diff --git a/apps/web/app/(app)/workspaces/[workspaceId]/surveys/[surveyId]/(analysis)/summary/actions.ts b/apps/web/app/(app)/workspaces/[workspaceId]/surveys/[surveyId]/(analysis)/summary/actions.ts
index de1b9d91ad..b6002cd8d0 100644
--- a/apps/web/app/(app)/workspaces/[workspaceId]/surveys/[surveyId]/(analysis)/summary/actions.ts
+++ b/apps/web/app/(app)/workspaces/[workspaceId]/surveys/[surveyId]/(analysis)/summary/actions.ts
@@ -2,7 +2,7 @@
 
 import { z } from "zod";
 import { ZId } from "@formbricks/types/common";
-import { OperationNotAllowedError, ResourceNotFoundError, UnknownError } from "@formbricks/types/errors";
+import { InvalidInputError, OperationNotAllowedError, ResourceNotFoundError } from "@formbricks/types/errors";
 import { getEmailTemplateHtml } from "@/app/(app)/workspaces/[workspaceId]/surveys/[surveyId]/(analysis)/summary/lib/emailTemplate";
 import { capturePostHogEvent } from "@/lib/posthog";
 import { getSurvey, updateSurvey } from "@/lib/survey/service";
@@ -176,7 +176,7 @@ export const generatePersonalLinksAction = authenticatedActionClient
     );
 
     if (!contactsResult || contactsResult.length === 0) {
-      throw new UnknownError("No contacts found for the selected segment");
+      throw new InvalidInputError("No contacts found for the selected segment");
     }
 
