diff --git a/.github/PULL_REQUEST_TEMPLATE.md b/.github/PULL_REQUEST_TEMPLATE.md
new file mode 100644
index 00000000000..23e3363b18c
--- /dev/null
+++ b/.github/PULL_REQUEST_TEMPLATE.md
@@ -0,0 +1,31 @@
+# Pull request template (#36400)
+
+## Summary
+
+<!-- What changed and why. One to three bullets. -->
+
+## Closing scope
+
+<!--
+Definition of Done (#36400):
+- Use **Closes #N** only when every Done-when box from issue N is ticked below
+  with the **exact** checkbox text from the issue, plus the gate/command output.
+- Use **Part of #N** for partial work and list what remains.
+- Issues without a ## Done when list must get one before being claimed.
+-->
+
+- Closes? / Part of? : <!-- e.g. Part of #36400  OR  Closes #36400 -->
+
+### Done when (copy from the issue; tick only what this PR completes)
+
+- [ ] <!-- paste each Done-when line from the issue; mark [x] when done -->
+
+### What remains (required for Part of #N)
+
+<!-- Bullet list, or "none — closing". -->
+
+## Test plan
+
+- [ ] Named gates run in the pinned Docker image (`./script/docker-exec.sh` / `./script/phpunit.sh`)
+- [ ] `php script/check-issue-close-scope.php --pr-body <this-body-file> --repo PurHur/php-compiler` (when using Closes #N)
+- [ ] Repro / Done-when commands pasted with output
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index be5a8cdce71..74bddb6f18b 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -18,6 +18,23 @@ Please follow [CODE_OF_CONDUCT.md](CODE_OF_CONDUCT.md) in all project interactio
 
 The sections below are for **maintainers and collaborators who have already coordinated** on other channels.
 
+### Definition of Done ([#36400](https://github.com/PurHur/php-compiler/issues/36400))
+
+An issue may be closed by a PR only when **all** of the following hold:
+
+1. Every checkbox under the issue’s `## Done when` section appears in the PR body as a **ticked** copy (`- [x]` + the same text), together with the command output that proves it.
+2. The named gates for that change class were run in the **pinned Docker image** (`./script/docker-exec.sh` / `./script/phpunit.sh`), and the transcript is in the PR.
+3. Partial work uses **`Part of #N`** (not `Closes #N`) and lists what remains. Issues without a `## Done when` list get one **before** being claimed.
+
+Gate (local / PR body check):
+
+```bash
+php script/check-issue-close-scope.php --self-test
+php script/check-issue-close-scope.php --pr-body /tmp/pr.md --repo PurHur/php-compiler
+```
+
+A `Closes #N` without the full ticked Done-when list **fails** the gate; `Part of #N` does not. Weekly audit: `php script/audit-closed-but-partial.php --days 7 --dry-run` (post with `--post --tracker 36379`; optional `--apply-labels`). See [`.github/PULL_REQUEST_TEMPLATE.md`](.github/PULL_REQUEST_TEMPLATE.md).
+
 ### Pull request process
 
 All submissions, including by project members, require review via GitHub pull requests. See [GitHub Help](https://help.github.com/articles/about-pull-requests/) for using pull requests.
diff --git a/script/audit-closed-but-partial.php b/script/audit-closed-but-partial.php
new file mode 100755
index 00000000000..6a1845ab539
--- /dev/null
+++ b/script/audit-closed-but-partial.php
@@ -0,0 +1,506 @@
+#!/usr/bin/env php
+<?php
+
+declare(strict_types=1);
+
+/**
+ * Weekly "closed but not done" audit (#36400).
+ *
+ * Finds issues closed in the last N days whose closing PR body looks partial
+ * ("remain", "follow-up", "partial", "part of") or whose Done-when boxes were
+ * not all ticked in that PR. Labels them `needs-respin` and can post one
+ * summary on the active Wave tracker.
