diff --git a/Frontend/src/App.jsx b/Frontend/src/App.jsx
index b9ed1bd..04f30af 100755
--- a/Frontend/src/App.jsx
+++ b/Frontend/src/App.jsx
@@ -5,6 +5,7 @@ import Navbar from "./components/navbar";
 import Footer from "./components/footer";
 import { GameTick } from "./components/GameTick";
 import { GameSync } from "./components/GameSync";
+import { OfflineReport } from "./components/OfflineReport";
 
 import navData from "./data/NavBarData.json";
 
@@ -15,6 +16,7 @@ function App() {
     <>
       <GameTick />
       <GameSync />
+      <OfflineReport />
       <Navbar
         navData={navData}
         toggleRain={toggleRain}
diff --git a/Frontend/src/__tests__/economy.test.ts b/Frontend/src/__tests__/economy.test.ts
index b394f7d..17af8d0 100644
--- a/Frontend/src/__tests__/economy.test.ts
+++ b/Frontend/src/__tests__/economy.test.ts
@@ -13,6 +13,8 @@ import {
   getClickMultiplierFromTree,
   getProductionMultiplierFromTree,
   getStartBonusFromTree,
+  offlineEfficiency,
+  computeOfflineGains,
   DEFAULT_STATE,
   DEFAULT_GENERATORS,
   DEFAULT_EVOLUTION_TREE,
@@ -364,3 +366,90 @@ describe("Evolution Tree", () => {
     });
   });
 });
+
+// --- Offline gains (courbe inversée) ---
+
+describe("offlineEfficiency", () => {
+  it("retourne 1.0 pour absence < 60s (pas offline)", () => {
+    expect(offlineEfficiency(30_000)).toBe(1);
+  });
+
+  it("retourne 1.0 pour absence de 10min (phase 100%)", () => {
+    expect(offlineEfficiency(10 * 60_000)).toBe(1);
+  });
+
+  it("retourne 1.0 à exactement 15min", () => {
+    expect(offlineEfficiency(15 * 60_000)).toBe(1);
+  });
+
+  it("retourne ~0.625 à 30min (milieu decay 1.0→0.25)", () => {
+    const eff = offlineEfficiency(30 * 60_000);
+    // 30min = 15min dans la phase decay (15min-1h = 45min total)
+    // t = 15/45 = 0.333 → eff = 1 - 0.333 * 0.75 = 0.75
+    expect(eff).toBeCloseTo(0.75, 1);
+  });
+
+  it("retourne 0.25 à exactement 1h (fin du decay)", () => {
+    expect(offlineEfficiency(60 * 60_000)).toBeCloseTo(0.25);
+  });
+
+  it("retourne ~0.125 à 1h30 (milieu 0.25→0)", () => {
+    const eff = offlineEfficiency(90 * 60_000);
+    expect(eff).toBeCloseTo(0.125, 1);
+  });
+
+  it("retourne 0 à exactement 2h", () => {
+    expect(offlineEfficiency(2 * 60 * 60_000)).toBe(0);
+  });
+
+  it("retourne 0 après 2h (cap)", () => {
+    expect(offlineEfficiency(5 * 60 * 60_000)).toBe(0);
+  });
+
+  it("courbe monotone décroissante", () => {
+    const points = [0, 10, 15, 30, 45, 60, 90, 120, 180].map(
+      (min) => offlineEfficiency(min * 60_000)
+    );
+    for (let i = 1; i < points.length; i++) {
+      expect(points[i]).toBeLessThanOrEqual(points[i - 1]);
+    }
+  });
+});
+
+describe("computeOfflineGains", () => {
+  const stateWithProd = {
+    ...DEFAULT_STATE,
+    generators: DEFAULT_STATE.generators.map((g, i) =>
+      i === 0 ? { ...g, owned: 10 } : g
+    ),
+    lastTick: 0,
+    lastOnline: 0,
+  };
+  const pps = DEFAULT_GENERATORS[0].baseProduction * 10; // 1/s
+
+  it("gains normaux si absence < 60s", () => {
+    const gains = computeOfflineGains(stateWithProd, 30_000);
+    // < threshold → computeIdleGains classique
+    expect(gains).toBeCloseTo(pps * 30);
+  });
+
+  it("gains < idle pur pour absence de 1h", () => {
+    const gains = computeOfflineGains(stateWithProd, 60 * 60_000);
+    const fullIdleGains = pps * 3600;
+    expect(gains).toBeLessThan(fullIdleGains);
+    expect(gains).toBeGreaterThan(0);
+  });
+
+  it("gains = 0 pour absence > 2h si prod constante", () => {
+    // > 2h : tout tombe à 0%, mais les premières 2h produisent encore
