diff --git a/.github/workflows/claude-review.yml b/.github/workflows/claude-review.yml
index 1d9c5917c..1056e6dab 100644
--- a/.github/workflows/claude-review.yml
+++ b/.github/workflows/claude-review.yml
@@ -79,14 +79,31 @@ jobs:
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           fetch-depth: 0
+          # On issue_comment events github.ref is the default branch, not the PR
+          # head, so checkout would fetch main. Check out the PR merge ref instead
+          # so the review actually runs against the PR code. This ref works for
+          # fork PRs with a read token; the fork guard below still governs whether
+          # AI review runs. pull_request events keep default behavior (no ref),
+          # which already checks out the PR merge commit.
+          ref: ${{ github.event_name == 'issue_comment' && format('refs/pull/{0}/merge', github.event.issue.number) || '' }}
 
       - name: Check PR size (cost control)
         if: steps.fork_check.outputs.is_fork != 'true'
         id: pr_size
         run: |
-          BASE_REF="${{ github.base_ref || github.event.repository.default_branch || 'main' }}"
-          FILES_CHANGED=$(git diff --name-only origin/${BASE_REF}...HEAD | wc -l)
-          LINES_CHANGED=$(git diff --stat origin/${BASE_REF}...HEAD | tail -1 | awk '{print $4+$6}')
+          # On issue_comment events github.base_ref is empty and the checked-out
+          # HEAD is the PR merge ref (refs/pull/N/merge), whose first parent is
+          # the PR's base tip — so diff HEAD^...HEAD to measure the PR changes.
+          # On pull_request events diff against the base branch as before.
+          if [ "${{ github.event_name }}" == "issue_comment" ]; then
+            DIFF_ARGS="HEAD^...HEAD"
+          else
+            BASE_REF="${{ github.base_ref || github.event.repository.default_branch || 'main' }}"
+            DIFF_ARGS="origin/${BASE_REF}...HEAD"
+          fi
+
+          FILES_CHANGED=$(git diff --name-only ${DIFF_ARGS} | wc -l)
+          LINES_CHANGED=$(git diff --stat ${DIFF_ARGS} | tail -1 | awk '{print $4+$6}')
 
           echo "files_changed=$FILES_CHANGED" >> $GITHUB_OUTPUT
           echo "lines_changed=$LINES_CHANGED" >> $GITHUB_OUTPUT
diff --git a/ods/tests/test-claude-review-pr-ref.sh b/ods/tests/test-claude-review-pr-ref.sh
new file mode 100644
index 000000000..02bf80473
--- /dev/null
+++ b/ods/tests/test-claude-review-pr-ref.sh
@@ -0,0 +1,82 @@
+#!/bin/bash
+# ============================================================================
+# claude-review.yml PR-head checkout test
+# ============================================================================
+# Regression for a subtle CI bug: on issue_comment events github.ref points at
+# the default branch and github.base_ref is empty, so the checkout fetched
+# main and the review silently reviewed main instead of the PR head.
+#
+# The workflow must:
+#   - check out the PR merge ref (refs/pull/N/merge) on issue_comment events
+#     via the checkout `ref:` expression, and
+#   - use HEAD^...HEAD for the size check on issue_comment (where base_ref is
+#     empty), keeping origin/<base>...HEAD for pull_request events.
+#
+# Usage: ./tests/test-claude-review-pr-ref.sh
+# ============================================================================
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+ROOT_DIR="$(cd "$SCRIPT_DIR/.." && pwd)"
+WORKFLOW="$ROOT_DIR/../.github/workflows/claude-review.yml"
+
+GREEN='\033[0;32m'
+RED='\033[0;31m'
+NC='\033[0m'
+
+PASSED=0
+FAILED=0
+
+pass() { echo -e "  ${GREEN}✓ PASS${NC} $1"; PASSED=$((PASSED + 1)); }
+fail() { echo -e "  ${RED}✗ FAIL${NC} $1"; FAILED=$((FAILED + 1)); }
+
+echo ""
+echo "╔═══════════════════════════════════════════════╗"
+echo "║   claude-review.yml PR-head checkout test     ║"
+echo "╚═══════════════════════════════════════════════╝"
+echo ""
+
+if [[ ! -f "$WORKFLOW" ]]; then
+    fail "claude-review.yml not found at $WORKFLOW"
+    echo ""; echo "Result: $PASSED passed, $FAILED failed"; exit 1
+fi
+pass "claude-review.yml exists"
+
+# --- 1. issue_comment path checks out the PR merge ref ------------------------
+if grep -q "format('refs/pull/{0}/merge', github.event.issue.number)" "$WORKFLOW"; then
+    pass "checkout ref resolves the PR merge ref on issue_comment events"
+else
+    fail "checkout must use format('refs/pull/{0}/merge', github.event.issue.number) on issue_comment"
+fi
+
+if grep -q "ref: \${{ github.event_name == 'issue_comment' && format('refs/pull/{0}/merge', github.event.issue.number) || '' }}" "$WORKFLOW"; then
+    pass "checkout ref is conditional on issue_comment and empty otherwise"
+else
+    fail "checkout ref must be conditional: refs/pull/N/merge on issue_comment, '' otherwise"
+fi
+
+# --- 2. Size check uses HEAD^...HEAD on issue_comment, base diff otherwise -----
+if grep -q 'DIFF_ARGS="HEAD^...HEAD"' "$WORKFLOW"; then
+    pass "issue_comment path diffs HEAD^...HEAD for the PR size check"
+else
+    fail "issue_comment path must set DIFF_ARGS=HEAD^...HEAD"
+fi
+
+if grep -q 'DIFF_ARGS="origin/\${BASE_REF}...HEAD"' "$WORKFLOW"; then
+    pass "pull_request path still diffs against origin/\${BASE_REF}...HEAD"
+else
+    fail "pull_request path must keep DIFF_ARGS=origin/\${BASE_REF}...HEAD"
+fi
+
+# --- 3. The two branches are keyed on github.event_name ------------------------
+if grep -q 'if \[ "\${{ github.event_name }}" == "issue_comment" \]; then' "$WORKFLOW"; then
+    pass "diff-arg selection branches on github.event_name"
+else
+    fail "diff-arg selection must branch on github.event_name == issue_comment"
+fi
+
+echo ""
+echo "Result: $PASSED passed, $FAILED failed"
+[[ $FAILED -eq 0 ]] || exit 1
+exit 0