+ *
+ * Usage:
+ *   php script/audit-closed-but-partial.php --days 7 --dry-run
+ *   php script/audit-closed-but-partial.php --days 7 --post --tracker 36379
+ *   php script/audit-closed-but-partial.php --days 7 --post --apply-labels
+ *   php script/audit-closed-but-partial.php --fixture FILE --dry-run
+ *
+ * Default: posts an umbrella child + tracker comment; pass --apply-labels to
+ * also stamp `needs-respin` on each flagged closed issue.
+ */
+
+$root = dirname(__DIR__);
+$days = 7;
+$dryRun = in_array('--dry-run', $argv, true);
+$post = in_array('--post', $argv, true);
+$applyLabels = in_array('--apply-labels', $argv, true);
+$tracker = 36379;
+$repo = 'PurHur/php-compiler';
+$fixture = null;
+$label = 'needs-respin';
+
+for ($i = 1; $i < $argc; $i++) {
+    $arg = $argv[$i];
+    if ($arg === '--days' && isset($argv[$i + 1])) {
+        $days = max(1, (int) $argv[++$i]);
+        continue;
+    }
+    if ($arg === '--tracker' && isset($argv[$i + 1])) {
+        $tracker = (int) $argv[++$i];
+        continue;
+    }
+    if ($arg === '--repo' && isset($argv[$i + 1])) {
+        $repo = $argv[++$i];
+        continue;
+    }
+    if ($arg === '--fixture' && isset($argv[$i + 1])) {
+        $fixture = $argv[++$i];
+        continue;
+    }
+    if ($arg === '--label' && isset($argv[$i + 1])) {
+        $label = $argv[++$i];
+        continue;
+    }
+}
+
+exit(main_audit($root, $days, $dryRun, $post, $applyLabels, $tracker, $repo, $fixture, $label));
+
+function main_audit(
+    string $root,
+    int $days,
+    bool $dryRun,
+    bool $post,
+    bool $applyLabels,
+    int $tracker,
+    string $repo,
+    ?string $fixture,
+    string $label
+): int {
+    $rows = $fixture !== null
+        ? load_fixture_rows($fixture)
+        : fetch_recently_closed_rows($repo, $days);
+
+    $flagged = [];
+    foreach ($rows as $row) {
+        $reason = classify_partial_close($row);
+        if ($reason === null) {
+            continue;
+        }
+        $flagged[] = [
+            'number' => (int) $row['number'],
+            'title' => (string) ($row['title'] ?? ''),
+            'reason' => $reason,
+            'prUrl' => (string) ($row['prUrl'] ?? ''),
+        ];
+    }
+
+    if ($flagged === []) {
+        fwrite(STDOUT, "audit-closed-but-partial: none flagged in last {$days} day(s)\n");
+        if ($post && !$dryRun) {
+            $body = build_audit_comment($days, $flagged, $repo, null);
+            post_tracker_comment($repo, $tracker, $body);
+            fwrite(STDOUT, "audit-closed-but-partial: posted empty-result summary on #{$tracker}\n");
+        }
+
+        return 0;
+    }
+
+    fwrite(STDOUT, 'audit-closed-but-partial: '.count($flagged)." candidate(s):\n");
+    foreach ($flagged as $f) {
+        fwrite(STDOUT, "  #{$f['number']}\t{$f['reason']}\t{$f['title']}\n");
+    }
+
+    if ($dryRun) {
+        fwrite(STDOUT, "audit-closed-but-partial: dry-run — not labeling or posting\n");
+
+        return 0;
+    }
+
+    $umbrella = null;
+    if ($post) {
+        $umbrella = create_umbrella_issue($repo, $days, $flagged);
+        fwrite(STDOUT, "audit-closed-but-partial: umbrella child #{$umbrella}\n");
+    }
+
+    if ($applyLabels) {
+        ensure_label($repo, $label);
+        foreach ($flagged as $f) {
+            apply_label($repo, $f['number'], $label);
+            fwrite(STDOUT, "audit-closed-but-partial: labeled #{$f['number']} {$label}\n");
+        }
+        if ($umbrella !== null) {
+            apply_label($repo, $umbrella, $label);