+    const gains = computeOfflineGains(stateWithProd, 3 * 60 * 60_000);
+    const gainsAt2h = computeOfflineGains(stateWithProd, 2 * 60 * 60_000);
+    // gains at 3h should equal gains at 2h (nothing added after 2h)
+    expect(gains).toBeCloseTo(gainsAt2h, 0);
+  });
+
+  it("retourne 0 si aucune production", () => {
+    const gains = computeOfflineGains({ ...DEFAULT_STATE, lastTick: 0 }, 60 * 60_000);
+    expect(gains).toBe(0);
+  });
+});
diff --git a/Frontend/src/components/OfflineReport.tsx b/Frontend/src/components/OfflineReport.tsx
new file mode 100644
index 0000000..b790ce2
--- /dev/null
+++ b/Frontend/src/components/OfflineReport.tsx
@@ -0,0 +1,65 @@
+// OfflineReport.tsx — Écran "Pendant ton absence..." affiché au retour offline
+
+import { useGameStore } from "../store/useGameStore";
+import { formatNumber } from "../utils/formatNumber";
+
+function formatDuration(ms: number): string {
+  const minutes = Math.floor(ms / 60_000);
+  if (minutes < 60) return `${minutes}min`;
+  const hours = Math.floor(minutes / 60);
+  const remainMinutes = minutes % 60;
+  return remainMinutes > 0 ? `${hours}h${remainMinutes}min` : `${hours}h`;
+}
+
+export function OfflineReport() {
+  const report = useGameStore((s) => s.offlineReport);
+  const dismiss = useGameStore((s) => s.dismissOfflineReport);
+
+  if (!report || !report.wasOffline) return null;
+
+  const effPercent = Math.round(report.efficiency * 100);
+
+  return (
+    <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/80 backdrop-blur-sm">
+      <div className="gp max-w-sm w-full mx-4 text-center">
+        <span className="gp-title text-lg!">Pendant ton absence...</span>
+
+        <div className="flex flex-col gap-3 mt-2">
+          <div className="flex justify-between">
+            <span className="gp-label">Durée</span>
+            <span className="gp-value">{formatDuration(report.duration)}</span>
+          </div>
+
+          <div className="flex justify-between">
+            <span className="gp-label">Efficacité marais</span>
+            <span className={`gp-value ${effPercent > 50 ? "gp-accent-green" : "gp-accent-amber"}`}>
+              {effPercent}%
+            </span>
+          </div>
+
+          <div className="gp-sep" />
+
+          <div className="flex justify-between items-center">
+            <span className="gp-label">Têtards récoltés</span>
+            <span className="gp-value gp-accent-green text-lg!">
+              +{formatNumber(report.gains)}
+            </span>
+          </div>
+
+          {report.efficiency < 0.5 && (
+            <p className="gp-label text-center">
+              Le marais s'endort sans toi... Joue activement pour maximiser ta production !
+            </p>
+          )}
+        </div>
+
+        <button
+          onClick={dismiss}
+          className="gp-btn gp-btn--buy w-full mt-3 py-2!"
+        >
+          Retour au marais
+        </button>
+      </div>
+    </div>
+  );
+}
diff --git a/Frontend/src/core/economy.ts b/Frontend/src/core/economy.ts
index bb792fc..3dd3c1e 100644
--- a/Frontend/src/core/economy.ts
+++ b/Frontend/src/core/economy.ts
@@ -26,6 +26,7 @@ export interface GameState {
   clickMultiplier: number;
   generators: Generator[];
   lastTick: number; // timestamp ms — lazy calc reference
+  lastOnline: number; // timestamp ms — dernière activité réelle (tick actif)
   prestigeCount: number;
   prestigeMultiplier: number; // legacy — conservé pour compat, remplacé par arbre
   ancestralDna: number;
@@ -95,6 +96,52 @@ export function getStartBonusFromTree(tree: EvolutionNode[]): number {
     .reduce((sum, n) => sum + n.value, 0);
 }
 