     capturePostHogEvent(
diff --git a/apps/web/i18n.lock b/apps/web/i18n.lock
index 623f510868..f7cd9154db 100644
--- a/apps/web/i18n.lock
+++ b/apps/web/i18n.lock
@@ -1859,6 +1859,7 @@ checksums:
     workspace/contacts/attribute_key_hint: 1a68c6f91e1a5cf9eff811e2e54e92b8
     workspace/contacts/attribute_key_placeholder: 31702e553b3f138a623dbaa42b6f878f
     workspace/contacts/attribute_key_required: 75f22558e9bafe7da2a549e75fab5f75
+    workspace/contacts/attribute_key_reserved_future_default: 2dbd2159bb6883bf56195448789ef72e
     workspace/contacts/attribute_key_safe_identifier_required: aece7d4708065ec5f110b82fc061621d
     workspace/contacts/attribute_label: a5c71bf158481233f8215dbd38cc196b
     workspace/contacts/attribute_label_placeholder: bf5106cb14d2ec0c21e7d8b4ab1f3a93
@@ -1893,6 +1894,7 @@ checksums:
     workspace/contacts/generate_personal_link: 9ac0865f6876d40fe858f94eae781eb8
     workspace/contacts/generate_personal_link_description: b9dbaf9e2d8362505b7e3cfa40f415a6
     workspace/contacts/invalid_csv_column_names: dcb8534e7d4c00b9ea7bdaf389f72328
+    workspace/contacts/invalid_csv_reserved_column_names: 6fef9d55e3dd298fea069404c9aaa474
     workspace/contacts/invalid_date_format: 5bad9730ac5a5bacd0792098f712b1c4
     workspace/contacts/invalid_number_format: bd0422507385f671c3046730a6febc64
     workspace/contacts/no_activity_yet: f88897ac05afd6bf8af0d4834ad24ffc
diff --git a/apps/web/locales/de-DE.json b/apps/web/locales/de-DE.json
index 7422e97042..e05268f087 100644
--- a/apps/web/locales/de-DE.json
+++ b/apps/web/locales/de-DE.json
@@ -1935,6 +1935,7 @@
       "attribute_key_hint": "Nur Kleinbuchstaben, Zahlen und Unterstriche. Muss mit einem Buchstaben beginnen.",
       "attribute_key_placeholder": "z. B. geburtsdatum",
       "attribute_key_required": "Schlüssel ist erforderlich",
+      "attribute_key_reserved_future_default": "Der Schlüssel ist für zukünftige Standardattribute reserviert ({reservedKeys}). Bitte wähle einen anderen Schlüssel.",
       "attribute_key_safe_identifier_required": "Schlüssel muss ein sicherer Identifikator sein: nur Kleinbuchstaben, Zahlen und Unterstriche, und muss mit einem Buchstaben beginnen",
       "attribute_label": "Bezeichnung",
       "attribute_label_placeholder": "z. B. Geburtsdatum",
@@ -1969,6 +1970,7 @@
       "generate_personal_link": "Persönlichen Link erstellen",
       "generate_personal_link_description": "Wähle eine veröffentlichte Umfrage aus, um einen personalisierten Link für diesen Kontakt zu erstellen.",
       "invalid_csv_column_names": "Ungültige CSV-Spaltennamen: {columns}. Spaltennamen, die zu neuen Attributen werden, dürfen nur Kleinbuchstaben, Zahlen und Unterstriche enthalten und müssen mit einem Buchstaben beginnen.",
+      "invalid_csv_reserved_column_names": "Reservierte CSV-Spaltennamen: {columns}. Diese Namen sind für zukünftige Standardattribute ({reservedKeys}) reserviert und können nicht als neue Attribute erstellt werden.",
       "invalid_date_format": "Ungültiges Datumsformat. Bitte verwende ein gültiges Datum.",
       "invalid_number_format": "Ungültiges Zahlenformat. Bitte gib eine gültige Zahl ein.",
       "no_activity_yet": "Noch keine Aktivität",
diff --git a/apps/web/locales/en-US.json b/apps/web/locales/en-US.json
index 7d444dc3dd..3e8e548dd6 100644
--- a/apps/web/locales/en-US.json
+++ b/apps/web/locales/en-US.json
@@ -1935,6 +1935,7 @@
       "attribute_key_hint": "Only lowercase letters, numbers, and underscores. Must start with a letter.",
       "attribute_key_placeholder": "e.g. date_of_birth",
       "attribute_key_required": "Key is required",
+      "attribute_key_reserved_future_default": "Key is reserved for future default attributes ({reservedKeys}). Please choose a different key.",
       "attribute_key_safe_identifier_required": "Key must be a safe identifier: only lowercase letters, numbers, and underscores, and must start with a letter",
       "attribute_label": "Label",
       "attribute_label_placeholder": "e.g. Date of Birth",
@@ -1969,6 +1970,7 @@
       "generate_personal_link": "Generate Personal Link",
       "generate_personal_link_description": "Select a published survey to generate a personalized link for this contact.",
       "invalid_csv_column_names": "Invalid CSV column name(s): {columns}. Column names that will become new attributes must only contain lowercase letters, numbers, and underscores, and must start with a letter.",
+      "invalid_csv_reserved_column_names": "Reserved CSV column name(s): {columns}. These names are reserved for future default attributes ({reservedKeys}) and cannot be created as new attributes.",
       "invalid_date_format": "Invalid date format. Please use a valid date.",
       "invalid_number_format": "Invalid number format. Please enter a valid number.",
       "no_activity_yet": "No activity yet",
diff --git a/apps/web/locales/es-ES.json b/apps/web/locales/es-ES.json
index 9533eb8984..bb3edbef43 100644
--- a/apps/web/locales/es-ES.json
+++ b/apps/web/locales/es-ES.json
@@ -1935,6 +1935,7 @@
       "attribute_key_hint": "Solo letras minúsculas, números y guiones bajos. Debe empezar con una letra.",
       "attribute_key_placeholder": "p. ej. fecha_de_nacimiento",
       "attribute_key_required": "La clave es obligatoria",
+      "attribute_key_reserved_future_default": "La clave está reservada para atributos predeterminados futuros ({reservedKeys}). Por favor, elige una clave diferente.",
       "attribute_key_safe_identifier_required": "La clave debe ser un identificador seguro: solo letras minúsculas, números y guiones bajos, y debe empezar con una letra",
       "attribute_label": "Etiqueta",
       "attribute_label_placeholder": "p. ej. fecha de nacimiento",
@@ -1969,6 +1970,7 @@
       "generate_personal_link": "Generar enlace personal",
       "generate_personal_link_description": "Selecciona una encuesta publicada para generar un enlace personalizado para este contacto.",
       "invalid_csv_column_names": "Nombre(s) de columna CSV no válido(s): {columns}. Los nombres de columna que se convertirán en nuevos atributos solo deben contener letras minúsculas, números y guiones bajos, y deben comenzar con una letra.",
+      "invalid_csv_reserved_column_names": "Nombre(s) de columna CSV reservado(s): {columns}. Estos nombres están reservados para atributos predeterminados futuros ({reservedKeys}) y no se pueden crear como nuevos atributos.",
       "invalid_date_format": "Formato de fecha no válido. Por favor, usa una fecha válida.",
       "invalid_number_format": "Formato de número no válido. Por favor, introduce un número válido.",
       "no_activity_yet": "Aún no hay actividad",
diff --git a/apps/web/locales/fr-FR.json b/apps/web/locales/fr-FR.json
index 5d5c19aba0..c3afb910fc 100644
--- a/apps/web/locales/fr-FR.json
+++ b/apps/web/locales/fr-FR.json
@@ -1935,6 +1935,7 @@
       "attribute_key_hint": "Uniquement des lettres minuscules, des chiffres et des underscores. Doit commencer par une lettre.",
       "attribute_key_placeholder": "ex. date_de_naissance",
       "attribute_key_required": "La clé est requise",
+      "attribute_key_reserved_future_default": "La clé est réservée pour les attributs par défaut futurs ({reservedKeys}). Veuillez choisir une clé différente.",
       "attribute_key_safe_identifier_required": "La clé doit être un identifiant sûr : uniquement des lettres minuscules, des chiffres et des underscores, et doit commencer par une lettre",
       "attribute_label": "Étiquette",
       "attribute_label_placeholder": "ex. Date de naissance",
@@ -1969,6 +1970,7 @@
       "generate_personal_link": "Générer un lien personnel",
       "generate_personal_link_description": "Sélectionnez une enquête publiée pour générer un lien personnalisé pour ce contact.",
       "invalid_csv_column_names": "Nom(s) de colonne CSV invalide(s) : {columns}. Les noms de colonnes qui deviendront de nouveaux attributs ne doivent contenir que des lettres minuscules, des chiffres et des underscores, et doivent commencer par une lettre.",
+      "invalid_csv_reserved_column_names": "Nom(s) de colonne CSV réservé(s) : {columns}. Ces noms sont réservés pour les attributs par défaut futurs ({reservedKeys}) et ne peuvent pas être créés en tant que nouveaux attributs.",
       "invalid_date_format": "Format de date invalide. Merci d'utiliser une date valide.",
       "invalid_number_format": "Format de nombre invalide. Veuillez saisir un nombre valide.",
       "no_activity_yet": "Aucune activité pour le moment",
diff --git a/apps/web/locales/hu-HU.json b/apps/web/locales/hu-HU.json
index f632e6b181..c8da598fa1 100644
--- a/apps/web/locales/hu-HU.json
+++ b/apps/web/locales/hu-HU.json
@@ -1935,6 +1935,7 @@
       "attribute_key_hint": "Csak ékezet nélküli kisbetűk, számok és aláhúzásjelek használhatók. Betűvel kell kezdődnie.",
       "attribute_key_placeholder": "például: szuletesi_ido",
       "attribute_key_required": "A kulcs kötelező",
+      "attribute_key_reserved_future_default": "A kulcs le van foglalva jövőbeli alapértelmezett attribútumok számára ({reservedKeys}). Kérem, válasszon egy másik kulcsot.",
       "attribute_key_safe_identifier_required": "A kulcs csak biztonságos azonosító lehet: csak ékezet nélküli kisbetűk, számok és aláhúzásjelek használhatók, és betűvel kell kezdődnie",
       "attribute_label": "Címke",
       "attribute_label_placeholder": "például: Születési idő",
@@ -1969,6 +1970,7 @@
       "generate_personal_link": "Személyes hivatkozás előállítása",
       "generate_personal_link_description": "Válasszon egy közzétett kérdőívet, hogy személyre szabott hivatkozást állítson elő ehhez a partnerhez.",
       "invalid_csv_column_names": "Érvénytelen CSV-oszlopnevek: {columns}. Az új attribútumokká váló oszlopnevek csak ékezet nélküli kisbetűket, számokat és aláhúzásjeleket tartalmazhatnak, valamint betűvel kell kezdődniük.",
+      "invalid_csv_reserved_column_names": "Fenntartott CSV oszlopnév/nevek: {columns}. Ezek a nevek le vannak foglalva jövőbeli alapértelmezett attribútumok számára ({reservedKeys}), és nem hozhatók létre új attribútumokként.",
       "invalid_date_format": "Érvénytelen dátumformátum. Használjon érvényes dátumot.",
       "invalid_number_format": "Érvénytelen számformátum. Adjon meg érvényes számot.",
       "no_activity_yet": "Még nincs tevékenység",
diff --git a/apps/web/locales/ja-JP.json b/apps/web/locales/ja-JP.json
index f585407b7e..001f9a288c 100644
--- a/apps/web/locales/ja-JP.json
+++ b/apps/web/locales/ja-JP.json
@@ -1935,6 +1935,7 @@
       "attribute_key_hint": "小文字のアルファベット、数字、アンダースコアのみ使用可能です。アルファベットで始める必要があります。",
       "attribute_key_placeholder": "例: date_of_birth",
       "attribute_key_required": "キーは必須です",
+      "attribute_key_reserved_future_default": "このキーは将来のデフォルト属性用に予約されています（{reservedKeys}）。別のキーを選択してください。",
       "attribute_key_safe_identifier_required": "キーは安全な識別子である必要があります: 小文字のアルファベット、数字、アンダースコアのみ使用可能で、アルファベットで始める必要があります",
       "attribute_label": "ラベル",
       "attribute_label_placeholder": "例: 生年月日",
@@ -1969,6 +1970,7 @@
       "generate_personal_link": "個人リンクを生成",
       "generate_personal_link_description": "公開されたフォームを選択して、この連絡先用のパーソナライズされたリンクを生成します。",
       "invalid_csv_column_names": "無効なCSV列名: {columns}。新しい属性となる列名は、小文字、数字、アンダースコアのみを含み、文字で始まる必要があります。",
+      "invalid_csv_reserved_column_names": "予約されたCSV列名: {columns}。これらの名前は将来のデフォルト属性（{reservedKeys}）用に予約されており、新しい属性として作成できません。",
       "invalid_date_format": "無効な日付形式です。有効な日付を使用してください。",
       "invalid_number_format": "無効な数値形式です。有効な数値を入力してください。",
       "no_activity_yet": "まだアクティビティがありません",
diff --git a/apps/web/locales/nl-NL.json b/apps/web/locales/nl-NL.json
index ebddca4dcf..fa7e5308c3 100644
--- a/apps/web/locales/nl-NL.json
+++ b/apps/web/locales/nl-NL.json
@@ -1935,6 +1935,7 @@
       "attribute_key_hint": "Alleen kleine letters, cijfers en onderstrepingstekens. Moet beginnen met een letter.",
       "attribute_key_placeholder": "bijv. geboortedatum",
       "attribute_key_required": "Sleutel is verplicht",
+      "attribute_key_reserved_future_default": "Sleutel is gereserveerd voor toekomstige standaardattributen ({reservedKeys}). Kies een andere sleutel.",
       "attribute_key_safe_identifier_required": "Sleutel moet een veilige identifier zijn: alleen kleine letters, cijfers en onderstrepingstekens, en moet beginnen met een letter",
       "attribute_label": "Label",
       "attribute_label_placeholder": "bijv. Geboortedatum",
@@ -1969,6 +1970,7 @@
       "generate_personal_link": "Persoonlijke link genereren",
       "generate_personal_link_description": "Selecteer een gepubliceerde enquête om een gepersonaliseerde link voor dit contact te genereren.",
       "invalid_csv_column_names": "Ongeldige CSV-kolomna(a)m(en): {columns}. Kolomnamen die nieuwe kenmerken worden, mogen alleen kleine letters, cijfers en underscores bevatten en moeten beginnen met een letter.",
+      "invalid_csv_reserved_column_names": "Gereserveerde CSV-kolomnaam/namen: {columns}. Deze namen zijn gereserveerd voor toekomstige standaardattributen ({reservedKeys}) en kunnen niet als nieuwe attributen worden aangemaakt.",
       "invalid_date_format": "Ongeldig datumformaat. Gebruik een geldige datum.",
       "invalid_number_format": "Ongeldig getalformaat. Voer een geldig getal in.",
       "no_activity_yet": "Nog geen activiteit",
