diff --git a/.github/scripts/collect-cache-health.sh b/.github/scripts/collect-cache-health.sh
new file mode 100755
index 000000000000..e6c19a70e687
--- /dev/null
+++ b/.github/scripts/collect-cache-health.sh
@@ -0,0 +1,65 @@
+#!/usr/bin/env bash
+# Scheduled collector: scans recent reviewer runs' LOGS for the CACHE_USAGE line
+# emitted by emit-cache-usage.sh, computes the warm-hit trend, and fails (a red
+# scheduled run = built-in alert) if caching looks regressed across a meaningful
+# sample.
+#
+# Reads only run logs (token counts) - no message content, no artifacts. Runs
+# entirely out of the review hot path.
+#
+# Tunables (env):
+#   LOOKBACK                  how many recent reviewer runs to scan (default 30)
+#   CACHE_CREATION_WARN_TOKENS a run is "cold" if creation exceeds this (default 15000)
+#   COLD_ALERT_PCT            alert if more than this % of the sample is cold (default 40)
+#   MIN_SAMPLE               need at least this many data-carrying runs to judge (default 10)
+set -euo pipefail
+
+REPO="${GITHUB_REPOSITORY:?GITHUB_REPOSITORY not set}"
+LOOKBACK="${LOOKBACK:-30}"
+WARN_TOKENS="${CACHE_CREATION_WARN_TOKENS:-15000}"
+COLD_ALERT_PCT="${COLD_ALERT_PCT:-40}"
+MIN_SAMPLE="${MIN_SAMPLE:-10}"
+
+mapfile -t RUNS < <(gh run list --repo "$REPO" --workflow claude-review.yml \
+  --status completed --limit "$LOOKBACK" --json databaseId --jq '.[].databaseId')
+
+scanned=0
+cold=0
+rows=""
+for id in "${RUNS[@]}"; do
+  line=$(gh run view "$id" --repo "$REPO" --log 2>/dev/null | grep -m1 'CACHE_USAGE' || true)
+  [ -n "$line" ] || continue
+  c=$(sed -n 's/.*creation=\([0-9][0-9]*\).*/\1/p' <<<"$line")
+  r=$(sed -n 's/.*read=\([0-9][0-9]*\).*/\1/p' <<<"$line")
+  [ -n "$c" ] && [ -n "$r" ] || continue
+  # no cache activity at all = no data, not a warm sample
+  [ "$c" -eq 0 ] && [ "$r" -eq 0 ] && continue
+  t=$(( c + r )); ratio=0; [ "$t" -gt 0 ] && ratio=$(( r * 100 / t ))
+  scanned=$(( scanned + 1 ))
+  [ "$c" -gt "$WARN_TOKENS" ] && cold=$(( cold + 1 ))
+  rows="${rows}| ${id} | ${c} | ${r} | ${ratio}% |"$'\n'
+done
+
+{
+  echo "### Reviewer cache monitor"
+  echo ""
+  echo "Scanned **${scanned}** data-carrying runs of the last ${LOOKBACK}."
+  echo ""
+  echo "| run | creation | read | warm% |"
+  echo "| --- | --- | --- | --- |"
+  printf '%s' "$rows"
+} >> "${GITHUB_STEP_SUMMARY:-/dev/stdout}"
+
+if [ "$scanned" -lt "$MIN_SAMPLE" ]; then
+  echo "Only ${scanned} runs carried cache data (< ${MIN_SAMPLE}); not enough to judge — skipping."
+  exit 0
+fi
+
+coldpct=$(( cold * 100 / scanned ))
+echo "cold=${cold}/${scanned} (${coldpct}%)  alert threshold=${COLD_ALERT_PCT}%"
+
+if [ "$coldpct" -gt "$COLD_ALERT_PCT" ]; then
+  echo "::error title=Reviewer cache regressed::${coldpct}% of the last ${scanned} reviewer runs were cold (> ${COLD_ALERT_PCT}%). Likely cause: a rules edit, an action/CLI bump, a model change, or the 1h TTL being lost."
