diff --git a/docs/ARCHITECTURE_GUIDE.md b/docs/ARCHITECTURE_GUIDE.md
index 11c44efd3..9a0bf3a2c 100644
--- a/docs/ARCHITECTURE_GUIDE.md
+++ b/docs/ARCHITECTURE_GUIDE.md
@@ -75,7 +75,7 @@ export type DayDietRepository = {
     limit?: number,
   ) => Promise<readonly DayDiet[]>
   fetchDayDietById: (dayId: DayDiet['id']) => Promise<DayDiet | null>
-  insertDayDiet: (newDay: NewDayDiet) => Promise<DayDiet | null>
+  insertDayDiet: (newDay: NewDayDiet) => Promise<DayDiet>
   updateDayDietById: (
     dayId: DayDiet['id'],
     newDay: NewDayDiet,
@@ -84,6 +84,8 @@ export type DayDietRepository = {
 }
 ```
 
+`insertDayDiet` returns the created `DayDiet` and surfaces errors instead of returning `null`; callers should treat failures as exceptions rather than nullable results.
+
 ### `infrastructure/supabase/supabaseDayGateway.ts`
 
 ```ts
@@ -253,8 +255,8 @@ export async function fetchTargetDay(
   await dayRepository.fetchDayDietByUserIdAndTargetDay(userId, targetDay);
 }
 
-export async function insertDayDiet(dayDiet: NewDayDiet): Promise<void> {
-  await showPromise(
+export async function insertDayDiet(dayDiet: NewDayDiet): Promise<DayDiet> {
+  return await showPromise(
     dayRepository.insertDayDiet(dayDiet),
     {
       loading: 'Criando dia de dieta...',
@@ -501,8 +503,8 @@ User operations with toast integration.
 ```typescript
 const dayRepository = createDayDietRepository()
 
-export async function insertDayDiet(dayDiet: NewDayDiet): Promise<void> {
-  await showPromise(
+export async function insertDayDiet(dayDiet: NewDayDiet): Promise<DayDiet> {
+  return await showPromise(
     dayRepository.insertDayDiet(dayDiet),
     {
       loading: 'Criando dia de dieta...',
diff --git a/src/modules/diet/day-diet/application/usecases/dayUseCases.ts b/src/modules/diet/day-diet/application/usecases/dayUseCases.ts
index 75888cf33..53b1dbcde 100644
--- a/src/modules/diet/day-diet/application/usecases/dayUseCases.ts
+++ b/src/modules/diet/day-diet/application/usecases/dayUseCases.ts
@@ -175,13 +175,11 @@ export function createDayUseCases(deps: {
             },
             { context: 'user-action' },
           )
-          if (insertedDayDiet !== null) {
-            dayCacheStore.upsertToCache(insertedDayDiet)
-          }
+          dayCacheStore.upsertToCache(insertedDayDiet)
           return insertedDayDiet
         } catch (error) {
           logging.error('DayDiet insert error:', error)
-          return null
+          throw error
         }
       },
       updateDayDietById: async (dayId: DayDiet['id'], dayDiet: NewDayDiet) => {
diff --git a/src/modules/diet/day-diet/domain/dayDietGateway.ts b/src/modules/diet/day-diet/domain/dayDietGateway.ts
index f2b18ff74..7d092a4ff 100644
--- a/src/modules/diet/day-diet/domain/dayDietGateway.ts
+++ b/src/modules/diet/day-diet/domain/dayDietGateway.ts
@@ -15,7 +15,7 @@ export type DayGateway = {
     limit?: number,
   ) => Promise<readonly DayDiet[]>
   fetchDayDietById: (dayId: DayDiet['id']) => Promise<DayDiet | null>
-  insertDayDiet: (newDay: NewDayDiet) => Promise<DayDiet | null>
+  insertDayDiet: (newDay: NewDayDiet) => Promise<DayDiet>
   updateDayDietById: (
     dayId: DayDiet['id'],
     newDay: NewDayDiet,
diff --git a/src/modules/diet/day-diet/domain/dayDietRepository.ts b/src/modules/diet/day-diet/domain/dayDietRepository.ts
index 38720ae45..b9c01aca6 100644
--- a/src/modules/diet/day-diet/domain/dayDietRepository.ts
+++ b/src/modules/diet/day-diet/domain/dayDietRepository.ts
@@ -15,7 +15,7 @@ export type DayRepository = {
     limit?: number,
   ) => Promise<readonly DayDiet[]>
   fetchDayDietById: (dayId: DayDiet['id']) => Promise<DayDiet | null>
