diff --git a/roles/environment/defaults/main.yml b/roles/environment/defaults/main.yml
index 69520c6..73b1511 100644
--- a/roles/environment/defaults/main.yml
+++ b/roles/environment/defaults/main.yml
@@ -9,6 +9,14 @@ environment_pacman_lock_timeout: 120
 environment_pacman_retries: 4
 environment_pacman_retry_delay: 15
 
+# Libraries the installer tools pull whose soname may have bumped past the ISO.
+# Each library's installed reverse-dependencies are upgraded together with the
+# tools so a current install onto an older ISO stays a consistent transaction
+# instead of a partial upgrade. Extend if a future transition breaks the install.
+environment_partial_upgrade_libs:
+  - nettle
+  - leancrypto
+
 # PCI vendor IDs -> vendor codes used by hardware detection.
 # Only vendors that drive distinct firmware/driver packages are mapped.
 environment_pci_vendor_map:
diff --git a/roles/environment/tasks/_prepare_installer.yml b/roles/environment/tasks/_prepare_installer.yml
index 667cc60..adf1f67 100644
--- a/roles/environment/tasks/_prepare_installer.yml
+++ b/roles/environment/tasks/_prepare_installer.yml
@@ -14,24 +14,52 @@
     timeout: "{{ environment_pacman_lock_timeout }}"
   changed_when: false
 
-- name: Setup Pacman
+- name: Resolve installer tools for the target OS
+  when: not (custom_iso | bool)
+  ansible.builtin.set_fact:
+    environment_installer_tools: >-
+      {{
+        ['glibc']
+        + (['lua', 'dnf'] if os in ['almalinux', 'fedora', 'rhel', 'rocky'] else [])
+        + (['debootstrap'] if os in ['debian', 'ubuntu', 'ubuntu-lts'] else [])
+        + (['debian-archive-keyring'] if os == 'debian' else [])
+        + (['ubuntu-keyring'] if os in ['ubuntu', 'ubuntu-lts'] else [])
+      }}
+
+- name: Query reverse-dependencies of transition-sensitive libraries
   when:
     - not (custom_iso | bool)
-    - item.os is not defined or os in item.os
+    - environment_partial_upgrade_libs | length > 0
+  ansible.builtin.command: "pacman -Qi {{ item }}"
+  loop: "{{ environment_partial_upgrade_libs }}"
+  register: environment_revdep_query
+  changed_when: false
+  failed_when: false
+
+# Co-upgrade each transition library with its installed reverse-deps so a soname
+# bump moves the whole closure in one transaction, not a partial upgrade.
+- name: Setup Pacman
+  when: not (custom_iso | bool)
+  vars:
+    environment_pacman_closure: >-
+      {{
+        (
+          environment_installer_tools
+          + (environment_revdep_query.results | default([])
+             | selectattr('rc', 'equalto', 0) | map(attribute='item') | list)
+          + (environment_revdep_query.results | default([])
+             | selectattr('rc', 'equalto', 0) | map(attribute='stdout')
+             | map('regex_search', 'Required By\s*:\s*(.+)', '\1')
+             | map('first') | map('split') | flatten)
+        )
+        | reject('equalto', 'None') | unique
+      }}
   community.general.pacman:
     update_cache: true
-    force: true
-    name: "{{ item.name }}"
+    name: "{{ environment_pacman_closure }}"
     state: latest
-  loop:
-    - { name: glibc }
-    - { name: lua, os: [almalinux, fedora, rhel, rocky] }
-    - { name: dnf, os: [almalinux, fedora, rhel, rocky] }
-    - { name: debootstrap, os: [debian, ubuntu, ubuntu-lts] }
-    - { name: debian-archive-keyring, os: [debian] }
-    - { name: ubuntu-keyring, os: [ubuntu, ubuntu-lts] }
-  loop_control:
-    label: "{{ item.name }}"
+  register: environment_tool_install
+  until: environment_tool_install is succeeded
   retries: "{{ environment_pacman_retries }}"
   delay: "{{ environment_pacman_retry_delay }}"
 
@@ -76,10 +104,8 @@
         opts: "ro,loop"
         state: mounted
 
-# Security note: RPM Sequoia signature policy is relaxed to allow
-# bootstrapping RHEL-family distros from the Arch ISO, where the
-# host rpm/dnf does not trust target distro GPG keys. Package
-# integrity is verified by the target system's own rpm after reboot.
+# RPM Sequoia signature policy is relaxed because the Arch ISO host does not
+# trust target-distro GPG keys; the target's own rpm re-verifies after reboot.
 - name: Create RPM macros directory
   when: is_rhel | bool
   ansible.builtin.file: