diff --git a/mcp-spring/mcp-spring-webflux/src/main/java/io/modelcontextprotocol/client/transport/WebClientStreamableHttpTransport.java b/mcp-spring/mcp-spring-webflux/src/main/java/io/modelcontextprotocol/client/transport/WebClientStreamableHttpTransport.java
index 154eb4703..f6eeb0b1f 100644
--- a/mcp-spring/mcp-spring-webflux/src/main/java/io/modelcontextprotocol/client/transport/WebClientStreamableHttpTransport.java
+++ b/mcp-spring/mcp-spring-webflux/src/main/java/io/modelcontextprotocol/client/transport/WebClientStreamableHttpTransport.java
@@ -18,6 +18,7 @@
 import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
 import org.springframework.http.codec.ServerSentEvent;
+import org.springframework.util.StringUtils;
 import org.springframework.web.reactive.function.client.ClientResponse;
 import org.springframework.web.reactive.function.client.WebClient;
 import org.springframework.web.reactive.function.client.WebClientResponseException;
@@ -279,8 +280,8 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage message) {
 				})
 				.bodyValue(message)
 				.exchangeToFlux(response -> {
-					if (transportSession
-						.markInitialized(response.headers().asHttpHeaders().getFirst(HttpHeaders.MCP_SESSION_ID))) {
+					String mcpSessionId = response.headers().asHttpHeaders().getFirst(HttpHeaders.MCP_SESSION_ID);
+					if (StringUtils.hasText(mcpSessionId) && transportSession.markInitialized(mcpSessionId)) {
 						// Once we have a session, we try to open an async stream for
 						// the server to send notifications and requests out-of-band.
 						reconnect(null).contextWrite(sink.contextView()).subscribe();
diff --git a/mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java b/mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java
index c73515938..fff8d5d97 100644
--- a/mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java
+++ b/mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java
@@ -12,6 +12,7 @@
 import java.net.http.HttpResponse.BodyHandler;
 import java.time.Duration;
 import java.util.List;
+import java.util.Objects;
 import java.util.Optional;
 import java.util.concurrent.CompletionException;
 import java.util.concurrent.atomic.AtomicReference;
@@ -442,8 +443,8 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage sentMessage) {
 					})).onErrorMap(CompletionException.class, t -> t.getCause()).onErrorComplete().subscribe();
 
 			})).flatMap(responseEvent -> {
-				if (transportSession.markInitialized(
-						responseEvent.responseInfo().headers().firstValue("mcp-session-id").orElseGet(() -> null))) {
+				String mcpSessionId = responseEvent.responseInfo().headers().firstValue("mcp-session-id").orElseGet(() -> null);
+				if (Objects.nonNull(mcpSessionId) && transportSession.markInitialized(mcpSessionId)) {
 					// Once we have a session, we try to open an async stream for
 					// the server to send notifications and requests out-of-band.