diff --git a/apps/api/src/lib/cooking.ts b/apps/api/src/lib/cooking.ts
index 38f2438..ab4fc53 100644
--- a/apps/api/src/lib/cooking.ts
+++ b/apps/api/src/lib/cooking.ts
@@ -1,16 +1,29 @@
 import type { FastifyInstance } from "fastify";
 import { and, eq, gt, isNull, sql } from "drizzle-orm";
 import { schema, markMilestone } from "@app/database";
-import { allocateFefo, updateCookingAssumptionProfile } from "@app/inventory-engine";
+import {
+  allocateFefo,
+  computeBalance,
+  updateCookingAssumptionProfile,
+  rollbackCookingAssumptionProfile,
+} from "@app/inventory-engine";
 import { scaleNutrition } from "@app/nutrition-engine";
 import type { Unit } from "@app/shared-types";
-import { cookingSessionStarted, cookingSessionCompleted } from "@app/analytics";
+import {
+  COOKING_SESSION_STATUSES,
+  MEAL_BOX_STATUSES,
+  MEAL_TYPES,
+} from "@app/shared-types";
+import {
+  cookingSessionStarted,
+  cookingSessionCompleted,
+  cookingSessionUndone,
+} from "@app/analytics";
 import { todayIso, emitEvent, trackProductAnalytics } from "./helpers.js";
 import { errors } from "./errors.js";
 import { loadFullRecipe } from "../routes/recipes.js";
 import { userLanguageTag } from "./contentLanguage.js";
 import { t } from "./i18n.js";
-import { MEAL_TYPES } from "@app/shared-types";
 
 export interface CompleteCookingInput {
   portionsCooked?: number;
@@ -191,7 +204,11 @@ export async function completeCookingSessionCore(
   // 1. FEFO-avdrag
   const deductions: Array<{ itemId: string; quantity: number; unit: string; name: string }> = [];
   if (input.deductInventory !== false) {
-    const factor = portionsCooked / recipe.portions;
+    // Partiell förbrukning (Fas 3 §6.3): om actualPortionsEaten anges drar vi
+    // endast råvaror för de faktiskt ätna portionerna. Rester/överskott stannar
+    // kvar som råvara i lagret tills de förbrukas på annat sätt.
+    const consumptionPortions = input.actualPortionsEaten ?? portionsCooked;
+    const factor = consumptionPortions / recipe.portions;
     const overrides = new Map(input.inventoryOverrides?.map((o) => [o.canonicalIngredientId, o]) ?? []);
 
     for (const ing of recipe.ingredients) {
@@ -405,3 +422,234 @@ export async function completeCookingSessionCore(
     })),
   };
 }
+
+export interface UndoCookingSessionResult {
+  ok: boolean;
+  reversedTransactions: number;
+  restoredItemIds: string[];
+  removedMealIds: string[];
+  discardedMealBoxIds: string[];
+}
+
+/**
+ * Ångra en completed cooking session (Fas 3 §6.3).
+ *
+ * - Ledger är append-only: befintliga transaktioner rörs inte; istället skrivs
+ *   reversal-transaktioner (type='correction') med motsatt delta.
+ * - Inventory-saldo räknas om från transaktionshistoriken; depletedAt nollas där
+ *   saldot blir > 0 igen.
+ * - meals och meal_boxes som skapades av sessionen tas bort resp. markeras
+ *   discarded; recipe_cooks-raden tas bort och cookCount backas.
+ * - Aktiveringsmilstolpar rörs inte (once-ever).
+ * - Antagandeprofiler rullas tillbaka deterministiskt via lastSessionAnswers.
+ * - undo_until = completedAt + 24 h, verkställt server-side i UTC.
+ */
+export async function undoCookingSession(
+  app: FastifyInstance,
+  session: typeof schema.cookingSessions.$inferSelect,
+  userId: string,
+  correlationId: string,
+): Promise<UndoCookingSessionResult> {
+  if (session.status !== "completed") {
+    throw errors.conflict("Sessionen måste vara avslutad för att kunna ångras.");
+  }
+  if (!session.completedAt) {
+    throw errors.internal("Sessionen saknar completedAt.");
+  }
+
+  const now = new Date();
+  const deadline = new Date(session.completedAt.getTime() + 24 * 60 * 60 * 1000);
+  if (now > deadline) {
+    const languageTag = await userLanguageTag(app.db, userId);
+    throw errors.conflict(t("cooked.undoWindowExpired", languageTag));
+  }
+
+  // 1. Reversera inventory-transaktioner (append-only).
+  const cookUses = await app.db
+    .select()
+    .from(schema.inventoryTransactions)
+    .where(
+      and(
+        eq(schema.inventoryTransactions.cookingSessionId, session.id),
+        eq(schema.inventoryTransactions.type, "cook_use"),
+      ),
+    );
+
+  const affectedItemIds = new Set<string>();
+  for (const tx of cookUses) {
+    await app.db.insert(schema.inventoryTransactions).values({
+      householdId: tx.householdId,
+      inventoryItemId: tx.inventoryItemId,
+      cookingSessionId: session.id,
+      type: "correction",
+      quantityDelta: -tx.quantityDelta,
+      unit: tx.unit,
+      refType: "cooking_session_undo",
+      refId: session.id,
+      actorUserId: userId,
+      note: "Ångrat cook_use",
+    });
+    affectedItemIds.add(tx.inventoryItemId);
+  }
+
+  // 2. Återställ inventory-saldon från transaktionerna.
+  for (const itemId of affectedItemIds) {
+    const txs = await app.db
+      .select({ type: schema.inventoryTransactions.type, quantityDelta: schema.inventoryTransactions.quantityDelta, unit: schema.inventoryTransactions.unit })
+      .from(schema.inventoryTransactions)
+      .where(eq(schema.inventoryTransactions.inventoryItemId, itemId));
+    const balance = computeBalance(txs);
+    await app.db
+      .update(schema.inventoryItems)
+      .set({
+        quantity: balance.balance,
+        depletedAt: balance.balance <= 1e-9 ? new Date() : null,
+        updatedAt: new Date(),
+      })
+      .where(eq(schema.inventoryItems.id, itemId));
+  }
+
+  // 3. Ta bort måltider.
+  const mealsToRemove = await app.db
+    .select()
+    .from(schema.meals)
+    .where(eq(schema.meals.cookingSessionId, session.id));
+  for (const meal of mealsToRemove) {
+    await emitEvent(app.db, {
+      type: "MEAL_REMOVED",
+      payload: { mealId: meal.id, recipeId: session.recipeId, source: "cooking_session_undo" },
+      userId: meal.userId,
+      householdId: session.householdId,
+      correlationId,
+    });
+  }
+  await app.db.delete(schema.meals).where(eq(schema.meals.cookingSessionId, session.id));
+
+  // 4. Markera matlådor som discarded.
+  const boxesToDiscard = await app.db
+    .select()
+    .from(schema.mealBoxes)
+    .where(eq(schema.mealBoxes.cookingSessionId, session.id));
+  for (const box of boxesToDiscard) {
+    await emitEvent(app.db, {
+      type: "MEAL_BOX_DISCARDED",
+      payload: { mealBoxId: box.id, portions: box.portions, source: "cooking_session_undo" },
+      userId,
+      householdId: session.householdId,
+      correlationId,
+    });
+  }
+  await app.db
+    .update(schema.mealBoxes)
+    .set({ status: "discarded", portionsRemaining: 0 })
+    .where(eq(schema.mealBoxes.cookingSessionId, session.id));
+
+  // 5. Ta bort recipe_cooks-raden och backa cookCount.
+  await app.db.delete(schema.recipeCooks).where(eq(schema.recipeCooks.cookingSessionId, session.id));
+  await app.db
+    .update(schema.recipes)
+    .set({ cookCount: sql`GREATEST(${schema.recipes.cookCount} - 1, 0)` })
+    .where(eq(schema.recipes.id, session.recipeId));
+
+  // 6. Rulla tillbaka antagandeprofiler.
+  const recipe = await loadFullRecipe(app, session.recipeId);
+  for (const ing of recipe.ingredients) {
+    if (ing.optional) continue;
+    const [existing] = await app.db
+      .select()
+      .from(schema.cookingAssumptionProfiles)
+      .where(
+        and(
+          eq(schema.cookingAssumptionProfiles.householdId, session.householdId),
+          eq(schema.cookingAssumptionProfiles.canonicalIngredientId, ing.canonicalIngredientId),
+        ),
+      )
+      .limit(1);
+    if (!existing) continue;
+
+    const rolledBack = rollbackCookingAssumptionProfile(session.id, {
+      averageEatenPortions: existing.averageEatenPortions,
+      averageLeftoverPortions: existing.averageLeftoverPortions,
+      observationCount: existing.observationCount,
+      lastSessionAnswers: existing.lastSessionAnswers,
+    });
+
+    if (rolledBack.observationCount === 0) {
+      await app.db
+        .delete(schema.cookingAssumptionProfiles)
+        .where(
+          and(
+            eq(schema.cookingAssumptionProfiles.householdId, session.householdId),
+            eq(schema.cookingAssumptionProfiles.canonicalIngredientId, ing.canonicalIngredientId),
+          ),
+        );
+    } else {
+      await app.db
+        .insert(schema.cookingAssumptionProfiles)
+        .values({
+          householdId: session.householdId,
+          canonicalIngredientId: ing.canonicalIngredientId,
+          averageEatenPortions: rolledBack.averageEatenPortions,
+          averageLeftoverPortions: rolledBack.averageLeftoverPortions,
+          observationCount: rolledBack.observationCount,
+          lastSessionAnswers: rolledBack.lastSessionAnswers,
+          updatedAt: new Date(),
+        })
+        .onConflictDoUpdate({
+          target: [
+            schema.cookingAssumptionProfiles.householdId,
+            schema.cookingAssumptionProfiles.canonicalIngredientId,
+          ],
+          set: {
+            averageEatenPortions: rolledBack.averageEatenPortions,
+            averageLeftoverPortions: rolledBack.averageLeftoverPortions,
+            observationCount: rolledBack.observationCount,
+            lastSessionAnswers: rolledBack.lastSessionAnswers,
+            updatedAt: new Date(),
+          },
+        });
+    }
+  }
+
+  // 7. Sätt sessionstatus till undone.
+  await app.db
+    .update(schema.cookingSessions)
+    .set({
+      status: "undone",
+      updatedAt: new Date(),
+    })
+    .where(eq(schema.cookingSessions.id, session.id));
+
+  await trackProductAnalytics(
+    app.db,
+    userId,
+    cookingSessionUndone({
+      householdId: session.householdId,
+      properties: {
+        cookingSessionId: session.id,
+        recipeId: session.recipeId,
+        reversedTransactions: cookUses.length,
+      },
+    }),
+  );
+
+  await emitEvent(app.db, {
+    type: "COOKING_SESSION_UNDONE",
+    payload: {
+      cookingSessionId: session.id,
+      recipeId: session.recipeId,
+      reversedTransactions: cookUses.length,
+    },
+    userId,
+    householdId: session.householdId,
+    correlationId,
+  });
+
+  return {
+    ok: true,
+    reversedTransactions: cookUses.length,
+    restoredItemIds: [...affectedItemIds],
+    removedMealIds: mealsToRemove.map((m) => m.id),
+    discardedMealBoxIds: boxesToDiscard.map((b) => b.id),
+  };
+}
diff --git a/apps/api/src/lib/i18n.ts b/apps/api/src/lib/i18n.ts
index 678cfb5..100bd12 100644
--- a/apps/api/src/lib/i18n.ts
+++ b/apps/api/src/lib/i18n.ts
@@ -34,6 +34,21 @@ const COOKED_PORTIONS_SUM_EXCEEDS_PLANNED: Record<string, string> = {
   sv: "Antalet ätna portioner och rester får inte överstiga det totala antalet portioner.",
 };
 
