diff --git a/roles/pi_musicmouse/defaults/main.yml b/roles/pi_musicmouse/defaults/main.yml
index 72ea230..c44756c 100644
--- a/roles/pi_musicmouse/defaults/main.yml
+++ b/roles/pi_musicmouse/defaults/main.yml
@@ -43,3 +43,7 @@ pi_musicmouse_force_config: false
 # section will not start without it - curriculum_file is validated as must-exist. Set to
 # "" for a host whose config has no tippen section.
 pi_musicmouse_curriculum_file: "tippen-curriculum.yml"
+
+# The commit the checkout task actually landed on, used to decide whether the venv
+# needs rebuilding. Empty under --check, where the git task reports without writing.
+pi_musicmouse_commit: "{{ pi_musicmouse_checkout.after | default('') }}"
diff --git a/roles/pi_musicmouse/tasks/main.yml b/roles/pi_musicmouse/tasks/main.yml
index 1b18a88..8fdf7b2 100644
--- a/roles/pi_musicmouse/tasks/main.yml
+++ b/roles/pi_musicmouse/tasks/main.yml
@@ -51,6 +51,7 @@
     version: "{{ musicmouse_version }}"
     accept_hostkey: true
     force: true
+  register: pi_musicmouse_checkout
 
 - name: Check the checkout has the layout this role knows
   ansible.builtin.stat:
@@ -77,18 +78,45 @@
     cmd: "/usr/local/bin/uv venv --python {{ pi_musicmouse_python_version }} /opt/musicmouse/.venv"
     creates: /opt/musicmouse/.venv/bin/python
 
+# Installing the backend is deliberately not a handler hanging off the checkout.
+# Tied to the checkout, one failed run strands the device: the notification is dropped
+# when the play fails, the next run finds the checkout already current and notifies
+# nothing, and the venv stays empty while the service crash-loops on
+# ModuleNotFoundError. Instead, decide from what is actually on the device - the commit
+# it has checked out, and whether the venv can import the package at all.
+#
+# uv's own output cannot answer this: `musicmouse` is a local path dependency, so every
+# run rebuilds and reinstalls it and always prints "Installed 1 package". Keying off
+# that restarts the service on every run and rescans the whole library for nothing.
+- name: Read which commit the venv was built from
+  ansible.builtin.command:
+    cmd: cat /opt/musicmouse/.venv/.installed-commit
+  register: pi_musicmouse_stamp
+  changed_when: false
+  failed_when: false
+
+- name: Check the venv can actually import the backend
+  ansible.builtin.command:
+    cmd: /opt/musicmouse/.venv/bin/python -c "import musicmouse"
+  register: pi_musicmouse_importable
+  changed_when: false
+  failed_when: false
+
 - name: Install the backend into the venv
-  # Every run, not on a checkout change via a handler. Tied to the checkout, one failed
-  # run strands the device: the notification is dropped when the play fails, the next
-  # run finds the checkout already current and notifies nothing, and the venv stays
-  # empty while the service crash-loops on ModuleNotFoundError. uv is fast when there
-  # is nothing to do, and its output says whether there was.
   ansible.builtin.command:
     cmd: "/usr/local/bin/uv pip install --python /opt/musicmouse/.venv/bin/python /opt/musicmouse/python-backend"
-  register: pi_musicmouse_install
-  changed_when: "'Installed' in pi_musicmouse_install.stderr"
+  when: >-
+    pi_musicmouse_importable.rc != 0
+    or pi_musicmouse_stamp.stdout | trim != pi_musicmouse_commit
+  changed_when: true # guarded by `when`, so reaching this means it installed something
   notify: Restart musicmouse
 
+- name: Record which commit the venv was built from
+  ansible.builtin.copy:
+    content: "{{ pi_musicmouse_commit }}\n"
+    dest: /opt/musicmouse/.venv/.installed-commit
+    mode: "0644"
+
 - name: Create media directory
   ansible.builtin.file:
     path: /media/musicmouse