feat: Add message history trimming to AI workflow builder (no-changelog) (#17829)

packages/@n8n/ai-workflow-builder.ee/src/constants.ts

@@ -0,0 +1,3 @@
+export const MAX_AI_BUILDER_PROMPT_LENGTH = 1000; // characters
+
+export const MAX_USER_MESSAGES = 10; // Maximum number of user messages to keep in the state

packages/@n8n/ai-workflow-builder.ee/src/test/workflow-state.test.ts

@@ -0,0 +1,154 @@
+import { HumanMessage, AIMessage as AssistantMessage, ToolMessage } from '@langchain/core/messages';
+import type { BaseMessage } from '@langchain/core/messages';
+
+import { createTrimMessagesReducer } from '../workflow-state';
+
+describe('createTrimMessagesReducer', () => {
+	it('should return messages unchanged when human messages are within limit', () => {
+		const reducer = createTrimMessagesReducer(3);
+		const messages: BaseMessage[] = [
+			new HumanMessage('User 1'),
+			new AssistantMessage('Assistant 1'),
+			new ToolMessage({ content: 'Tool 1', tool_call_id: '1' }),
+			new ToolMessage({ content: 'Tool 2', tool_call_id: '2' }),
+			new AssistantMessage('Assistant 2'),
+		];
+
+		const result = reducer(messages);
+		expect(result).toEqual(messages);
+		expect(result.length).toBe(5);
+	});
+
+	it('should trim messages when human messages exceed limit', () => {
+		const reducer = createTrimMessagesReducer(3);
+		const messages: BaseMessage[] = [
+			new HumanMessage('User 1'),
+			new AssistantMessage('Assistant 1'),
+			new ToolMessage({ content: 'Tool 1', tool_call_id: '1' }),
+			new HumanMessage('User 2'),
+			new AssistantMessage('Assistant 2'),
+			new HumanMessage('User 3'),
+			new AssistantMessage('Assistant 3'),
+			new HumanMessage('User 4'),
+			new AssistantMessage('Assistant 4'),
+		];
+
+		const result = reducer(messages);
+
+		// Should keep only the last 3 HumanMessages
+		const humanMessages = result.filter((msg) => msg instanceof HumanMessage);
+		expect(humanMessages.length).toBe(3);
+
+		// Should start with HumanMessage
+		expect(result[0]).toBeInstanceOf(HumanMessage);
+		expect((result[0] as HumanMessage).content).toBe('User 2');
+
+		// Should preserve messages between HumanMessages
+		expect(result.length).toBe(6); // User 2, Assistant 2, User 3, Assistant 3, User 4, Assistant 4
+	});
+
+	it('should handle typical conversation pattern', () => {
+		const reducer = createTrimMessagesReducer(2);
+		const messages: BaseMessage[] = [
+			new HumanMessage('User 1'),
+			new AssistantMessage('Assistant 1'),
+			new ToolMessage({ content: 'Tool 1', tool_call_id: '1' }),
+			new ToolMessage({ content: 'Tool 2', tool_call_id: '2' }),
+			new AssistantMessage('Assistant 2'),
+			new HumanMessage('User 2'),
+			new AssistantMessage('Assistant 3'),
+			new ToolMessage({ content: 'Tool 3', tool_call_id: '3' }),
+			new ToolMessage({ content: 'Tool 4', tool_call_id: '4' }),
+			new AssistantMessage('Assistant 4'),
+			new HumanMessage('User 3'),
+			new AssistantMessage('Assistant 5'),
+			new ToolMessage({ content: 'Tool 5', tool_call_id: '5' }),
+			new ToolMessage({ content: 'Tool 6', tool_call_id: '6' }),
+			new AssistantMessage('Assistant 6'),
+		];
+
+		const result = reducer(messages);
+
+		// Should keep only the last 2 HumanMessages
+		const humanMessages = result.filter((msg) => msg instanceof HumanMessage);
+		expect(humanMessages.length).toBe(2);
+
+		// Should start with HumanMessage
+		expect(result[0]).toBeInstanceOf(HumanMessage);
+		expect((result[0] as HumanMessage).content).toBe('User 2');
+
+		// Should include all messages from User 2 onwards
+		expect(result.length).toBe(10);
+		expect(result.map((m) => m.content)).toEqual([
+			'User 2',
+			'Assistant 3',
+			'Tool 3',
+			'Tool 4',
+			'Assistant 4',
+			'User 3',
+			'Assistant 5',
+			'Tool 5',
+			'Tool 6',
+			'Assistant 6',
+		]);
+	});
+
+	it('should handle edge case with exactly maxUserMessages', () => {
+		const reducer = createTrimMessagesReducer(2);
+		const messages: BaseMessage[] = [
+			new HumanMessage('User 1'),
+			new AssistantMessage('Assistant 1'),
+			new HumanMessage('User 2'),
+			new AssistantMessage('Assistant 2'),
+		];
+
+		const result = reducer(messages);
+		expect(result).toEqual(messages);
+		expect(result.length).toBe(4);
+	});
+
+	it('should handle empty array', () => {
+		const reducer = createTrimMessagesReducer(5);
+		const messages: BaseMessage[] = [];
+
+		const result = reducer(messages);
+		expect(result).toEqual([]);
+	});
+
+	it('should handle array with no HumanMessages', () => {
+		const reducer = createTrimMessagesReducer(5);
+		const messages: BaseMessage[] = [
+			new AssistantMessage('Assistant 1'),
+			new ToolMessage({ content: 'Tool 1', tool_call_id: '1' }),
+			new AssistantMessage('Assistant 2'),
+		];
+
+		const result = reducer(messages);
+		expect(result).toEqual(messages);
+	});
+
+	it('should handle maxUserMessages = 1', () => {
+		const reducer = createTrimMessagesReducer(1);
+		const messages: BaseMessage[] = [
+			new HumanMessage('User 1'),
+			new AssistantMessage('Assistant 1'),
+			new HumanMessage('User 2'),
+			new AssistantMessage('Assistant 2'),
+			new HumanMessage('User 3'),
+			new AssistantMessage('Assistant 3'),
+		];
+
+		const result = reducer(messages);
+
+		// Should keep only the last HumanMessage
+		const humanMessages = result.filter((msg) => msg instanceof HumanMessage);
+		expect(humanMessages.length).toBe(1);
+
+		// Should start with User 3
+		expect(result[0]).toBeInstanceOf(HumanMessage);
+		expect((result[0] as HumanMessage).content).toBe('User 3');
+
+		// Should only include User 3 and Assistant 3
+		expect(result.length).toBe(2);
+	});
+});

