diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 9d37d7dc..41af1487 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -61,6 +61,8 @@ jobs:
         run: node tools/ci/link-check.mjs
       - name: identity grep of the public export list
         run: bash tools/ci/identity-check.sh
+      - name: no conflict markers in tracked files
+        run: bash tools/ci/no-conflict-markers.sh
       - name: copied sources are re-stamped before a build
         run: bash tools/ci/copied-sources-check.sh
       - name: pinned guest programs match their manifest and are built only by pin-guests.sh
diff --git a/tools/ci/install-hooks.sh b/tools/ci/install-hooks.sh
new file mode 100755
index 00000000..f138ab7c
--- /dev/null
+++ b/tools/ci/install-hooks.sh
@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+# Installs the repository's git hooks into this checkout (pre-push: no conflict markers, the site builds).
+set -euo pipefail
+cd "$(dirname "$0")/../.."
+cat > .git/hooks/pre-push <<'HOOK'
+#!/usr/bin/env bash
+set -e
+cd "$(git rev-parse --show-toplevel)"
+bash tools/ci/no-conflict-markers.sh
+(cd site && node build.mjs >/dev/null) || { echo "pre-push: the site build fails; fix it before pushing" >&2; exit 1; }
+HOOK
+chmod +x .git/hooks/pre-push; echo "pre-push hook installed"
diff --git a/tools/ci/no-conflict-markers.sh b/tools/ci/no-conflict-markers.sh
new file mode 100755
index 00000000..de3c2913
--- /dev/null
+++ b/tools/ci/no-conflict-markers.sh
@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+# No tracked file may carry a merge conflict marker. 5 October 2026: a marker in the generated site/journey.json broke
+# `node build.mjs` silently for two merges and a half-merged home page went live for a few minutes. Runs in CI and
+# before every push from the main checkout (git hook installed by tools/ci/install-hooks.sh).
+set -euo pipefail
+cd "$(dirname "$0")/../.."
+hits="$(git ls-files -z | xargs -0 grep -lE '^(<<<<<<< |=======$|>>>>>>> )' 2>/dev/null | grep -vE '^tools/ci/no-conflict-markers\.sh$' || true)"
+if [ -n "$hits" ]; then echo "conflict markers in:"; echo "$hits" | sed 's/^/  /'; exit 1; fi
+echo "no conflict markers in tracked files"