diff --git a/apps/web/locales/pt-BR.json b/apps/web/locales/pt-BR.json
index 5fd8524267..8b263048d3 100644
--- a/apps/web/locales/pt-BR.json
+++ b/apps/web/locales/pt-BR.json
@@ -1935,6 +1935,7 @@
       "attribute_key_hint": "Apenas letras minúsculas, números e underscores. Deve começar com uma letra.",
       "attribute_key_placeholder": "ex: data_de_nascimento",
       "attribute_key_required": "A chave é obrigatória",
+      "attribute_key_reserved_future_default": "A chave está reservada para atributos padrão futuros ({reservedKeys}). Por favor, escolha uma chave diferente.",
       "attribute_key_safe_identifier_required": "A chave deve ser um identificador seguro: apenas letras minúsculas, números e underscores, e deve começar com uma letra",
       "attribute_label": "Etiqueta",
       "attribute_label_placeholder": "ex: Data de nascimento",
@@ -1969,6 +1970,7 @@
       "generate_personal_link": "Gerar link pessoal",
       "generate_personal_link_description": "Selecione uma pesquisa publicada para gerar um link personalizado para este contato.",
       "invalid_csv_column_names": "Nome(s) de coluna CSV inválido(s): {columns}. Os nomes de colunas que se tornarão novos atributos devem conter apenas letras minúsculas, números e sublinhados, e devem começar com uma letra.",
+      "invalid_csv_reserved_column_names": "Nome(s) de coluna CSV reservado(s): {columns}. Esses nomes estão reservados para atributos padrão futuros ({reservedKeys}) e não podem ser criados como novos atributos.",
       "invalid_date_format": "Formato de data inválido. Por favor, use uma data válida.",
       "invalid_number_format": "Formato de número inválido. Por favor, insira um número válido.",
       "no_activity_yet": "Nenhuma atividade ainda",
diff --git a/apps/web/locales/pt-PT.json b/apps/web/locales/pt-PT.json
index 8ba8ee40bd..fe0b4bd31b 100644
--- a/apps/web/locales/pt-PT.json
+++ b/apps/web/locales/pt-PT.json
@@ -1935,6 +1935,7 @@
       "attribute_key_hint": "Apenas letras minúsculas, números e sublinhados. Deve começar com uma letra.",
       "attribute_key_placeholder": "ex. data_de_nascimento",
       "attribute_key_required": "A chave é obrigatória",
+      "attribute_key_reserved_future_default": "A chave está reservada para atributos padrão futuros ({reservedKeys}). Por favor, escolhe uma chave diferente.",
       "attribute_key_safe_identifier_required": "A chave deve ser um identificador seguro: apenas letras minúsculas, números e sublinhados, e deve começar com uma letra",
       "attribute_label": "Etiqueta",
       "attribute_label_placeholder": "ex. Data de nascimento",
@@ -1969,6 +1970,7 @@
       "generate_personal_link": "Gerar Link Pessoal",
       "generate_personal_link_description": "Selecione um inquérito publicado para gerar um link personalizado para este contacto.",
       "invalid_csv_column_names": "Nome(s) de coluna CSV inválido(s): {columns}. Os nomes de colunas que se tornarão novos atributos devem conter apenas letras minúsculas, números e underscores, e devem começar com uma letra.",
+      "invalid_csv_reserved_column_names": "Nome(s) de coluna CSV reservado(s): {columns}. Estes nomes estão reservados para atributos padrão futuros ({reservedKeys}) e não podem ser criados como novos atributos.",
       "invalid_date_format": "Formato de data inválido. Por favor, usa uma data válida.",
       "invalid_number_format": "Formato de número inválido. Por favor, introduz um número válido.",
       "no_activity_yet": "Ainda sem atividade",
diff --git a/apps/web/locales/ro-RO.json b/apps/web/locales/ro-RO.json
index 740cf219a0..366fd4a365 100644
--- a/apps/web/locales/ro-RO.json
+++ b/apps/web/locales/ro-RO.json
@@ -1935,6 +1935,7 @@
       "attribute_key_hint": "Doar litere mici, cifre și caractere de subliniere. Trebuie să înceapă cu o literă.",
       "attribute_key_placeholder": "ex: date_of_birth",
       "attribute_key_required": "Cheia este obligatorie",
+      "attribute_key_reserved_future_default": "Cheia este rezervată pentru atribute implicite viitoare ({reservedKeys}). Te rugăm să alegi o cheie diferită.",
       "attribute_key_safe_identifier_required": "Cheia trebuie să fie un identificator sigur: doar litere mici, cifre și caractere de subliniere, și trebuie să înceapă cu o literă",
       "attribute_label": "Etichetă",
       "attribute_label_placeholder": "ex: Data nașterii",
@@ -1969,6 +1970,7 @@
       "generate_personal_link": "Generează link personal",
       "generate_personal_link_description": "Selectați un sondaj publicat pentru a genera un link personalizat pentru acest contact.",
       "invalid_csv_column_names": "Nume de coloană CSV nevalide: {columns}. Numele coloanelor care vor deveni atribute noi trebuie să conțină doar litere mici, cifre și caractere de subliniere și trebuie să înceapă cu o literă.",
+      "invalid_csv_reserved_column_names": "Nume de coloană CSV rezervate: {columns}. Aceste nume sunt rezervate pentru atribute implicite viitoare ({reservedKeys}) și nu pot fi create ca atribute noi.",
       "invalid_date_format": "Format de dată invalid. Te rugăm să folosești o dată validă.",
       "invalid_number_format": "Format de număr invalid. Te rugăm să introduci un număr valid.",
       "no_activity_yet": "Nicio activitate încă",
diff --git a/apps/web/locales/ru-RU.json b/apps/web/locales/ru-RU.json
index 037edbd56d..1021c92c2b 100644
--- a/apps/web/locales/ru-RU.json
+++ b/apps/web/locales/ru-RU.json
@@ -1935,6 +1935,7 @@
       "attribute_key_hint": "Только строчные буквы, цифры и символы подчёркивания. Должен начинаться с буквы.",
       "attribute_key_placeholder": "например, date_of_birth",
       "attribute_key_required": "Ключ обязателен",
+      "attribute_key_reserved_future_default": "Ключ зарезервирован для будущих атрибутов по умолчанию ({reservedKeys}). Пожалуйста, выбери другой ключ.",
       "attribute_key_safe_identifier_required": "Ключ должен быть безопасным идентификатором: только строчные буквы, цифры и символы подчёркивания, и должен начинаться с буквы",
       "attribute_label": "Метка",
       "attribute_label_placeholder": "например, дата рождения",
@@ -1969,6 +1970,7 @@
       "generate_personal_link": "Сгенерировать персональную ссылку",
       "generate_personal_link_description": "Выберите опубликованный опрос, чтобы сгенерировать персональную ссылку для этого контакта.",
       "invalid_csv_column_names": "Недопустимые имена столбцов в CSV: {columns}. Имена столбцов, которые станут новыми атрибутами, должны содержать только строчные буквы, цифры и подчёркивания, а также начинаться с буквы.",
+      "invalid_csv_reserved_column_names": "Зарезервированные названия столбцов CSV: {columns}. Эти названия зарезервированы для будущих атрибутов по умолчанию ({reservedKeys}) и не могут быть созданы как новые атрибуты.",
       "invalid_date_format": "Неверный формат даты. Пожалуйста, используйте корректную дату.",
       "invalid_number_format": "Неверный формат числа. Пожалуйста, введите корректное число.",
       "no_activity_yet": "Пока нет активности",
diff --git a/apps/web/locales/sv-SE.json b/apps/web/locales/sv-SE.json
index ef65468bc9..1b1abb2cc1 100644
--- a/apps/web/locales/sv-SE.json
+++ b/apps/web/locales/sv-SE.json
@@ -1935,6 +1935,7 @@
       "attribute_key_hint": "Endast små bokstäver, siffror och understreck. Måste börja med en bokstav.",
       "attribute_key_placeholder": "t.ex. date_of_birth",
       "attribute_key_required": "Nyckel krävs",
+      "attribute_key_reserved_future_default": "Nyckeln är reserverad för framtida standardattribut ({reservedKeys}). Välj en annan nyckel.",
       "attribute_key_safe_identifier_required": "Nyckeln måste vara en säker identifierare: endast små bokstäver, siffror och understreck, och måste börja med en bokstav",
       "attribute_label": "Etikett",
       "attribute_label_placeholder": "t.ex. Födelsedatum",
@@ -1969,6 +1970,7 @@
       "generate_personal_link": "Generera personlig länk",
       "generate_personal_link_description": "Välj en publicerad enkät för att generera en personlig länk för denna kontakt.",
       "invalid_csv_column_names": "Ogiltiga CSV-kolumnnamn: {columns}. Kolumnnamn som ska bli nya attribut får bara innehålla små bokstäver, siffror och understreck, och måste börja med en bokstav.",
+      "invalid_csv_reserved_column_names": "Reserverade CSV-kolumnnamn: {columns}. Dessa namn är reserverade för framtida standardattribut ({reservedKeys}) och kan inte skapas som nya attribut.",
       "invalid_date_format": "Ogiltigt datumformat. Ange ett giltigt datum.",
       "invalid_number_format": "Ogiltigt nummerformat. Ange ett giltigt nummer.",
       "no_activity_yet": "Ingen aktivitet än",
diff --git a/apps/web/locales/tr-TR.json b/apps/web/locales/tr-TR.json
index e5b37a7a76..67775528a9 100644
--- a/apps/web/locales/tr-TR.json
+++ b/apps/web/locales/tr-TR.json
@@ -1935,6 +1935,7 @@
       "attribute_key_hint": "Yalnızca küçük harfler, rakamlar ve alt çizgiler. Bir harfle başlamalıdır.",
       "attribute_key_placeholder": "örn. dogum_tarihi",
       "attribute_key_required": "Anahtar gereklidir",
+      "attribute_key_reserved_future_default": "Anahtar, gelecekteki varsayılan özellikler için ayrılmıştır ({reservedKeys}). Lütfen farklı bir anahtar seçin.",
       "attribute_key_safe_identifier_required": "Anahtar güvenli bir tanımlayıcı olmalıdır: yalnızca küçük harfler, rakamlar ve alt çizgiler içermeli ve bir harfle başlamalıdır",
       "attribute_label": "Etiket",
       "attribute_label_placeholder": "örn. Doğum Tarihi",
@@ -1969,6 +1970,7 @@
       "generate_personal_link": "Kişisel Bağlantı Oluştur",
       "generate_personal_link_description": "Bu kişi için kişiselleştirilmiş bir bağlantı oluşturmak üzere yayınlanmış bir anket seç.",
       "invalid_csv_column_names": "Geçersiz CSV sütun adı/adları: {columns}. Yeni özellik olacak sütun adları yalnızca küçük harf, rakam ve alt çizgi içerebilir ve bir harfle başlamalıdır.",
+      "invalid_csv_reserved_column_names": "Ayrılmış CSV sütun adı/adları: {columns}. Bu adlar gelecekteki varsayılan özellikler ({reservedKeys}) için ayrılmıştır ve yeni özellik olarak oluşturulamaz.",
       "invalid_date_format": "Geçersiz tarih formatı. Lütfen geçerli bir tarih kullanın.",
       "invalid_number_format": "Geçersiz sayı formatı. Lütfen geçerli bir sayı girin.",
       "no_activity_yet": "Henüz aktivite yok",
diff --git a/apps/web/locales/zh-Hans-CN.json b/apps/web/locales/zh-Hans-CN.json
index ca4fd3496c..e7e590069d 100644
--- a/apps/web/locales/zh-Hans-CN.json
+++ b/apps/web/locales/zh-Hans-CN.json
@@ -1935,6 +1935,7 @@
       "attribute_key_hint": "仅允许小写字母、数字和下划线，且必须以字母开头。",
       "attribute_key_placeholder": "例如：date_of_birth",
       "attribute_key_required": "键为必填项",
+      "attribute_key_reserved_future_default": "该键已保留用于未来的默认属性（{reservedKeys}）。请选择其他键。",
       "attribute_key_safe_identifier_required": "键必须为安全标识符：仅允许小写字母、数字和下划线，且必须以字母开头",
       "attribute_label": "标签",
       "attribute_label_placeholder": "例如：出生日期",
@@ -1969,6 +1970,7 @@
       "generate_personal_link": "生成个人链接",
       "generate_personal_link_description": "选择一个已发布的调查，为此联系人生成个性化链接。",
       "invalid_csv_column_names": "无效的 CSV 列名：{columns}。作为新属性的列名只能包含小写字母、数字和下划线，并且必须以字母开头。",
+      "invalid_csv_reserved_column_names": "CSV 列名已被保留：{columns}。这些名称已保留用于未来的默认属性（{reservedKeys}），无法创建为新属性。",
       "invalid_date_format": "日期格式无效。请使用有效日期。",
       "invalid_number_format": "数字格式无效。请输入有效的数字。",
       "no_activity_yet": "暂无活动",
diff --git a/apps/web/locales/zh-Hant-TW.json b/apps/web/locales/zh-Hant-TW.json
index bbe53ff8f1..18c780597f 100644
--- a/apps/web/locales/zh-Hant-TW.json
+++ b/apps/web/locales/zh-Hant-TW.json
@@ -1935,6 +1935,7 @@
       "attribute_key_hint": "僅限小寫字母、數字和底線，且必須以字母開頭。",
       "attribute_key_placeholder": "例如：date_of_birth",
       "attribute_key_required": "金鑰為必填項目",
+      "attribute_key_reserved_future_default": "此鍵已保留供未來預設屬性使用（{reservedKeys}）。請選擇其他鍵。",
       "attribute_key_safe_identifier_required": "金鑰必須為安全識別字：僅限小寫字母、數字和底線，且必須以字母開頭",
       "attribute_label": "標籤",
       "attribute_label_placeholder": "例如：出生日期",
@@ -1969,6 +1970,7 @@
       "generate_personal_link": "產生個人連結",
       "generate_personal_link_description": "選擇一個已發佈的問卷，為此聯絡人產生個人化連結。",
       "invalid_csv_column_names": "無效的 CSV 欄位名稱：{columns}。作為新屬性的欄位名稱只能包含小寫字母、數字和底線，且必須以字母開頭。",
+      "invalid_csv_reserved_column_names": "保留的 CSV 欄位名稱：{columns}。這些名稱已保留供未來預設屬性使用（{reservedKeys}），無法建立為新屬性。",
       "invalid_date_format": "日期格式無效。請使用有效的日期。",
       "invalid_number_format": "數字格式無效。請輸入有效的數字。",
       "no_activity_yet": "尚無活動",
diff --git a/apps/web/modules/api/v2/management/contact-attribute-keys/lib/contact-attribute-key.ts b/apps/web/modules/api/v2/management/contact-attribute-keys/lib/contact-attribute-key.ts
index 3dccbaeeda..5eddb03ffe 100644
--- a/apps/web/modules/api/v2/management/contact-attribute-keys/lib/contact-attribute-key.ts
+++ b/apps/web/modules/api/v2/management/contact-attribute-keys/lib/contact-attribute-key.ts
@@ -10,6 +10,10 @@ import {
   TGetContactAttributeKeysFilter,
 } from "@/modules/api/v2/management/contact-attribute-keys/types/contact-attribute-keys";
 import { ApiErrorResponseV2 } from "@/modules/api/v2/types/api-error";
+import {
+  getReservedFutureDefaultAttributeKeyIssue,
+  isReservedFutureDefaultAttributeKey,
+} from "@/modules/ee/contacts/lib/attribute-key-policy";
 
