diff --git a/src/messages/en.ts b/src/messages/en.ts
index 61bf7e4..d87def5 100644
--- a/src/messages/en.ts
+++ b/src/messages/en.ts
@@ -47,6 +47,20 @@ export default {
   },
   validation: {
     max_char:
-      'no character available | 1 character max | {max} character maximum'
+      'no character available | 1 character max | {max} character maximum',
+    transaction_name_required: 'Transaction name is required.',
+    amount_required: 'Amount is required.',
+    amount_must_be_positive: 'Amount must be greater than 0.',
+    amount_too_large: 'Amount must be less than {max}.',
+    currency_required: 'Currency is required.',
+    date_required: 'Date is required.',
+    pay_by_required: 'Payer is required.',
+    pay_for_required: 'At least one share must be assigned.',
+    user_id_required: 'A username is required.',
+    passwords_mismatch: 'Passwords must match.',
+    account_name_required: 'Account name is required.',
+    account_needs_one_user: 'At least one person must be added to the account.',
+    aliases_must_be_unique: 'Aliases must be unique.',
+    user_ids_must_be_unique: 'Usernames must be unique.'
   }
 }
diff --git a/src/messages/fr.ts b/src/messages/fr.ts
index be53fbe..7262c79 100644
--- a/src/messages/fr.ts
+++ b/src/messages/fr.ts
@@ -64,6 +64,20 @@ export default {
     pseudo: 'identifiant'
   },
   validation: {
-    max_char: 'limite atteinte | 1 caractère maximum | {max} caractères maximum'
+    max_char: 'limite atteinte | 1 caractère maximum | {max} caractères maximum',
+    transaction_name_required: 'Le nom de la dépense est requis.',
+    amount_required: 'Le montant est requis.',
+    amount_must_be_positive: 'Le montant doit être supérieur à 0.',
+    amount_too_large: 'Le montant doit être inférieur à {max}.',
+    currency_required: 'La devise est requise.',
+    date_required: 'La date est requise.',
+    pay_by_required: 'Le payeur est requis.',
+    pay_for_required: 'Au moins une part doit être attribuée.',
+    user_id_required: 'Un pseudo est obligatoire.',
+    passwords_mismatch: 'Les mots de passe doivent être identiques.',
+    account_name_required: 'Le nom du compte est requis.',
+    account_needs_one_user: 'Au moins une personne doit être ajoutée au compte.',
+    aliases_must_be_unique: 'Les alias doivent être uniques.',
+    user_ids_must_be_unique: 'Les identifiants doivent être uniques.'
   }
 }
diff --git a/src/schemas/account.ts b/src/schemas/account.ts
new file mode 100644
index 0000000..73b430c
--- /dev/null
+++ b/src/schemas/account.ts
@@ -0,0 +1,20 @@
+import { type } from 'arktype'
+
+export const accountFormSchema = type({
+  name: 'unknown',
+  aliases: 'unknown',
+  userIds: 'unknown'
+}).narrow((data, ctx) => {
+  if (typeof data.name !== 'string' || data.name.trim().length === 0) {
+    ctx.reject({ message: 'validation.account_name_required', path: ['name'] })
+  }
+  if (!Array.isArray(data.aliases) || data.aliases.length === 0) {
+    ctx.reject({ message: 'validation.account_needs_one_user', path: ['aliases'] })
+  } else if (new Set(data.aliases).size !== data.aliases.length) {
+    ctx.reject({ message: 'validation.aliases_must_be_unique', path: ['aliases'] })
+  }
+  if (Array.isArray(data.userIds) && new Set(data.userIds).size !== data.userIds.length) {
+    ctx.reject({ message: 'validation.user_ids_must_be_unique', path: ['userIds'] })
+  }
+  return !ctx.hasError()
+})
diff --git a/src/schemas/transaction.ts b/src/schemas/transaction.ts
new file mode 100644
index 0000000..1b3f1a4
--- /dev/null
+++ b/src/schemas/transaction.ts
@@ -0,0 +1,24 @@
+import { type } from 'arktype'
+
+export const MAX_TRANSACTION_AMOUNT = 1_000_000
+
+export const transactionFormSchema = type({
+  name: 'unknown',
+  amount: 'unknown',
+  payBy: 'unknown'
+}).narrow((data, ctx) => {
+  if (typeof data.name !== 'string' || data.name.trim().length === 0) {
+    ctx.reject({ message: 'validation.transaction_name_required', path: ['name'] })
+  }
+  if (typeof data.amount !== 'number' || Number.isNaN(data.amount)) {
+    ctx.reject({ message: 'validation.amount_required', path: ['amount'] })
+  } else if (data.amount <= 0) {
+    ctx.reject({ message: 'validation.amount_must_be_positive', path: ['amount'] })
+  } else if (data.amount > MAX_TRANSACTION_AMOUNT) {
+    ctx.reject({ message: 'validation.amount_too_large', path: ['amount'] })
+  }
+  if (typeof data.payBy !== 'string' || data.payBy.length === 0) {
+    ctx.reject({ message: 'validation.pay_by_required', path: ['payBy'] })
+  }
+  return !ctx.hasError()
+})
diff --git a/src/schemas/user.ts b/src/schemas/user.ts
new file mode 100644
index 0000000..10af7cb
--- /dev/null
+++ b/src/schemas/user.ts
@@ -0,0 +1,15 @@
+import { type } from 'arktype'
+
+export const signupFormSchema = type({
+  userId: 'unknown',
+  password: 'unknown',
+  confirmPassword: 'unknown'
+}).narrow((data, ctx) => {
+  if (typeof data.userId !== 'string' || data.userId.trim().length === 0) {
+    ctx.reject({ message: 'validation.user_id_required', path: ['userId'] })
+  }
+  if (data.password !== data.confirmPassword) {
+    ctx.reject({ message: 'validation.passwords_mismatch', path: ['confirmPassword'] })
+  }
+  return !ctx.hasError()
+})
diff --git a/src/utils/validate.ts b/src/utils/validate.ts
new file mode 100644
index 0000000..b48e38c
--- /dev/null
+++ b/src/utils/validate.ts
@@ -0,0 +1,29 @@
+import { ArkErrors } from 'arktype'
+import type { Type } from 'arktype'
+import type { Composer } from 'vue-i18n'
+import queueNotifService from '@/services/QueueNotifService'
+
+type TranslateFn = Composer['t']
+type MessageParams = Record<string, unknown>
+export type ParamsByPath = Record<string, MessageParams>
+
+export function validateAndNotify(
+  schema: Type,
+  data: unknown,
+  t: TranslateFn,
+  paramsByPath: ParamsByPath = {}
+): boolean {
+  const result = schema(data)
+  if (!(result instanceof ArkErrors)) return true
+
+  const seen = new Set<string>()
+  for (const error of result) {
+    const pathKey = error.path.join('.')
+    // Skip duplicate field-level errors at the same path so the user gets one toast per field.
+    if (seen.has(pathKey)) continue
+    seen.add(pathKey)
+    const params = paramsByPath[pathKey] ?? {}
+    queueNotifService.error(t(error.message, params))
+  }
+  return false
+}