SDK_FORBIDDEN_CONSTRUCT

SDK_FORBIDDEN_CONSTRUCT

Error message

Arrow functions are not allowed. Use fromAi() directly instead of ($) => $.fromAi()

What it means

SDK_FORBIDDEN_CONSTRUCT for ArrowFunctionExpression is a lint issue. The FORBIDDEN_NODE_TYPES map assigns arrow functions the message 'Use fromAi() directly instead of ($) => $.fromAi()'. The SDK builder interpreter rejects arrow functions entirely; fromAi() must be called directly as a parameter value.

Source

Thrown at packages/@n8n/workflow-sdk/src/lint/sdk/workflow-sdk-lint.ts:217

					...locationOf(stmt),
					lintTarget: 'sdk',
				}),
			);
		}
	}

	const branchCounts = new Map<string, { count: number; line?: number; column?: number }>();

	walkAst(
		ast,
		(node, parent) => {
			if (node.type === 'ImportDeclaration') return;

			const forbidden = FORBIDDEN_NODE_TYPES[node.type];
			if (forbidden) {
				issues.push(
					lintIssue({
						code: 'SDK_FORBIDDEN_CONSTRUCT',
						message: forbidden,
						...locationOf(node),
						lintTarget: 'sdk',
					}),
				);
			}

			if (node.type === 'Identifier' && DANGEROUS_GLOBALS.has(node.name)) {
				const isPropertyName =
					parent?.type === 'MemberExpression' && parent.property === node && !parent.computed;
				const isObjectKey = parent?.type === 'Property' && parent.key === node;
				// Mirrors the interpreter: safe global methods (e.g. JSON.stringify) are allowed.
				const isSafeMethodObject =
					parent?.type === 'MemberExpression' &&
					parent.object === node &&
					!parent.computed &&
					parent.property.type === 'Identifier' &&
					getSafeJSONMethod(node.name, parent.property.name) !== undefined;

View on GitHub (pinned to 5ac6606e81)

Solutions

Use fromAi() directly as the parameter value, e.g. fromAi: fromAi('x').
Replace array callbacks with declarative builder calls or move the logic to a Code node.
Inline any lambda logic so the SDK source has no arrow expressions.

Example fix

// before
export default workflow().addNode(httpRequest({
  parameters: { prompt: ($) => $.fromAi('prompt') },
}));
// after
export default workflow().addNode(httpRequest({
  parameters: { prompt: fromAi('prompt') },
}));

Defensive patterns

Strategy: validation

Validate before calling

import { lintWorkflowSource } from '@n8n/workflow-sdk/lint/lint-workflow-source';
const issues = lintWorkflowSource(sdkSource);
if (issues.some((i) => i.code === 'SDK_FORBIDDEN_CONSTRUCT' && i.message.includes('Arrow functions'))) {
  throw new Error('Arrow functions are forbidden in SDK code; call fromAi() directly.');
}

Prevention

Use fromAi() directly as the parameter value, never ($) => $.fromAi().
Avoid array callbacks (.map(x => ...)) in SDK source — move transforms to a Code node.
Lint generated SDK code to catch arrow functions.

When it happens

Trigger: fromAi: ($) => $.fromAi('x'), callbacks like .map((x) => x), or any () => ... / (x) => ... expression in SDK builder code.

Common situations: Agent wraps fromAi in an arrow callback; developer uses functional array methods; LLM emits a lambda for a helper.

Related errors

AI-assisted analysis of n8n-io/n8n@5ac6606e81 (2026-08-12). Data as JSON: /api/errors/b73f6e5259defe87. Report an issue: GitHub.