fix: reject duplicate streamable HTTP initialize requests

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

@@ -447,6 +447,15 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 					return;
 				}
 
+				String sessionId = request.getHeader(HttpHeaders.MCP_SESSION_ID);
+				if (sessionId != null && this.sessions.containsKey(sessionId)) {
+					this.responseError(response, HttpServletResponse.SC_BAD_REQUEST,
+							McpError.builder(McpSchema.ErrorCodes.INVALID_REQUEST)
+								.message("Duplicate initialize request for active session: " + sessionId)
+								.build());
+					return;
+				}
+
 				McpSchema.InitializeRequest initializeRequest = jsonMapper.convertValue(jsonrpcRequest.params(),
 						new TypeRef<McpSchema.InitializeRequest>() {
 						});

mcp-test/src/test/java/io/modelcontextprotocol/server/transport/HttpServletStreamableServerTransportProviderTests.java

@@ -0,0 +1,87 @@
+/*
+ * Copyright 2026-2026 the original author or authors.
+ */
+
+package io.modelcontextprotocol.server.transport;
+
+import io.modelcontextprotocol.server.McpServer;
+import io.modelcontextprotocol.spec.HttpHeaders;
+import io.modelcontextprotocol.spec.McpSchema;
+import io.modelcontextprotocol.spec.ProtocolVersions;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.mock.web.MockHttpServletRequest;
+import org.springframework.mock.web.MockHttpServletResponse;
+
+import static io.modelcontextprotocol.server.transport.HttpServletStreamableServerTransportProvider.APPLICATION_JSON;
+import static io.modelcontextprotocol.server.transport.HttpServletStreamableServerTransportProvider.TEXT_EVENT_STREAM;
+import static io.modelcontextprotocol.util.McpJsonMapperUtils.JSON_MAPPER;
+import static org.assertj.core.api.Assertions.assertThat;
+
+class HttpServletStreamableServerTransportProviderTests {
+
+	private static final String MESSAGE_ENDPOINT = "/mcp/message";
+
+	@Test
+	void shouldRejectDuplicateInitializeForActiveSession() throws Exception {
+		var transportProvider = HttpServletStreamableServerTransportProvider.builder()
+			.mcpEndpoint(MESSAGE_ENDPOINT)
+			.build();
+		var server = McpServer.sync(transportProvider).serverInfo("test-server", "1.0.0").build();
+
+		try {
+			MockHttpServletResponse initialResponse = initialize(transportProvider, null, "init-1",
+					ProtocolVersions.MCP_2025_11_25, "initial-client");
+			assertThat(initialResponse.getStatus()).isEqualTo(200);
+
+			String sessionId = initialResponse.getHeader(HttpHeaders.MCP_SESSION_ID);
+			assertThat(sessionId).isNotBlank();
+
+			MockHttpServletResponse initializedResponse = sendMessage(transportProvider, sessionId,
+					new McpSchema.JSONRPCNotification(McpSchema.METHOD_NOTIFICATION_INITIALIZED),
+					ProtocolVersions.MCP_2025_11_25);
+			assertThat(initializedResponse.getStatus()).isEqualTo(202);
+
+			MockHttpServletResponse duplicateResponse = initialize(transportProvider, sessionId, "init-2",
+					ProtocolVersions.MCP_2024_11_05, "duplicate-client");
+
+			assertThat(duplicateResponse.getStatus()).isEqualTo(400);
+			assertThat(duplicateResponse.getHeader(HttpHeaders.MCP_SESSION_ID)).isNull();
+			assertThat(duplicateResponse.getContentAsString()).contains("Duplicate initialize");
+		}
+		finally {
+			server.closeGracefully();
+		}
+	}
+
+	private static MockHttpServletResponse initialize(HttpServletStreamableServerTransportProvider transportProvider,
+			String sessionId, String requestId, String protocolVersion, String clientName) throws Exception {
+		McpSchema.InitializeRequest initializeRequest = McpSchema.InitializeRequest
+			.builder(protocolVersion, McpSchema.ClientCapabilities.builder().roots(true).build(),
+					McpSchema.Implementation.builder(clientName, "1.0.0").build())
+			.build();
+		McpSchema.JSONRPCRequest jsonRpcRequest = new McpSchema.JSONRPCRequest(McpSchema.METHOD_INITIALIZE, requestId,
+				initializeRequest);
+		return sendMessage(transportProvider, sessionId, jsonRpcRequest, protocolVersion);
+	}
+
+	private static MockHttpServletResponse sendMessage(HttpServletStreamableServerTransportProvider transportProvider,
+			String sessionId, McpSchema.JSONRPCMessage message, String protocolVersion) throws Exception {
+		byte[] content = JSON_MAPPER.writeValueAsBytes(message);
+
+		MockHttpServletRequest request = new MockHttpServletRequest("POST", MESSAGE_ENDPOINT);
+		request.setContent(content);
+		request.addHeader("Content-Type", APPLICATION_JSON);
+		request.addHeader("Content-Length", Integer.toString(content.length));
+		request.addHeader("Accept", APPLICATION_JSON + ", " + TEXT_EVENT_STREAM);
+		request.addHeader(HttpHeaders.PROTOCOL_VERSION, protocolVersion);
+		if (sessionId != null) {
+			request.addHeader(HttpHeaders.MCP_SESSION_ID, sessionId);
+		}
+
+		MockHttpServletResponse response = new MockHttpServletResponse();
+		transportProvider.service(request, response);
+		return response;
+	}
+
+}