diff --git a/app/models/room/messages_count.rb b/app/models/room/messages_count.rb
index 53254ac..948724e 100644
--- a/app/models/room/messages_count.rb
+++ b/app/models/room/messages_count.rb
@@ -9,27 +9,9 @@ class Room::MessagesCount
 
   class << self
     def install!(connection = ActiveRecord::Base.connection)
-      uninstall!(connection)
-      connection.execute <<~SQL
-        CREATE TRIGGER #{INSERT_TRIGGER} AFTER INSERT ON messages
-        BEGIN
-          UPDATE rooms SET messages_count = messages_count + 1 WHERE id = NEW.room_id;
-        END
-      SQL
-      connection.execute <<~SQL
-        CREATE TRIGGER #{DELETE_TRIGGER} AFTER DELETE ON messages
-        BEGIN
-          UPDATE rooms SET messages_count = messages_count - 1 WHERE id = OLD.room_id;
-        END
-      SQL
-      connection.execute <<~SQL
-        CREATE TRIGGER #{UPDATE_TRIGGER} AFTER UPDATE OF room_id ON messages
-        WHEN OLD.room_id IS NOT NEW.room_id
-        BEGIN
-          UPDATE rooms SET messages_count = messages_count - 1 WHERE id = OLD.room_id;
-          UPDATE rooms SET messages_count = messages_count + 1 WHERE id = NEW.room_id;
-        END
-      SQL
+      with_immediate_write(connection) do
+        replace_triggers!(connection)
+      end
     end
 
     def uninstall!(connection = ActiveRecord::Base.connection)
@@ -47,13 +29,21 @@ class Room::MessagesCount
     end
 
     # schema.rb does not dump SQLite triggers; reinstall after schema:load.
+    # Repair rechecks, rebuilds counts, and installs triggers in one write txn
+    # so concurrent writers and concurrent boot repairs cannot observe a gap.
     def ensure!(connection = ActiveRecord::Base.connection)
       return unless connection.adapter_name.match?(/sqlite/i)
       return unless connection.data_source_exists?(:rooms)
       return unless connection.column_exists?(:rooms, :messages_count)
-      return if TRIGGERS.all? { |name| trigger_installed?(connection, name) }
+      return if triggers_installed?(connection)
 
-      install!(connection)
+      with_immediate_write(connection) do
+        next if triggers_installed?(connection)
+
+        uninstall!(connection)
+        backfill!(connection)
+        create_triggers!(connection)
+      end
     end
 
     def trigger_installed?(connection, name)
@@ -61,5 +51,46 @@ class Room::MessagesCount
         "SELECT 1 FROM sqlite_master WHERE type = 'trigger' AND name = #{connection.quote(name)}"
       ).present?
     end
+
+    def triggers_installed?(connection = ActiveRecord::Base.connection)
+      TRIGGERS.all? { |name| trigger_installed?(connection, name) }
+    end
+
+    private
+      def with_immediate_write(connection)
+        if connection.transaction_open?
+          yield
+        else
+          connection.raw_connection.transaction(:immediate) { yield }
+        end
+      end
+
+      def replace_triggers!(connection)
+        uninstall!(connection)
+        create_triggers!(connection)
+      end
+
+      def create_triggers!(connection)
+        connection.execute <<~SQL
+          CREATE TRIGGER #{INSERT_TRIGGER} AFTER INSERT ON messages
+          BEGIN
+            UPDATE rooms SET messages_count = messages_count + 1 WHERE id = NEW.room_id;
+          END
+        SQL
+        connection.execute <<~SQL
+          CREATE TRIGGER #{DELETE_TRIGGER} AFTER DELETE ON messages
+          BEGIN
+            UPDATE rooms SET messages_count = messages_count - 1 WHERE id = OLD.room_id;
+          END
+        SQL
+        connection.execute <<~SQL
+          CREATE TRIGGER #{UPDATE_TRIGGER} AFTER UPDATE OF room_id ON messages
+          WHEN OLD.room_id IS NOT NEW.room_id
+          BEGIN
+            UPDATE rooms SET messages_count = messages_count - 1 WHERE id = OLD.room_id;
+            UPDATE rooms SET messages_count = messages_count + 1 WHERE id = NEW.room_id;
+          END
+        SQL
+      end
   end
 end
diff --git a/test/models/room/messages_count_lifecycle_test.rb b/test/models/room/messages_count_lifecycle_test.rb
index 295e178..fff8a3f 100644
--- a/test/models/room/messages_count_lifecycle_test.rb
+++ b/test/models/room/messages_count_lifecycle_test.rb
@@ -21,25 +21,24 @@ class Room::MessagesCountLifecycleTest < ActiveSupport::TestCase
     Room::MessagesCount.backfill!
   end
 