+const COOKED_UNDO_WINDOW_EXPIRED: Record<string, string> = {
+  da: "Det er ikke længere muligt at fortryde denne madlavningssession. Tidsfristen på 24 timer er udløbet.",
+  de: "Dieser Kochvorgang kann nicht mehr rückgängig gemacht werden. Das 24-Stunden-Fenster ist abgelaufen.",
+  en: "This cooking session can no longer be undone. The 24-hour window has expired.",
+  es: "Ya no se puede deshacer esta sesión de cocina. Ha expirado la ventana de 24 horas.",
+  fi: "Tätä ruoanlaittokertaa ei voi enää kumota. 24 tunnin ikkuna on umpeutunut.",
+  fr: "Cette session de cuisine ne peut plus être annulée. La fenêtre de 24 heures a expiré.",
+  it: "Non è più possibile annullare questa sessione di cucina. La finestra di 24 ore è scaduta.",
+  nb: "Denne matlagingsøkten kan ikke lenger angres. Vinduet på 24 timer har utløpt.",
+  nl: "Deze kooksessie kan niet meer ongedaan worden gemaakt. Het venster van 24 uur is verstreken.",
+  pl: "Tej sesji gotowania nie można już cofnąć. Okno 24-godzinne wygasło.",
+  pt: "Esta sessão de cozinha já não pode ser desfeita. A janela de 24 horas expirou.",
+  sv: "Denna matlagningssession kan inte längre ångras. 24-timmarsfönstret har löpt ut.",
+};
+
 function resolve(catalog: Record<string, string>, languageTag: string): string {
   const lang = (languageTag.split("-")[0] ?? "sv").toLowerCase();
   return catalog[lang] ?? catalog["en"] ?? catalog["sv"]!;
@@ -44,6 +59,10 @@ export function t(key: string, languageTag: string): string {
     return resolve(COOKED_PORTIONS_SUM_EXCEEDS_PLANNED, languageTag);
   }
 
+  if (key === "cooked.undoWindowExpired") {
+    return resolve(COOKED_UNDO_WINDOW_EXPIRED, languageTag);
+  }
+
   if (key !== "onboarding.householdDefaultName") {
     // No other server-side keys are supported yet; fall back to a safe default.
     return HOUSEHOLD_DEFAULT_NAMES["sv"]!;
diff --git a/apps/api/src/routes/cooking-sessions.ts b/apps/api/src/routes/cooking-sessions.ts
index d89e8fe..cc5f63c 100644
--- a/apps/api/src/routes/cooking-sessions.ts
+++ b/apps/api/src/routes/cooking-sessions.ts
@@ -10,7 +10,7 @@ import { errors, parse } from "../lib/errors.js";
 import { requireActiveHousehold, requireMembership } from "../lib/helpers.js";
 import { cookingSessionStarted, cookingSessionCancelled } from "@app/analytics";
 import { trackProductAnalytics } from "../lib/helpers.js";
-import { completeCookingSession } from "../lib/cooking.js";
+import { completeCookingSession, undoCookingSession } from "../lib/cooking.js";
 import { z } from "zod";
 
 /**
@@ -103,7 +103,7 @@ export async function cookingSessionRoutes(app: FastifyInstance) {
     const params = z.object({ id: z.uuid() }).parse(req.params);
     const body = z.object({ reason: z.string().max(200).optional() }).parse(req.body ?? {});
     const session = await getOwnedSession(app, params.id, req.userId);
-    if (session.status === "completed" || session.status === "cancelled") {
+    if (session.status === "completed" || session.status === "cancelled" || session.status === "undone") {
       throw errors.conflict("Sessionen är redan avslutad.");
     }
 
@@ -196,6 +196,18 @@ export async function cookingSessionRoutes(app: FastifyInstance) {
     };
   });
 