 export const getContactAttributeKeys = reactCache(
   async (workspaceIds: string[], params: TGetContactAttributeKeysFilter) => {
@@ -45,6 +49,13 @@ export const createContactAttributeKey = async (
 ): Promise<Result<ContactAttributeKey, ApiErrorResponseV2>> => {
   const { workspaceId, name, description, key, dataType } = contactAttributeKey;
 
+  if (isReservedFutureDefaultAttributeKey(key)) {
+    return err({
+      type: "bad_request",
+      details: [{ field: "key", issue: getReservedFutureDefaultAttributeKeyIssue([key]) }],
+    });
+  }
+
   try {
     const prismaData: Prisma.ContactAttributeKeyCreateInput = {
       workspace: {
diff --git a/apps/web/modules/api/v2/management/contact-attribute-keys/lib/tests/contact-attribute-key.test.ts b/apps/web/modules/api/v2/management/contact-attribute-keys/lib/tests/contact-attribute-key.test.ts
index 7f2cc84b81..a86ee2190e 100644
--- a/apps/web/modules/api/v2/management/contact-attribute-keys/lib/tests/contact-attribute-key.test.ts
+++ b/apps/web/modules/api/v2/management/contact-attribute-keys/lib/tests/contact-attribute-key.test.ts
@@ -105,6 +105,28 @@ describe("createContactAttributeKey", () => {
     }
   });
 
+  test("returns bad request when key is reserved for future defaults", async () => {
+    const result = await createContactAttributeKey({
+      ...inputContactAttributeKey,
+      key: "user_id",
+    });
+    expect(result.ok).toBe(false);
+    expect(prisma.contactAttributeKey.create).not.toHaveBeenCalled();
+
+    if (!result.ok) {
+      expect(result.error).toStrictEqual({
+        type: "bad_request",
+        details: [
+          {
+            field: "key",
+            issue:
+              "Reserved attribute key(s): user_id. These keys are reserved for the v5.1 safe-identifier default attribute migration and cannot be created as custom attributes.",
+          },
+        ],
+      });
+    }
+  });
+
   test("returns conflict error when key already exists", async () => {
     const errToThrow = new Prisma.PrismaClientKnownRequestError("Mock error message", {
       code: PrismaErrorType.UniqueConstraintViolation,
diff --git a/apps/web/modules/api/v2/management/contact-attribute-keys/types/contact-attribute-keys.ts b/apps/web/modules/api/v2/management/contact-attribute-keys/types/contact-attribute-keys.ts
index 6e812c74fa..36771a9ed3 100644
--- a/apps/web/modules/api/v2/management/contact-attribute-keys/types/contact-attribute-keys.ts
+++ b/apps/web/modules/api/v2/management/contact-attribute-keys/types/contact-attribute-keys.ts
@@ -2,6 +2,10 @@ import { z } from "zod";
 import { ZContactAttributeKey } from "@formbricks/database/zod/contact-attribute-keys";
 import { isSafeIdentifier } from "@/lib/utils/safe-identifier";
 import { ZGetFilter } from "@/modules/api/v2/types/api-filter";
+import {
+  getReservedFutureDefaultAttributeKeyIssue,
+  isReservedFutureDefaultAttributeKey,
+} from "@/modules/ee/contacts/lib/attribute-key-policy";
 
 export const ZGetContactAttributeKeysFilter = ZGetFilter.extend({})
   .refine(
@@ -38,6 +42,14 @@ export const ZContactAttributeKeyInput = ZContactAttributeKey.pick({
         path: ["key"],
       });
     }
+
+    if (isReservedFutureDefaultAttributeKey(data.key)) {
+      ctx.addIssue({
+        code: "custom",
+        message: getReservedFutureDefaultAttributeKeyIssue([data.key]),
+        path: ["key"],
+      });
+    }
   })
   .meta({
     id: "contactAttributeKeyInput",
@@ -65,6 +77,14 @@ export const ZContactAttributeKeyCreateInput = ZContactAttributeKey.pick({
         path: ["key"],
       });
     }
+
+    if (isReservedFutureDefaultAttributeKey(data.key)) {
+      ctx.addIssue({
+        code: "custom",
+        message: getReservedFutureDefaultAttributeKeyIssue([data.key]),
+        path: ["key"],
+      });
+    }
   })
   .meta({
     id: "contactAttributeKeyCreateInput",
diff --git a/apps/web/modules/ee/contacts/api/v1/management/contact-attribute-keys/[contactAttributeKeyId]/lib/contact-attribute-key.ts b/apps/web/modules/ee/contacts/api/v1/management/contact-attribute-keys/[contactAttributeKeyId]/lib/contact-attribute-key.ts
index 494e7181f6..41b4c06027 100644
--- a/apps/web/modules/ee/contacts/api/v1/management/contact-attribute-keys/[contactAttributeKeyId]/lib/contact-attribute-key.ts
+++ b/apps/web/modules/ee/contacts/api/v1/management/contact-attribute-keys/[contactAttributeKeyId]/lib/contact-attribute-key.ts
@@ -3,8 +3,12 @@ import { cache as reactCache } from "react";
 import { prisma } from "@formbricks/database";
 import { ZId } from "@formbricks/types/common";
 import { TContactAttributeKey } from "@formbricks/types/contact-attribute-key";
-import { DatabaseError } from "@formbricks/types/errors";
+import { DatabaseError, InvalidInputError } from "@formbricks/types/errors";
 import { validateInputs } from "@/lib/utils/validate";
+import {
+  getReservedFutureDefaultAttributeKeyIssue,
+  isReservedFutureDefaultAttributeKey,
+} from "@/modules/ee/contacts/lib/attribute-key-policy";
 import {
   TContactAttributeKeyUpdateInput,
   ZContactAttributeKeyUpdateInput,
@@ -56,6 +60,10 @@ export const updateContactAttributeKey = async (
 ): Promise<TContactAttributeKey | null> => {
   validateInputs([contactAttributeKeyId, ZId], [data, ZContactAttributeKeyUpdateInput]);
 
+  if (data.key && isReservedFutureDefaultAttributeKey(data.key)) {
+    throw new InvalidInputError(getReservedFutureDefaultAttributeKeyIssue([data.key]));
+  }
+
   try {
     const contactAttributeKey = await prisma.contactAttributeKey.update({
       where: {
diff --git a/apps/web/modules/ee/contacts/api/v1/management/contact-attribute-keys/[contactAttributeKeyId]/types/contact-attribute-keys.ts b/apps/web/modules/ee/contacts/api/v1/management/contact-attribute-keys/[contactAttributeKeyId]/types/contact-attribute-keys.ts
index 1d55a8ec25..35872932d4 100644
--- a/apps/web/modules/ee/contacts/api/v1/management/contact-attribute-keys/[contactAttributeKeyId]/types/contact-attribute-keys.ts
+++ b/apps/web/modules/ee/contacts/api/v1/management/contact-attribute-keys/[contactAttributeKeyId]/types/contact-attribute-keys.ts
@@ -1,12 +1,21 @@
 import { z } from "zod";
 import { ZContactAttributeDataType } from "@formbricks/types/contact-attribute-key";
 import { isSafeIdentifier } from "@/lib/utils/safe-identifier";
+import {
+  RESERVED_FUTURE_DEFAULT_ATTRIBUTE_KEY_VALIDATION_MESSAGE,
+  isReservedFutureDefaultAttributeKey,
+} from "@/modules/ee/contacts/lib/attribute-key-policy";
 
 export const ZContactAttributeKeyCreateInput = z.object({
-  key: z.string().refine((val) => isSafeIdentifier(val), {
-    error:
-      "Key must be a safe identifier: only lowercase letters, numbers, and underscores, and must start with a letter",
-  }),
+  key: z
+    .string()
+    .refine((val) => isSafeIdentifier(val), {
+      error:
+        "Key must be a safe identifier: only lowercase letters, numbers, and underscores, and must start with a letter",
+    })
+    .refine((val) => !isReservedFutureDefaultAttributeKey(val), {
+      error: RESERVED_FUTURE_DEFAULT_ATTRIBUTE_KEY_VALIDATION_MESSAGE,
+    }),
   description: z.string().optional(),
   type: z.enum(["custom"]),
   dataType: ZContactAttributeDataType.optional(),
@@ -24,6 +33,9 @@ export const ZContactAttributeKeyUpdateInput = z.object({
       error:
         "Key must be a safe identifier: only lowercase letters, numbers, and underscores, and must start with a letter",
     })
+    .refine((val) => !isReservedFutureDefaultAttributeKey(val), {
+      error: RESERVED_FUTURE_DEFAULT_ATTRIBUTE_KEY_VALIDATION_MESSAGE,
+    })
     .optional(),
   dataType: ZContactAttributeDataType.optional(),
 });
diff --git a/apps/web/modules/ee/contacts/api/v1/management/contact-attribute-keys/lib/contact-attribute-keys.test.ts b/apps/web/modules/ee/contacts/api/v1/management/contact-attribute-keys/lib/contact-attribute-keys.test.ts
index dfea7c9201..1509e53233 100644
--- a/apps/web/modules/ee/contacts/api/v1/management/contact-attribute-keys/lib/contact-attribute-keys.test.ts
+++ b/apps/web/modules/ee/contacts/api/v1/management/contact-attribute-keys/lib/contact-attribute-keys.test.ts
@@ -3,7 +3,7 @@ import { beforeEach, describe, expect, test, vi } from "vitest";
 import { prisma } from "@formbricks/database";
 import { PrismaErrorType } from "@formbricks/database/types/error";
 import { TContactAttributeKeyType } from "@formbricks/types/contact-attribute-key";
-import { DatabaseError, OperationNotAllowedError } from "@formbricks/types/errors";
+import { DatabaseError, InvalidInputError, OperationNotAllowedError } from "@formbricks/types/errors";
 import { MAX_ATTRIBUTE_CLASSES_PER_ENVIRONMENT } from "@/lib/constants";
 import { TContactAttributeKeyCreateInput } from "@/modules/ee/contacts/api/v1/management/contact-attribute-keys/[contactAttributeKeyId]/types/contact-attribute-keys";
 import { createContactAttributeKey, getContactAttributeKeys } from "./contact-attribute-keys";
@@ -144,6 +144,17 @@ describe("createContactAttributeKey", () => {
     expect(prisma.contactAttributeKey.create).not.toHaveBeenCalled();
   });
 
