fix(Basic LLM Chain Node): Parse plain object output from structured output parser

packages/@n8n/nodes-langchain/nodes/chains/ChainLLM/methods/promptUtils.ts

@@ -191,5 +191,13 @@ export const getAgentStepsParser =
 			return parsedOutput;
 		}
 
+		if (typeof steps === 'object' && steps !== null && !Array.isArray(steps)) {
+			const parsedOutput = (await outputParser.parse(JSON.stringify(steps))) as Record<
+				string,
+				unknown
+			>;
+			return parsedOutput;
+		}
+
 		throw new Error('Failed to parse agent steps');
 	};

packages/@n8n/nodes-langchain/nodes/chains/ChainLLM/test/promptUtils.test.ts

@@ -349,5 +349,31 @@ describe('promptUtils', () => {
 
 			await expect(parser(steps)).rejects.toThrow('Failed to parse agent steps');
 		});
+
+		it('should parse plain object via outputParser (regression for #29903)', async () => {
+			const parser = getAgentStepsParser(mockOutputParser);
+			const plainObject = { output: { isValid: false, message: 'error' } };
+
+			const parsedResult = { isValid: false, message: 'error' };
+			mockOutputParser.parse.mockResolvedValue(parsedResult);
+
+			const result = await parser(plainObject as unknown as AgentFinish);
+
+			expect(mockOutputParser.parse).toHaveBeenCalledWith(JSON.stringify(plainObject));
+			expect(result).toEqual(parsedResult);
+		});
+
+		it('should parse plain object with arbitrary shape via outputParser', async () => {
+			const parser = getAgentStepsParser(mockOutputParser);
+			const plainObject = { foo: 'bar', count: 42 };
+
+			const parsedResult = { foo: 'bar', count: 42 };
+			mockOutputParser.parse.mockResolvedValue(parsedResult);
+
+			const result = await parser(plainObject as unknown as AgentFinish);
+
+			expect(mockOutputParser.parse).toHaveBeenCalledWith(JSON.stringify(plainObject));
+			expect(result).toEqual(parsedResult);
+		});
 	});
 });