+  /**
+   * Ångra en completed session inom 24 h.
+   * Ledger är append-only: reverseringstransaktioner skrivs, befintliga
+   * transaktioner rörs inte.
+   */
+  app.post("/v1/cooking-sessions/:id/undo", auth, async (req) => {
+    const { id } = parse(idParamSchema, req.params);
+    const session = await getOwnedSession(app, id, req.userId);
+    const result = await undoCookingSession(app, session, req.userId, req.correlationId);
+    return result;
+  });
+
   /** Lista hushållets aktiva sessioner. */
   app.get("/v1/cooking-sessions", auth, async (req) => {
     const householdId = await requireActiveHousehold(app.db, req.userId);
diff --git a/apps/api/test/cooking-sessions.test.ts b/apps/api/test/cooking-sessions.test.ts
index 1a21c3e..fb08509 100644
--- a/apps/api/test/cooking-sessions.test.ts
+++ b/apps/api/test/cooking-sessions.test.ts
@@ -6,6 +6,7 @@ import { loadConfig } from "../src/config.js";
 import { createDatabase, closeDatabase, schema } from "@app/database";
 import { cancelTimedOutCookingSessions } from "@app/database";
 import { computeBalance } from "@app/inventory-engine";
+import type { Unit } from "@app/shared-types";
 
 describe("cooking sessions", () => {
   const testDb = createDatabase(process.env.TEST_DATABASE_URL!);
@@ -17,6 +18,36 @@ describe("cooking sessions", () => {
   let recipeId: string;
   const email = "cooking-session-test@example.invalid";
 
