diff --git a/mcp/src/main/java/io/modelcontextprotocol/spec/McpClientSession.java b/mcp/src/main/java/io/modelcontextprotocol/spec/McpClientSession.java
index f7db3d7..6ac8def 100644
--- a/mcp/src/main/java/io/modelcontextprotocol/spec/McpClientSession.java
+++ b/mcp/src/main/java/io/modelcontextprotocol/spec/McpClientSession.java
@@ -35,6 +35,7 @@ import java.util.function.Function;
  *
  * @author Christian Tzolov
  * @author Dariusz Jędrzejczyk
+ * @author Yanming Zhou
  */
 public class McpClientSession implements McpSession {
 
@@ -146,13 +147,20 @@ public class McpClientSession implements McpSession {
 
 	private void handle(McpSchema.JSONRPCMessage message) {
 		if (message instanceof McpSchema.JSONRPCResponse response) {
-			logger.debug("Received Response: {}", response);
-			var sink = pendingResponses.remove(response.id());
-			if (sink == null) {
-				logger.warn("Unexpected response for unknown id {}", response.id());
+			logger.debug("Received response: {}", response);
+			if (response.id() != null) {
+				var sink = pendingResponses.remove(response.id());
+				if (sink == null) {
+					logger.warn("Unexpected response for unknown id {}", response.id());
+				}
+				else {
+					sink.success(response);
+				}
 			}
 			else {
-				sink.success(response);
+				logger.error("Discarded MCP request response without session id. "
+						+ "This is an indication of a bug in the request sender code that can lead to memory "
+						+ "leaks as pending requests will never be completed.");
 			}
 		}
 		else if (message instanceof McpSchema.JSONRPCRequest request) {
diff --git a/mcp/src/main/java/io/modelcontextprotocol/spec/McpServerSession.java b/mcp/src/main/java/io/modelcontextprotocol/spec/McpServerSession.java
index e6a0c8b..747b454 100644
--- a/mcp/src/main/java/io/modelcontextprotocol/spec/McpServerSession.java
+++ b/mcp/src/main/java/io/modelcontextprotocol/spec/McpServerSession.java
@@ -204,13 +204,20 @@ public class McpServerSession implements McpLoggableSession {
 			// TODO handle errors for communication to without initialization happening
 			// first
 			if (message instanceof McpSchema.JSONRPCResponse response) {
-				logger.debug("Received Response: {}", response);
-				var sink = pendingResponses.remove(response.id());
-				if (sink == null) {
-					logger.warn("Unexpected response for unknown id {}", response.id());
+				logger.debug("Received response: {}", response);
+				if (response.id() != null) {
+					var sink = pendingResponses.remove(response.id());
+					if (sink == null) {
+						logger.warn("Unexpected response for unknown id {}", response.id());
+					}
+					else {
+						sink.success(response);
+					}
 				}
 				else {
-					sink.success(response);
+					logger.error("Discarded MCP request response without session id. "
+							+ "This is an indication of a bug in the request sender code that can lead to memory "
+							+ "leaks as pending requests will never be completed.");
 				}
 				return Mono.empty();
 			}
diff --git a/mcp/src/main/java/io/modelcontextprotocol/spec/McpStreamableServerSession.java b/mcp/src/main/java/io/modelcontextprotocol/spec/McpStreamableServerSession.java
index af29ce0..53b56c7 100644
--- a/mcp/src/main/java/io/modelcontextprotocol/spec/McpStreamableServerSession.java
+++ b/mcp/src/main/java/io/modelcontextprotocol/spec/McpStreamableServerSession.java
@@ -21,6 +21,7 @@ import io.modelcontextprotocol.common.McpTransportContext;
 import io.modelcontextprotocol.server.McpAsyncServerExchange;
 import io.modelcontextprotocol.server.McpNotificationHandler;
 import io.modelcontextprotocol.server.McpRequestHandler;
+import io.modelcontextprotocol.spec.McpSchema.ErrorCodes;
 import io.modelcontextprotocol.util.Assert;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
@@ -33,6 +34,7 @@ import reactor.core.publisher.MonoSink;
  * capability without the insight into the transport-specific details of HTTP handling.
  *
  * @author Dariusz Jędrzejczyk
+ * @author Yanming Zhou
  */
 public class McpStreamableServerSession implements McpLoggableSession {
 
@@ -214,19 +216,30 @@ public class McpStreamableServerSession implements McpLoggableSession {
 	 */
 	public Mono<Void> accept(McpSchema.JSONRPCResponse response) {
 		return Mono.defer(() -> {
-			var stream = this.requestIdToStream.get(response.id());
-			if (stream == null) {
-				return Mono.error(new McpError("Unexpected response for unknown id " + response.id())); // TODO
-																										// JSONize
-			}
-			// TODO: encapsulate this inside the stream itself
-			var sink = stream.pendingResponses.remove(response.id());
-			if (sink == null) {
-				return Mono.error(new McpError("Unexpected response for unknown id " + response.id())); // TODO
-																										// JSONize
+			logger.debug("Received response: {}", response);
+
+			if (response.id() != null) {
+				var stream = this.requestIdToStream.get(response.id());
+				if (stream == null) {
+					return Mono.error(McpError.builder(ErrorCodes.INTERNAL_ERROR)
+						.message("Unexpected response for unknown id " + response.id())
+						.build());
+				}
+				// TODO: encapsulate this inside the stream itself
+				var sink = stream.pendingResponses.remove(response.id());
+				if (sink == null) {
+					return Mono.error(McpError.builder(ErrorCodes.INTERNAL_ERROR)
+						.message("Unexpected response for unknown id " + response.id())
+						.build());
+				}
+				else {
+					sink.success(response);
+				}
 			}
 			else {
-				sink.success(response);
+				logger.error("Discarded MCP request response without session id. "
+						+ "This is an indication of a bug in the request sender code that can lead to memory "
+						+ "leaks as pending requests will never be completed.");
 			}
 			return Mono.empty();
 		});