diff --git a/.github/Gemfile.pages b/.github/Gemfile.pages
new file mode 100644
index 0000000..0f76f1a
--- /dev/null
+++ b/.github/Gemfile.pages
@@ -0,0 +1,14 @@
+# The Gemfile GitHub Pages actually builds with — not the one in the repo root.
+#
+# `pages-build-deployment` ignores our Gemfile and runs the `github-pages` gem
+# (currently 232, which pins Jekyll 3.10). Jekyll 3.10 forgives less Liquid than
+# the Jekyll 4 we develop against: `where_exp` takes exactly one comparison and
+# then demands end-of-string, so `and` / `or` inside the expression — a Jekyll 4
+# feature — parses locally and dies on deploy.
+#
+# CI builds against this file so it predicts the deploy. Do not pin a version
+# here: tracking whatever `github-pages` currently resolves to is the point.
+
+source "https://rubygems.org"
+
+gem "github-pages", group: :jekyll_plugins
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d155990..68e8727 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -3,8 +3,11 @@ on:
   pull_request:
   push:
     branches: [main]
+
 jobs:
+  # The Jekyll we develop against. Fast, and matches a local `jekyll serve`.
   build:
+    name: Jekyll 4 (what we develop on)
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
@@ -13,3 +16,20 @@ jobs:
           ruby-version: '3.2'
           bundler-cache: true
       - run: bundle exec jekyll build
+
+  # The build that actually predicts the deploy. GitHub Pages ignores the root
+  # Gemfile and runs the `github-pages` gem (Jekyll 3.10), which accepts less
+  # Liquid than Jekyll 4 — see .github/Gemfile.pages. Without this job, CI stays
+  # green while pages-build-deployment fails and the site silently serves stale.
+  pages-parity:
+    name: GitHub Pages (what actually deploys)
+    runs-on: ubuntu-latest
+    env:
+      BUNDLE_GEMFILE: ${{ github.workspace }}/.github/Gemfile.pages
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.2'
+          bundler-cache: true
+      - run: bundle exec jekyll build
diff --git a/.gitignore b/.gitignore
index 119286a..ac51552 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,4 +4,5 @@ _site/
 vendor/
 .bundle/
 Gemfile.lock
+Gemfile.pages.lock
 .DS_Store
diff --git a/roadmap.md b/roadmap.md
index b727de2..f0350ba 100644
--- a/roadmap.md
+++ b/roadmap.md
@@ -162,9 +162,16 @@ Two rules keep it honest:
 
 **Maturity describes the conversation, not the code.** Nothing gets promoted by age.
 
-{% assign live = site.data.roadmap.items | where_exp: "i", "i.deferred == false and i.state == 'open'" %}
+{% comment %}
+  One comparison per `where_exp`, deliberately. GitHub Pages pins Jekyll 3.10, whose `where_exp`
+  parses a single comparison and then demands end-of-string; `and` / `or` inside the expression is
+  a Jekyll 4 feature. A local Jekyll 4 build is therefore not evidence the Pages build passes —
+  the multi-clause version of these lines failed with "Expected end_of_string but found id".
+{% endcomment %}
+{% assign open_items = site.data.roadmap.items | where_exp: "i", "i.state == 'open'" %}
+{% assign live = open_items | where_exp: "i", "i.deferred == false" %}
+{% assign parked = open_items | where_exp: "i", "i.deferred" %}
 {% assign done = site.data.roadmap.items | where_exp: "i", "i.state == 'closed'" %}
-{% assign parked = site.data.roadmap.items | where_exp: "i", "i.deferred and i.state == 'open'" %}
 {% assign groups = "ready,consensus,discussing,raised" | split: "," %}
 {% for g in groups %}
 {% assign bucket = live | where: "maturity", g %}