diff --git a/apps/api/src/lib/cooking.ts b/apps/api/src/lib/cooking.ts
index 7a5dbd5..db1d32a 100644
--- a/apps/api/src/lib/cooking.ts
+++ b/apps/api/src/lib/cooking.ts
@@ -97,7 +97,13 @@ export async function completeCookingSession(
     app,
     session,
     userId,
-    { ...input, portionsCooked: plannedPortions, mealBoxPortions },
+    {
+      ...input,
+      portionsCooked: plannedPortions,
+      mealBoxPortions,
+      actualPortionsEaten,
+      leftoverEstimatePortions,
+    },
     correlationId,
   );
 
diff --git a/apps/api/test/cooking-sessions.test.ts b/apps/api/test/cooking-sessions.test.ts
index 59e0de5..ae0f092 100644
--- a/apps/api/test/cooking-sessions.test.ts
+++ b/apps/api/test/cooking-sessions.test.ts
@@ -1030,6 +1030,94 @@ describe("cooking sessions", () => {
     await testDb.db.delete(schema.recipes).where(eq(schema.recipes.id, testRecipeId));
   });
 
+  it("defaults leftover estimate to meal box portions: 3 eaten + 1 box = 100% deduction", async () => {
+    const [recipe] = await testDb.db
+      .insert(schema.recipes)
+      .values({
+        slug: `default-leftover-${Date.now()}`,
+        titleSv: "Default leftover test",
+        descriptionSv: "",
+        cuisine: "international",
+        mealTypes: ["dinner"],
+        tags: [],
+        methods: [],
+        equipment: [],
+        difficulty: "easy",
+        prepTimeMinutes: 5,
+        cookTimeMinutes: 10,
+        totalTimeMinutes: 15,
+        portions: 4,
+        nutritionPerPortion: { kcal: 100, proteinG: 5, fatG: 3, carbsG: 12, saturatedFatG: 1, fiberG: 1, sugarG: 2, saltG: 0.1 },
+        allergens: [],
+        spiceLevel: 0,
+        dna: { cuisine: "international", vegetables: [], flavorProfile: [], spiceLevel: 0, method: "stovetop", timeMinutes: 15, calories: 100, proteinGrams: 5 },
+        status: "published",
+        verificationStatus: "unverified",
+        sourceType: "own_editorial",
+        creatorDisplayName: "Test",
+      })
+      .returning();
+    const testRecipeId = recipe!.id;
+    await testDb.db.insert(schema.recipeIngredients).values({
+      recipeId: testRecipeId,
+      canonicalIngredientId: "potato",
+      displayNameSv: "Potatis",
+      quantity: 400,
+      unit: "GRAM",
+      optional: false,
+      sortOrder: 0,
+    });
+
+    await testDb.db
+      .delete(schema.inventoryItems)
+      .where(and(eq(schema.inventoryItems.householdId, householdId), eq(schema.inventoryItems.canonicalIngredientId, "potato")));
+    const itemId = await createItemWithPurchase("potato", "Potatis", 400, "GRAM");
+
+    const start = await app.inject({
+      method: "POST",
+      url: `/v1/recipes/${testRecipeId}/cook/start`,
+      headers: { authorization: `Bearer ${token}` },
+      payload: { startNow: true, portions: 4 },
+    });
+    const sessionId = (JSON.parse(start.body) as { session: { id: string } }).session.id;
+
+    const complete = await app.inject({
+      method: "POST",
+      url: `/v1/cooking-sessions/${sessionId}/complete`,
+      headers: { authorization: `Bearer ${token}` },
+      // leftoverEstimatePortions UTELÄMNAD – wrapper ska defaulta till mealBoxPortions=1.
+      payload: { mealBoxPortions: 1, actualPortionsEaten: 3 },
+    });
+    expect(complete.statusCode).toBe(200);
+
+    // 3 ätna + 1 matlåda = 4 tillagade → allt 400 g borta.
+    const item = await testDb.db
+      .select({ quantity: schema.inventoryItems.quantity })
+      .from(schema.inventoryItems)
+      .where(eq(schema.inventoryItems.id, itemId))
+      .limit(1);
+    expect(item[0]!.quantity).toBeCloseTo(0, 1);
+
+    // Lådan ska ha 1 portion.
+    const boxes = await testDb.db.select().from(schema.mealBoxes).where(eq(schema.mealBoxes.cookingSessionId, sessionId));
+    expect(boxes.length).toBe(1);
+    expect(boxes[0]!.portions).toBe(1);
+
+    // Invariant.
+    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);
+
+    await testDb.db
+      .delete(schema.inventoryItems)
+      .where(and(eq(schema.inventoryItems.householdId, householdId), eq(schema.inventoryItems.canonicalIngredientId, "potato")));
+    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)