diff --git a/README.md b/README.md
index eedce41dcf..8d7bce3153 100644
--- a/README.md
+++ b/README.md
@@ -246,7 +246,7 @@ new tab in the browser.
 To see local email previews, check out http://localhost:3000/rails/mailers
 
 ### Running Tests
- - run the ruby test suite `bin/rails spec`
+ - run the ruby test suite `bin/rails spec`. It builds the JS and CSS bundles first; a bare `bundle exec rspec` does not, so run `npm run build` and `npm run build:css` (or `bin/setup`) once before using it
  - run the javascript test suite `npm run test`
 
 If you have trouble running tests, check out CI scripts in [`.github/workflows/`](.github/workflows/) for sample commands.
diff --git a/spec/rails_helper.rb b/spec/rails_helper.rb
index 005c9d840d..67ed69e0a9 100644
--- a/spec/rails_helper.rb
+++ b/spec/rails_helper.rb
@@ -42,6 +42,15 @@
   abort e.to_s.strip
 end
 
+# app/assets/builds/ is gitignored and only populated by `npm run build` and
+# `npm run build:css`. Until then any layout that links those bundles fails,
+# and for tailwind.css with a misleading AssetNotPrecompiledError that points
+# at manifest.js. Fail early with the real fix instead.
+missing_assets = %w[application.js all_casa_admin.js tailwind.css].reject { |asset| Rails.root.join("app/assets/builds", asset).exist? }
+if missing_assets.any?
+  abort "Missing built assets: #{missing_assets.join(", ")}. Run `npm run build` and `npm run build:css`, or use `bin/rails spec`, which builds them."
+end
+
 ci_environment = (ENV["GITHUB_ACTIONS"] || ENV["CI"]).present?
 
 RSpec.configure do |config|