fix(MCP Client Tool Node): Stringify tool result (#14554)

packages/@n8n/nodes-langchain/nodes/mcp/McpClientTool/McpClientTool.node.test.ts

@@ -57,7 +57,7 @@ describe('McpClientTool', () => {
 			jest.resetAllMocks();
 		});
 
-		it('should return a valid toolkit with usable tools', async () => {
+		it('should return a valid toolkit with usable tools (that returns a string)', async () => {
 			jest.spyOn(Client.prototype, 'connect').mockResolvedValue();
 			jest
 				.spyOn(Client.prototype, 'callTool')
@@ -93,7 +93,7 @@ describe('McpClientTool', () => {
 			expect(tools).toHaveLength(2);
 
 			const toolCallResult = await tools[0].invoke({ input: 'foo' });
-			expect(toolCallResult).toEqual([{ type: 'text', text: 'result from tool' }]);
+			expect(toolCallResult).toEqual(JSON.stringify([{ type: 'text', text: 'result from tool' }]));
 		});
 
 		it('should support selecting tools to expose', async () => {

packages/@n8n/nodes-langchain/nodes/mcp/McpTrigger/McpServer.ts

@@ -120,7 +120,8 @@ export class McpServer {
 					return {
 						name: tool.name,
 						description: tool.description,
-						inputSchema: zodToJsonSchema(tool.schema),
+						// Allow additional properties on tool call input
+						inputSchema: zodToJsonSchema(tool.schema, { removeAdditionalStrategy: 'strict' }),
 					};
 				}),
 			};

packages/@n8n/nodes-langchain/utils/logWrapper.ts

@@ -395,7 +395,9 @@ export function logWrapper<
 
 						logAiEvent(executeFunctions, 'ai-tool-called', { ...inputData, response });
 						executeFunctions.addOutputData(connectionType, index, [[{ json: { response } }]]);
-						return response;
+
+						if (typeof response === 'string') return response;
+						return JSON.stringify(response);
 					};
 				}
 			}