diff --git a/modele-social/README.md b/modele-social/README.md
index 2a6cadbce..02e875d4b 100644
--- a/modele-social/README.md
+++ b/modele-social/README.md
@@ -2,27 +2,28 @@
 
 Ce paquet contient les règles publicodes utilisées sur https://mon-entreprise.fr
 pour le calcul des cotisations sociales, des impôts et des droits sociaux.
+
 ### Installation
+
 ```
-npm install publicodes modele-social 
+npm install publicodes modele-social
 ```
 
 ### Exemple d'utilisation
-```js
-import Engine, { formatValue } from "publicodes";
-import rules from "modele-social";
 
-const engine = new Engine(rules);
+```js
+import Engine, { formatValue } from 'publicodes'
+import rules from 'modele-social'
+
+const engine = new Engine(rules)
 
 const net = engine
-  .setSituation({
-    "contrat salarié . rémunération . brut de base": "3000 €/mois",
-  })
-  .evaluate("contrat salarié . rémunération . net");
-
-console.log(formatValue(net));
+    .setSituation({
+        'contrat salarié . rémunération . brut de base': '3000 €/mois',
+    })
+    .evaluate('contrat salarié . rémunération . net')
 
+console.log(formatValue(net))
 ```
 
-
 👉 **[Voir le tutoriel complet](https://mon-entreprise.fr/int%C3%A9gration/biblioth%C3%A8que-de-calcul)**
diff --git a/mon-entreprise/source/App.tsx b/mon-entreprise/source/App.tsx
index 79080655d..b66e9a37c 100644
--- a/mon-entreprise/source/App.tsx
+++ b/mon-entreprise/source/App.tsx
@@ -4,14 +4,13 @@ import Header from 'Components/layout/Header'
 import Route404 from 'Components/Route404'
 import 'Components/ui/index.css'
 import {
-	engineOptions,
+	engineFactory,
 	EngineProvider,
+	Rules,
 	SituationProvider,
 } from 'Components/utils/EngineContext'
 import { SitePathsContext } from 'Components/utils/SitePathsContext'
 import 'iframe-resizer'
-import { DottedName } from 'modele-social'
-import Engine, { Rule } from 'publicodes'
 import { useContext, useMemo } from 'react'
 import { Helmet } from 'react-helmet'
 import { useTranslation } from 'react-i18next'
@@ -70,16 +69,13 @@ const middlewares = [createSentryMiddleware(Sentry as any)]
 
 type RootProps = {
 	basename: ProviderProps['basename']
-	rules: Record<DottedName, Rule>
+	rules: Rules
 }
 
 export default function Root({ basename, rules }: RootProps) {
 	const { language } = useTranslation().i18n
 	const paths = constructLocalizedSitePath(language as 'fr' | 'en')
-	const engine = useMemo(() => new Engine(rules, engineOptions), [
-		rules,
-		engineOptions,
-	])
+	const engine = useMemo(() => engineFactory(rules), [rules])
 	return (
 		<Provider
 			basename={basename}
diff --git a/mon-entreprise/source/components/SchemeComparaison.tsx b/mon-entreprise/source/components/SchemeComparaison.tsx
index 4005f54eb..3744bbeaa 100644
--- a/mon-entreprise/source/components/SchemeComparaison.tsx
+++ b/mon-entreprise/source/components/SchemeComparaison.tsx
@@ -9,8 +9,6 @@ import SeeAnswersButton from 'Components/conversation/SeeAnswersButton'
 import Value from 'Components/EngineValue'
 import InfoBulle from 'Components/ui/InfoBulle'
 import revenusSVG from 'Images/revenus.svg'
-import { DottedName } from 'modele-social'
-import Engine from 'publicodes'
 import { useCallback, useMemo, useState } from 'react'
 import emoji from 'react-easy-emoji'
 import { Trans } from 'react-i18next'
@@ -18,7 +16,7 @@ import { useSelector } from 'react-redux'
 import { situationSelector } from 'Selectors/simulationSelectors'
 import dirigeantComparaison from '../pages/Simulateurs/configs/rémunération-dirigeant.yaml'
 import './SchemeComparaison.css'