+        }
+    } else {
+        fwrite(STDOUT, "audit-closed-but-partial: skipping per-issue labels (pass --apply-labels to opt in)\n");
+    }
+
+    if ($post) {
+        $body = build_audit_comment($days, $flagged, $repo, $umbrella);
+        post_tracker_comment($repo, $tracker, $body);
+        fwrite(STDOUT, "audit-closed-but-partial: posted summary on #{$tracker}\n");
+    }
+
+    return 0;
+}
+
+/**
+ * @return list<array<string,mixed>>
+ */
+function load_fixture_rows(string $path): array
+{
+    if (!is_readable($path)) {
+        fwrite(STDERR, "audit-closed-but-partial: fixture not readable: {$path}\n");
+        exit(1);
+    }
+    $data = json_decode((string) file_get_contents($path), true);
+    if (!is_array($data)) {
+        fwrite(STDERR, "audit-closed-but-partial: fixture must be a JSON array\n");
+        exit(1);
+    }
+
+    return $data;
+}
+
+/**
+ * @return list<array<string,mixed>>
+ */
+function fetch_recently_closed_rows(string $repo, int $days): array
+{
+    $since = (new DateTimeImmutable('now', new DateTimeZone('UTC')))
+        ->modify('-'.$days.' days')
+        ->format('Y-m-d');
+    $cmd = 'gh issue list --repo '.escapeshellarg($repo)
+        .' --state closed --limit 100 --search '.escapeshellarg("closed:>={$since}")
+        .' --json number,title,closedAt,url 2>/dev/null';
+    $json = shell_exec($cmd);
+    if ($json === null || $json === '') {
+        fwrite(STDERR, "audit-closed-but-partial: gh issue list failed\n");
+        exit(1);
+    }
+    $issues = json_decode($json, true);
+    if (!is_array($issues)) {
+        return [];
+    }
+
+    $rows = [];
+    foreach ($issues as $issue) {
+        $n = (int) ($issue['number'] ?? 0);
+        if ($n <= 0) {
+            continue;
+        }
+        $pr = find_closing_pr($repo, $n);
+        $issueBody = fetch_issue_body_raw($repo, $n);
+        $rows[] = [
+            'number' => $n,
+            'title' => (string) ($issue['title'] ?? ''),
+            'closedAt' => (string) ($issue['closedAt'] ?? ''),
+            'closingPrBody' => $pr['body'] ?? '',
+            'prUrl' => $pr['url'] ?? '',
+            'issueBody' => $issueBody ?? '',
+        ];
+    }
+
+    return $rows;
+}
+
+/**
+ * @return array{body?:string,url?:string}
+ */
+function find_closing_pr(string $repo, int $issueNumber): array
+{
+    // Prefer timeline events via gh api
+    $cmd = 'gh api repos/'.escapeshellarg($repo).'/issues/'.$issueNumber.'/timeline --paginate 2>/dev/null';
+    // gh api with escapeshellarg on repo breaks the path — build carefully
+    $cmd = 'gh api '.escapeshellarg("repos/{$repo}/issues/{$issueNumber}/timeline").' --paginate 2>/dev/null';
+    $json = shell_exec($cmd);
+    if ($json === null || $json === '') {
+        return [];
+    }
+    // --paginate may concatenate arrays; try decode
+    $events = json_decode($json, true);
+    if (!is_array($events)) {
+        return [];
+    }
+    foreach (array_reverse($events) as $ev) {
+        if (($ev['event'] ?? '') !== 'closed' && ($ev['event'] ?? '') !== 'cross-referenced') {
+            // look for connected PR
+        }
+        $src = $ev['source'] ?? null;
+        if (is_array($src) && isset($src['issue']['pull_request'])) {
+            $prNum = (int) ($src['issue']['number'] ?? 0);
+            if ($prNum > 0) {
+                return fetch_pr($repo, $prNum);
+            }
+        }
+    }
+    // Fallback: search PRs that mention Closes #N
+    $cmd = 'gh pr list --repo '.escapeshellarg($repo)