+  async function createItemWithPurchase(canonicalIngredientId: string, displayName: string, quantity: number, unit: Unit) {
+    const [location] = await testDb.db
+      .select({ id: schema.storageLocations.id })
+      .from(schema.storageLocations)
+      .where(eq(schema.storageLocations.householdId, householdId))
+      .limit(1);
+    const [item] = await testDb.db
+      .insert(schema.inventoryItems)
+      .values({
+        householdId,
+        canonicalIngredientId,
+        displayName,
+        quantity,
+        unit,
+        storageLocationId: location!.id,
+        source: "manual_search",
+      })
+      .returning();
+    await testDb.db.insert(schema.inventoryTransactions).values({
+      householdId,
+      inventoryItemId: item!.id,
+      type: "purchase",
+      quantityDelta: quantity,
+      unit,
+      refType: "test_setup",
+      actorUserId: userId,
+    });
+    return item!.id;
+  }
+
   async function cleanup() {
     const existing = await testDb.db
       .select({ id: schema.users.id })
@@ -582,4 +613,419 @@ describe("cooking sessions", () => {
     const timedOut = await cancelTimedOutCookingSessions(testDb.db);
     expect(timedOut.some((s: { id: string }) => s.id === sessionId)).toBe(true);
   });
+
+  it("undo restores inventory, removes meals, discards meal boxes and deletes recipe_cooks", async () => {
+    const recipe = (await app.inject({
+      method: "GET",
+      url: `/v1/recipes/${recipeId}`,
+      headers: { authorization: `Bearer ${token}` },
+    })).json() as { ingredients: Array<{ canonicalIngredientId: string; optional?: boolean }> };
+    const firstNonOptional = recipe.ingredients.find((i) => i.canonicalIngredientId && !i.optional)?.canonicalIngredientId;
+
+    // Sätt upp ett känt lager om receptet har en icke-valfri ingrediens.
+    let itemId: string | undefined;
+    if (firstNonOptional) {
+      itemId = await createItemWithPurchase(firstNonOptional, "Testvara", 1000, "GRAM");
+    }
+
+    const start = await app.inject({
+      method: "POST",
+      url: `/v1/recipes/${recipeId}/cook/start`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: { startNow: true, portions: 4 },
+    });
+    const sessionId = (JSON.parse(start.body) as { session: { id: string } }).session.id;
+
+    const complete = await app.inject({
+      method: "POST",
+      url: `/v1/cooking-sessions/${sessionId}/complete`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: { mealBoxPortions: 1, actualPortionsEaten: 3, leftoverEstimatePortions: 1 },
+    });
+    expect(complete.statusCode).toBe(200);
+
+    const txsBefore = await testDb.db
+      .select({ count: count(schema.inventoryTransactions.id) })
+      .from(schema.inventoryTransactions)
+      .where(eq(schema.inventoryTransactions.cookingSessionId, sessionId));
+
+    const undo = await app.inject({
+      method: "POST",
+      url: `/v1/cooking-sessions/${sessionId}/undo`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: {},
+    });
+    expect(undo.statusCode).toBe(200);
+    const undoBody = JSON.parse(undo.body) as { ok: boolean; removedMealIds: string[]; discardedMealBoxIds: string[] };
+    expect(undoBody.ok).toBe(true);
+
+    // Sessionstatus = undone.
+    const session = await testDb.db
+      .select()
+      .from(schema.cookingSessions)
+      .where(eq(schema.cookingSessions.id, sessionId))
+      .limit(1);
+    expect(session[0]!.status).toBe("undone");
+
+    // Append-only: fler transaktioner efter undo.
+    const txsAfter = await testDb.db
+      .select({ count: count(schema.inventoryTransactions.id) })
+      .from(schema.inventoryTransactions)
+      .where(eq(schema.inventoryTransactions.cookingSessionId, sessionId));
+    expect(Number(txsAfter[0]!.count)).toBeGreaterThan(Number(txsBefore[0]!.count));
+
+    // Meals borttagna.
+    const meals = await testDb.db.select({ count: count(schema.meals.id) }).from(schema.meals).where(eq(schema.meals.cookingSessionId, sessionId));
+    expect(Number(meals[0]!.count)).toBe(0);
+
+    // Matlådor markerade discarded.
+    const boxes = await testDb.db.select().from(schema.mealBoxes).where(eq(schema.mealBoxes.cookingSessionId, sessionId));
+    expect(boxes.length).toBe(undoBody.discardedMealBoxIds.length);
+    for (const box of boxes) expect(box.status).toBe("discarded");
+
+    // recipe_cooks borttagen och cookCount backad.
+    const cooks = await testDb.db.select().from(schema.recipeCooks).where(eq(schema.recipeCooks.cookingSessionId, sessionId));
+    expect(cooks.length).toBe(0);
+
+    // Inventory-transaktionsinvariant.
+    if (itemId) {
+      const item = await testDb.db
+        .select({ quantity: schema.inventoryItems.quantity })
+        .from(schema.inventoryItems)
+        .where(eq(schema.inventoryItems.id, itemId))
+        .limit(1);
+      const itemTxs = await testDb.db
+        .select({ type: schema.inventoryTransactions.type, quantityDelta: schema.inventoryTransactions.quantityDelta, unit: schema.inventoryTransactions.unit })
+        .from(schema.inventoryTransactions)
+        .where(eq(schema.inventoryTransactions.inventoryItemId, itemId));
+      const balance = computeBalance(itemTxs);
+      expect(Math.abs(balance.balance - item[0]!.quantity)).toBeLessThan(1e-6);
+    }
+
+    // Analytics.
+    const analytics = await testDb.db
+      .select()
+      .from(schema.productAnalyticsEvents)
+      .where(eq(schema.productAnalyticsEvents.userId, userId))
+      .orderBy(schema.productAnalyticsEvents.occurredAt);
+    const props = (e: (typeof analytics)[number]) => (e.properties ?? {}) as { cookingSessionId?: string };
+    expect(analytics.some((e) => e.eventName === "cooking_session_undone" && props(e).cookingSessionId === sessionId)).toBe(true);
+
+    if (itemId) {
+      await testDb.db
+        .delete(schema.inventoryItems)
+        .where(eq(schema.inventoryItems.id, itemId));
+    }
+  });
+
+  it("undo is rejected after 24h window", async () => {
+    const start = await app.inject({
+      method: "POST",
+      url: `/v1/recipes/${recipeId}/cook/start`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: { startNow: true },
+    });
+    const sessionId = (JSON.parse(start.body) as { session: { id: string } }).session.id;
+
+    await app.inject({
+      method: "POST",
+      url: `/v1/cooking-sessions/${sessionId}/complete`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: { mealBoxPortions: 0 },
+    });
+
+    // Simulera 25 h gammal completed session.
+    await testDb.db
+      .update(schema.cookingSessions)
+      .set({ completedAt: new Date(Date.now() - 25 * 60 * 60 * 1000) })
+      .where(eq(schema.cookingSessions.id, sessionId));
+
+    const undo = await app.inject({
+      method: "POST",
+      url: `/v1/cooking-sessions/${sessionId}/undo`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: {},
+    });
+    expect(undo.statusCode).toBe(409);
+  });
+
+  it("partial consumption deducts only actual portions eaten from inventory", async () => {
+    // Skapa ett enkelt recept med en enda icke-valfri ingrediens.
+    const [recipe] = await testDb.db
+      .insert(schema.recipes)
+      .values({
+        slug: `partial-${Date.now()}`,
+        titleSv: "Partial test",
+        descriptionSv: "",
+        cuisine: "international",
+        mealTypes: ["dinner"],
+        tags: [],
+        methods: [],
+        equipment: [],
+        difficulty: "easy",
+        prepTimeMinutes: 5,
+        cookTimeMinutes: 10,
+        totalTimeMinutes: 15,
+        portions: 4,
+        nutritionPerPortion: { kcal: 100, proteinG: 5, fatG: 3, carbsG: 12, saturatedFatG: 1, fiberG: 1, sugarG: 2, saltG: 0.1 },
+        allergens: [],
+        spiceLevel: 0,
+        dna: { cuisine: "international", vegetables: [], flavorProfile: [], spiceLevel: 0, method: "stovetop", timeMinutes: 15, calories: 100, proteinGrams: 5 },
+        status: "published",
+        verificationStatus: "unverified",
+        sourceType: "own_editorial",
+        creatorDisplayName: "Test",
+      })
+      .returning();
+    const testRecipeId = recipe!.id;
+    await testDb.db.insert(schema.recipeIngredients).values({
+      recipeId: testRecipeId,
+      canonicalIngredientId: "pasta_dry",
+      displayNameSv: "Pasta",
+      quantity: 400,
+      unit: "GRAM",
+      optional: false,
+      sortOrder: 0,
+    });
+
+    await testDb.db
+      .delete(schema.inventoryItems)
+      .where(and(eq(schema.inventoryItems.householdId, householdId), eq(schema.inventoryItems.canonicalIngredientId, "pasta_dry")));
+    const itemId = await createItemWithPurchase("pasta_dry", "Pasta", 400, "GRAM");
+
+    const start = await app.inject({
+      method: "POST",
+      url: `/v1/recipes/${testRecipeId}/cook/start`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: { startNow: true, portions: 4 },
+    });
+    const sessionId = (JSON.parse(start.body) as { session: { id: string } }).session.id;
+
+    await app.inject({
+      method: "POST",
+      url: `/v1/cooking-sessions/${sessionId}/complete`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: { mealBoxPortions: 0, actualPortionsEaten: 2, leftoverEstimatePortions: 0 },
+    });
+
+    // 4 portioner → 400 g, 2 ätna → 200 g, alltså 200 g kvar.
+    const afterComplete = await testDb.db
+      .select({ quantity: schema.inventoryItems.quantity })
+      .from(schema.inventoryItems)
+      .where(eq(schema.inventoryItems.id, itemId))
+      .limit(1);
+    expect(afterComplete[0]!.quantity).toBeCloseTo(200, 1);
+
+    // Undo återställer.
+    const undo = await app.inject({
+      method: "POST",
+      url: `/v1/cooking-sessions/${sessionId}/undo`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: {},
+    });
+    expect(undo.statusCode).toBe(200);
+    const afterUndo = await testDb.db
+      .select({ quantity: schema.inventoryItems.quantity })
+      .from(schema.inventoryItems)
+      .where(eq(schema.inventoryItems.id, itemId))
+      .limit(1);
+    expect(afterUndo[0]!.quantity).toBeCloseTo(400, 1);
+
+    // Städa testreceptet och lagerposten.
+    await testDb.db
+      .delete(schema.inventoryItems)
+      .where(and(eq(schema.inventoryItems.householdId, householdId), eq(schema.inventoryItems.canonicalIngredientId, "pasta_dry")));
+    await testDb.db.delete(schema.recipeIngredients).where(eq(schema.recipeIngredients.recipeId, testRecipeId));
+    await testDb.db.delete(schema.recipes).where(eq(schema.recipes.id, testRecipeId));
+  });
+
+  it("undo + new complete maintains inventory invariant", async () => {
+    const [recipe] = await testDb.db
+      .insert(schema.recipes)
+      .values({
+        slug: `invariant-${Date.now()}`,
+        titleSv: "Invariant test",
+        descriptionSv: "",
+        cuisine: "international",
+        mealTypes: ["dinner"],
+        tags: [],
+        methods: [],
+        equipment: [],
+        difficulty: "easy",
+        prepTimeMinutes: 5,
+        cookTimeMinutes: 10,
+        totalTimeMinutes: 15,
+        portions: 4,
+        nutritionPerPortion: { kcal: 100, proteinG: 5, fatG: 3, carbsG: 12, saturatedFatG: 1, fiberG: 1, sugarG: 2, saltG: 0.1 },
+        allergens: [],
+        spiceLevel: 0,
+        dna: { cuisine: "international", vegetables: [], flavorProfile: [], spiceLevel: 0, method: "stovetop", timeMinutes: 15, calories: 100, proteinGrams: 5 },
+        status: "published",
+        verificationStatus: "unverified",
+        sourceType: "own_editorial",
+        creatorDisplayName: "Test",
+      })
+      .returning();
+    const testRecipeId = recipe!.id;
+    await testDb.db.insert(schema.recipeIngredients).values({
+      recipeId: testRecipeId,
+      canonicalIngredientId: "rice_white",
+      displayNameSv: "Ris",
+      quantity: 400,
+      unit: "GRAM",
+      optional: false,
+      sortOrder: 0,
+    });
+
+    await testDb.db
+      .delete(schema.inventoryItems)
+      .where(and(eq(schema.inventoryItems.householdId, householdId), eq(schema.inventoryItems.canonicalIngredientId, "rice_white")));
+    const itemId = await createItemWithPurchase("rice_white", "Ris", 400, "GRAM");
+
+    async function assertInvariant() {
+      const item = await testDb.db
+        .select({ quantity: schema.inventoryItems.quantity })
+        .from(schema.inventoryItems)
+        .where(eq(schema.inventoryItems.id, itemId))
+        .limit(1);
+      const txs = await testDb.db
+        .select({ type: schema.inventoryTransactions.type, quantityDelta: schema.inventoryTransactions.quantityDelta, unit: schema.inventoryTransactions.unit })
+        .from(schema.inventoryTransactions)
+        .where(eq(schema.inventoryTransactions.inventoryItemId, itemId));
+      const balance = computeBalance(txs);
+      expect(Math.abs(balance.balance - item[0]!.quantity)).toBeLessThan(1e-6);
+    }
+
+    const start = await app.inject({
+      method: "POST",
+      url: `/v1/recipes/${testRecipeId}/cook/start`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: { startNow: true, portions: 4 },
+    });
+    const sessionId = (JSON.parse(start.body) as { session: { id: string } }).session.id;
+
+    await app.inject({
+      method: "POST",
+      url: `/v1/cooking-sessions/${sessionId}/complete`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: { mealBoxPortions: 0, actualPortionsEaten: 4, leftoverEstimatePortions: 0 },
+    });
+    await assertInvariant();
+
+    await app.inject({
+      method: "POST",
+      url: `/v1/cooking-sessions/${sessionId}/undo`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: {},
+    });
+    await assertInvariant();
+
+    // Ny session + complete efter undo ska bibehålla invarianten.
+    const start2 = await app.inject({
