fix: handle root exceptions in McpToolCallback

- Sync and Async McpToolCallback can now handle exceptions where
  cause == null instead of throwing a NPE.

Author: Kehrlann

mcp/common/src/main/java/org/springframework/ai/mcp/AsyncMcpToolCallback.java

@@ -122,7 +122,7 @@ public String call(String functionInput) {
 			}).block();
 		}
 		catch (Exception ex) {
-			throw new ToolExecutionException(this.getToolDefinition(), ex.getCause());
+			throw new ToolExecutionException(this.getToolDefinition(), ex);
 		}
 
 	}

mcp/common/src/main/java/org/springframework/ai/mcp/SyncMcpToolCallback.java

@@ -131,7 +131,7 @@ public String call(String functionInput) {
 		}
 		catch (Exception ex) {
 			logger.error("Exception while tool calling: ", ex);
-			throw new ToolExecutionException(this.getToolDefinition(), ex.getCause());
+			throw new ToolExecutionException(this.getToolDefinition(), ex);
 		}
 
 	}

mcp/common/src/test/java/org/springframework/ai/mcp/AsyncMcpToolCallbackTest.java

@@ -0,0 +1,38 @@
+package org.springframework.ai.mcp;
+
+import io.modelcontextprotocol.client.McpAsyncClient;
+import io.modelcontextprotocol.spec.McpSchema;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+import org.springframework.ai.tool.execution.ToolExecutionException;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.when;
+
+@ExtendWith(MockitoExtension.class)
+class AsyncMcpToolCallbackTest {
+
+	@Mock
+	private McpAsyncClient mcpClient;
+
+	@Mock
+	private McpSchema.Tool tool;
+
+	@Test
+	void callShouldHandleExceptions() {
+		when(this.tool.name()).thenReturn("testTool");
+		var clientInfo = new McpSchema.Implementation("testClient", "1.0.0");
+		when(this.mcpClient.getClientInfo()).thenReturn(clientInfo);
+		when(this.mcpClient.callTool(any(McpSchema.CallToolRequest.class))).thenThrow(new RuntimeException("Testing tool error"));
+
+		AsyncMcpToolCallback callback = new AsyncMcpToolCallback(this.mcpClient, this.tool);
+		assertThatThrownBy(() -> callback.call("{\"param\":\"value\"}"))
+				.isInstanceOf(ToolExecutionException.class)
+				.rootCause()
+				.hasMessage("Testing tool error");
+	}
+
+}

mcp/common/src/test/java/org/springframework/ai/mcp/SyncMcpToolCallbackTests.java

@@ -29,8 +29,10 @@
 import org.mockito.junit.jupiter.MockitoExtension;
 
 import org.springframework.ai.chat.model.ToolContext;
+import org.springframework.ai.tool.execution.ToolExecutionException;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
@@ -94,4 +96,18 @@ void callShouldIgnoreToolContext() {
 		assertThat(response).isNotNull();
 	}
 
+	@Test
+	void callShouldWrapExceptions() {
+		when(this.tool.name()).thenReturn("testTool");
+		var clientInfo = new Implementation("testClient", "1.0.0");
+		when(this.mcpClient.getClientInfo()).thenReturn(clientInfo);
+		when(this.mcpClient.callTool(any(CallToolRequest.class))).thenThrow(new RuntimeException("Testing tool error"));
+
+		SyncMcpToolCallback callback = new SyncMcpToolCallback(this.mcpClient, this.tool);
+		assertThatThrownBy(() -> callback.call("{\"param\":\"value\"}"))
+				.isInstanceOf(ToolExecutionException.class)
+				.rootCause()
+				.hasMessage("Testing tool error");
+	}
+
 }