+        .' --state merged --limit 20 --search '.escapeshellarg((string) $issueNumber)
+        .' --json number,url,body 2>/dev/null';
+    $list = json_decode((string) shell_exec($cmd), true);
+    if (!is_array($list)) {
+        return [];
+    }
+    foreach ($list as $pr) {
+        $body = (string) ($pr['body'] ?? '');
+        if (preg_match('/\b(?:close[sd]?|fix(?:e[sd])?|resolve[sd]?)\s+#'.$issueNumber.'\b/i', $body)) {
+            return ['body' => $body, 'url' => (string) ($pr['url'] ?? '')];
+        }
+    }
+
+    return [];
+}
+
+/**
+ * @return array{body?:string,url?:string}
+ */
+function fetch_pr(string $repo, int $number): array
+{
+    $cmd = 'gh pr view '.escapeshellarg((string) $number)
+        .' --repo '.escapeshellarg($repo)
+        .' --json body,url 2>/dev/null';
+    $data = json_decode((string) shell_exec($cmd), true);
+    if (!is_array($data)) {
+        return [];
+    }
+
+    return ['body' => (string) ($data['body'] ?? ''), 'url' => (string) ($data['url'] ?? '')];
+}
+
+function fetch_issue_body_raw(string $repo, int $number): ?string
+{
+    $cmd = 'gh issue view '.escapeshellarg((string) $number)
+        .' --repo '.escapeshellarg($repo)
+        .' --json body -q .body 2>/dev/null';
+    $out = shell_exec($cmd);
+    if ($out === null || $out === '') {
+        return null;
+    }
+
+    return $out;
+}
+
+/**
+ * @param array<string,mixed> $row
+ */
+function classify_partial_close(array $row): ?string
+{
+    $prBody = (string) ($row['closingPrBody'] ?? '');
+    $issueBody = (string) ($row['issueBody'] ?? '');
+    $n = (int) ($row['number'] ?? 0);
+
+    if ($prBody === '') {
+        return null; // no closing PR found — skip (manual close)
+    }
+
+    // Explicit partial language in the closing PR (narrow patterns — avoid "partial" in prose titles)
+    if (preg_match('/\bPart of #\d+\b/i', $prBody)
+        || preg_match('/\b(?:follow-?up|remain(?:s|ing)?)\b/i', $prBody)
+        || preg_match('/\bpartial\s+(?:merge|only|slice|fix|land|work)\b/i', $prBody)
+    ) {
+        // Only flag when the same PR also claims Closes #N for this issue
+        if (preg_match('/\b(?:close[sd]?|fix(?:e[sd])?|resolve[sd]?)\s+#'.$n.'\b/i', $prBody)) {
+            return 'closing PR both Closes #'.$n.' and uses partial/follow-up language';
+        }
+    }
+
+    // Closes #N without ticked Done-when
+    if ($issueBody !== '' && preg_match('/\b(?:close[sd]?|fix(?:e[sd])?|resolve[sd]?)\s+#'.$n.'\b/i', $prBody)) {
+        $required = audit_extract_done_when_items($issueBody);
+        if ($required === []) {
+            return 'Closes #'.$n.' but issue has no Done-when checklist';
+        }
+        $ticked = audit_extract_ticked_items($prBody);
+        $tickedSet = array_fill_keys($ticked, true);
+        foreach ($required as $item) {
+            if (!isset($tickedSet[$item])) {
+                return 'Closes #'.$n.' without ticked Done-when item: '.$item;
+            }
+        }
+    }
+
+    return null;
+}
+
+/** @return list<string> */
+function audit_extract_done_when_items(string $issueBody): array
+{
+    if (!preg_match('/^##\s+Done when\s*$/mi', $issueBody, $m, PREG_OFFSET_CAPTURE)) {
+        return [];
+    }
+    $start = (int) $m[0][1] + strlen($m[0][0]);
+    $rest = substr($issueBody, $start);
+    if (preg_match('/^##\s+/m', $rest, $next, PREG_OFFSET_CAPTURE)) {
+        $rest = substr($rest, 0, (int) $next[0][1]);
+    }
+    $items = [];
+    if (preg_match_all('/^\s*-\s*\[([ xX])\]\s*(.+?)\s*$/m', $rest, $boxes, PREG_SET_ORDER)) {
