fix(core): Run node-type-gated validators in MCP workflow build tools (#31247)

packages/@n8n/ai-workflow-builder.ee/src/code-builder/handlers/parse-validate-handler.ts

@@ -9,6 +9,7 @@
 import type { Logger } from '@n8n/backend-common';
 import { parseWorkflowCodeToBuilder, validateWorkflow, workflow } from '@n8n/workflow-sdk';
 import type { WorkflowJSON } from '@n8n/workflow-sdk';
+import type { INodeTypes } from 'n8n-workflow';
 
 import type { ParseAndValidateResult, ValidationWarning } from '../types';
 import { stripImportStatements } from '../utils/extract-code';
@@ -31,6 +32,13 @@ export interface ParseValidateHandlerConfig {
 	logger?: Logger;
 	/** Whether to generate pin data for new nodes. Defaults to true. */
 	generatePinData?: boolean;
+	/**
+	 * Optional node-type provider used to unlock the provider-gated validators
+	 * in `validateWorkflow` (input-index checks, AI input-type support checks,
+	 * main-output index checks, etc.). Without it those validators silently
+	 * skip — agent-built workflows then pass validation despite real defects.
+	 */
+	nodeTypesProvider?: INodeTypes;
 }
 
 /**
@@ -51,10 +59,12 @@ interface ValidationIssue {
 export class ParseValidateHandler {
 	private logger?: Logger;
 	private generatePinData: boolean;
+	private nodeTypesProvider?: INodeTypes;
 
 	constructor(config: ParseValidateHandlerConfig = {}) {
 		this.logger = config.logger;
 		this.generatePinData = config.generatePinData ?? true;
+		this.nodeTypesProvider = config.nodeTypesProvider;
 	}
 
 	/**
@@ -153,7 +163,9 @@ export class ParseValidateHandler {
 			'info',
 		);
 
-		const jsonValidation = validateWorkflow(json);
+		const jsonValidation = validateWorkflow(json, {
+			nodeTypesProvider: this.nodeTypesProvider,
+		});
 		this.collectValidationIssues(
 			jsonValidation.errors,
 			allWarnings,
@@ -224,7 +236,9 @@ export class ParseValidateHandler {
 			const json = builder.toJSON();
 
 			// Run JSON-based validation for additional checks
-			const validationResult = validateWorkflow(json);
+			const validationResult = validateWorkflow(json, {
+				nodeTypesProvider: this.nodeTypesProvider,
+			});
 
 			// Collect JSON validation errors as warnings for agent self-correction
 			this.collectValidationIssues(

packages/@n8n/ai-workflow-builder.ee/src/code-builder/handlers/test/parse-validate-handler.test.ts

@@ -561,7 +561,9 @@ describe('ParseValidateHandler', () => {
 
 			expect(result.map((w) => w.code)).toEqual(['GRAPH_ERR', 'JSON_ERR']);
 			expect(mockFromJSON).toHaveBeenCalledWith(nonEmptyJson);
-			expect(mockValidateWorkflow).toHaveBeenCalledWith(nonEmptyJson);
+			expect(mockValidateWorkflow).toHaveBeenCalledWith(nonEmptyJson, {
+				nodeTypesProvider: undefined,
+			});
 		});
 	});
 });

packages/cli/src/modules/mcp/__tests__/create-workflow-from-code.tool.test.ts

@@ -115,7 +115,7 @@ describe('create-workflow-from-code MCP tool', () => {
 			return { description: {} };
 		}) as typeof nodeTypes.getByNameAndVersion);
 
-		mockParseAndValidate.mockResolvedValue({ workflow: mockWorkflowJson });
+		mockParseAndValidate.mockResolvedValue({ workflow: mockWorkflowJson, warnings: [] });
 		mockStripImportStatements.mockImplementation((code: string) => code);
 		mockAutoPopulateNodeCredentials.mockResolvedValue({ assignments: [], skippedHttpNodes: [] });
 