-  test "ensure! installs missing triggers without changing existing counts" do
+  test "ensure! installs missing triggers and keeps accurate counts" do
     before = @room.reload.messages_count
     Room::MessagesCount.uninstall!
 
-    assert_not Room::MessagesCount.trigger_installed?(ActiveRecord::Base.connection, Room::MessagesCount::INSERT_TRIGGER)
+    assert_not Room::MessagesCount.triggers_installed?
 
     Room::MessagesCount.ensure!
 
-    assert Room::MessagesCount::TRIGGERS.all? { |name|
-      Room::MessagesCount.trigger_installed?(ActiveRecord::Base.connection, name)
-    }
+    assert Room::MessagesCount.triggers_installed?
     assert_equal before, @room.reload.messages_count
+    assert_equal @room.messages.count, @room.messages_count
 
     assert_difference -> { @room.reload.messages_count }, +1 do
       @room.messages.create!(creator: users(:jason), body: "After ensure", client_message_id: "count-ensure")
     end
   end
 
-  test "ensure! repairs a partial trigger install" do
+  test "ensure! repairs a partial trigger install and backfills drifted counts" do
     Room::MessagesCount.uninstall!
     ActiveRecord::Base.connection.execute <<~SQL
       CREATE TRIGGER #{Room::MessagesCount::INSERT_TRIGGER} AFTER INSERT ON messages
@@ -51,11 +50,147 @@ class Room::MessagesCountLifecycleTest < ActiveSupport::TestCase
     assert Room::MessagesCount.trigger_installed?(ActiveRecord::Base.connection, Room::MessagesCount::INSERT_TRIGGER)
     assert_not Room::MessagesCount.trigger_installed?(ActiveRecord::Base.connection, Room::MessagesCount::DELETE_TRIGGER)
 
+    # Delete fires with no delete trigger → counter drifts high.
+    drifted = @room.messages.create!(creator: users(:jason), body: "Drift", client_message_id: "count-partial-drift")
+    ActiveRecord::Base.connection.execute("DELETE FROM messages WHERE id = #{drifted.id}")
+    assert_operator @room.reload.messages_count, :>, @room.messages.count
+
     Room::MessagesCount.ensure!
 