+  test("should throw InvalidInputError when key is reserved for future defaults", async () => {
+    await expect(
+      createContactAttributeKey(workspaceId, {
+        ...createInput,
+        key: "user_id",
+      })
+    ).rejects.toThrow(InvalidInputError);
+    expect(prisma.contactAttributeKey.count).not.toHaveBeenCalled();
+    expect(prisma.contactAttributeKey.create).not.toHaveBeenCalled();
+  });
+
   test("should throw DatabaseError if Prisma create fails", async () => {
     vi.mocked(prisma.contactAttributeKey.count).mockResolvedValue(0);
     const errorMessage = "Prisma create error";
diff --git a/apps/web/modules/ee/contacts/api/v1/management/contact-attribute-keys/lib/contact-attribute-keys.ts b/apps/web/modules/ee/contacts/api/v1/management/contact-attribute-keys/lib/contact-attribute-keys.ts
index 2f77a85eac..e4e637ef0f 100644
--- a/apps/web/modules/ee/contacts/api/v1/management/contact-attribute-keys/lib/contact-attribute-keys.ts
+++ b/apps/web/modules/ee/contacts/api/v1/management/contact-attribute-keys/lib/contact-attribute-keys.ts
@@ -3,10 +3,14 @@ import { cache as reactCache } from "react";
 import { prisma } from "@formbricks/database";
 import { PrismaErrorType } from "@formbricks/database/types/error";
 import { TContactAttributeKey } from "@formbricks/types/contact-attribute-key";
-import { DatabaseError, OperationNotAllowedError } from "@formbricks/types/errors";
+import { DatabaseError, InvalidInputError, OperationNotAllowedError } from "@formbricks/types/errors";
 import { MAX_ATTRIBUTE_CLASSES_PER_ENVIRONMENT } from "@/lib/constants";
 import { formatSnakeCaseToTitleCase } from "@/lib/utils/safe-identifier";
 import { TContactAttributeKeyCreateInput } from "@/modules/ee/contacts/api/v1/management/contact-attribute-keys/[contactAttributeKeyId]/types/contact-attribute-keys";
+import {
+  getReservedFutureDefaultAttributeKeyIssue,
+  isReservedFutureDefaultAttributeKey,
+} from "@/modules/ee/contacts/lib/attribute-key-policy";
 
 export const getContactAttributeKeys = reactCache(
   async (workspaceIds: string[]): Promise<TContactAttributeKey[]> => {
@@ -29,6 +33,10 @@ export const createContactAttributeKey = async (
   workspaceId: string,
   data: TContactAttributeKeyCreateInput
 ): Promise<TContactAttributeKey | null> => {
+  if (isReservedFutureDefaultAttributeKey(data.key)) {
+    throw new InvalidInputError(getReservedFutureDefaultAttributeKeyIssue([data.key]));
+  }
+
   const contactAttributeKeysCount = await prisma.contactAttributeKey.count({
     where: {
       workspaceId,
diff --git a/apps/web/modules/ee/contacts/api/v2/management/contacts/bulk/lib/contact.ts b/apps/web/modules/ee/contacts/api/v2/management/contacts/bulk/lib/contact.ts
index 4809687313..60b8d431b4 100644
--- a/apps/web/modules/ee/contacts/api/v2/management/contacts/bulk/lib/contact.ts
+++ b/apps/web/modules/ee/contacts/api/v2/management/contacts/bulk/lib/contact.ts
@@ -6,6 +6,10 @@ import { TContactAttributeDataType } from "@formbricks/types/contact-attribute-k
 import { Result, err, ok } from "@formbricks/types/error-handlers";
 import { isSafeIdentifier } from "@/lib/utils/safe-identifier";
 import { ApiErrorResponseV2 } from "@/modules/api/v2/types/api-error";
+import {
+  getReservedFutureDefaultAttributeKeyIssue,
+  isReservedFutureDefaultAttributeKey,
+} from "@/modules/ee/contacts/lib/attribute-key-policy";
 import { prepareAttributeColumnsForStorage } from "@/modules/ee/contacts/lib/attribute-storage";
 import { detectAttributeDataType } from "@/modules/ee/contacts/lib/detect-attribute-type";
 import { TContactBulkUploadContact } from "@/modules/ee/contacts/types/contact";
@@ -545,6 +549,22 @@ export const upsertBulkContacts = async (
     });
   }
 
+  const reservedNewKeys = attributeKeys.filter(
+    (key) => !existingKeySet.has(key) && isReservedFutureDefaultAttributeKey(key)
+  );
+
+  if (reservedNewKeys.length > 0) {
+    return err({
+      type: "bad_request",
+      details: [
+        {
+          field: "attributes",
+          issue: getReservedFutureDefaultAttributeKeyIssue(reservedNewKeys),
+        },
+      ],
+    });
+  }
+
   // Type Detection Phase
   const attributeValuesByKey = buildAttributeValuesByKey(contacts);
   const attributeTypeMap = determineAttributeTypes(attributeValuesByKey, existingAttributeKeys);
diff --git a/apps/web/modules/ee/contacts/api/v2/management/contacts/bulk/lib/tests/contact.test.ts b/apps/web/modules/ee/contacts/api/v2/management/contacts/bulk/lib/tests/contact.test.ts
index 6fcbb41f82..7d1fecdf54 100644
--- a/apps/web/modules/ee/contacts/api/v2/management/contacts/bulk/lib/tests/contact.test.ts
+++ b/apps/web/modules/ee/contacts/api/v2/management/contacts/bulk/lib/tests/contact.test.ts
@@ -347,6 +347,42 @@ describe("upsertBulkContacts", () => {
     expect(prisma.$executeRaw).toHaveBeenCalled();
   });
 
+  test("should return bad request when payload creates reserved future default keys", async () => {
+    const mockContacts = [
+      {
+        attributes: [
+          { attributeKey: { key: "email", name: "Email" }, value: "john@example.com" },
+          { attributeKey: { key: "user_id", name: "User Id" }, value: "user-123" },
+        ],
+      },
+    ];
+
+    const mockParsedEmails = ["john@example.com"];
+
+    vi.mocked(prisma.contactAttribute.findMany).mockResolvedValueOnce([]);
+    vi.mocked(prisma.contact.findMany).mockResolvedValueOnce([]);
+    vi.mocked(prisma.contactAttributeKey.findMany).mockResolvedValueOnce([
+      { id: "attr-key-email", key: "email", workspaceId: mockWorkspaceId, name: "Email" },
+    ] as any);
+
+    const result = await upsertBulkContacts(mockContacts, mockWorkspaceId, mockParsedEmails);
+    expect(result.ok).toBe(false);
+    expect(prisma.contact.createMany).not.toHaveBeenCalled();
+
+    if (!result.ok) {
+      expect(result.error).toStrictEqual({
+        type: "bad_request",
+        details: [
+          {
+            field: "attributes",
+            issue:
+              "Reserved attribute key(s): user_id. These keys are reserved for the v5.1 safe-identifier default attribute migration and cannot be created as custom attributes.",
+          },
+        ],
+      });
+    }
+  });
+
   test("should update attribute key names when they change", async () => {
     // Mock data: a contact with an attribute that has a new name for an existing key
     const mockContacts = [
diff --git a/apps/web/modules/ee/contacts/attributes/actions.ts b/apps/web/modules/ee/contacts/attributes/actions.ts
index 7bc9c01272..18dc7533ee 100644
--- a/apps/web/modules/ee/contacts/attributes/actions.ts
+++ b/apps/web/modules/ee/contacts/attributes/actions.ts
@@ -10,6 +10,10 @@ import { checkAuthorizationUpdated } from "@/lib/utils/action-client/action-clie
 import { getOrganizationIdFromWorkspaceId } from "@/lib/utils/helper";
 import { isSafeIdentifier } from "@/lib/utils/safe-identifier";
 import { withAuditLogging } from "@/modules/ee/audit-logs/lib/handler";
+import {
+  RESERVED_FUTURE_DEFAULT_ATTRIBUTE_KEY_VALIDATION_MESSAGE,
+  isReservedFutureDefaultAttributeKey,
+} from "@/modules/ee/contacts/lib/attribute-key-policy";
 import {
   createContactAttributeKey,
   deleteContactAttributeKey,
@@ -19,10 +23,15 @@ import {
 
 const ZCreateContactAttributeKeyAction = z.object({
   workspaceId: ZId,
-  key: z.string().refine((val) => isSafeIdentifier(val), {
-    error:
-      "Key must be a safe identifier: only lowercase letters, numbers, and underscores, and must start with a letter",
-  }),
+  key: z
+    .string()
+    .refine((val) => isSafeIdentifier(val), {
+      error:
+        "Key must be a safe identifier: only lowercase letters, numbers, and underscores, and must start with a letter",
+    })
+    .refine((val) => !isReservedFutureDefaultAttributeKey(val), {
+      error: RESERVED_FUTURE_DEFAULT_ATTRIBUTE_KEY_VALIDATION_MESSAGE,
+    }),
   name: z.string().optional(),
   description: z.string().optional(),
   dataType: ZContactAttributeDataType.optional(),
diff --git a/apps/web/modules/ee/contacts/attributes/components/create-attribute-modal.tsx b/apps/web/modules/ee/contacts/attributes/components/create-attribute-modal.tsx
index 07d75bf34f..ce82212ee8 100644
--- a/apps/web/modules/ee/contacts/attributes/components/create-attribute-modal.tsx
+++ b/apps/web/modules/ee/contacts/attributes/components/create-attribute-modal.tsx
@@ -8,6 +8,10 @@ import { useTranslation } from "react-i18next";
 import { TContactAttributeDataType } from "@formbricks/types/contact-attribute-key";
 import { getFormattedErrorMessage } from "@/lib/utils/helper";
 import { formatSnakeCaseToTitleCase, isSafeIdentifier, toSafeIdentifier } from "@/lib/utils/safe-identifier";
+import {
+  RESERVED_FUTURE_DEFAULT_ATTRIBUTE_SAFE_IDENTIFIER_KEYS_TEXT,
+  isReservedFutureDefaultAttributeKey,
+} from "@/modules/ee/contacts/lib/attribute-key-policy";
 import { Button } from "@/modules/ui/components/button";
 import {
   Dialog,
@@ -93,6 +97,14 @@ export function CreateAttributeModal({ workspaceId }: Readonly<CreateAttributeMo
       );
       return false;
     }
+    if (isReservedFutureDefaultAttributeKey(key)) {
+      setKeyError(
+        t("workspace.contacts.attribute_key_reserved_future_default", {
+          reservedKeys: RESERVED_FUTURE_DEFAULT_ATTRIBUTE_SAFE_IDENTIFIER_KEYS_TEXT,
+        })
+      );
+      return false;
+    }
     setKeyError("");
     return true;
   };
diff --git a/apps/web/modules/ee/contacts/components/csv-table.tsx b/apps/web/modules/ee/contacts/components/csv-table.tsx
index 26c140a64e..b358a2d37d 100644
--- a/apps/web/modules/ee/contacts/components/csv-table.tsx
+++ b/apps/web/modules/ee/contacts/components/csv-table.tsx
@@ -10,33 +10,37 @@ export const CsvTable = ({ data }: CsvTableProps) => {
   }
 
