From 2b685c33b68feacad613665f74a87093a788dfcf Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mike=20Repe=C4=87?= <mike.repec@n8n.io>
Date: Tue, 6 Oct 2026 13:07:49 +0000
Subject: [PATCH] test(core): Run only expression tests once per engine in the
 workflow package (no-changelog) (#40242)

---
 packages/workflow/test/setup-vm-evaluator.ts | 31 ++++++++++-
 packages/workflow/vitest.config.ts           | 56 ++++++++++++++------
 2 files changed, 69 insertions(+), 18 deletions(-)

diff --git a/packages/workflow/test/setup-vm-evaluator.ts b/packages/workflow/test/setup-vm-evaluator.ts
index 0919b0e20b9..8f92f0624f9 100644
--- a/packages/workflow/test/setup-vm-evaluator.ts
+++ b/packages/workflow/test/setup-vm-evaluator.ts
@@ -1,9 +1,36 @@
 import { Expression } from '../src/expression';
 
-// Only runs when N8N_EXPRESSION_ENGINE is set to 'vm' or 'quickjs'.
+// The engine projects (vitest.config.ts) run the tests that evaluate
+// expressions once per engine and set N8N_EXPRESSION_ENGINE; the default
+// project runs everything else once and sets it to ''.
+const engine = (process.env.N8N_EXPRESSION_ENGINE || undefined) as
+	| 'vm'
+	| 'legacy'
+	| 'quickjs'
+	| undefined;
+
+// A test that evaluates an expression outside the engine projects would pass
+// against one engine only, so it fails here until it is added to ENGINE_TESTS.
+// With no engine initialised every entry point (resolveSimpleParameterValue,
+// resolveWithoutWorkflow) ends in the legacy evaluator, so that is the guard.
+// vi.mock is hoisted above every other statement, so the factory reads the
+// environment itself instead of the `engine` constant below.
+vi.mock('../src/expression-evaluator-proxy', async (importOriginal) => {
+	const original = await importOriginal<typeof import('../src/expression-evaluator-proxy')>();
+	if (process.env.N8N_EXPRESSION_ENGINE) return original;
+
+	return {
+		...original,
+		evaluateExpression: () => {
+			throw new Error(
+				`${expect.getState().testPath} evaluates an expression. Add it to ENGINE_TESTS in vitest.config.ts so it runs once per engine.`,
+			);
+		},
+	};
+});
+
 // Initializes the expression evaluator once per vitest worker before all tests,
 // and disposes it after.
-const engine = process.env.N8N_EXPRESSION_ENGINE as 'vm' | 'quickjs' | undefined;
 if (engine === 'vm' || engine === 'quickjs') {
 	beforeAll(async () => {
 		await Expression.initExpressionEngine({
diff --git a/packages/workflow/vitest.config.ts b/packages/workflow/vitest.config.ts
index a011d7ab377..abe06fda7cd 100644
--- a/packages/workflow/vitest.config.ts
+++ b/packages/workflow/vitest.config.ts
@@ -1,11 +1,43 @@
 import { defineConfig } from 'vitest/config';
 import { createBaseInlineConfig } from '@n8n/vitest-config/node';
 
+// Tests that evaluate expressions, so their outcome depends on the engine.
+// Only these run once per engine project; everything else runs once in the
+// default project. A test outside this list that evaluates an expression fails
+// there (see test/setup-vm-evaluator.ts), so the list cannot drift silently.
+const ENGINE_TESTS = [
+	'test/expression.test.ts',
+	'test/expression-array-proxy-semantics.test.ts',
+	'test/expression-compatibility.test.ts',
+	'test/expression-engine-start-failure.test.ts',
+	'test/expression-item-accessor-engine-parity.test.ts',
+	'test/expression-items-engine-parity.test.ts',
+	'test/expression-nested-data.test.ts',
+	'test/expression-vm-errors.test.ts',
+	'test/expression-with-isolate.test.ts',
+	'test/ExpressionExtensions/*.test.ts',
+	'test/native-evaluation-fuzz.test.ts',
+	'test/native-evaluation-parity.test.ts',
+	'test/webhook-description-fields.test.ts',
+	'test/workflow.test.ts',
+	'test/workflow-data-proxy.test.ts',
+	'test/workflow-expression.test.ts',
+];
+
 const { reporters, outputFile, ...sharedTestConfig } = createBaseInlineConfig({
 	include: ['test/**/*.test.ts'],
 	setupFiles: ['./test/setup-vm-evaluator.ts'],
 });
 
+const engineProject = (engine: 'vm' | 'legacy' | 'quickjs') => ({
+	test: {
+		...sharedTestConfig,
+		include: ENGINE_TESTS,
+		name: `${engine}-engine`,
+		env: { N8N_EXPRESSION_ENGINE: engine },
+	},
+});
+
 export default defineConfig({
 	test: {
 		reporters,
@@ -14,24 +46,16 @@ export default defineConfig({
 			{
 				test: {
 					...sharedTestConfig,
-					name: 'vm-engine',
-					env: { N8N_EXPRESSION_ENGINE: 'vm' },
-				},
-			},
-			{
-				test: {
-					...sharedTestConfig,
-					name: 'legacy-engine',
-					env: { N8N_EXPRESSION_ENGINE: 'legacy' },
-				},
-			},
-			{
-				test: {
-					...sharedTestConfig,
-					name: 'quickjs-engine',
-					env: { N8N_EXPRESSION_ENGINE: 'quickjs' },
+					exclude: [...(sharedTestConfig.exclude ?? []), ...ENGINE_TESTS],
+					name: 'default',
+					// Explicitly no engine, so a shell N8N_EXPRESSION_ENGINE cannot turn
+					// this project into a fourth engine run and disable the guard.
+					env: { N8N_EXPRESSION_ENGINE: '' },
 				},
 			},
+			engineProject('vm'),
+			engineProject('legacy'),
+			engineProject('quickjs'),
 		],
 	},
 });