-import { engineOptions, useEngine } from './utils/EngineContext'
+import { useEngine } from './utils/EngineContext'
 import useSimulationConfig from './utils/useSimulationConfig'
 
 type SchemeComparaisonProps = {
@@ -45,13 +43,12 @@ export default function SchemeComparaison({
 		setConversationStarted,
 	])
 
-	const parsedRules = engine.getParsedRules()
 	const situation = useSelector(situationSelector)
 	const displayResult =
 		useSelector(situationSelector)['entreprise . charges'] != undefined
 	const assimiléEngine = useMemo(
 		() =>
-			new Engine<DottedName>(parsedRules, engineOptions).setSituation({
+			engine.shallowCopy().setSituation({
 				...situation,
 				dirigeant: "'assimilé salarié'",
 			}),
@@ -59,7 +56,7 @@ export default function SchemeComparaison({
 	)
 	const autoEntrepreneurEngine = useMemo(
 		() =>
-			new Engine<DottedName>(parsedRules, engineOptions).setSituation({
+			engine.shallowCopy().setSituation({
 				...situation,
 				dirigeant: "'auto-entrepreneur'",
 			}),
@@ -67,7 +64,7 @@ export default function SchemeComparaison({
 	)
 	const indépendantEngine = useMemo(
 		() =>
-			new Engine<DottedName>(parsedRules, engineOptions).setSituation({
+			engine.shallowCopy().setSituation({
 				...situation,
 				dirigeant: "'indépendant'",
 			}),
diff --git a/mon-entreprise/source/components/utils/EngineContext.tsx b/mon-entreprise/source/components/utils/EngineContext.tsx
index 16318ce92..7891de9cb 100644
--- a/mon-entreprise/source/components/utils/EngineContext.tsx
+++ b/mon-entreprise/source/components/utils/EngineContext.tsx
@@ -1,14 +1,12 @@
-import Engine from 'publicodes'
+import Engine, { Rule } from 'publicodes'
 import React, { createContext, useContext } from 'react'
 import { DottedName } from 'modele-social'
 import i18n from '../../locales/i18n'
 
-export const EngineContext = createContext<Engine>(new Engine({}))
-export const EngineProvider = EngineContext.Provider
+export type Rules = Record<DottedName, Rule>
 
 const unitsTranslations = Object.entries(i18n.getResourceBundle('fr', 'units'))
-
-export const engineOptions = {
+const engineOptions = {
 	getUnitKey(unit: string): string {
 		const key = unitsTranslations
 			.find(([, trans]) => trans === unit)?.[0]
@@ -19,6 +17,12 @@ export const engineOptions = {
 		return i18n?.t(`units:${unit}`, { count })
 	},
 }
+export function engineFactory(rules: Rules) {
+	return new Engine(rules, engineOptions)
+}
+
+export const EngineContext = createContext<Engine>(new Engine())
+export const EngineProvider = EngineContext.Provider
 
 export function useEngine(): Engine<DottedName> {
 	return useContext(EngineContext) as Engine<DottedName>
diff --git a/mon-entreprise/source/pages/Simulateurs/AidesEmbauche.tsx b/mon-entreprise/source/pages/Simulateurs/AidesEmbauche.tsx
index 193b0fa74..e03cc1c1e 100644
--- a/mon-entreprise/source/pages/Simulateurs/AidesEmbauche.tsx
+++ b/mon-entreprise/source/pages/Simulateurs/AidesEmbauche.tsx
@@ -10,7 +10,7 @@ import { useSimulationProgress } from 'Components/utils/useNextQuestion'
 import { useParamsFromSituation } from 'Components/utils/useSearchParamsSimulationSharing'
 import useSimulationConfig from 'Components/utils/useSimulationConfig'
 import { DottedName } from 'modele-social'
-import Engine, { formatValue } from 'publicodes'
+import { formatValue } from 'publicodes'
 import { partition } from 'ramda'
 import { useContext } from 'react'
 import { Trans, useTranslation } from 'react-i18next'
@@ -229,7 +229,7 @@ function Results() {
 	const progress = useSimulationProgress()