-  insertDayDiet: (newDay: NewDayDiet) => Promise<DayDiet | null> // TODO: Remove nullability from insertDay
+  insertDayDiet: (newDay: NewDayDiet) => Promise<DayDiet>
   updateDayDietById: (
     dayId: DayDiet['id'],
     newDay: NewDayDiet,
diff --git a/src/modules/diet/day-diet/infrastructure/guest/guestDayGateway.ts b/src/modules/diet/day-diet/infrastructure/guest/guestDayGateway.ts
index f3c2e818d..0bc3d7d87 100644
--- a/src/modules/diet/day-diet/infrastructure/guest/guestDayGateway.ts
+++ b/src/modules/diet/day-diet/infrastructure/guest/guestDayGateway.ts
@@ -75,7 +75,7 @@ async function fetchDayDietsByUserIdBeforeDate(
   return dayDiets
 }
 
-async function insertDayDiet(newDay: NewDayDiet): Promise<DayDiet | null> {
+async function insertDayDiet(newDay: NewDayDiet): Promise<DayDiet> {
   const dayDiet = promoteDayDiet(newDay, { id: generateDayId() })
 
   updateGuestDatabase((db) => ({
diff --git a/src/modules/diet/day-diet/infrastructure/supabase/supabaseDayGateway.ts b/src/modules/diet/day-diet/infrastructure/supabase/supabaseDayGateway.ts
index 994d9c430..4c7b2641b 100644
--- a/src/modules/diet/day-diet/infrastructure/supabase/supabaseDayGateway.ts
+++ b/src/modules/diet/day-diet/infrastructure/supabase/supabaseDayGateway.ts
@@ -371,7 +371,7 @@ async function fetchDayDietsByUserIdBeforeDate(
   return dayDTOs.map((dto) => dayToDomain(dto))
 }
 
-async function insertDayDiet(newDay: NewDayDiet): Promise<DayDiet | null> {
+async function insertDayDiet(newDay: NewDayDiet): Promise<DayDiet> {
   const newDayDTO = dayToInsertDTO(newDay)
 
   const { data: dayDTO, error } = await supabase
@@ -384,6 +384,14 @@ async function insertDayDiet(newDay: NewDayDiet): Promise<DayDiet | null> {
     throw wrapErrorWithStack(error)
   }
 
+  // Supabase types mark `data` as non-null after `.single()`, but keep a runtime guard for safety.
+  // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
+  if (dayDTO === null) {
+    throw new Error('Failed to insert day diet: Supabase returned no data', {
+      cause: { newDayDTO },
+    })
+  }
+
   return dayToDomain(dayDTO)
 }
 
diff --git a/src/modules/diet/day-diet/tests/infrastructure/guestDayGateway.test.ts b/src/modules/diet/day-diet/tests/infrastructure/guestDayGateway.test.ts
new file mode 100644
index 000000000..c70c868a7
--- /dev/null
+++ b/src/modules/diet/day-diet/tests/infrastructure/guestDayGateway.test.ts
@@ -0,0 +1,37 @@
+import { beforeEach, describe, expect, it } from 'vitest'
+
+import { createNewDayDiet } from '~/modules/diet/day-diet/domain/dayDiet'
+import { createDefaultMeals } from '~/modules/diet/day-diet/domain/defaultMeals'
+import { createGuestDayGateway } from '~/modules/diet/day-diet/infrastructure/guest/guestDayGateway'
+import {
+  getGuestDatabase,
+  resetGuestDatabase,
+} from '~/shared/guest/guestDatabase'
+
+describe('guestDayGateway.insertDayDiet', () => {
+  beforeEach(() => {
+    resetGuestDatabase()
+  })
+
+  it('returns the created day diet and persists it', async () => {
+    const gateway = createGuestDayGateway()
+    const userId = getGuestDatabase().user.uuid
+    const newDayDiet = createNewDayDiet({
+      target_day: '2099-01-02',
+      user_id: userId,
+      meals: createDefaultMeals(),
+    })
+
+    const insertedDayDiet = await gateway.insertDayDiet(newDayDiet)
+
+    expect(insertedDayDiet).toMatchObject({
+      target_day: '2099-01-02',
+      user_id: userId,
+      __type: 'DayDiet',
+    })
+    expect(typeof insertedDayDiet.id).toBe('number')
+    expect(
+      getGuestDatabase().dayDiets.some((day) => day.id === insertedDayDiet.id),
+    ).toBe(true)
+  })
+})