fix(core): Allow .trim() in workflow-sdk parsing and coerce stringified patches (#29111)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/@n8n/instance-ai/src/tools/workflows/__tests__/build-workflow-schema.test.ts

@@ -0,0 +1,33 @@
+import { buildWorkflowInputSchema } from '../build-workflow.tool';
+
+describe('buildWorkflowInputSchema.patches coercion', () => {
+	const patch = { old_str: 'foo', new_str: 'bar' };
+
+	it('accepts a native array of patches', () => {
+		const parsed = buildWorkflowInputSchema.parse({ patches: [patch] });
+		expect(parsed.patches).toEqual([patch]);
+	});
+
+	it('accepts a JSON-stringified array of patches', () => {
+		const parsed = buildWorkflowInputSchema.parse({ patches: JSON.stringify([patch]) });
+		expect(parsed.patches).toEqual([patch]);
+	});
+
+	it('rejects a non-JSON string with a helpful array-expected error', () => {
+		const result = buildWorkflowInputSchema.safeParse({ patches: 'not-json' });
+		expect(result.success).toBe(false);
+		if (!result.success) {
+			expect(result.error.issues[0].path).toEqual(['patches']);
+		}
+	});
+
+	it('rejects a stringified object (not an array)', () => {
+		const result = buildWorkflowInputSchema.safeParse({ patches: JSON.stringify(patch) });
+		expect(result.success).toBe(false);
+	});
+
+	it('leaves patches undefined when not provided', () => {
+		const parsed = buildWorkflowInputSchema.parse({});
+		expect(parsed.patches).toBeUndefined();
+	});
+});

packages/@n8n/instance-ai/src/tools/workflows/build-workflow.tool.ts

@@ -15,13 +15,25 @@ const patchSchema = z.object({
 	new_str: z.string().describe('Replacement string'),
 });
 
+// Coerce JSON-stringified arrays into arrays. The model sometimes sends `patches`
+// as a JSON string because the payload contains escaped code. Leave non-strings
+// untouched so Zod can validate them normally.
+function coercePatches(value: unknown): unknown {
+	if (typeof value !== 'string') return value;
+	try {
+		return JSON.parse(value);
+	} catch {
+		return value;
+	}
+}
+
 export const buildWorkflowInputSchema = z.object({
 	code: z
 		.string()
 		.optional()
 		.describe('Full TypeScript workflow code using @n8n/workflow-sdk. Required for new workflows.'),
 	patches: z
-		.array(patchSchema)
+		.preprocess(coercePatches, z.array(patchSchema))
 		.optional()
 		.describe(
 			'Array of {old_str, new_str} replacements to apply to existing workflow code. ' +

packages/@n8n/workflow-sdk/src/ast-interpreter/interpreter.test.ts

@@ -932,6 +932,32 @@ describe('AST Interpreter', () => {
 		});
 	});
 
+	describe('String.trim', () => {
+		let sdkFunctions: SDKFunctions;
+
+		beforeEach(() => {
+			sdkFunctions = createMockSDKFunctions();
+		});
+
+		it('should allow "  abc  ".trim()', () => {
+			const code = 'export default "  abc  ".trim();';
+			const result = interpretSDKCode(code, sdkFunctions);
+			expect(result).toBe('abc');
+		});
+
+		it('should allow trim on a template literal', () => {
+			const code = 'export default `\n  hello\n`.trim();';
+			const result = interpretSDKCode(code, sdkFunctions);
+			expect(result).toBe('hello');
+		});
+
+		it('should allow trim on a variable holding a string', () => {
+			const code = 'const padded = "  x  "; export default padded.trim();';
+			const result = interpretSDKCode(code, sdkFunctions);
+			expect(result).toBe('x');
+		});
+	});
+
 	describe('expr(placeholder(...)) error', () => {
 		it('should throw clear error when expr receives a PlaceholderValue', () => {
 			const funcs: SDKFunctions = {

packages/@n8n/workflow-sdk/src/ast-interpreter/validators.ts

@@ -377,6 +377,7 @@ export function getSafeJSONMethod(
  */
 const SAFE_STRING_METHODS: Record<string, (str: string, ...args: unknown[]) => unknown> = {
 	repeat: (str: string, count: unknown) => str.repeat(count as number),
+	trim: (str: string) => str.trim(),
 };
 
 /**