+      method: "POST",
+      url: `/v1/recipes/${testRecipeId}/cook/start`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: { startNow: true, portions: 4 },
+    });
+    const sessionId2 = (JSON.parse(start2.body) as { session: { id: string } }).session.id;
+    await app.inject({
+      method: "POST",
+      url: `/v1/cooking-sessions/${sessionId2}/complete`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: { mealBoxPortions: 0, actualPortionsEaten: 2, leftoverEstimatePortions: 0 },
+    });
+    await assertInvariant();
+
+    await testDb.db
+      .delete(schema.inventoryItems)
+      .where(and(eq(schema.inventoryItems.householdId, householdId), eq(schema.inventoryItems.canonicalIngredientId, "rice_white")));
+    await testDb.db.delete(schema.recipeIngredients).where(eq(schema.recipeIngredients.recipeId, testRecipeId));
+    await testDb.db.delete(schema.recipes).where(eq(schema.recipes.id, testRecipeId));
+  });
+
+  it("legacy /cook followed by undo works", async () => {
+    const cook = await app.inject({
+      method: "POST",
+      url: `/v1/recipes/${recipeId}/cook`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: { portionsCooked: 4, mealBoxPortions: 1, deductInventory: true },
+    });
+    expect(cook.statusCode).toBe(200);
+    const { sessionId } = JSON.parse(cook.body) as { sessionId: string };
+
+    const undo = await app.inject({
+      method: "POST",
+      url: `/v1/cooking-sessions/${sessionId}/undo`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: {},
+    });
+    expect(undo.statusCode).toBe(200);
+
+    const session = await testDb.db
+      .select()
+      .from(schema.cookingSessions)
+      .where(eq(schema.cookingSessions.id, sessionId))
+      .limit(1);
+    expect(session[0]!.status).toBe("undone");
+
+    const boxes = await testDb.db.select().from(schema.mealBoxes).where(eq(schema.mealBoxes.cookingSessionId, sessionId));
+    expect(boxes.every((b) => b.status === "discarded")).toBe(true);
+  });
+
+  it("undo rolls back cooking assumption profiles", async () => {
+    await testDb.db.delete(schema.cookingAssumptionProfiles).where(eq(schema.cookingAssumptionProfiles.householdId, householdId));
+
+    const recipe = (await app.inject({
+      method: "GET",
+      url: `/v1/recipes/${recipeId}`,
+      headers: { authorization: `Bearer ${token}` },
+    })).json() as { ingredients: Array<{ canonicalIngredientId: string; optional?: boolean }> };
+    const firstNonOptionalIngredientId = recipe.ingredients.find((i) => i.canonicalIngredientId && !i.optional)?.canonicalIngredientId;
+    expect(firstNonOptionalIngredientId).toBeDefined();
+
+    const cook = await app.inject({
+      method: "POST",
+      url: `/v1/recipes/${recipeId}/cook`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: { portionsCooked: 4, actualPortionsEaten: 3, leftoverEstimatePortions: 1, deductInventory: true },
+    });
+    const { sessionId } = JSON.parse(cook.body) as { sessionId: string };
+
+    await app.inject({
+      method: "POST",
+      url: `/v1/cooking-sessions/${sessionId}/undo`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: {},
+    });
+
+    const profile = await testDb.db
+      .select()
+      .from(schema.cookingAssumptionProfiles)
+      .where(
+        and(
+          eq(schema.cookingAssumptionProfiles.householdId, householdId),
+          eq(schema.cookingAssumptionProfiles.canonicalIngredientId, firstNonOptionalIngredientId!),
+        ),
+      )
+      .limit(1);
+    expect(profile.length).toBe(0);
+  });
+
+  it("cannot undo a non-completed session", async () => {
+    const start = await app.inject({
+      method: "POST",
+      url: `/v1/recipes/${recipeId}/cook/start`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: { startNow: true },
+    });
+    const sessionId = (JSON.parse(start.body) as { session: { id: string } }).session.id;
+
+    const undo = await app.inject({
+      method: "POST",
+      url: `/v1/cooking-sessions/${sessionId}/undo`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: {},
+    });
+    expect(undo.statusCode).toBe(409);
+  });
 });
diff --git a/apps/mobile/src/locales/da/common.json b/apps/mobile/src/locales/da/common.json
index 081e830..8b2fb66 100644
--- a/apps/mobile/src/locales/da/common.json
+++ b/apps/mobile/src/locales/da/common.json
@@ -383,5 +383,6 @@
   "scan.diff.noChanges": "Ingen forskelle fundet.",
   "scan.diff.undo": "Fortryd",
   "scan.diff.undoHint": "Hver ændring kan fortrydes fra varedetaljevisningen.",
-  "cooked.portionsSumExceedsPlanned": "Antal spiste portioner og rester må ikke overstige det samlede antal portioner."
+  "cooked.portionsSumExceedsPlanned": "Antal spiste portioner og rester må ikke overstige det samlede antal portioner.",
+  "cooked.undoWindowExpired": "Det er ikke længere muligt at fortryde denne madlavningssession. Tidsfristen på 24 timer er udløbet."
 }
diff --git a/apps/mobile/src/locales/de/common.json b/apps/mobile/src/locales/de/common.json
index 639fb00..0e4eef3 100644
--- a/apps/mobile/src/locales/de/common.json
+++ b/apps/mobile/src/locales/de/common.json
@@ -383,5 +383,6 @@
   "scan.diff.noChanges": "Keine Unterschiede gefunden.",
   "scan.diff.undo": "Rückgängig",
   "scan.diff.undoHint": "Jede Änderung kann in der Artikeldetailansicht rückgängig gemacht werden.",
-  "cooked.portionsSumExceedsPlanned": "Gegessene Portionen und Reste dürfen die Gesamtanzahl der Portionen nicht überschreiten."
+  "cooked.portionsSumExceedsPlanned": "Gegessene Portionen und Reste dürfen die Gesamtanzahl der Portionen nicht überschreiten.",
+  "cooked.undoWindowExpired": "Dieser Kochvorgang kann nicht mehr rückgängig gemacht werden. Das 24-Stunden-Fenster ist abgelaufen."
 }
diff --git a/apps/mobile/src/locales/en/common.json b/apps/mobile/src/locales/en/common.json
index f706744..59e73ed 100644
--- a/apps/mobile/src/locales/en/common.json
+++ b/apps/mobile/src/locales/en/common.json
@@ -383,5 +383,6 @@
   "scan.diff.noChanges": "No differences found.",
   "scan.diff.undo": "Undo",
   "scan.diff.undoHint": "Each change can be undone from the item detail view.",
-  "cooked.portionsSumExceedsPlanned": "Eaten portions and leftovers cannot exceed the total number of portions."
+  "cooked.portionsSumExceedsPlanned": "Eaten portions and leftovers cannot exceed the total number of portions.",
+  "cooked.undoWindowExpired": "This cooking session can no longer be undone. The 24-hour window has expired."
 }
diff --git a/apps/mobile/src/locales/es/common.json b/apps/mobile/src/locales/es/common.json
index e766a68..e383886 100644
--- a/apps/mobile/src/locales/es/common.json
+++ b/apps/mobile/src/locales/es/common.json
@@ -383,5 +383,6 @@
   "scan.diff.noChanges": "No se encontraron diferencias.",
   "scan.diff.undo": "Deshacer",
   "scan.diff.undoHint": "Cada cambio se puede deshacer desde la vista de detalle del producto.",
-  "cooked.portionsSumExceedsPlanned": "Las raciones comidas y las sobras no pueden superar el número total de raciones."
+  "cooked.portionsSumExceedsPlanned": "Las raciones comidas y las sobras no pueden superar el número total de raciones.",
+  "cooked.undoWindowExpired": "Ya no se puede deshacer esta sesión de cocina. Ha expirado la ventana de 24 horas."
 }
diff --git a/apps/mobile/src/locales/fi/common.json b/apps/mobile/src/locales/fi/common.json
index a4a9c91..90898ff 100644
--- a/apps/mobile/src/locales/fi/common.json
+++ b/apps/mobile/src/locales/fi/common.json
@@ -383,5 +383,6 @@
   "scan.diff.noChanges": "Eroja ei löytynyt.",
   "scan.diff.undo": "Kumoa",
   "scan.diff.undoHint": "Jokainen muutos voidaan kumota tuotteen tietonäkymästä.",
-  "cooked.portionsSumExceedsPlanned": "Syödyt annokset ja tähteet eivät voi ylittää annosten kokonaismäärää."
+  "cooked.portionsSumExceedsPlanned": "Syödyt annokset ja tähteet eivät voi ylittää annosten kokonaismäärää.",
+  "cooked.undoWindowExpired": "Tätä ruoanlaittokertaa ei voi enää kumota. 24 tunnin ikkuna on umpeutunut."
 }
diff --git a/apps/mobile/src/locales/fr/common.json b/apps/mobile/src/locales/fr/common.json
index 12acf10..054f9bb 100644
--- a/apps/mobile/src/locales/fr/common.json
+++ b/apps/mobile/src/locales/fr/common.json
@@ -383,5 +383,6 @@
   "scan.diff.noChanges": "Aucune différence trouvée.",
   "scan.diff.undo": "Annuler",
   "scan.diff.undoHint": "Chaque modification peut être annulée depuis la vue détail de l'article.",
-  "cooked.portionsSumExceedsPlanned": "Les portions mangées et les restes ne peuvent pas dépasser le nombre total de portions."
+  "cooked.portionsSumExceedsPlanned": "Les portions mangées et les restes ne peuvent pas dépasser le nombre total de portions.",
+  "cooked.undoWindowExpired": "Cette session de cuisine ne peut plus être annulée. La fenêtre de 24 heures a expiré."
 }
diff --git a/apps/mobile/src/locales/it/common.json b/apps/mobile/src/locales/it/common.json
index 07d7bc6..718ea4c 100644
--- a/apps/mobile/src/locales/it/common.json
+++ b/apps/mobile/src/locales/it/common.json
@@ -383,5 +383,6 @@
   "scan.diff.noChanges": "Nessuna differenza trovata.",
   "scan.diff.undo": "Annulla",
   "scan.diff.undoHint": "Ogni modifica può essere annullata dalla vista dettaglio dell'articolo.",
-  "cooked.portionsSumExceedsPlanned": "Le porzioni mangiate e gli avanzi non possono superare il numero totale di porzioni."
+  "cooked.portionsSumExceedsPlanned": "Le porzioni mangiate e gli avanzi non possono superare il numero totale di porzioni.",
+  "cooked.undoWindowExpired": "Non è più possibile annullare questa sessione di cucina. La finestra di 24 ore è scaduta."
 }
diff --git a/apps/mobile/src/locales/nb/common.json b/apps/mobile/src/locales/nb/common.json
index f3a6fde..ad50d0c 100644
--- a/apps/mobile/src/locales/nb/common.json
+++ b/apps/mobile/src/locales/nb/common.json
@@ -383,5 +383,6 @@
   "scan.diff.noChanges": "Ingen forskjeller funnet.",
   "scan.diff.undo": "Angre",
   "scan.diff.undoHint": "Hver endring kan angres fra varedetaljvisningen.",
-  "cooked.portionsSumExceedsPlanned": "Antall spiste porsjoner og rester kan ikke overstige det totale antallet porsjoner."
+  "cooked.portionsSumExceedsPlanned": "Antall spiste porsjoner og rester kan ikke overstige det totale antallet porsjoner.",
+  "cooked.undoWindowExpired": "Denne matlagingsøkten kan ikke lenger angres. Vinduet på 24 timer har utløpt."
 }
diff --git a/apps/mobile/src/locales/nl/common.json b/apps/mobile/src/locales/nl/common.json
index d6b3cfb..663adf0 100644
--- a/apps/mobile/src/locales/nl/common.json
+++ b/apps/mobile/src/locales/nl/common.json
@@ -383,5 +383,6 @@
   "scan.diff.noChanges": "Geen verschillen gevonden.",
   "scan.diff.undo": "Ongedaan maken",
   "scan.diff.undoHint": "Elke wijziging kan ongedaan worden gemaakt vanuit de detailweergave.",
-  "cooked.portionsSumExceedsPlanned": "Gegeten porties en restjes mogen het totaal aantal porties niet overschrijden."
+  "cooked.portionsSumExceedsPlanned": "Gegeten porties en restjes mogen het totaal aantal porties niet overschrijden.",
+  "cooked.undoWindowExpired": "Deze kooksessie kan niet meer ongedaan worden gemaakt. Het venster van 24 uur is verstreken."
 }
diff --git a/apps/mobile/src/locales/pl/common.json b/apps/mobile/src/locales/pl/common.json
index 805038c..c9ec0d5 100644
--- a/apps/mobile/src/locales/pl/common.json
+++ b/apps/mobile/src/locales/pl/common.json
@@ -397,5 +397,6 @@
   "scan.diff.noChanges": "Nie znaleziono różnic.",
   "scan.diff.undo": "Cofnij",
   "scan.diff.undoHint": "Każdą zmianę można cofnąć z widoku szczegółów produktu.",
-  "cooked.portionsSumExceedsPlanned": "Zjedzone porcje i resztki nie mogą przekroczyć całkowitej liczby porcji."
+  "cooked.portionsSumExceedsPlanned": "Zjedzone porcje i resztki nie mogą przekroczyć całkowitej liczby porcji.",
+  "cooked.undoWindowExpired": "Tej sesji gotowania nie można już cofnąć. Okno 24-godzinne wygasło."
 }
diff --git a/apps/mobile/src/locales/pt/common.json b/apps/mobile/src/locales/pt/common.json
index 1a5db6d..fb6dbbf 100644
--- a/apps/mobile/src/locales/pt/common.json
+++ b/apps/mobile/src/locales/pt/common.json
@@ -383,5 +383,6 @@
   "scan.diff.noChanges": "Nenhuma diferença encontrada.",
   "scan.diff.undo": "Desfazer",
   "scan.diff.undoHint": "Cada alteração pode ser desfeita a partir da vista de detalhes do item.",
-  "cooked.portionsSumExceedsPlanned": "As porções comidas e as sobras não podem ultrapassar o número total de porções."
+  "cooked.portionsSumExceedsPlanned": "As porções comidas e as sobras não podem ultrapassar o número total de porções.",
+  "cooked.undoWindowExpired": "Esta sessão de cozinha já não pode ser desfeita. A janela de 24 horas expirou."
 }
diff --git a/apps/mobile/src/locales/sv/common.json b/apps/mobile/src/locales/sv/common.json
index 315931f..9920f77 100644
--- a/apps/mobile/src/locales/sv/common.json
+++ b/apps/mobile/src/locales/sv/common.json
@@ -383,5 +383,6 @@
   "scan.diff.noChanges": "Inga skillnader hittades.",
   "scan.diff.undo": "Ångra",
   "scan.diff.undoHint": "Varje ändring kan ångras från varans detaljvy.",
-  "cooked.portionsSumExceedsPlanned": "Antalet ätna portioner och rester får inte överstiga det totala antalet portioner."
+  "cooked.portionsSumExceedsPlanned": "Antalet ätna portioner och rester får inte överstiga det totala antalet portioner.",
+  "cooked.undoWindowExpired": "Denna matlagningssession kan inte längre ångras. 24-timmarsfönstret har löpt ut."
 }
diff --git a/docs/FAS3-COOKING-SESSIONS-AUDIT.md b/docs/FAS3-COOKING-SESSIONS-AUDIT.md
index 5a2e736..6a5b9be 100644
--- a/docs/FAS3-COOKING-SESSIONS-AUDIT.md
+++ b/docs/FAS3-COOKING-SESSIONS-AUDIT.md
@@ -140,7 +140,7 @@ Att lägga till `prepared_food_batches` skulle skapa:
 ---
 