-    assert Room::MessagesCount::TRIGGERS.all? { |name|
-      Room::MessagesCount.trigger_installed?(ActiveRecord::Base.connection, name)
-    }
+    assert Room::MessagesCount.triggers_installed?
+    assert_equal @room.messages.count, @room.reload.messages_count
+  end
+
+  test "ensure! backfills writes that landed while the insert trigger was missing" do
+    Room::MessagesCount.uninstall!
+    before_count = @room.messages.count
+    before_cached = @room.reload.messages_count
+    assert_equal before_count, before_cached
+
+    path = File.expand_path(ActiveRecord::Base.connection_db_config.database)
+    now = Time.current.utc.strftime("%Y-%m-%d %H:%M:%S.%6N")
+    ActiveRecord::Base.connection_pool.release_connection
+
+    SQLite3::Database.new(path) do |db|
+      db.busy_timeout = 5_000
+      db.execute(
+        "INSERT INTO messages (room_id, creator_id, client_message_id, created_at, updated_at) VALUES (?, ?, ?, ?, ?)",
+        [ @room.id, users(:david).id, "count-missing-insert", now, now ]
+      )
+    end
+
+    assert_equal before_count + 1, @room.messages.count
+    assert_equal before_cached, @room.reload.messages_count
+
+    Room::MessagesCount.ensure!
+
+    assert Room::MessagesCount.triggers_installed?
+    assert_equal @room.messages.count, @room.reload.messages_count
+    assert_equal before_cached + 1, @room.messages_count
+  end
+
+  test "concurrent ensure! repairs leave triggers installed and counts accurate" do
+    Room::MessagesCount.uninstall!
+    path = File.expand_path(ActiveRecord::Base.connection_db_config.database)
+    now = Time.current.utc.strftime("%Y-%m-%d %H:%M:%S.%6N")
+    ActiveRecord::Base.connection_pool.release_connection
+
+    SQLite3::Database.new(path) do |db|
+      db.busy_timeout = 5_000
+      db.execute(
+        "INSERT INTO messages (room_id, creator_id, client_message_id, created_at, updated_at) VALUES (?, ?, ?, ?, ?)",
+        [ @room.id, users(:david).id, "count-concurrent-drift", now, now ]
+      )
+    end
+
+    assert_not_equal @room.messages.count, @room.reload.messages_count
+
+    errors = []
+    errors_mutex = Mutex.new
+
+    workers = 2.times.map do
+      Thread.new do
+        ActiveRecord::Base.connection_pool.with_connection do |connection|
+          Room::MessagesCount.ensure!(connection)
+        end
+      rescue => error
+        errors_mutex.synchronize { errors << error }
+      end
+    end
+
+    workers.each(&:join)
+
+    assert_empty errors, -> { errors.map(&:full_message).join("\n") }
+    assert Room::MessagesCount.triggers_installed?
+    assert_equal @room.messages.count, @room.reload.messages_count
+  end
+
+  test "writes concurrent with ensure! repair leave accurate counts" do
+    Room::MessagesCount.uninstall!
+    path = File.expand_path(ActiveRecord::Base.connection_db_config.database)
+    now = Time.current.utc.strftime("%Y-%m-%d %H:%M:%S.%6N")
+    ActiveRecord::Base.connection_pool.release_connection
+
+    errors = []
+    errors_mutex = Mutex.new
+
+    ensure_thread = Thread.new do
+      ActiveRecord::Base.connection_pool.with_connection do |connection|
+        Room::MessagesCount.ensure!(connection)
+      end
+    rescue => error
+      errors_mutex.synchronize { errors << error }
+    end
+
+    insert_thread = Thread.new do
+      SQLite3::Database.new(path) do |db|
+        db.busy_timeout = 10_000
+        db.execute(
+          "INSERT INTO messages (room_id, creator_id, client_message_id, created_at, updated_at) VALUES (?, ?, ?, ?, ?)",
+          [ @room.id, users(:david).id, "count-during-repair", now, now ]
+        )
+      end
+    rescue => error
+      errors_mutex.synchronize { errors << error }
+    end
+
+    [ ensure_thread, insert_thread ].each(&:join)
+
+    assert_empty errors, -> { errors.map(&:full_message).join("\n") }
+    assert Room::MessagesCount.triggers_installed?
+    assert_equal @room.messages.count, @room.reload.messages_count
+    assert Message.exists?(client_message_id: "count-during-repair")
+  end
+
+  test "install! never exposes a partial trigger set to other connections" do
+    Room::MessagesCount.uninstall!
+    path = File.expand_path(ActiveRecord::Base.connection_db_config.database)
+    ActiveRecord::Base.connection_pool.release_connection
+
+    stop = false
+    partial_snapshots = []
+    snapshots_mutex = Mutex.new
+
+    watcher = Thread.new do
+      SQLite3::Database.new(path) do |db|
+        db.busy_timeout = 5_000
+        until stop
+          names = db.execute(
+            "SELECT name FROM sqlite_master WHERE type = 'trigger' AND name IN (#{Room::MessagesCount::TRIGGERS.map { "'#{it}'" }.join(", ")})"
+          ).flatten
+          if names.any? && names.sort != Room::MessagesCount::TRIGGERS.sort
+            snapshots_mutex.synchronize { partial_snapshots << names.sort }
+          end
+        end
+      end
+    end
+
+    25.times { Room::MessagesCount.install! }
+    stop = true
+    watcher.join
+
+    assert_empty partial_snapshots
+    assert Room::MessagesCount.triggers_installed?
   end
 
   test "foreign SQLite connections keep the counter in step" do