packages/@n8n/ai-workflow-builder.ee/src/workflow-builder-agent.ts

@@ -12,8 +12,10 @@ import type {
 	NodeExecutionSchema,
 } from 'n8n-workflow';
 
+import { MAX_AI_BUILDER_PROMPT_LENGTH } from '@/constants';
+
 import { conversationCompactChain } from './chains/conversation-compact';
-import { LLMServiceError } from './errors';
+import { LLMServiceError, ValidationError } from './errors';
 import { createAddNodeTool } from './tools/add-node.tool';
 import { createConnectNodesTool } from './tools/connect-nodes.tool';
 import { createNodeDetailsTool } from './tools/node-details.tool';
@@ -191,6 +193,17 @@ export class WorkflowBuilderAgent {
 	}
 
 	async *chat(payload: ChatPayload, userId?: string, abortSignal?: AbortSignal) {
+		// Check for the message maximum length
+		if (payload.message.length > MAX_AI_BUILDER_PROMPT_LENGTH) {
+			this.logger?.warn('Message exceeds maximum length', {
+				messageLength: payload.message.length,
+				maxLength: MAX_AI_BUILDER_PROMPT_LENGTH,
+			});
+
+			throw new ValidationError(
+				`Message exceeds maximum length of ${MAX_AI_BUILDER_PROMPT_LENGTH} characters`,
+			);
+		}
 		const agent = this.createWorkflow().compile({ checkpointer: this.checkpointer });
 		const workflowId = payload.workflowContext?.currentWorkflow?.id;
 		// Generate thread ID from workflowId and userId

packages/@n8n/ai-workflow-builder.ee/src/workflow-state.ts

@@ -1,5 +1,9 @@
 import type { BaseMessage } from '@langchain/core/messages';
+import { HumanMessage } from '@langchain/core/messages';
 import { Annotation, messagesStateReducer } from '@langchain/langgraph';
+import type { BinaryOperator } from '@langchain/langgraph/dist/channels/binop';
+
+import { MAX_USER_MESSAGES } from '@/constants';
 
 import type { SimpleWorkflow, WorkflowOperation } from './types/workflow';
 import type { ChatPayload } from './workflow-builder-agent';
@@ -32,9 +36,44 @@ function operationsReducer(
 	return [...(current ?? []), ...update];
 }
 
+// Creates a reducer that trims the message history to keep only the last `maxUserMessages` HumanMessage instances
+export function createTrimMessagesReducer(maxUserMessages: number) {
+	return (current: BaseMessage[]): BaseMessage[] => {
+		// Count HumanMessage instances and remember their indices
+		const humanMessageIndices: number[] = [];
+		current.forEach((msg, index) => {
+			if (msg instanceof HumanMessage) {
+				humanMessageIndices.push(index);
+			}
+		});
+
+		// If we have fewer than or equal to maxUserMessages, return as is
+		if (humanMessageIndices.length <= maxUserMessages) {
+			return current;
+		}
+
+		// Find the index of the first HumanMessage that we want to keep
+		const startHumanMessageIndex =
+			humanMessageIndices[humanMessageIndices.length - maxUserMessages];
+
+		// Slice from that HumanMessage onwards
+		return current.slice(startHumanMessageIndex);
+	};
+}
+
+// Utility function to combine multiple message reducers into one.
+function combineMessageReducers(...reducers: Array<BinaryOperator<BaseMessage[], BaseMessage[]>>) {
+	return (current: BaseMessage[], update: BaseMessage[]): BaseMessage[] => {
+		return reducers.reduce((acc, reducer) => reducer(acc, update), current);
+	};
+}
+
 export const WorkflowState = Annotation.Root({
 	messages: Annotation<BaseMessage[]>({
-		reducer: messagesStateReducer,
+		reducer: combineMessageReducers(
+			messagesStateReducer,
+			createTrimMessagesReducer(MAX_USER_MESSAGES),
+		),
 		default: () => [],
 	}),
 	// // The original prompt from the user.