diff --git a/lib/mv/vereinfacht/changes/sync_contact.ex b/lib/mv/vereinfacht/changes/sync_contact.ex
index 4ea6cc8..99875e0 100644
--- a/lib/mv/vereinfacht/changes/sync_contact.ex
+++ b/lib/mv/vereinfacht/changes/sync_contact.ex
@@ -7,20 +7,57 @@ defmodule Mv.Vereinfacht.Changes.SyncContact do
   Runs in `after_transaction` so the member is persisted first. API failures are logged
   but do not block the member operation. Requires Vereinfacht to be configured
   (Mv.Config.vereinfacht_configured?/0).
+
+  Only runs when relevant data changed: on create always; on update only when
+  first_name, last_name, email, street, house_number, postal_code, or city changed,
+  or when the member has no vereinfacht_contact_id yet (to avoid unnecessary API calls).
   """
   use Ash.Resource.Change
 
   require Logger
 
+  @synced_attributes [
+    :first_name,
+    :last_name,
+    :email,
+    :street,
+    :house_number,
+    :postal_code,
+    :city
+  ]
+
   @impl true
   def change(changeset, _opts, _context) do
-    if Mv.Config.vereinfacht_configured?() do
+    if Mv.Config.vereinfacht_configured?() and sync_relevant?(changeset) do
       Ash.Changeset.after_transaction(changeset, &sync_after_transaction/2)
     else
       changeset
     end
   end
 
+  defp sync_relevant?(changeset) do
+    case changeset.action_type do
+      :create -> true
+      :update -> relevant_update?(changeset)
+      _ -> false
+    end
+  end
+
+  defp relevant_update?(changeset) do
+    any_synced_attr_changed? =
+      Enum.any?(@synced_attributes, &Ash.Changeset.changing_attribute?(changeset, &1))
+
+    record = changeset.data
+    no_contact_id_yet? = record && blank_contact_id?(record.vereinfacht_contact_id)
+
+    any_synced_attr_changed? or no_contact_id_yet?
+  end
+
+  defp blank_contact_id?(nil), do: true
+  defp blank_contact_id?(""), do: true
+  defp blank_contact_id?(s) when is_binary(s), do: String.trim(s) == ""
+  defp blank_contact_id?(_), do: false
+
   # Ash calls after_transaction with (changeset, result) only - 2 args.
   defp sync_after_transaction(_changeset, {:ok, member}) do
     case Mv.Vereinfacht.sync_member(member) do