 ### Steg 3c – Partiell förbrukning §6.3 + undo_until
-**Mål:** Om användaren säger att de bara åt 3 av 4 portioner, ska FEFO-avdraget justeras så att motsvarande råvaror återstår.
+**Mål:** Om användaren säger att de bara åt 3 av 4 portioner, ska FEFO-avdraget justeras så att motsvarande råvaror återstår. En completed session kan ångras inom 24 h via append-only reverseringstransaktioner.
 
 **Ändringar:**
 1. Vid complete: använd `actualPortionsEaten` för att räkna om råvaror.
@@ -149,24 +149,40 @@ Att lägga till `prepared_food_batches` skulle skapa:
 2. Lagra ursprungligt FEFO-avdrag i `cooking_sessions.plannedDeductions` (jsonb).
 3. `POST /v1/cooking-sessions/:id/undo`:
    - Status måste vara `completed`.
-   - För varje `inventory_transactions` med `cookingSessionId`: skapa en `correction` med motsatt delta.
-   - För varje `meal` med `cookingSessionId`: radera (eller markera som cancelled).
-   - För varje `meal_box` med `cookingSessionId`: sätt status `discarded`.
-   - Sätt `cooking_sessions.status = 'cancelled'`.
-4. Lägg `cookingSessionId` i inventory item detail så användaren kan ångra enskilda transaktioner därifrån också.
+   - `undo_until = completedAt + 24 h` (server-side, UTC). Efter fönstret: 409 med i18n-nyckeln `cooked.undoWindowExpired`.
+   - För varje `inventory_transactions.type = 'cook_use'` med `cookingSessionId`: skapa en `correction` med motsatt delta (append-only; befintliga rader rörs inte).
+   - Lagersaldo räknas om från transaktionshistoriken; `depletedAt` nollas där saldot blir > 0 igen.
+   - För varje `meal` med `cookingSessionId`: radera och emitta `MEAL_REMOVED`.
+   - För varje `meal_box` med `cookingSessionId`: sätt status `discarded`, `portionsRemaining = 0`, emitta `MEAL_BOX_DISCARDED`.
+   - Ta bort `recipe_cooks`-raden och backa `recipes.cookCount` med `GREATEST(cookCount - 1, 0)`.
+   - Sätt `cooking_sessions.status = 'undone'` (TEXT, valideras mot `COOKING_SESSION_STATUSES` i shared-types; ingen pgEnum, inga nya tabeller).
+   - Emitta `COOKING_SESSION_UNDONE` och analytics-event `cooking_session_undone` (consent-gatat via `trackProductAnalytics`, ingen fritext).
+   - Aktiveringsmilstolpar backas INTE (once-ever).
+4. Antagandeprofiler rullas tillbaka deterministiskt via `lastSessionAnswers.sessionId`:
+   - `rollbackCookingAssumptionProfile(sessionId, existingProfile)` är en ren funktion i `packages/inventory-engine/src/cooking-profiles.ts`.
+   - Den tar bort det aktuella sessions-id:t från `lastSessionAnswers`, räknar om EMA från scratch i kronologisk ordning och sätter `observationCount = history.length`.
+   - Om historiken blir tom raderas profilen (inte nollas); det förhindrar dubbelräkning vid undo + ny complete.
+5. Lägg `cookingSessionId` i inventory item detail så användaren kan ångra enskilda transaktioner därifrån också.
 
 **Berörda filer:**
+- `apps/api/src/lib/cooking.ts` (`completeCookingSession`, `undoCookingSession`)
 - `apps/api/src/routes/cooking-sessions.ts`
-- `apps/api/src/routes/inventory.ts` (visa cookingSessionId i transaktionslistan)
-- `packages/inventory-engine/src/fefo.ts` (ev. helper för omräkning)
-- `apps/mobile/src/app/cooking/[id].tsx`
+- `apps/api/src/lib/i18n.ts`
+- `packages/inventory-engine/src/cooking-profiles.ts`
+- `packages/shared-types/src/enums.ts`, `packages/shared-types/src/analytics.ts`
+- `packages/events/src/index.ts`
+- `packages/analytics/src/builders.ts`
+- `apps/mobile/src/locales/*/common.json` (+12)
 
-**Migration:** ingår i 0011 (kolumn cookingSessionId)
+**Migration:** 0014_expand_event_types.sql (nya domänhändelser i `event_type`-enum)
 
 **Testplan:**
 - Laga 4 portioner, ät 3 → lager innehåller 25 % kvar av varje ingrediens
 - Undo → allt återställs
-- Invariant: `computeBalance(transaktioner) === item.quantity` efter complete och efter undo
+- Invariant: `computeBalance(transaktioner) === item.quantity` efter (a) complete, (b) undo, (c) undo + ny complete
+- Append-only-bevis: antalet transaktioner ökar vid undo
+- Undo efter 24 h → 409 med lokaliserat fel
+- Legacy `/cook` → undo fungerar via samma kodväg
 
 ---
 
diff --git a/infrastructure/migrations/0014_expand_event_types.sql b/infrastructure/migrations/0014_expand_event_types.sql
new file mode 100644
index 0000000..e8297a5
--- /dev/null
+++ b/infrastructure/migrations/0014_expand_event_types.sql
@@ -0,0 +1,4 @@
+-- Expandera event_type-enum med nya domänhändelser för 3c (undo/partiell förbrukning).
+ALTER TYPE "event_type" ADD VALUE IF NOT EXISTS 'MEAL_BOX_DISCARDED';
+ALTER TYPE "event_type" ADD VALUE IF NOT EXISTS 'MEAL_REMOVED';
+ALTER TYPE "event_type" ADD VALUE IF NOT EXISTS 'COOKING_SESSION_UNDONE';
diff --git a/infrastructure/migrations/meta/_journal.json b/infrastructure/migrations/meta/_journal.json
index 8c01350..5e75501 100644
--- a/infrastructure/migrations/meta/_journal.json
+++ b/infrastructure/migrations/meta/_journal.json
@@ -92,6 +92,13 @@
       "when": 1786049332540,
       "tag": "0013_dark_the_anarchist",
       "breakpoints": true
+    },
+    {
+      "idx": 13,
+      "version": "7",
+      "when": 1786055900000,
+      "tag": "0014_expand_event_types",
+      "breakpoints": true
     }
   ]
 }
\ No newline at end of file
diff --git a/packages/analytics/src/builders.ts b/packages/analytics/src/builders.ts
index 69d6150..843e912 100644
--- a/packages/analytics/src/builders.ts
+++ b/packages/analytics/src/builders.ts
@@ -57,6 +57,7 @@ export const recipeSaved = builder("recipe_saved");
 export const cookingSessionStarted = builder("cooking_session_started");
 export const cookingSessionCompleted = builder("cooking_session_completed");
 export const cookingSessionCancelled = builder("cooking_session_cancelled");
+export const cookingSessionUndone = builder("cooking_session_undone");
 export const leftoversCreated = builder("leftovers_created");
 export const substitutionConfirmed = builder("substitution_confirmed");
 
diff --git a/packages/events/src/index.ts b/packages/events/src/index.ts
index bd5e8eb..e68910d 100644
--- a/packages/events/src/index.ts
+++ b/packages/events/src/index.ts
@@ -42,6 +42,13 @@ export interface EventPayloadMap {
   WEEK_PLAN_UPDATED: { weekPlanId: string; reason?: string | null };
   MEAL_BOX_CREATED: { mealBoxId: string; portions: number };
   MEAL_BOX_CONSUMED: { mealBoxId: string; portions: number };
+  MEAL_BOX_DISCARDED: { mealBoxId: string; portions: number; source: string };
+  MEAL_REMOVED: { mealId: string; recipeId: string; source: string };
+  COOKING_SESSION_UNDONE: {
+    cookingSessionId: string;
+    recipeId: string;
+    reversedTransactions: number;
+  };
 }
 
 /** Kontroll i kompileringstid att kartan täcker alla EventType. */
diff --git a/packages/inventory-engine/src/cooking-profiles.ts b/packages/inventory-engine/src/cooking-profiles.ts
index f83b6d0..48f648d 100644
--- a/packages/inventory-engine/src/cooking-profiles.ts
+++ b/packages/inventory-engine/src/cooking-profiles.ts
@@ -55,6 +55,51 @@ export function updateCookingAssumptionProfile(
   return { averageEatenPortions, averageLeftoverPortions, observationCount, lastSessionAnswers };
 }
 