 	const baseEngine = useEngine()
 	const aidesEngines = aides.map((aide) => {
-		const engine = new Engine(baseEngine.parsedRules)
+		const engine = baseEngine.shallowCopy()
 		engine.setSituation({ ...aide.situation, ...baseEngine.parsedSituation })
 		const isActive =
 			typeof engine.evaluate(aide.dottedName).nodeValue === 'number'
diff --git a/mon-entreprise/test/real-rules.test.js b/mon-entreprise/test/real-rules.test.js
index dad097818..09ca149e1 100644
--- a/mon-entreprise/test/real-rules.test.js
+++ b/mon-entreprise/test/real-rules.test.js
@@ -6,7 +6,7 @@ import rules from 'modele-social'
 // les variables dans les tests peuvent être exprimées relativement à l'espace de nom de la règle,
 // comme dans sa formule
 let parsedRules = parsePublicodes(rules)
-const engine = new Engine(parsedRules)
+const engine = new Engine(rules)
 let runExamples = (examples, rule) =>
 	examples.map((ex) => {
 		const expected = ex['valeur attendue']
diff --git a/mon-entreprise/test/regressions/simulations.jest.js b/mon-entreprise/test/regressions/simulations.jest.js
index 4b1777478..4c020a6d9 100644
--- a/mon-entreprise/test/regressions/simulations.jest.js
+++ b/mon-entreprise/test/regressions/simulations.jest.js
@@ -6,9 +6,8 @@
 // renamed the test configuration may be adapted but the persisted snapshot will remain unchanged).
 
 /* eslint-disable no-undef */
-import Engine from 'publicodes'
 import rules from '../../../modele-social'
-import { engineOptions } from '../../source/components/utils/EngineContext'
+import { engineFactory } from '../../source/components/utils/EngineContext'
 import aideDéclarationConfig from '../../source/pages/Gérer/AideDéclarationIndépendant/config.yaml'
 import artisteAuteurConfig from '../../source/pages/Simulateurs/configs/artiste-auteur.yaml'
 import autoentrepreneurConfig from '../../source/pages/Simulateurs/configs/auto-entrepreneur.yaml'
@@ -26,7 +25,7 @@ import remunerationDirigeantSituations from './simulations-rémunération-dirige
 import employeeSituations from './simulations-salarié.yaml'
 
 const roundResult = (arr) => arr.map((x) => Math.round(x))
-const engine = new Engine(rules, engineOptions)
+const engine = engineFactory(rules)
 const runSimulations = (situations, targets, baseSituation = {}) =>
 	Object.entries(situations).map(([name, situations]) =>
 		situations.forEach((situation) => {
diff --git a/mon-entreprise/test/useSearchParamsSimulationSharing.test.js b/mon-entreprise/test/useSearchParamsSimulationSharing.test.js
index 02ee5e9bf..d24d0a3e9 100644
--- a/mon-entreprise/test/useSearchParamsSimulationSharing.test.js
+++ b/mon-entreprise/test/useSearchParamsSimulationSharing.test.js
@@ -26,14 +26,13 @@ describe('identifiant court', () => {
 })
 
 describe('useSearchParamsSimulationSharing', () => {
-	const someRules = parsePublicodes(`
+	const engine = new Engine(`
 rule with:
 	identifiant court: panta
 	formule: 0
 rule without:
 	formule: 0
 	`)
-	const engine = new Engine(someRules)
 	const dottedNameParamName = getRulesParamNames(engine.getParsedRules())
 
 	describe('getSearchParamsFromSituation', () => {
@@ -92,7 +91,7 @@ rule without:
 })
 
 describe('useSearchParamsSimulationSharing hook', () => {
-	const someRules = parsePublicodes(`
+	const parsedRules = parsePublicodes(`
 rule with:
 	identifiant court: panta
 	formule: 0
@@ -100,9 +99,7 @@ rule without:
 	formule: 0
 	`)
 
-	const dottedNameParamName = getRulesParamNames(
-		new Engine(someRules).getParsedRules()
-	)
+	const dottedNameParamName = getRulesParamNames(parsedRules)
