diff --git a/OldKotlin/realm-models/src/main/kotlin/com/infomaniak/mail/data/api/IntOrStringSerializer.kt b/OldKotlin/realm-models/src/main/kotlin/com/infomaniak/mail/data/api/IntOrStringSerializer.kt
new file mode 100644
index 00000000000..98bb51e0034
--- /dev/null
+++ b/OldKotlin/realm-models/src/main/kotlin/com/infomaniak/mail/data/api/IntOrStringSerializer.kt
@@ -0,0 +1,52 @@
+/*
+ * Infomaniak Mail - Android
+ * Copyright (C) 2026 Infomaniak Network SA
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package com.infomaniak.mail.data.api
+
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.SerializationException
+import kotlinx.serialization.descriptors.PrimitiveKind
+import kotlinx.serialization.descriptors.PrimitiveSerialDescriptor
+import kotlinx.serialization.descriptors.SerialDescriptor
+import kotlinx.serialization.encoding.Decoder
+import kotlinx.serialization.encoding.Encoder
+import kotlinx.serialization.json.JsonDecoder
+import kotlinx.serialization.json.JsonPrimitive
+import kotlinx.serialization.json.intOrNull
+
+object IntOrStringSerializer : KSerializer<String> {
+
+    override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("IntOrString", PrimitiveKind.STRING)
+
+    override fun deserialize(decoder: Decoder): String {
+        return if (decoder is JsonDecoder) {
+            val jsonPrimitive = decoder.decodeJsonElement() as? JsonPrimitive
+                ?: throw SerializationException("Expected a primitive element for id")
+            if (jsonPrimitive.isString) {
+                jsonPrimitive.content
+            } else {
+                jsonPrimitive.intOrNull?.toString() ?: jsonPrimitive.content
+            }
+        } else {
+            decoder.decodeString()
+        }
+    }
+
+    override fun serialize(encoder: Encoder, value: String) {
+        encoder.encodeString(value)
+    }
+}
diff --git a/OldKotlin/realm-models/src/main/kotlin/com/infomaniak/mail/data/models/correspondent/Contact.kt b/OldKotlin/realm-models/src/main/kotlin/com/infomaniak/mail/data/models/correspondent/Contact.kt
index cd9a7643f93..15db60deb7e 100644
--- a/OldKotlin/realm-models/src/main/kotlin/com/infomaniak/mail/data/models/correspondent/Contact.kt
+++ b/OldKotlin/realm-models/src/main/kotlin/com/infomaniak/mail/data/models/correspondent/Contact.kt
@@ -19,6 +19,7 @@
 
 package com.infomaniak.mail.data.models.correspondent
 
+import com.infomaniak.mail.data.api.IntOrStringSerializer
 import io.realm.kotlin.ext.realmListOf
 import io.realm.kotlin.serializers.RealmListKSerializer
 import io.realm.kotlin.types.RealmList
@@ -28,6 +29,8 @@ import kotlinx.serialization.UseSerializers
 
 @Serializable
 data class Contact(
+    @Serializable(with = IntOrStringSerializer::class)
+    val id: String? = null,
     val name: String = "",
     val emails: RealmList<String> = realmListOf(),
     val avatar: String? = null,
diff --git a/OldKotlin/realm-models/src/main/kotlin/com/infomaniak/mail/data/models/correspondent/MergedContact.kt b/OldKotlin/realm-models/src/main/kotlin/com/infomaniak/mail/data/models/correspondent/MergedContact.kt
index b04f34e17cd..60dda35a66d 100644
--- a/OldKotlin/realm-models/src/main/kotlin/com/infomaniak/mail/data/models/correspondent/MergedContact.kt
+++ b/OldKotlin/realm-models/src/main/kotlin/com/infomaniak/mail/data/models/correspondent/MergedContact.kt
@@ -46,6 +46,9 @@ class MergedContact() : RealmObject, Correspondent, ContactAutocompletable {
     var comesFromApi: Boolean = false // In opposition to coming from the phone's address book
         private set
 
+    var remoteContactId: String? = null
+        private set
+
     var remoteContactGroupIds: RealmList<Int> = realmListOf()
 
     var addressbookIds: RealmList<Int?> = realmListOf()
@@ -78,6 +81,7 @@ class MergedContact() : RealmObject, Correspondent, ContactAutocompletable {
 
         this.contactedTimes = apiContact.contactedTimes?.get(email)
         this.other = apiContact.other
+        this.remoteContactId = apiContact.id
 
         this.comesFromApi = comesFromApi
 
@@ -113,6 +117,7 @@ class MergedContact() : RealmObject, Correspondent, ContactAutocompletable {
             avatar = apiContact.avatar
             comesFromApi = true
         }
+        remoteContactId = apiContact.id
         addressbookIds += realmListOf(apiContact.addressbookId)
         remoteContactGroupIds += apiContact.remoteContactGroupIds
     }
diff --git a/app/src/main/java/com/infomaniak/mail/data/cache/RealmDatabase.kt b/app/src/main/java/com/infomaniak/mail/data/cache/RealmDatabase.kt
index 0448ff0d957..4b4d13c881e 100644
--- a/app/src/main/java/com/infomaniak/mail/data/cache/RealmDatabase.kt
+++ b/app/src/main/java/com/infomaniak/mail/data/cache/RealmDatabase.kt
@@ -202,7 +202,7 @@ object RealmDatabase {
     private object RealmConfig {
 
         //region Configurations versions
-        const val USER_INFO_SCHEMA_VERSION = 5L
+        const val USER_INFO_SCHEMA_VERSION = 6L
         const val MAILBOX_INFO_SCHEMA_VERSION = 19L
         const val MAILBOX_CONTENT_SCHEMA_VERSION = 37L
         //endregion
diff --git a/app/src/main/java/com/infomaniak/mail/ui/MainViewModel.kt b/app/src/main/java/com/infomaniak/mail/ui/MainViewModel.kt
index 593c8024f5b..86ade845785 100644
--- a/app/src/main/java/com/infomaniak/mail/ui/MainViewModel.kt
+++ b/app/src/main/java/com/infomaniak/mail/ui/MainViewModel.kt
@@ -307,6 +307,11 @@ class MainViewModel @Inject constructor(
 
     //region Merged Contacts
     val mergedContactsLive: LiveData<MergedContactDictionary> = avatarMergedContactData.mergedContactLiveData
+
+    fun shouldShowAddToContacts(recipient: Recipient): Boolean {
+        val remoteContactId = mergedContactController.getMergedContactFromEmail(recipient.email)?.remoteContactId
+        return remoteContactId.isNullOrBlank()
+    }
     //endregion
 
     //region Share Thread URL
diff --git a/app/src/main/java/com/infomaniak/mail/ui/main/thread/DetailedContactBottomSheetDialog.kt b/app/src/main/java/com/infomaniak/mail/ui/main/thread/DetailedContactBottomSheetDialog.kt
index b3409727062..331af385ce1 100644
--- a/app/src/main/java/com/infomaniak/mail/ui/main/thread/DetailedContactBottomSheetDialog.kt
+++ b/app/src/main/java/com/infomaniak/mail/ui/main/thread/DetailedContactBottomSheetDialog.kt
@@ -63,6 +63,7 @@ class DetailedContactBottomSheetDialog : ActionsBottomSheetDialog() {
         val bimi = navigationArgs.bimi
         containerInfoCertified.isVisible = bimi?.isCertified == true
         contactDetails.setCorrespondent(navigationArgs.recipient, bimi)
+        addToContacts.isVisible = mainViewModel.shouldShowAddToContacts(navigationArgs.recipient)
 
         setupListeners()