diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 13c4fbba..0dc132d2 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -1,9 +1,19 @@
 name: split
 
-on: [push]
+on:
+  push:
+  pull_request:
+  schedule:
+    # Gemfile.lock is not committed, so new dependency releases can break the
+    # build without any change here. Build main weekly to catch that early.
+    - cron: "0 3 * * 1"
 
 jobs:
   test:
+    # Branches in this repository are already built on push, so only build
+    # pull requests opened from forks to avoid running the matrix twice.
+    if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name != github.repository
+
     strategy:
       matrix:
         ruby: ["4.0", "3.4", "3.3", "3.2", "3.1"]