesXAccelBufferingHeader() throws Exception {
+		prepareAsyncServerBuilder().serverInfo("test-server", "1.0.0").build();
+		var sessionId = initializeSession(httpClient);
+
+		var get = HttpRequest.newBuilder()
+			.uri(URI.create("http://localhost:" + PORT + MESSAGE_ENDPOINT))
+			.header("Accept", "text/event-stream")
+			.header(HttpHeaders.MCP_SESSION_ID, sessionId)
+			.GET()
+			.build();
+
+		var response = httpClient.send(get, HttpResponse.BodyHandlers.ofInputStream());
+		try {
+			assertThat(response.headers().firstValue("X-Accel-Buffering")).contains("no");
+		}
+		finally {
+			response.body().close();
+		}
+	}
+
+	/**
+	 * https://github.com/modelcontextprotocol/java-sdk/issues/293 - same header is also
+	 * required on the SSE response a POST request opens for a streamed tool call.
+	 */
+	@Test
+	void toolCallSseResponseIncludesXAccelBufferingHeader() throws Exception {
+		prepareAsyncServerBuilder().serverInfo("test-server", "1.0.0")
+			.capabilities(McpSchema.ServerCapabilities.builder().tools(true).build())
+			.tools(McpServerFeatures.AsyncToolSpecification.builder()
+				.tool(McpSchema.Tool.builder("echo", EMPTY_JSON_SCHEMA).description("returns immediately").build())
+				.callHandler((exchange,
+						request) -> Mono.just(McpSchema.CallToolResult.builder()
+							.content(List.of(McpSchema.TextContent.builder("ok").build()))
+							.isError(false)
+							.build()))
+				.build())
+			.build();
+		var sessionId = initializeSession(httpClient);
+
+		var post = HttpRequest.newBuilder()
+			.uri(URI.create("http://localhost:" + PORT + MESSAGE_ENDPOINT))
+			.header("Content-Type", "application/json")
+			.header("Accept", "text/event-stream, application/json")
+			.header(HttpHeaders.MCP_SESSION_ID, sessionId)
+			.POST(HttpRequest.BodyPublishers
+				.ofString("{\"jsonrpc\":\"2.0\",\"id\":\"call-1\",\"method\":\"tools/call\","
+						+ "\"params\":{\"name\":\"echo\",\"arguments\":{}}}"))
+			.build();
+
+		var response = httpClient.send(post, HttpResponse.BodyHandlers.ofInputStream());
+		try {
+			assertThat(response.headers().firstValue("X-Accel-Buffering")).contains("no");
+		}
+		finally {
+			response.body().close();
+		}
+	}
+
 	@Override
 	protected void prepareClients(int port, String mcpEndpoint) {
 	}