diff --git a/.github/workflows/render-and-publish.yml b/.github/workflows/render-and-publish.yml
index de274ae3..c881b2fb 100644
--- a/.github/workflows/render-and-publish.yml
+++ b/.github/workflows/render-and-publish.yml
@@ -1,4 +1,7 @@
 on:
+  pull_request:
+    branches:
+      - main
   push:
     branches:
       - main
@@ -49,8 +52,11 @@ jobs:
           key: cljdeps-${{ hashFiles('deps.edn') }}
           restore-keys: cljdeps-
 
+      - name: Test strict Clay failure detection
+        run: scripts/test-render-clay-strict.sh
+
       - name: Build the content notebooks
-        run: clojure -M:clay -A:markdown
+        run: scripts/render-clay-strict.sh
 
       - name: Install lsof and apt prerequisites for Janqua
         run: sudo apt-get update && sudo apt-get install -y lsof gnupg lsb-release
@@ -91,6 +97,7 @@ jobs:
           path: site/_site
 
   deploy:
+    if: github.event_name == 'push' && github.ref == 'refs/heads/main'
     needs: build
     permissions:
       pages: write
diff --git a/README.md b/README.md
index b6efeb18..4e07be3c 100644
--- a/README.md
+++ b/README.md
@@ -310,6 +310,13 @@ Goal: Minimize friction in authoring while ensuring publishable reproducibility.
 
 The site is built and deployed using GitHub Actions with two workflows:
 
+Before opening a pull request, run `scripts/pre-publish-gate.sh` from the
+repository root. The same strict Clay renderer is used locally and in the full
+GitHub Pages workflow. It requires Java 21, uses the runner's 1 MiB JVM thread
+stack, fails when any source prints `Clay FAILED:`, then renders the full site
+with Quarto. `scripts/test-render-clay-strict.sh` reproduces the historical
+stack-overflow signal and checks that CI rejects it with an error annotation.
+
 - **Full Build and Publish**: Triggered on pushes to `main`.
   Rebuilds all notebooks with Clay, renders the entire site with Quarto, and deploys to GitHub Pages.
   See [.github/workflows/render-and-publish.yml](.github/workflows/render-and-publish.yml).
diff --git a/scripts/pre-publish-gate.sh b/scripts/pre-publish-gate.sh
new file mode 100755
index 00000000..580656be
--- /dev/null
+++ b/scripts/pre-publish-gate.sh
@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+repo_root="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+cd "$repo_root"
+
+scripts/render-clay-strict.sh
+quarto render site
diff --git a/scripts/render-clay-strict.sh b/scripts/render-clay-strict.sh
new file mode 100755
index 00000000..4ab529a2
--- /dev/null
+++ b/scripts/render-clay-strict.sh
@@ -0,0 +1,41 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+repo_root="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+cd "$repo_root"
+
+java_version="$({ java -XshowSettings:properties -version; } 2>&1 \
+  | awk -F= '/java.specification.version/ {gsub(/[[:space:]]/, "", $2); print $2; exit}')"
+if [[ "$java_version" != "21" ]]; then
+  echo "Expected Java 21 to match GitHub Pages, found Java ${java_version:-unknown}." >&2
+  exit 1
+fi
+
+# GitHub's Ubuntu runner uses a 1 MiB JVM thread stack. macOS commonly uses
+# 2 MiB, which can hide recursive parser/regex failures until publication.
+export JAVA_TOOL_OPTIONS="${JAVA_TOOL_OPTIONS:+${JAVA_TOOL_OPTIONS} }-Xss1m"
+
+log_file="$(mktemp "${TMPDIR:-/tmp}/clay-render.XXXXXX.log")"
+trap 'rm -f "$log_file"' EXIT
+
+set +e
+clojure -M:clay -A:markdown "$@" 2>&1 | tee "$log_file"
+clay_status="${PIPESTATUS[0]}"
+set -e
+
+if [[ "$clay_status" -ne 0 ]]; then
+  exit "$clay_status"
+fi
+
+# Clay reports individual source failures but can still return success.
+if grep -q 'Clay FAILED:' "$log_file"; then
+  echo "Clay reported source failures despite exiting successfully:" >&2
+  grep 'Clay FAILED:' "$log_file" >&2
+  if [[ "${GITHUB_ACTIONS:-}" == "true" ]]; then
+    while IFS= read -r failure; do
+      echo "::error title=Clay source render failed::${failure}" >&2
+    done < <(grep 'Clay FAILED:' "$log_file")
+  fi
+  exit 1
+fi
diff --git a/scripts/test-render-clay-strict.sh b/scripts/test-render-clay-strict.sh
new file mode 100755
index 00000000..015a87ff
--- /dev/null
+++ b/scripts/test-render-clay-strict.sh
@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+repo_root="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+fixture_bin="$(mktemp -d "${TMPDIR:-/tmp}/clay-strict-fixture.XXXXXX")"
+output_file="$(mktemp "${TMPDIR:-/tmp}/clay-strict-output.XXXXXX.log")"
+trap 'rm -rf "$fixture_bin"; rm -f "$output_file"' EXIT
+
+cat >"$fixture_bin/java" <<'EOF'
+#!/usr/bin/env bash
+echo '    java.specification.version = 21' >&2
+EOF
+
+cat >"$fixture_bin/clojure" <<'EOF'
+#!/usr/bin/env bash
+echo 'Clay FAILED: src/language_learning/vocabulary_estimation/beta_binomial_first_pass.clj'
+echo 'java.lang.StackOverflowError'
+exit 0
+EOF
+
+chmod +x "$fixture_bin/java" "$fixture_bin/clojure"
+
+set +e
+GITHUB_ACTIONS=true PATH="$fixture_bin:$PATH" \
+  "$repo_root/scripts/render-clay-strict.sh" >"$output_file" 2>&1
+result_status=$?
+set -e
+
+if [[ "$result_status" -ne 1 ]]; then
+  sed -n '1,120p' "$output_file" >&2
+  echo "Expected the strict Clay renderer to reject the historical failure; got status $result_status." >&2
+  exit 1
+fi
+
+grep -Fq 'java.lang.StackOverflowError' "$output_file"
+grep -Fq 'Clay reported source failures despite exiting successfully:' "$output_file"
+grep -Fq '::error title=Clay source render failed::Clay FAILED: src/language_learning/vocabulary_estimation/beta_binomial_first_pass.clj' "$output_file"
+
+echo 'Strict Clay regression passed: the historical stack-overflow signal was rejected with a GitHub error annotation.'