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HttpServletStreamableServerTransportProvider: GET stream sends no status or headers until the first event

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Valutazione

Difficoltà
2/5
Tempo stimato
1-3 ore
Idoneità per principianti
86/100
Tipo di issue
Bug
Chiarezza
Specificata chiaramente
Stato di attività
Attiva
Stack tecnologico
java
Ambito
api, backend

Direzione di ricerca

Inizia in HttpServletStreamableServerTransportProvider#doGet ed esegui la riproduzione fornita StreamableGetStreamHeadersTest.java sul transport servlet. Il lavoro è completato quando la richiesta GET termina con stato 200 e un Content-Type text/event-stream prima che venga inviato qualsiasi evento o ping di keep-alive.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

Bug description
After initialize, a GET to the MCP endpoint with Accept: text/event-stream gets no status line and no headers. doGet sets the SSE headers and starts async but doesn't flush, so nothing is sent until the server writes its first event to the stream. If the server has nothing to send, the client never gets a response.

Clients that wait for headers time out. Node's fetch (undici) gives up after its default headersTimeout of 300 s with UND_ERR_HEADERS_TIMEOUT. mcp-remote then retries twice and stops ("Maximum reconnection attempts (2) exceeded"). After that the server has no stream to the client, and server-initiated requests fail with Stream unavailable for session ... (see also #1061).

With keepAliveInterval set, the first ping sends the headers, but only after one interval.

The spec says the server must either return Content-Type: text/event-stream for this GET or return 405 (Streamable HTTP, 2025-11-25).

Environment

  • Where it was seen: mcp-core 1.1.1, Spring Boot 4.0.5 (embedded Tomcat), Java 25.0.3, macOS 27.0.1. Client: Claude Desktop through mcp-remote 0.14.3, protocol 2025-11-25.
  • Reproduced with the test below: mcp-core 1.1.1 and 2.0.1, tomcat-embed-core 11.0.22, JDK 26.0.1, macOS 14.7.4. The doGet code is the same on main (1cf7903).
  • No keep-alive configured.

Steps to reproduce

  1. POST initialize and note the Mcp-Session-Id.
  2. POST notifications/initialized. You get 202.
  3. GET the endpoint with Accept: text/event-stream and the session ID.
curl -sN --max-time 20 -D - http://127.0.0.1:8080/mcp -H 'Accept: text/event-stream' -H "Mcp-Session-Id: $SID"

Actual: 0 bytes received in 20 s.

Expected behavior
200 with Content-Type: text/event-stream sent as soon as the GET stream opens, before any event is ready.

Minimal Complete Reproducible example
Dependencies: io.modelcontextprotocol.sdk:mcp, org.apache.tomcat.embed:tomcat-embed-core:11.0.22, org.junit.jupiter:junit-jupiter. The test fails with java.util.concurrent.TimeoutException on 1.1.1 and 2.0.1. If .keepAliveInterval(Duration.ofSeconds(1)) is added to the builder, it passes: the headers arrive with the first ping.

StreamableGetStreamHeadersTest.java 
package repro;

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertTrue;

import java.io.InputStream;
import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.nio.file.Files;
import java.util.concurrent.TimeUnit;

import org.apache.catalina.Context;
import org.apache.catalina.startup.Tomcat;
import org.junit.jupiter.api.Test;

import io.modelcontextprotocol.server.McpAsyncServer;
import io.modelcontextprotocol.server.McpServer;
import io.modelcontextprotocol.server.transport.HttpServletStreamableServerTransportProvider;

class StreamableGetStreamHeadersTest {

	@Test
	void getStreamSendsHeadersBeforeFirstEvent() throws Exception {
		HttpServletStreamableServerTransportProvider transport = HttpServletStreamableServerTransportProvider.builder()
			.build();
		McpAsyncServer server = McpServer.async(transport).serverInfo("repro", "1").build();

		Tomcat tomcat = new Tomcat();
		tomcat.setBaseDir(Files.createTempDirectory("tomcat").toString());
		tomcat.setPort(0);
		Context context = tomcat.addContext("", null);
		Tomcat.addServlet(context, "mcp", transport).setAsyncSupported(true);
		context.addServletMappingDecoded("/*", "mcp");
		tomcat.start();

		try {
			URI endpoint = URI.create("http://127.0.0.1:" + tomcat.getConnector().getLocalPort() + "/mcp");
			HttpClient client = HttpClient.newHttpClient();

			String sessionId = client.send(post(endpoint, null, """
					{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-11-25",\
					"capabilities":{},"clientInfo":{"name":"repro","version":"1"}}}"""),
					HttpResponse.BodyHandlers.discarding())
				.headers()
				.firstValue("Mcp-Session-Id")
				.orElseThrow();
			assertEquals(202, client.send(post(endpoint, sessionId, """
					{"jsonrpc":"2.0","method":"notifications/initialized"}"""),
					HttpResponse.BodyHandlers.discarding())
				.statusCode());

			HttpRequest get = HttpRequest.newBuilder(endpoint)
				.header("Accept", "text/event-stream")
				.header("Mcp-Session-Id", sessionId)
				.GET()
				.build();

			// Completes once the status line and headers arrive. Fails with TimeoutException.
			HttpResponse<InputStream> response = client.sendAsync(get, HttpResponse.BodyHandlers.ofInputStream())
				.get(5, TimeUnit.SECONDS);

			assertEquals(200, response.statusCode());
			assertTrue(response.headers().firstValue("Content-Type").orElse("").startsWith("text/event-stream"));
			response.body().close();
		}
		finally {
			server.closeGracefully().block();
			tomcat.stop();
			tomcat.destroy();
		}
	}

	private static HttpRequest post(URI endpoint, String sessionId, String body) {
		HttpRequest.Builder builder = HttpRequest.newBuilder(endpoint)
			.header("Accept", "application/json, text/event-stream")
			.header("Content-Type", "application/json")
			.POST(HttpRequest.BodyPublishers.ofString(body));
		if (sessionId != null) {
			builder.header("Mcp-Session-Id", sessionId);
		}
		return builder.build();
	}

}
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