+// --- Offline gains (courbe inversée) ---
+
+const OFFLINE_THRESHOLD = 60_000; // 60s — en-dessous = idle normal, au-dessus = offline
+const OFFLINE_FULL_MS = 15 * 60_000; // 0-15min : 100%
+const OFFLINE_DECAY_END_MS = 60 * 60_000; // 15min-1h : 100% → 25%
+const OFFLINE_ZERO_MS = 2 * 60 * 60_000; // 1h-2h : 25% → 0%
+const OFFLINE_FLOOR = 0.25; // plancher de la phase de decay
+
+// Retourne le multiplicateur d'efficacité offline (1.0 → 0.0)
+// basé sur le temps d'absence en ms
+export function offlineEfficiency(elapsedMs: number): number {
+  if (elapsedMs <= OFFLINE_THRESHOLD) return 1; // pas offline
+  if (elapsedMs <= OFFLINE_FULL_MS) return 1; // 0-15min : 100%
+  if (elapsedMs <= OFFLINE_DECAY_END_MS) {
+    // 15min-1h : linéaire 1.0 → 0.25
+    const t = (elapsedMs - OFFLINE_FULL_MS) / (OFFLINE_DECAY_END_MS - OFFLINE_FULL_MS);
+    return 1 - t * (1 - OFFLINE_FLOOR);
+  }
+  if (elapsedMs <= OFFLINE_ZERO_MS) {
+    // 1h-2h : linéaire 0.25 → 0.0
+    const t = (elapsedMs - OFFLINE_DECAY_END_MS) / (OFFLINE_ZERO_MS - OFFLINE_DECAY_END_MS);
+    return OFFLINE_FLOOR * (1 - t);
+  }
+  return 0; // >2h : rien
+}
+
+// Calcule les gains offline avec la courbe dégressive
+// Intègre la courbe par tranches de 1 minute pour plus de précision
+export function computeOfflineGains(state: GameState, now: number): number {
+  const elapsed = now - state.lastTick;
+  if (elapsed <= OFFLINE_THRESHOLD) return computeIdleGains(state, now);
+
+  const pps = totalProductionPerSecond(state);
+  if (pps <= 0) return 0;
+
+  // Intégration par tranches de 60s
+  const STEP = 60_000;
+  let total = 0;
+  for (let t = 0; t < elapsed; t += STEP) {
+    const chunk = Math.min(STEP, elapsed - t);
+    const eff = offlineEfficiency(t + chunk / 2); // milieu de la tranche
+    total += pps * (chunk / 1000) * eff;
+  }
+  return total;
+}
+
 // --- Core economy (mis à jour pour intégrer l'arbre) ---
 
 // Coût d'achat du N-ième générateur : baseCost × 1.15^owned
@@ -183,6 +230,7 @@ export function applyPrestige(state: GameState): GameState {
     ancestralDna: state.ancestralDna + dnaGained,
     lifetimeTadpoles: 0,
     lastTick: Date.now(),
+    lastOnline: Date.now(),
     // evolutionTree persiste — jamais reset
   };
 }
@@ -201,6 +249,7 @@ export const DEFAULT_STATE: GameState = {
   clickMultiplier: 1,
   generators: DEFAULT_GENERATORS,
   lastTick: Date.now(),
+  lastOnline: Date.now(),
   prestigeCount: 0,
   prestigeMultiplier: 1,
   ancestralDna: 0,
diff --git a/Frontend/src/store/useGameStore.ts b/Frontend/src/store/useGameStore.ts
index 90e7f4f..59d87aa 100644
--- a/Frontend/src/store/useGameStore.ts
+++ b/Frontend/src/store/useGameStore.ts
@@ -14,18 +14,23 @@ import {
   canPrestige as canPrestigeCheck,
   totalProductionPerSecond,
   generatorCost as genCost,
+  computeOfflineGains,
+  offlineEfficiency,
 } from "../core/economy";
 
 const SAVE_KEY = "clickerz_state";
+const OFFLINE_THRESHOLD = 60_000; // 60s — same as economy.ts
 
 function loadLocalState(): GameState {
   try {
     const raw = localStorage.getItem(SAVE_KEY);
-    if (!raw) return { ...DEFAULT_STATE, lastTick: Date.now() };
+    if (!raw) return { ...DEFAULT_STATE, lastTick: Date.now(), lastOnline: Date.now() };
     const saved = JSON.parse(raw) as GameState;
+    // Backfill lastOnline for old saves
+    if (!saved.lastOnline) saved.lastOnline = saved.lastTick;
     return applyIdleGains(saved, Date.now());
   } catch {
-    return { ...DEFAULT_STATE, lastTick: Date.now() };
+    return { ...DEFAULT_STATE, lastTick: Date.now(), lastOnline: Date.now() };
   }
 }
 
@@ -33,11 +38,22 @@ function saveLocal(state: GameState): void {
   localStorage.setItem(SAVE_KEY, JSON.stringify(state));
 }
 
