From c49241c8549612f2b1d0ab1384a54dbe4196e8b9 Mon Sep 17 00:00:00 2001
From: Michael Vorburger <mike@vorburger.ch>
Date: Wed, 8 Oct 2025 01:04:47 +0200
Subject: [PATCH] fix: Avoid NPE in McpClientSession

---
 .../io/modelcontextprotocol/spec/McpClientSession.java | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mcp-core/src/main/java/io/modelcontextprotocol/spec/McpClientSession.java b/mcp-core/src/main/java/io/modelcontextprotocol/spec/McpClientSession.java
index 0ba7ab3b8..109ac0dbf 100644
--- a/mcp-core/src/main/java/io/modelcontextprotocol/spec/McpClientSession.java
+++ b/mcp-core/src/main/java/io/modelcontextprotocol/spec/McpClientSession.java
@@ -283,7 +283,15 @@ public <T> Mono<T> sendRequest(String method, Object requestParams, TypeRef<T> t
 					deliveredResponseSink.complete();
 				}
 				else {
-					deliveredResponseSink.next(this.transport.unmarshalFrom(jsonRpcResponse.result(), typeRef));
+					var result = jsonRpcResponse.result();
+					var unmarshalled = this.transport.unmarshalFrom(result, typeRef);
+					// https://github.com/modelcontextprotocol/java-sdk/issues/605
+					if (unmarshalled != null)
+						deliveredResponseSink.next(unmarshalled);
+					else
+						// NB: next() cannot be called with null
+						deliveredResponseSink
+							.error(new IllegalStateException("Failed to unmarshal response: " + jsonRpcResponse));
 				}
 			}
 		});