@@ -225,6 +225,30 @@ describe('create-workflow-from-code MCP tool', () => {
 			expect(result.isError).toBeUndefined();
 		});
 
+		test('surfaces validation warnings in the response', async () => {
+			const warning = {
+				code: 'INVALID_OUTPUT_INDEX',
+				message: "'Fetch Google' has a connection from its error output (index 1).",
+				nodeName: 'Fetch Google',
+			};
+			mockParseAndValidate.mockResolvedValue({ workflow: mockWorkflowJson, warnings: [warning] });
+
+			const result = await callHandler({ code: 'const wf = ...' });
+
+			const response = parseResult(result);
+			expect(response.warnings).toEqual([warning]);
+			expect(result.isError).toBeUndefined();
+		});
+
+		test('omits the warnings field when validation produced none', async () => {
+			mockParseAndValidate.mockResolvedValue({ workflow: mockWorkflowJson, warnings: [] });
+
+			const result = await callHandler({ code: 'const wf = ...' });
+
+			const response = parseResult(result);
+			expect(response).not.toHaveProperty('warnings');
+		});
+
 		test('sets correct workflow entity defaults', async () => {
 			await callHandler({ code: 'const wf = ...' });
 
@@ -257,6 +281,7 @@ describe('create-workflow-from-code MCP tool', () => {
 
 		test('falls back to "Untitled Workflow" when neither name nor code name exists', async () => {
 			mockParseAndValidate.mockResolvedValue({
+				warnings: [],
 				workflow: { ...mockWorkflowJson, name: undefined },
 			});
 
@@ -530,6 +555,7 @@ describe('create-workflow-from-code MCP tool', () => {
 
 			test('rejects workflow whose data table id does not exist', async () => {
 				mockParseAndValidate.mockResolvedValue({
+					warnings: [],
 					workflow: {
 						...mockWorkflowJson,
 						nodes: [dataTableNode(dataTableLocator('id', 'missing'))],
@@ -547,6 +573,7 @@ describe('create-workflow-from-code MCP tool', () => {
 
 			test('rejects workflow whose data table name does not exist', async () => {
 				mockParseAndValidate.mockResolvedValue({
+					warnings: [],
 					workflow: {
 						...mockWorkflowJson,
 						nodes: [dataTableNode(dataTableLocator('name', 'missing-table'))],
@@ -567,6 +594,7 @@ describe('create-workflow-from-code MCP tool', () => {
 					count: 1,
 				});
 				mockParseAndValidate.mockResolvedValue({
+					warnings: [],
 					workflow: {
 						...mockWorkflowJson,
 						nodes: [dataTableNode(dataTableLocator('id', 'dt-existing'))],
@@ -591,6 +619,7 @@ describe('create-workflow-from-code MCP tool', () => {
 					count: 1,
 				});
 				mockParseAndValidate.mockResolvedValue({
+					warnings: [],
 					workflow: {
 						...mockWorkflowJson,
 						nodes: [dataTableNode(dataTableLocator('id', 'dt-existing'))],
@@ -615,6 +644,7 @@ describe('create-workflow-from-code MCP tool', () => {
 
 			test('skips validation when dataTableId is an expression', async () => {
 				mockParseAndValidate.mockResolvedValue({
+					warnings: [],
 					workflow: {
 						...mockWorkflowJson,
 						nodes: [dataTableNode(dataTableLocator('id', '={{ $json.id }}'))],
@@ -661,6 +691,7 @@ describe('create-workflow-from-code MCP tool', () => {
 				});
 
 				mockParseAndValidate.mockResolvedValue({
+					warnings: [],
 					workflow: {
 						...mockWorkflowJson,
 						nodes: [httpNodeWithGithub('6CoUMkVOJRNsbmr2')],
@@ -686,6 +717,7 @@ describe('create-workflow-from-code MCP tool', () => {
 				);
 