+export interface OfflineReport {
+  wasOffline: boolean;
+  duration: number; // ms
+  gains: number;
+  efficiency: number; // 0-1 average
+}
+
 interface GameStore {
   // State
   state: GameState;
   playSeconds: number;
-  ready: boolean; // true once the authoritative state is loaded
+  ready: boolean;
+
+  // Offline report (shown once after load)
+  offlineReport: OfflineReport | null;
+  dismissOfflineReport: () => void;
 
   // Derived (recalculated on tick)
   canPrestige: boolean;
@@ -51,24 +67,69 @@ interface GameStore {
   prestige: () => void;
   reset: () => void;
   loadFromServer: (serverState: GameState) => void;
-  initGuest: () => void; // fallback when no server save (guest mode)
+  initGuest: () => void;
   generatorCost: typeof genCost;
 }
 
-// Start with DEFAULT_STATE — game is NOT ready until loadFromServer or initGuest
-const initialState = { ...DEFAULT_STATE, lastTick: Date.now() };
+function hydrateWithOffline(saved: GameState, now: number): { state: GameState; report: OfflineReport | null } {
+  // Backfill lastOnline for old saves
+  if (!saved.lastOnline) saved.lastOnline = saved.lastTick;
+
+  const elapsed = now - saved.lastTick;
+
+  if (elapsed <= OFFLINE_THRESHOLD) {
+    // Normal idle — no offline report
+    const hydrated = applyIdleGains(saved, now);
+    return {
+      state: { ...hydrated, lastOnline: now },
+      report: null,
+    };
+  }
+
+  // Offline — use degraded curve
+  const gains = computeOfflineGains(saved, now);
+  const pps = totalProductionPerSecond(saved);
+  const fullGains = pps * (elapsed / 1000);
+  const avgEfficiency = fullGains > 0 ? gains / fullGains : 0;
+
+  const hydrated: GameState = {
+    ...saved,
+    resources: saved.resources + gains,
+    lifetimeTadpoles: saved.lifetimeTadpoles + gains,
+    lastTick: now,
+    lastOnline: now,
+  };
+
+  return {
+    state: hydrated,
+    report: {
+      wasOffline: true,
+      duration: elapsed,
+      gains,
+      efficiency: avgEfficiency,
+    },
+  };
+}
+
+const initialState = { ...DEFAULT_STATE, lastTick: Date.now(), lastOnline: Date.now() };
 
 export const useGameStore = create<GameStore>((set, get) => ({
   state: initialState,
   playSeconds: 0,
   ready: false,
+  offlineReport: null,
   canPrestige: false,
   productionPerSecond: 0,
 
+  dismissOfflineReport: () => set({ offlineReport: null }),
+
   tick: () => {
     if (!get().ready) return;
+    const now = Date.now();
     set((s) => {
-      const updated = applyIdleGains(s.state, Date.now());
+      const updated = applyIdleGains(s.state, now);
+      // Mark as actively online
+      updated.lastOnline = now;
       saveLocal(updated);
       return {
         state: updated,
@@ -134,37 +195,40 @@ export const useGameStore = create<GameStore>((set, get) => ({
   },
 
   reset: () => {
-    const fresh = { ...DEFAULT_STATE, lastTick: Date.now() };
+    const fresh = { ...DEFAULT_STATE, lastTick: Date.now(), lastOnline: Date.now() };
     saveLocal(fresh);
     set({
       state: fresh,
       playSeconds: 0,
       ready: true,
+      offlineReport: null,
       canPrestige: false,
       productionPerSecond: 0,
     });
   },
 
-  // Server save loaded — this IS the authority
   loadFromServer: (serverState: GameState) => {
-    const hydrated = applyIdleGains(serverState, Date.now());
-    saveLocal(hydrated); // mirror to localStorage as cache
+    const { state: hydrated, report } = hydrateWithOffline(serverState, Date.now());
+    saveLocal(hydrated);
     set({
       state: hydrated,
       ready: true,
+      offlineReport: report,
       canPrestige: canPrestigeCheck(hydrated),
       productionPerSecond: totalProductionPerSecond(hydrated),
     });
   },
 
-  // Guest mode — no server save, use localStorage or fresh state
   initGuest: () => {
     const local = loadLocalState();
+    const { state: hydrated, report } = hydrateWithOffline(local, Date.now());
+    saveLocal(hydrated);
     set({
-      state: local,
+      state: hydrated,
       ready: true,
-      canPrestige: canPrestigeCheck(local),
-      productionPerSecond: totalProductionPerSecond(local),
+      offlineReport: report,
+      canPrestige: canPrestigeCheck(hydrated),
+      productionPerSecond: totalProductionPerSecond(hydrated),
     });
   },