+/**
+ * Återställ en antagandeprofil genom att ta bort ett specifikt sessions-id
+ * från historiken och räkna om medelvärdena deterministiskt från resterande
+ * observationer. Ren funktion – alla beroenden är explicita argument.
+ */
+export function rollbackCookingAssumptionProfile(
+  sessionId: string,
+  existing: ExistingProfile,
+): {
+  averageEatenPortions: number | null;
+  averageLeftoverPortions: number | null;
+  observationCount: number;
+  lastSessionAnswers: Array<{ sessionId: string; eaten: number; leftovers: number; date: string }>;
+} {
+  const history = (existing.lastSessionAnswers ?? []).filter((a) => a.sessionId !== sessionId);
+
+  if (history.length === 0) {
+    return {
+      averageEatenPortions: null,
+      averageLeftoverPortions: null,
+      observationCount: 0,
+      lastSessionAnswers: [],
+    };
+  }
+
+  // Räkna om EMA från scratch i kronologisk ordning (äldst först).
+  const chronological = [...history].reverse();
+  let avgEaten = chronological[0]!.eaten;
+  let avgLeftovers = chronological[0]!.leftovers;
+
+  for (let i = 1; i < chronological.length; i++) {
+    const alpha = 1 / (i + 1);
+    const a = chronological[i]!;
+    avgEaten = round2(avgEaten + alpha * (a.eaten - avgEaten));
+    avgLeftovers = round2(avgLeftovers + alpha * (a.leftovers - avgLeftovers));
+  }
+
+  return {
+    averageEatenPortions: avgEaten,
+    averageLeftoverPortions: avgLeftovers,
+    observationCount: history.length,
+    lastSessionAnswers: history,
+  };
+}
+
 function clamp(n: number, min: number, max: number): number {
   return Math.max(min, Math.min(max, n));
 }
diff --git a/packages/inventory-engine/test/cooking-profiles.test.ts b/packages/inventory-engine/test/cooking-profiles.test.ts
index bf6b8ea..2a33dbe 100644
--- a/packages/inventory-engine/test/cooking-profiles.test.ts
+++ b/packages/inventory-engine/test/cooking-profiles.test.ts
@@ -1,5 +1,8 @@
 import { describe, expect, it } from "vitest";
-import { updateCookingAssumptionProfile } from "../src/cooking-profiles.js";
+import {
+  updateCookingAssumptionProfile,
+  rollbackCookingAssumptionProfile,
+} from "../src/cooking-profiles.js";
 
 describe("updateCookingAssumptionProfile", () => {
   it("initializes profile from first observation", () => {
@@ -61,3 +64,104 @@ describe("updateCookingAssumptionProfile", () => {
     expect(existing.lastSessionAnswers![0]!.sessionId).toBe("s11");
   });
 });
+
+describe("rollbackCookingAssumptionProfile", () => {
+  it("returns null averages and zero count when history becomes empty", () => {
+    const existing = updateCookingAssumptionProfile(
+      {
+        householdId: "h1",
+        canonicalIngredientId: "i1",
+        plannedPortions: 4,
+        actualPortionsEaten: 3,
+        leftoverEstimatePortions: 1,
+        sessionId: "s1",
+        date: "2026-08-07",
+      },
+      { averageEatenPortions: null, averageLeftoverPortions: null, observationCount: 0 },
+    );
+    const rolled = rollbackCookingAssumptionProfile("s1", existing);
+    expect(rolled.averageEatenPortions).toBeNull();
+    expect(rolled.averageLeftoverPortions).toBeNull();
+    expect(rolled.observationCount).toBe(0);
+    expect(rolled.lastSessionAnswers).toHaveLength(0);
+  });
+
+  it("recalculates averages deterministically from remaining history", () => {
+    let existing: {
+      averageEatenPortions: number | null;
+      averageLeftoverPortions: number | null;
+      observationCount: number;
+      lastSessionAnswers?: Array<{ sessionId: string; eaten: number; leftovers: number; date: string }> | null;
+    } = { averageEatenPortions: null, averageLeftoverPortions: null, observationCount: 0 };
+
+    // Three chronological observations.
+    const observations = [
+      { sessionId: "s1", eaten: 4, leftovers: 0 },
+      { sessionId: "s2", eaten: 2, leftovers: 1 },
+      { sessionId: "s3", eaten: 3, leftovers: 1 },
+    ];
+    for (const obs of observations) {
+      existing = updateCookingAssumptionProfile(
+        {
+          householdId: "h1",
+          canonicalIngredientId: "i1",
+          plannedPortions: 4,
+          actualPortionsEaten: obs.eaten,
+          leftoverEstimatePortions: obs.leftovers,
+          sessionId: obs.sessionId,
+          date: "2026-08-07",
+        },
+        existing,
+      );
+    }
+
+    // Roll back the middle observation and recompute from scratch.
+    const rolled = rollbackCookingAssumptionProfile("s2", existing);
+    expect(rolled.lastSessionAnswers).toHaveLength(2);
+    expect(rolled.lastSessionAnswers!.map((a) => a.sessionId)).toContain("s1");
+    expect(rolled.lastSessionAnswers!.map((a) => a.sessionId)).toContain("s3");
+
+    // Recompute EMA manually from s1 then s3.
+    // s1: avgEaten=4, avgLeftovers=0
+    // s3: alpha=1/2 => eaten=4+0.5*(3-4)=3.5, leftovers=0+0.5*(1-0)=0.5
+    expect(rolled.averageEatenPortions).toBe(3.5);
+    expect(rolled.averageLeftoverPortions).toBe(0.5);
+    expect(rolled.observationCount).toBe(2);
+  });
+
+  it("is idempotent: rolling back the same session twice leaves history unchanged", () => {
+    let existing: {
+      averageEatenPortions: number | null;
+      averageLeftoverPortions: number | null;
+      observationCount: number;
+      lastSessionAnswers?: Array<{ sessionId: string; eaten: number; leftovers: number; date: string }> | null;
+    } = { averageEatenPortions: null, averageLeftoverPortions: null, observationCount: 0 };
+    existing = updateCookingAssumptionProfile(
+      {
+        householdId: "h1",
+        canonicalIngredientId: "i1",
+        plannedPortions: 4,
+        actualPortionsEaten: 3,
+        leftoverEstimatePortions: 1,
+        sessionId: "s1",
+        date: "2026-08-07",
+      },
+      existing,
+    );
+    existing = updateCookingAssumptionProfile(
+      {
+        householdId: "h1",
+        canonicalIngredientId: "i1",
+        plannedPortions: 4,
+        actualPortionsEaten: 2,
+        leftoverEstimatePortions: 0,
+        sessionId: "s2",
+        date: "2026-08-07",
+      },
+      existing,
+    );
+    const first = rollbackCookingAssumptionProfile("s1", existing);
+    const second = rollbackCookingAssumptionProfile("s1", first);
+    expect(second).toEqual(first);
+  });
+});
diff --git a/packages/shared-types/src/analytics.ts b/packages/shared-types/src/analytics.ts
index 7583e0c..422711c 100644
--- a/packages/shared-types/src/analytics.ts
+++ b/packages/shared-types/src/analytics.ts
@@ -53,6 +53,7 @@ export const ANALYTICS_COOKING_EVENT_NAMES = [
   "cooking_session_started",
   "cooking_session_completed",
   "cooking_session_cancelled",
+  "cooking_session_undone",
   "leftovers_created",
   "substitution_confirmed",
 ] as const;
diff --git a/packages/shared-types/src/enums.ts b/packages/shared-types/src/enums.ts
index 00be2d8..9c9edd2 100644
--- a/packages/shared-types/src/enums.ts
+++ b/packages/shared-types/src/enums.ts
@@ -413,6 +413,9 @@ export const EVENT_TYPES = [
   "WEEK_PLAN_UPDATED",
   "MEAL_BOX_CREATED",
   "MEAL_BOX_CONSUMED",
+  "MEAL_BOX_DISCARDED",
+  "MEAL_REMOVED",
+  "COOKING_SESSION_UNDONE",
 ] as const;
 export type EventType = (typeof EVENT_TYPES)[number];
 
@@ -433,6 +436,20 @@ export const MEAL_LOG_SOURCES = [
 ] as const;
 export type MealLogSource = (typeof MEAL_LOG_SOURCES)[number];
 
+/** Cooking session lifecycle (Fas 3 §6). TEXT i databasen, valideras mot denna const. */
+export const COOKING_SESSION_STATUSES = [
+  "planned",
+  "started",
+  "completed",
+  "cancelled",
+  "undone",
+] as const;
+export type CookingSessionStatus = (typeof COOKING_SESSION_STATUSES)[number];
+
+/** Matlådestatuser (spec §24). TEXT i databasen, valideras mot denna const. */
+export const MEAL_BOX_STATUSES = ["available", "consumed", "discarded"] as const;
+export type MealBoxStatus = (typeof MEAL_BOX_STATUSES)[number];
+
 // ---------------------------------------------------------------------------
 // Smakprofil & feedback (spec §30)
 // ---------------------------------------------------------------------------