   const columns = Object.keys(data[0]);
-
   return (
     <div className="w-full overflow-x-auto rounded-md">
-      <div
-        className="sticky top-0 z-10 grid gap-2 border-b-2 border-slate-100 bg-slate-100 px-3 py-2 text-left"
-        style={{ gridTemplateColumns: `repeat(${columns.length}, minmax(100px, 1fr))` }}>
-        {columns.map((header, index) => (
-          <span
-            key={index}
-            className="overflow-hidden text-ellipsis whitespace-nowrap text-xs font-semibold capitalize leading-tight">
-            {header.replace(/_/g, " ")}
-          </span>
-        ))}
-      </div>
-
-      {data.map((row, rowIndex) => (
-        <div
-          key={rowIndex}
-          className="grid gap-2 border-b border-gray-200 bg-white px-3 py-2 text-left leading-tight last:border-b-0"
-          style={{ gridTemplateColumns: `repeat(${columns.length}, minmax(100px, 1fr))` }}>
-          {columns.map((header, colIndex) => (
-            <span key={colIndex} className="overflow-hidden text-ellipsis whitespace-nowrap text-xs">
-              {row[header]}
-            </span>
+      <table className="w-max min-w-full border-separate border-spacing-0 text-left text-xs">
+        <thead>
+          <tr className="bg-slate-100">
+            {columns.map((header) => (
+              <th
+                key={header}
+                scope="col"
+                className="sticky top-0 z-10 min-w-[120px] border-b-2 border-slate-200 bg-slate-100 px-3 py-2 font-semibold">
+                {header}
+              </th>
+            ))}
+          </tr>
+        </thead>
+        <tbody>
+          {data.map((row, rowIndex) => (
+            <tr key={rowIndex} className="bg-white">
+              {columns.map((header) => (
+                <td
+                  key={`${rowIndex}-${header}`}
+                  className="min-w-[120px] border-b border-slate-200 px-3 py-2">
+                  <span className="block overflow-hidden text-ellipsis whitespace-nowrap">
+                    {row[header] ?? ""}
+                  </span>
+                </td>
+              ))}
+            </tr>
           ))}
-        </div>
-      ))}
+        </tbody>
+      </table>
     </div>
   );
 };
diff --git a/apps/web/modules/ee/contacts/components/upload-contacts-attribute-combobox.tsx b/apps/web/modules/ee/contacts/components/upload-contacts-attribute-combobox.tsx
index bce9749d01..3f1f10eff0 100644
--- a/apps/web/modules/ee/contacts/components/upload-contacts-attribute-combobox.tsx
+++ b/apps/web/modules/ee/contacts/components/upload-contacts-attribute-combobox.tsx
@@ -4,6 +4,10 @@ import { ChevronDownIcon } from "lucide-react";
 import { useEffect, useMemo } from "react";
 import { useTranslation } from "react-i18next";
 import { isSafeIdentifier } from "@/lib/utils/safe-identifier";
+import {
+  RESERVED_FUTURE_DEFAULT_ATTRIBUTE_SAFE_IDENTIFIER_KEYS_TEXT,
+  isReservedFutureDefaultAttributeKey,
+} from "@/modules/ee/contacts/lib/attribute-key-policy";
 import { Button } from "@/modules/ui/components/button";
 import {
   Command,
@@ -41,6 +45,8 @@ export const UploadContactsAttributeCombobox = ({
   currentKey,
 }: ITagsComboboxProps) => {
   const { t } = useTranslation();
+  const normalizedSearchValue = searchValue.trim();
+
   useEffect(() => {
     // reset search value and value when closing the combobox
     if (!open) {
@@ -50,20 +56,56 @@ export const UploadContactsAttributeCombobox = ({
 
   // Check if the search value is a valid safe identifier for creating new attributes
   const isValidNewKey = useMemo(() => {
-    if (!searchValue) return false;
-    return isSafeIdentifier(searchValue.trim());
-  }, [searchValue]);
+    if (!normalizedSearchValue) return false;
+    return isSafeIdentifier(normalizedSearchValue);
+  }, [normalizedSearchValue]);
+
+  const isReservedNewKey = useMemo(() => {
+    if (!normalizedSearchValue) return false;
+    return isReservedFutureDefaultAttributeKey(normalizedSearchValue);
+  }, [normalizedSearchValue]);
 
   const existingKeyMatch = useMemo(() => {
     return keys.find((tag) => tag?.label?.toLowerCase().includes(searchValue?.toLowerCase()));
   }, [keys, searchValue]);
 
   const handleCreateKey = () => {
-    if (isValidNewKey && !existingKeyMatch) {
-      createKey(searchValue.trim());
+    if (isValidNewKey && !existingKeyMatch && !isReservedNewKey) {
+      createKey(normalizedSearchValue);
     }
   };
 
+  const renderCreateOptionContent = () => {
+    if (isValidNewKey && !isReservedNewKey) {
+      return (
+        <button
+          onClick={handleCreateKey}
+          className="h-8 w-full text-left hover:cursor-pointer hover:bg-slate-50 disabled:cursor-not-allowed disabled:opacity-50"
+          disabled={!!existingKeyMatch}>
+          + {t("common.add")} {normalizedSearchValue}
+        </button>
+      );
+    }
+
+    if (isReservedNewKey) {
+      return (
+        <div className="flex flex-col py-1 text-xs text-slate-500">
+          <span className="text-red-500">
+            {t("workspace.contacts.attribute_key_reserved_future_default", {
+              reservedKeys: RESERVED_FUTURE_DEFAULT_ATTRIBUTE_SAFE_IDENTIFIER_KEYS_TEXT,
+            })}
+          </span>
+        </div>
+      );
+    }
+
+    return (
+      <div className="flex flex-col py-1 text-xs text-slate-500">
+        <span className="text-red-500">{t("workspace.contacts.attribute_key_safe_identifier_required")}</span>
+      </div>
+    );
+  };
+
   return (
     <Popover open={open} onOpenChange={setOpen}>
       <PopoverTrigger asChild>
@@ -135,22 +177,7 @@ export const UploadContactsAttributeCombobox = ({
                 );
               })}
               {searchValue !== "" && !keys.some((tag) => tag.label === searchValue) && (
-                <CommandItem value="_create">
-                  {isValidNewKey ? (
-                    <button
-                      onClick={handleCreateKey}
-                      className="h-8 w-full text-left hover:cursor-pointer hover:bg-slate-50 disabled:cursor-not-allowed disabled:opacity-50"
-                      disabled={!!existingKeyMatch}>
-                      + Add {searchValue}
-                    </button>
-                  ) : (
-                    <div className="flex flex-col py-1 text-xs text-slate-500">
-                      <span className="text-red-500">
-                        {t("workspace.contacts.attribute_key_safe_identifier_required")}
-                      </span>
-                    </div>
-                  )}
-                </CommandItem>
+                <CommandItem value="_create">{renderCreateOptionContent()}</CommandItem>
               )}
             </CommandGroup>
           </CommandList>
diff --git a/apps/web/modules/ee/contacts/components/upload-contacts-button.tsx b/apps/web/modules/ee/contacts/components/upload-contacts-button.tsx
index dd0388026c..ffc3e21fc9 100644
--- a/apps/web/modules/ee/contacts/components/upload-contacts-button.tsx
+++ b/apps/web/modules/ee/contacts/components/upload-contacts-button.tsx
@@ -13,6 +13,10 @@ import { isSafeIdentifier } from "@/lib/utils/safe-identifier";
 import { createContactsFromCSVAction } from "@/modules/ee/contacts/actions";
 import { CsvTable } from "@/modules/ee/contacts/components/csv-table";
 import { UploadContactsAttributes } from "@/modules/ee/contacts/components/upload-contacts-attribute";
+import {
+  RESERVED_FUTURE_DEFAULT_ATTRIBUTE_SAFE_IDENTIFIER_KEYS_TEXT,
+  isReservedFutureDefaultAttributeKey,
+} from "@/modules/ee/contacts/lib/attribute-key-policy";
 import { TContactCSVUploadResponse, ZContactCSVUploadResponse } from "@/modules/ee/contacts/types/contact";
 import { Alert } from "@/modules/ui/components/alert";
 import { Button } from "@/modules/ui/components/button";
@@ -257,7 +261,6 @@ export const UploadContactsCSVButton = ({
 
   useEffect(() => {
     const matches: Record<string, string> = {};
-    const invalidColumns: string[] = [];
 
     for (const columnName of csvColumns) {
       let matched = false;
@@ -270,25 +273,66 @@ export const UploadContactsCSVButton = ({
       }
 
       if (!matched) {
-        // This column will become a new attribute - validate it's a safe identifier
-        if (!isSafeIdentifier(columnName)) {
-          invalidColumns.push(columnName);
-        }
         matches[columnName] = columnName;
       }
     }
 
     setAttributeMap(matches);
+  }, [contactAttributeKeys, csvColumns]);
+
+  useEffect(() => {
+    if (!csvColumns.length || !csvResponse.length) {
+      return;
+    }
+
+    const invalidColumns: string[] = [];
+    const reservedColumns: string[] = [];
+
+    for (const columnName of csvColumns) {
+      const mappedAttribute = attributeMap[columnName];
+      if (!mappedAttribute) {
+        continue;
+      }
+
+      const mapsToExistingAttribute = contactAttributeKeys.some(
+        (attributeKey) => attributeKey.id === mappedAttribute
+      );
+
+      if (mapsToExistingAttribute) {
+        continue;
+      }
+
+      if (!isSafeIdentifier(mappedAttribute)) {
+        invalidColumns.push(columnName);
+        continue;
+      }
+
+      if (isReservedFutureDefaultAttributeKey(mappedAttribute)) {
+        reservedColumns.push(columnName);
+      }
+    }
+
+    const errorMessages: string[] = [];
 
