fix: retain streamable HTTP session on response write failure

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

@@ -755,7 +755,6 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage message, String messageId
 				}
 				catch (Exception e) {
 					logger.error("Failed to send message to session {}: {}", this.sessionId, e.getMessage());
-					HttpServletStreamableServerTransportProvider.this.sessions.remove(this.sessionId);
 					this.asyncContext.complete();
 				}
 				finally {

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

@@ -0,0 +1,132 @@
+/*
+ * Copyright 2024-2026 the original author or authors.
+ */
+
+package io.modelcontextprotocol.server;
+
+import java.io.IOException;
+import java.io.PrintWriter;
+import java.io.StringWriter;
+import java.nio.charset.StandardCharsets;
+import java.util.List;
+import java.util.Map;
+
+import io.modelcontextprotocol.server.transport.HttpServletStreamableServerTransportProvider;
+import io.modelcontextprotocol.spec.HttpHeaders;
+import io.modelcontextprotocol.spec.McpSchema;
+import io.modelcontextprotocol.spec.ProtocolVersions;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.Timeout;
+
+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;
+
+@Timeout(15)
+class HttpServletStreamableSessionFailureTests {
+
+	private static final String MCP_ENDPOINT = "/mcp";
+
+	@Test
+	void postStreamWriteFailureShouldNotRemoveSession() throws Exception {
+		HttpServletStreamableServerTransportProvider transport = HttpServletStreamableServerTransportProvider.builder()
+			.mcpEndpoint(MCP_ENDPOINT)
+			.build();
+
+		var tool = McpSchema.Tool.builder("test-tool").description("Test tool").build();
+		var toolSpecification = McpServerFeatures.SyncToolSpecification.builder()
+			.tool(tool)
+			.callHandler((exchange, request) -> McpSchema.CallToolResult.builder()
+				.content(List.of(McpSchema.TextContent.builder("tool response").build()))
+				.isError(false)
+				.build())
+			.build();
+		var server = McpServer.sync(transport)
+			.serverInfo("test-server", "1.0.0")
+			.capabilities(McpSchema.ServerCapabilities.builder().tools(false).build())
+			.tools(toolSpecification)
+			.build();
+
+		try {
+			MockHttpServletResponse initializeResponse = new MockHttpServletResponse();
+			transport.service(postRequest(initializeRequest(), null), initializeResponse);
+
+			String sessionId = initializeResponse.getHeader(HttpHeaders.MCP_SESSION_ID);
+			assertThat(sessionId).isNotBlank();
+
+			CheckErrorResponse failedWriteResponse = new CheckErrorResponse();
+			transport.service(postRequest(toolCallRequest("first-call"), sessionId), failedWriteResponse);
+
+			assertThat(failedWriteResponse.getWrittenContent()).contains("event: message");
+
+			MockHttpServletResponse subsequentResponse = new MockHttpServletResponse();
+			transport.service(postRequest(toolCallRequest("second-call"), sessionId), subsequentResponse);
+
+			assertThat(subsequentResponse.getStatus()).isNotEqualTo(404);
+			assertThat(subsequentResponse.getContentAsString()).doesNotContain("Session not found");
+		}
+		finally {
+			server.close();
+			transport.closeGracefully().block();
+		}
+	}
+
+	private static MockHttpServletRequest postRequest(McpSchema.JSONRPCMessage message, String sessionId)
+			throws IOException {
+		MockHttpServletRequest request = new MockHttpServletRequest("POST", MCP_ENDPOINT);
+		byte[] content = JSON_MAPPER.writeValueAsBytes(message);
+		request.setContent(content);
+		request.setCharacterEncoding(StandardCharsets.UTF_8.name());
+		request.addHeader("Accept", APPLICATION_JSON + ", " + TEXT_EVENT_STREAM);
+		request.addHeader("Content-Type", APPLICATION_JSON);
+		request.addHeader("Content-Length", Integer.toString(content.length));
+		request.addHeader(HttpHeaders.PROTOCOL_VERSION, ProtocolVersions.MCP_2025_11_25);
+		request.setAsyncSupported(true);
+		if (sessionId != null) {
+			request.addHeader(HttpHeaders.MCP_SESSION_ID, sessionId);
+		}
+		return request;
+	}
+
+	private static McpSchema.JSONRPCRequest initializeRequest() {
+		var clientInfo = McpSchema.Implementation.builder("test-client", "1.0.0").build();
+		var initializeRequest = McpSchema.InitializeRequest
+			.builder(ProtocolVersions.MCP_2025_11_25, McpSchema.ClientCapabilities.builder().build(), clientInfo)
+			.build();
+		return new McpSchema.JSONRPCRequest(McpSchema.METHOD_INITIALIZE, "init", initializeRequest);
+	}
+
+	private static McpSchema.JSONRPCRequest toolCallRequest(String id) {
+		var callToolRequest = McpSchema.CallToolRequest.builder("test-tool").arguments(Map.of()).build();
+		return new McpSchema.JSONRPCRequest(McpSchema.METHOD_TOOLS_CALL, id, callToolRequest);
+	}
+
+	private static final class CheckErrorResponse extends MockHttpServletResponse {
+
+		private final StringWriter content = new StringWriter();
+
+		private final PrintWriter writer = new PrintWriter(this.content) {
+			@Override
+			public boolean checkError() {
+				super.checkError();
+				return true;
+			}
+		};
+
+		@Override
+		public PrintWriter getWriter() {
+			return this.writer;
+		}
+
+		String getWrittenContent() {
+			this.writer.flush();
+			return this.content.toString();
+		}
+
+	}
+
+}