 	let setSearchParams
 
 	beforeEach(() => {
diff --git a/publicodes/core/source/index.ts b/publicodes/core/source/index.ts
index 13d540abc..e61b1b199 100644
--- a/publicodes/core/source/index.ts
+++ b/publicodes/core/source/index.ts
@@ -67,6 +67,7 @@ export type ParsedRules<Name extends string> = Record<
 	Name,
 	RuleNode & { dottedName: Name }
 >
+
 export default class Engine<Name extends string = string> {
 	parsedRules: ParsedRules<Name>
 	parsedSituation: Record<string, ASTNode> = {}
@@ -75,25 +76,11 @@ export default class Engine<Name extends string = string> {
 	options: Options
 
 	constructor(
-		rules: string | Record<string, Rule> | ParsedRules<Name> = {},
+		rules: string | Record<string, Rule> = {},
 		options: Partial<Options> = {}
 	) {
 		this.options = { ...options, logger: options.logger ?? console }
-		if (typeof rules === 'string') {
-			rules = parsePublicodes(rules, this.options) as ParsedRules<Name>
-		}
-		const firstRuleObject = Object.values(rules)[0] as Rule | RuleNode
-		if (
-			typeof firstRuleObject !== 'object' ||
-			firstRuleObject == null ||
-			!('nodeKind' in firstRuleObject)
-		) {
-			rules = parsePublicodes(
-				rules as Record<string, Rule>,
-				this.options
-			) as ParsedRules<Name>
-		}
-		this.parsedRules = rules as ParsedRules<Name>
+		this.parsedRules = parsePublicodes(rules, this.options) as ParsedRules<Name>
 		this.replacements = getReplacements(this.parsedRules)
 	}
 
@@ -150,6 +137,10 @@ export default class Engine<Name extends string = string> {
 		return this.parsedRules
 	}
 
+	getOptions(): Options {
+		return this.options
+	}
+
 	evaluate<N extends ASTNode = ASTNode>(value: N): N & EvaluatedNode
 	evaluate(value: PublicodesExpression): EvaluatedNode
 	evaluate(value: PublicodesExpression | ASTNode): EvaluatedNode {
@@ -180,6 +171,17 @@ export default class Engine<Name extends string = string> {
 		this.cache.nodes.set(value, evaluatedNode)
 		return evaluatedNode
 	}
+
+	/**
+	 * Shallow Engine instance copy. Keeps references to the original Engine instance attributes.
+	 */
+	shallowCopy(): Engine<Name> {
+		const newEngine = new Engine<Name>()
+		newEngine.options = this.options
+		newEngine.parsedRules = this.parsedRules
+		newEngine.replacements = this.replacements
+		return newEngine
+	}
 }
 
 /**
diff --git a/publicodes/core/test/inversion.test.js b/publicodes/core/test/inversion.test.js
index f0c2089d9..2de306c83 100644
--- a/publicodes/core/test/inversion.test.js
+++ b/publicodes/core/test/inversion.test.js
@@ -51,7 +51,7 @@ describe('inversions', () => {
               assiette: brut
               taux: 77%
 
-        brut:          
+        brut:
           formule:
             inversion numérique:
               unité: €
@@ -147,7 +147,7 @@ describe('inversions', () => {
           formule:
             produit:
               assiette: assiette
-              taux: 
+              taux:
                 variations:
                   - si: cadre
                     alors: 80%
diff --git a/publicodes/ui-react/source/mecanisms/common.tsx b/publicodes/ui-react/source/mecanisms/common.tsx
index d96967084..6e4dd8c06 100644
--- a/publicodes/ui-react/source/mecanisms/common.tsx
+++ b/publicodes/ui-react/source/mecanisms/common.tsx
@@ -64,7 +64,7 @@ export const NodeValuePointer = ({ data, unit }: NodeValuePointerProps) => {
 			}}
 		>
 			{formatValue(simplifyNodeUnit({ nodeValue: data, unit }), {
-				formatUnit: engine?.options?.formatUnit,
+				formatUnit: engine?.getOptions()?.formatUnit,
 			})}
 		</small>
 	)