-    // Show error for invalid column names that would become new attributes
     if (invalidColumns.length > 0) {
-      setError(
+      errorMessages.push(
         t("workspace.contacts.invalid_csv_column_names", {
           columns: invalidColumns.join(", "),
         })
       );
     }
-  }, [contactAttributeKeys, csvColumns, t]);
+
+    if (reservedColumns.length > 0) {
+      errorMessages.push(
+        t("workspace.contacts.invalid_csv_reserved_column_names", {
+          columns: reservedColumns.join(", "),
+          reservedKeys: RESERVED_FUTURE_DEFAULT_ATTRIBUTE_SAFE_IDENTIFIER_KEYS_TEXT,
+        })
+      );
+    }
+
+    setError(errorMessages.join("\n"));
+  }, [attributeMap, contactAttributeKeys, csvColumns, csvResponse.length, t]);
 
   useEffect(() => {
     if (error && errorContainerRef.current) {
@@ -304,7 +348,7 @@ export const UploadContactsCSVButton = ({
     const exampleData = [
       {
         email: "user1@example.com",
-        userId: "1001",
+        user_id: "1001",
         first_name: "John",
         last_name: "Doe",
         age: "28",
@@ -313,7 +357,7 @@ export const UploadContactsCSVButton = ({
       },
       {
         email: "user2@example.com",
-        userId: "1002",
+        user_id: "1002",
         first_name: "Jane",
         last_name: "Smith",
         age: "34",
@@ -322,7 +366,7 @@ export const UploadContactsCSVButton = ({
       },
       {
         email: "user3@example.com",
-        userId: "1003",
+        user_id: "1003",
         first_name: "Mark",
         last_name: "Jones",
         age: "45",
@@ -331,7 +375,7 @@ export const UploadContactsCSVButton = ({
       },
       {
         email: "user4@example.com",
-        userId: "1004",
+        user_id: "1004",
         first_name: "Emily",
         last_name: "Brown",
         age: "22",
@@ -340,7 +384,7 @@ export const UploadContactsCSVButton = ({
       },
       {
         email: "user5@example.com",
-        userId: "1005",
+        user_id: "1005",
         first_name: "David",
         last_name: "Wilson",
         age: "31",
diff --git a/apps/web/modules/ee/contacts/lib/attribute-key-policy.ts b/apps/web/modules/ee/contacts/lib/attribute-key-policy.ts
new file mode 100644
index 0000000000..d7e43f1c67
--- /dev/null
+++ b/apps/web/modules/ee/contacts/lib/attribute-key-policy.ts
@@ -0,0 +1,39 @@
+// Keep these keys reserved until the v5.1 migration moves default contact attributes
+// from camelCase to safe identifiers with backward compatibility aliases.
+// This is a preventive guardrail only (no schema/data migration in v5).
+export const RESERVED_FUTURE_DEFAULT_ATTRIBUTE_SAFE_IDENTIFIER_KEYS = [
+  "user_id",
+  "first_name",
+  "last_name",
+] as const;
+
+export const RESERVED_FUTURE_DEFAULT_ATTRIBUTE_SAFE_IDENTIFIER_KEYS_TEXT =
+  RESERVED_FUTURE_DEFAULT_ATTRIBUTE_SAFE_IDENTIFIER_KEYS.join(", ");
+
+export const RESERVED_FUTURE_DEFAULT_ATTRIBUTE_KEY_VALIDATION_MESSAGE = `Key is reserved for the v5.1 safe-identifier default attribute migration. Reserved keys: ${RESERVED_FUTURE_DEFAULT_ATTRIBUTE_SAFE_IDENTIFIER_KEYS_TEXT}.`;
+
+const RESERVED_FUTURE_DEFAULT_ATTRIBUTE_SAFE_IDENTIFIER_KEY_SET: ReadonlySet<string> = new Set(
+  RESERVED_FUTURE_DEFAULT_ATTRIBUTE_SAFE_IDENTIFIER_KEYS
+);
+
+const normalizeKey = (key: string): string => key.trim().toLowerCase();
+
+export const isReservedFutureDefaultAttributeKey = (key: string): boolean => {
+  return RESERVED_FUTURE_DEFAULT_ATTRIBUTE_SAFE_IDENTIFIER_KEY_SET.has(normalizeKey(key));
+};
+
+export const getReservedFutureDefaultAttributeKeys = (keys: string[]): string[] => {
+  const normalized = keys
+    .map(normalizeKey)
+    .filter((key) => key.length > 0 && isReservedFutureDefaultAttributeKey(key));
+
+  return Array.from(new Set(normalized));
+};
+
+export const getReservedFutureDefaultAttributeKeyIssue = (keys: string[]): string => {
+  const reservedKeys = getReservedFutureDefaultAttributeKeys(keys);
+
+  return `Reserved attribute key(s): ${reservedKeys.join(
+    ", "
+  )}. These keys are reserved for the v5.1 safe-identifier default attribute migration and cannot be created as custom attributes.`;
+};
diff --git a/apps/web/modules/ee/contacts/lib/attributes.test.ts b/apps/web/modules/ee/contacts/lib/attributes.test.ts
index 7579beedfb..e771e79e8a 100644
--- a/apps/web/modules/ee/contacts/lib/attributes.test.ts
+++ b/apps/web/modules/ee/contacts/lib/attributes.test.ts
@@ -221,6 +221,30 @@ describe("updateAttributes", () => {
     );
   });
 
+  test("skips creating reserved future default attributes", async () => {
+    vi.mocked(getContactAttributeKeys).mockResolvedValue([attributeKeys[1]]);
+    vi.mocked(getContactAttributes).mockResolvedValue({ email: "jane@example.com" });
+    vi.mocked(hasEmailAttribute).mockResolvedValue(false);
+    vi.mocked(hasUserIdAttribute).mockResolvedValue(false);
+    vi.mocked(prisma.$transaction).mockResolvedValue(undefined);
+    vi.mocked(prisma.contactAttribute.deleteMany).mockResolvedValue({ count: 0 });
+
+    const result = await updateAttributes(contactId, userId, workspaceId, {
+      email: "john@example.com",
+      user_id: "future-safe",
+    });
+
+    expect(result.success).toBe(true);
+    expect(result.errors).toContainEqual({
+      code: "reserved_attribute_keys",
+      params: {
+        issue:
+          "Reserved attribute key(s): user_id. These keys are reserved for the v5.1 safe-identifier default attribute migration and cannot be created as custom attributes.",
+      },
+    });
+    expect(prisma.contactAttributeKey.create).not.toHaveBeenCalled();
+  });
+
   test("returns success with only email attribute", async () => {
     vi.mocked(getContactAttributeKeys).mockResolvedValue([attributeKeys[1]]); // email key
     vi.mocked(getContactAttributes).mockResolvedValue({ email: "existing@example.com" });
diff --git a/apps/web/modules/ee/contacts/lib/attributes.ts b/apps/web/modules/ee/contacts/lib/attributes.ts
index 84e98ebd9b..440b01c7c1 100644
--- a/apps/web/modules/ee/contacts/lib/attributes.ts
+++ b/apps/web/modules/ee/contacts/lib/attributes.ts
@@ -6,6 +6,10 @@ import { TContactAttributeKey } from "@formbricks/types/contact-attribute-key";
 import { MAX_ATTRIBUTE_CLASSES_PER_ENVIRONMENT } from "@/lib/constants";
 import { formatSnakeCaseToTitleCase, isSafeIdentifier } from "@/lib/utils/safe-identifier";
 import { validateInputs } from "@/lib/utils/validate";
+import {
+  getReservedFutureDefaultAttributeKeyIssue,
+  isReservedFutureDefaultAttributeKey,
+} from "@/modules/ee/contacts/lib/attribute-key-policy";
 import { prepareNewSDKAttributeForStorage } from "@/modules/ee/contacts/lib/attribute-storage";
 import { getContactAttributeKeys } from "@/modules/ee/contacts/lib/contact-attribute-keys";
 import {
@@ -38,6 +42,7 @@ const MESSAGE_TEMPLATES: Record<string, string> = {
   userid_already_exists: "The userId already exists for this environment and was not updated.",
   invalid_attribute_keys:
     "Skipped creating attribute(s) with invalid key(s): {keys}. Keys must only contain lowercase letters, numbers, and underscores, and must start with a letter.",
+  reserved_attribute_keys: "{issue}",
   attribute_limit_exceeded:
     "Could not create {count} new attribute(s) as it would exceed the maximum limit of {limit} attribute classes. Existing attributes were updated successfully.",
   new_attribute_created: "Created new attribute '{key}' with type '{dataType}'",
@@ -304,12 +309,15 @@ export const updateAttributes = async (
     // Validate that new attribute keys are safe identifiers
     const validNewAttributes: typeof newAttributes = [];
     const invalidKeys: string[] = [];
+    const reservedKeys: string[] = [];
 
     for (const attr of newAttributes) {
-      if (isSafeIdentifier(attr.key)) {
-        validNewAttributes.push(attr);
-      } else {
+      if (!isSafeIdentifier(attr.key)) {
         invalidKeys.push(attr.key);
+      } else if (isReservedFutureDefaultAttributeKey(attr.key)) {
+        reservedKeys.push(attr.key);
+      } else {
+        validNewAttributes.push(attr);
       }
     }
 
@@ -325,6 +333,17 @@ export const updateAttributes = async (
       );
     }
 
+    if (reservedKeys.length > 0) {
+      errors.push({
+        code: "reserved_attribute_keys",
+        params: { issue: getReservedFutureDefaultAttributeKeyIssue(reservedKeys) },
+      });
+      logger.warn(
+        { workspaceId, reservedKeys },
+        "SDK tried to create reserved future default attribute keys - skipping"
+      );
+    }
+
     if (validNewAttributes.length > 0) {
       const totalAttributeClassesLength = contactAttributeKeys.length + validNewAttributes.length;
 
diff --git a/apps/web/modules/ee/contacts/lib/contact-attribute-keys.test.ts b/apps/web/modules/ee/contacts/lib/contact-attribute-keys.test.ts
index e4f8ccc223..361aa730c3 100644
--- a/apps/web/modules/ee/contacts/lib/contact-attribute-keys.test.ts
+++ b/apps/web/modules/ee/contacts/lib/contact-attribute-keys.test.ts
@@ -147,6 +147,13 @@ describe("createContactAttributeKey", () => {
     await expect(createContactAttributeKey({ workspaceId, key: "email" })).rejects.toThrow(InvalidInputError);
   });
 
+  test("throws InvalidInputError when key is reserved for future defaults", async () => {
+    await expect(createContactAttributeKey({ workspaceId, key: "user_id" })).rejects.toThrow(
+      InvalidInputError
+    );
+    expect(prisma.contactAttributeKey.create).not.toHaveBeenCalled();
+  });
+
   test("rethrows unknown prisma error codes", async () => {
     const err = Object.assign(new Error("Some prisma error"), { code: PrismaErrorType.RecordDoesNotExist });
     vi.mocked(prisma.contactAttributeKey.create).mockRejectedValue(err);
diff --git a/apps/web/modules/ee/contacts/lib/contact-attribute-keys.ts b/apps/web/modules/ee/contacts/lib/contact-attribute-keys.ts
index 8d46ad70ea..1722d0696c 100644
--- a/apps/web/modules/ee/contacts/lib/contact-attribute-keys.ts
+++ b/apps/web/modules/ee/contacts/lib/contact-attribute-keys.ts
@@ -4,6 +4,10 @@ import { PrismaErrorType } from "@formbricks/database/types/error";
 import { TContactAttributeDataType, TContactAttributeKey } from "@formbricks/types/contact-attribute-key";
 import { InvalidInputError, OperationNotAllowedError, ResourceNotFoundError } from "@formbricks/types/errors";
 import { formatSnakeCaseToTitleCase } from "@/lib/utils/safe-identifier";
+import {
+  getReservedFutureDefaultAttributeKeyIssue,
+  isReservedFutureDefaultAttributeKey,
+} from "./attribute-key-policy";
 
 export const getContactAttributeKeys = reactCache(
   async (workspaceId: string): Promise<TContactAttributeKey[]> => {
@@ -31,6 +35,10 @@ export const createContactAttributeKey = async (data: {
   description?: string;
   dataType?: TContactAttributeDataType;
 }): Promise<TContactAttributeKey> => {
+  if (isReservedFutureDefaultAttributeKey(data.key)) {
+    throw new InvalidInputError(getReservedFutureDefaultAttributeKeyIssue([data.key]));
+  }
+
   try {
     const contactAttributeKey = await prisma.contactAttributeKey.create({
       data: {
diff --git a/apps/web/modules/ee/contacts/lib/contacts.test.ts b/apps/web/modules/ee/contacts/lib/contacts.test.ts
index 8a1ec75d36..6ccec00f37 100644
--- a/apps/web/modules/ee/contacts/lib/contacts.test.ts
+++ b/apps/web/modules/ee/contacts/lib/contacts.test.ts
@@ -537,6 +537,22 @@ describe("Contacts Lib", () => {
       ).rejects.toThrow(ValidationError);
     });
 