+  exit 1
+fi
+echo "Reviewer cache healthy: ${coldpct}% cold over ${scanned} runs."
diff --git a/.github/scripts/emit-cache-usage.sh b/.github/scripts/emit-cache-usage.sh
new file mode 100755
index 000000000000..82a2f1866b1f
--- /dev/null
+++ b/.github/scripts/emit-cache-usage.sh
@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+# In-run, minimal: emit the reviewer's prompt-cache token counts to the run log.
+#
+# Token COUNTS ONLY (no message content) -> safe in public logs. Source is the
+# action's execution file (always written, independent of `show_full_output`).
+#
+# Analysis, thresholds and alerting live OUT of the review hot path: the
+# scheduled collector (.github/workflows/reviewer-cache-monitor.yml) scans these
+# lines across recent runs to detect caching regressions over time.
+set -euo pipefail
+
+f="${EXEC_FILE:-}"
+if [ -z "$f" ] || [ ! -s "$f" ]; then
+  echo "emit-cache-usage: no execution file, skipping"
+  exit 0
+fi
+
+jq -r '
+  [.[] | select(.type=="assistant") | .message.usage // empty] as $u
+  | (($u | map(.cache_creation_input_tokens // 0) | add) // 0) as $c
+  | (($u | map(.cache_read_input_tokens // 0) | add) // 0) as $r
+  | (([.[] | select(.type=="result")] | last | .total_cost_usd) // 0) as $cost
+  | "CACHE_USAGE creation=\($c) read=\($r) cost=\($cost)"' "$f"
diff --git a/.github/workflows/claude-review.yml b/.github/workflows/claude-review.yml
index 4cfc0f46fcf2..efd3fd1b06e3 100644
--- a/.github/workflows/claude-review.yml
+++ b/.github/workflows/claude-review.yml
@@ -110,6 +110,15 @@ jobs:
           STRUCTURED_OUTPUT: ${{ steps.code-review.outputs.structured_output }}
         run: postCodeReviewResults.sh "$PR_NUMBER"
 
+      # Monitoring (emit only): print prompt-cache token counts to the log so the
+      # out-of-band collector (.github/workflows/reviewer-cache-monitor.yml) can
+      # track cache health across runs. Counts only - no message content.
+      - name: Emit reviewer cache usage
+        if: always() && steps.code-review.outputs.execution_file != ''
+        env:
+          EXEC_FILE: ${{ steps.code-review.outputs.execution_file }}
+        run: .github/scripts/emit-cache-usage.sh
+
       - name: Run Claude Code (docs)
         if: steps.set-authorized.outputs.IS_AUTHORIZED == 'true' && steps.filter.outputs.docs == 'true'
         uses: anthropics/claude-code-action@4d7e1f0cd85743fdc93b1c8040ab54395da024e2 # v1.0.149
diff --git a/.github/workflows/reviewer-cache-monitor.yml b/.github/workflows/reviewer-cache-monitor.yml
new file mode 100644
index 000000000000..2de9fe5886bc
--- /dev/null
+++ b/.github/workflows/reviewer-cache-monitor.yml
@@ -0,0 +1,39 @@
+name: Reviewer cache monitor
+
+# Out-of-band collector for reviewer prompt-cache health. Scans recent
+# "PR Reviews with Claude Code" runs' logs for the CACHE_USAGE line and fails
+# (a red scheduled run = built-in alert) if caching has regressed across a
+# meaningful sample. Reads logs only - no message content, no artifacts.
+
+on:
+  schedule:
+    - cron: "23 * * * *" # hourly
+  workflow_dispatch:
+    inputs:
+      lookback:
+        description: How many recent reviewer runs to scan
+        required: false
+        default: "30"
+
+permissions:
+  contents: read
+  actions: read
+
+concurrency:
+  group: reviewer-cache-monitor
+  cancel-in-progress: true
+
+jobs:
+  collect:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 1
+
+      - name: Scan reviewer runs for cache health
+        env:
+          GH_TOKEN: ${{ github.token }}
+          LOOKBACK: ${{ github.event.inputs.lookback || '30' }}
+        run: .github/scripts/collect-cache-health.sh