 				mockParseAndValidate.mockResolvedValue({
+					warnings: [],
 					workflow: {
 						...mockWorkflowJson,
 						nodes: [httpNodeWithGithub('ghost')],
@@ -706,6 +738,7 @@ describe('create-workflow-from-code MCP tool', () => {
 				]);
 
 				mockParseAndValidate.mockResolvedValue({
+					warnings: [],
 					workflow: {
 						...mockWorkflowJson,
 						nodes: [httpNodeWithGithub('in-project-cred')],

packages/cli/src/modules/mcp/tools/workflow-builder/create-workflow-from-code.tool.ts

@@ -114,6 +114,22 @@ const outputSchema = {
 		.describe(
 			'Actionable hint for recovering from the error. When present, follow the suggested action before retrying.',
 		),
+	warnings: z
+		.array(
+			z.object({
+				code: z.string().describe('The warning code identifying the type of warning'),
+				message: z.string().describe('The warning message'),
+				nodeName: z.string().optional().describe('The node that triggered the warning'),
+				parameterPath: z
+					.string()
+					.optional()
+					.describe('The parameter path that triggered the warning'),
+			}),
+		)
+		.optional()
+		.describe(
+			'Validation warnings emitted while parsing the submitted code. Surface these to the user so they can correct the workflow.',
+		),
 	error: z
 		.string()
 		.optional()
@@ -195,7 +211,10 @@ export const createCreateWorkflowFromCodeTool = (
 				'@n8n/ai-workflow-builder'
 			);
 
-			const handler = new ParseValidateHandler({ generatePinData: false });
+			const handler = new ParseValidateHandler({
+				generatePinData: false,
+				nodeTypesProvider: nodeTypes,
+			});
 			const strippedCode = stripImportStatements(code);
 			const result = await handler.parseAndValidate(strippedCode);
 
@@ -297,7 +316,7 @@ export const createCreateWorkflowFromCodeTool = (
 					: undefined,
 			].filter((note): note is string => note !== undefined);
 
-			const output = {
+			const baseOutput = {
 				workflowId: savedWorkflow.id,
 				name: savedWorkflow.name,
 				nodeCount: savedWorkflow.nodes.length,
@@ -310,6 +329,8 @@ export const createCreateWorkflowFromCodeTool = (
 				},
 				note: notes.length ? notes.join(' ') : undefined,
 			};
+			const output =
+				result.warnings.length > 0 ? { ...baseOutput, warnings: result.warnings } : baseOutput;
 
 			return {
 				content: [{ type: 'text', text: JSON.stringify(output, null, 2) }],

packages/cli/src/modules/mcp/tools/workflow-builder/update-workflow.tool.ts

@@ -414,7 +414,10 @@ export const createUpdateWorkflowTool = (
 			}
 
 			const { ParseValidateHandler } = await import('@n8n/ai-workflow-builder');
-			const validator = new ParseValidateHandler({ generatePinData: false });
+			const validator = new ParseValidateHandler({
+				generatePinData: false,
+				nodeTypesProvider: nodeTypes,
+			});
 			const validationWarnings = validator.validateJSON({
 				name: workflowUpdateData.name,
 				nodes: workflowUpdateData.nodes,

packages/cli/src/modules/mcp/tools/workflow-builder/validate-workflow-code.tool.ts

@@ -80,7 +80,10 @@ export const createValidateWorkflowCodeTool = (
 			const { ParseValidateHandler, stripImportStatements } = await import(
 				'@n8n/ai-workflow-builder'
 			);
-			const handler = new ParseValidateHandler({ generatePinData: false });
+			const handler = new ParseValidateHandler({
+				generatePinData: false,
+				nodeTypesProvider: nodeTypes,
+			});
 			const strippedCode = stripImportStatements(code);
 			const result = await handler.parseAndValidate(strippedCode);