+    test("throws ValidationError when CSV creates reserved future default keys", async () => {
+      const reservedCsvData = [{ email: "john@example.com", user_id: "user-1" }];
+      const attributeMap = { email: "email", user_id: "user_id" };
+
+      vi.mocked(prisma.contact.findMany).mockResolvedValueOnce([]);
+      vi.mocked(prisma.contactAttribute.findMany).mockResolvedValue([]);
+      vi.mocked(prisma.contactAttributeKey.findMany).mockResolvedValueOnce([
+        { key: "email", id: "key-1", dataType: "string" },
+      ] as any);
+
+      await expect(
+        createContactsFromCSV(reservedCsvData as any, mockWorkspaceId, "skip", attributeMap)
+      ).rejects.toThrow(ValidationError);
+      expect(prisma.contactAttributeKey.createMany).not.toHaveBeenCalled();
+    });
+
     test("throws DatabaseError on Prisma error", async () => {
       const attributeMap = { email: "email" };
       const prismaError = new Prisma.PrismaClientKnownRequestError("DB Error", {
diff --git a/apps/web/modules/ee/contacts/lib/contacts.ts b/apps/web/modules/ee/contacts/lib/contacts.ts
index 0842c1f2b7..05a8d8191e 100644
--- a/apps/web/modules/ee/contacts/lib/contacts.ts
+++ b/apps/web/modules/ee/contacts/lib/contacts.ts
@@ -9,6 +9,10 @@ import { DatabaseError, ValidationError } from "@formbricks/types/errors";
 import { ITEMS_PER_PAGE } from "@/lib/constants";
 import { formatSnakeCaseToTitleCase, isSafeIdentifier } from "@/lib/utils/safe-identifier";
 import { validateInputs } from "@/lib/utils/validate";
+import {
+  getReservedFutureDefaultAttributeKeyIssue,
+  getReservedFutureDefaultAttributeKeys,
+} from "@/modules/ee/contacts/lib/attribute-key-policy";
 import { prepareAttributeColumnsForStorage } from "@/modules/ee/contacts/lib/attribute-storage";
 import { getContactSurveyLink } from "@/modules/ee/contacts/lib/contact-survey-link";
 import { detectAttributeDataType } from "@/modules/ee/contacts/lib/detect-attribute-type";
@@ -412,6 +416,11 @@ const createMissingAttributeKeys = async (
     );
   }
 
+  const reservedKeys = getReservedFutureDefaultAttributeKeys(missingKeys);
+  if (reservedKeys.length > 0) {
+    throw new ValidationError(getReservedFutureDefaultAttributeKeyIssue(reservedKeys));
+  }
+
   // Deduplicate by lowercase to avoid creating duplicates like "firstName" and "firstname"
   const uniqueMissingKeys = new Map<string, string>();
   for (const key of missingKeys) {
diff --git a/apps/web/modules/ee/contacts/segments/actions.ts b/apps/web/modules/ee/contacts/segments/actions.ts
index c06e53e136..4ace9586cb 100644
--- a/apps/web/modules/ee/contacts/segments/actions.ts
+++ b/apps/web/modules/ee/contacts/segments/actions.ts
@@ -2,7 +2,7 @@
 
 import { z } from "zod";
 import { ZId } from "@formbricks/types/common";
-import { OperationNotAllowedError, ResourceNotFoundError } from "@formbricks/types/errors";
+import { InvalidInputError, OperationNotAllowedError, ResourceNotFoundError } from "@formbricks/types/errors";
 import { ZSegmentCreateInput, ZSegmentFilters, ZSegmentUpdateInput } from "@formbricks/types/segment";
 import { getOrganization } from "@/lib/organization/service";
 import { capturePostHogEvent } from "@/lib/posthog";
@@ -49,7 +49,7 @@ export const createSegmentAction = authenticatedActionClient.inputSchema(ZSegmen
       const surveyWorkspaceId = await getWorkspaceIdFromSurveyId(parsedInput.surveyId);
 
       if (surveyWorkspaceId !== parsedInput.workspaceId) {
-        throw new Error("Survey and segment are not in the same workspace");
+        throw new InvalidInputError("Survey and segment are not in the same workspace");
       }
     }
 
@@ -82,7 +82,7 @@ export const createSegmentAction = authenticatedActionClient.inputSchema(ZSegmen
     if (!parsedFilters.success) {
       const errMsg =
         parsedFilters.error.issues.find((issue) => issue.code === "custom")?.message || "Invalid filters";
-      throw new Error(errMsg);
+      throw new InvalidInputError(errMsg);
     }
 
     const segment = await createSegment(parsedInput);
@@ -139,7 +139,7 @@ export const updateSegmentAction = authenticatedActionClient.inputSchema(ZUpdate
       if (!parsedFilters.success) {
         const errMsg =
           parsedFilters.error.issues.find((issue) => issue.code === "custom")?.message || "Invalid filters";
-        throw new Error(errMsg);
+        throw new InvalidInputError(errMsg);
       }
 
       await checkForRecursiveSegmentFilter(parsedFilters.data, parsedInput.segmentId);
@@ -169,7 +169,7 @@ export const loadNewSegmentAction = authenticatedActionClient
     const segmentWorkspaceId = await getWorkspaceIdFromSegmentId(parsedInput.segmentId);
 
     if (surveyWorkspaceId !== segmentWorkspaceId) {
-      throw new Error("Segment and survey are not in the same workspace");
+      throw new InvalidInputError("Segment and survey are not in the same workspace");
     }
 
     const organizationId = await getOrganizationIdFromSurveyId(parsedInput.surveyId);
diff --git a/apps/web/modules/ee/contacts/segments/components/add-filter-modal.tsx b/apps/web/modules/ee/contacts/segments/components/add-filter-modal.tsx
index 8b02b4410d..80facf2a8f 100644
--- a/apps/web/modules/ee/contacts/segments/components/add-filter-modal.tsx
+++ b/apps/web/modules/ee/contacts/segments/components/add-filter-modal.tsx
@@ -184,22 +184,33 @@ export function AddFilterModal({
     [t]
   );
 
+  const contactAttributeKeysForPicker = useMemo(() => {
+    // `userId` is represented by the person filter (fingerprint icon), so hide it from attribute entries.
+    return contactAttributeKeys.filter((attributeKey) => attributeKey.key !== "userId");
+  }, [contactAttributeKeys]);
+
   const contactAttributeKeysFiltered = useMemo(() => {
-    if (!contactAttributeKeys) return [];
+    if (!contactAttributeKeysForPicker) return [];
 
-    if (!searchValue) return contactAttributeKeys;
+    if (!searchValue) return contactAttributeKeysForPicker;
 
-    return contactAttributeKeys.filter((attributeKey) => {
+    return contactAttributeKeysForPicker.filter((attributeKey) => {
       const attributeValueToSeach = attributeKey.name ?? attributeKey.key;
       return attributeValueToSeach.toLowerCase().includes(searchValue.toLowerCase());
     });
-  }, [contactAttributeKeys, searchValue]);
+  }, [contactAttributeKeysForPicker, searchValue]);
 
   const contactAttributeFiltered = useMemo(() => {
-    const contactAttributes = [{ name: "userId" }];
+    const personIdentifiers = [{ id: "userId", label: t("common.user_id") }];
 
-    return contactAttributes.filter((ca) => ca.name.toLowerCase().includes(searchValue.toLowerCase()));
-  }, [searchValue]);
+    return personIdentifiers.filter((personIdentifier) => {
+      const query = searchValue.toLowerCase();
+      return (
+        personIdentifier.id.toLowerCase().includes(query) ||
+        personIdentifier.label.toLowerCase().includes(query)
+      );
+    });
+  }, [searchValue, t]);
 
   const segmentsFiltered = useMemo(() => {
     if (!segments) return [];
@@ -283,10 +294,10 @@ export function AddFilterModal({
 
             {filters.contactAttributeFiltered.map((personAttribute) => (
               <FilterButton
-                key={personAttribute.name}
-                data-testid={`filter-btn-person-${personAttribute.name}`}
+                key={personAttribute.id}
+                data-testid={`filter-btn-person-${personAttribute.id}`}
                 icon={<FingerprintIcon className="h-4 w-4" />}
-                label={personAttribute.name}
+                label={personAttribute.label}
                 onClick={() => {
                   handleAddFilter({
                     type: "person",
diff --git a/apps/web/modules/workspaces/settings/lib/workspace.ts b/apps/web/modules/workspaces/settings/lib/workspace.ts
index 7e64497c64..553d90fe1c 100644
--- a/apps/web/modules/workspaces/settings/lib/workspace.ts
+++ b/apps/web/modules/workspaces/settings/lib/workspace.ts
@@ -9,6 +9,8 @@ import { TWorkspace, TWorkspaceUpdateInput, ZWorkspaceUpdateInput } from "@formb
 import { validateInputs } from "@/lib/utils/validate";
 import { deleteFilesByWorkspaceId } from "@/modules/storage/service";
 
+// Keep v5 defaults aligned with current production camelCase keys.
+// Safe-identifier migration (with backwards compatibility) is intentionally deferred to v5.1.
 const DEFAULT_CONTACT_ATTRIBUTE_KEYS: Prisma.ContactAttributeKeyCreateWithoutWorkspaceInput[] = [
   {
     key: "userId",
@@ -151,7 +153,7 @@ export const deleteWorkspace = async (workspaceId: string): Promise<TWorkspace>
     if (workspace) {
       const s3Result = await deleteFilesByWorkspaceId(workspaceId, []);
 
-      if (!s3Result.ok) {
+      if (!s3Result.ok && "error" in s3Result) {
         // fail silently because we don't want to throw an error if the files are not deleted
         logger.error(s3Result.error, "Error deleting S3 files");
       }
diff --git a/packages/database/src/seed.ts b/packages/database/src/seed.ts
index d1fa162f99..ddce33bd7d 100644
--- a/packages/database/src/seed.ts
+++ b/packages/database/src/seed.ts
@@ -449,6 +449,8 @@ async function main(): Promise<void> {
     },
   });
 
+  // Keep seed defaults aligned with production v5 camelCase keys.
+  // Safe-identifier migration is deferred to v5.1.
   // Contact attribute keys for the workspace
   const defaultAttributeKeys = [
     { name: "Email", key: "email", isUnique: true, type: "default" as const },