+        foreach ($boxes as $box) {
+            $text = audit_normalize($box[2]);
+            if ($text !== '') {
+                $items[] = $text;
+            }
+        }
+    }
+
+    return $items;
+}
+
+/** @return list<string> */
+function audit_extract_ticked_items(string $prBody): array
+{
+    $items = [];
+    if (preg_match_all('/^\s*-\s*\[[xX]\]\s*(.+?)\s*$/m', $prBody, $boxes, PREG_SET_ORDER)) {
+        foreach ($boxes as $box) {
+            $text = audit_normalize($box[1]);
+            if ($text !== '') {
+                $items[] = $text;
+            }
+        }
+    }
+
+    return $items;
+}
+
+function audit_normalize(string $text): string
+{
+    $text = html_entity_decode($text, ENT_QUOTES | ENT_HTML5, 'UTF-8');
+    $text = preg_replace('/\s+/u', ' ', trim($text)) ?? trim($text);
+
+    return $text;
+}
+
+/**
+ * @param list<array{number:int,title:string,reason:string,prUrl:string}> $flagged
+ */
+function build_audit_comment(int $days, array $flagged, string $repo, ?int $umbrella): string
+{
+    $date = gmdate('Y-m-d');
+    $lines = [];
+    $lines[] = "## Closed-but-partial audit ({$date}, last {$days} days) — #36400";
+    $lines[] = '';
+    if ($flagged === []) {
+        $lines[] = 'No closed issues in the window matched partial-close heuristics (`Closes #N` + partial/follow-up language, or `Closes #N` without a full ticked Done-when copy).';
+        $lines[] = '';
+        $lines[] = 'Label `needs-respin` was not applied.';
+
+        return implode("\n", $lines)."\n";
+    }
+    $lines[] = 'Flagged **'.count($flagged).'** issue(s):';
+    $lines[] = '';
+    foreach ($flagged as $f) {
+        $link = "https://github.com/{$repo}/issues/{$f['number']}";
+        $pr = $f['prUrl'] !== '' ? " — PR: {$f['prUrl']}" : '';
+        $lines[] = "- [#{$f['number']}]({$link}) — {$f['reason']}{$pr}";
+    }
+    $lines[] = '';
+    if ($umbrella !== null) {
+        $lines[] = "Umbrella respin child: [#{$umbrella}](https://github.com/{$repo}/issues/{$umbrella}) (queue for fleet — do not mass-reopen here).";
+    } else {
+        $lines[] = 'Fleet: open a fresh child (or re-open) for each entry before claiming that scope again.';
+    }
+
+    return implode("\n", $lines)."\n";
+}
+
+/**
+ * @param list<array{number:int,title:string,reason:string,prUrl:string}> $flagged
+ */
+function create_umbrella_issue(string $repo, int $days, array $flagged): int
+{
+    $date = gmdate('Y-m-d');
+    $title = "Respin queue from #36400 closed-but-partial audit ({$date})";
+    $bodyLines = [];
+    $bodyLines[] = "Parent: #36400 / tracker #36379";
+    $bodyLines[] = '';
+    $bodyLines[] = "First weekly audit found **".count($flagged)."** closed issues in the last {$days} days whose closing PR looks partial under the #36400 heuristics.";
+    $bodyLines[] = '';
+    $bodyLines[] = '## Queue';
+    $bodyLines[] = '';
+    foreach ($flagged as $f) {
+        $bodyLines[] = "- #{$f['number']} — {$f['reason']} — {$f['title']}";
+    }
+    $bodyLines[] = '';
+    $bodyLines[] = '## Done when';
+    $bodyLines[] = '';
+    $bodyLines[] = '- [ ] Each queue entry either has an open successor child covering the remaining Done-when, or is confirmed complete and removed from this list';
+    $bodyLines[] = '- [ ] No new `Closes #N` lands without a full ticked Done-when copy (`script/check-issue-close-scope.php`)';
+    $body = implode("\n", $bodyLines)."\n";
+    $tmp = tempnam(sys_get_temp_dir(), 'umbrella36400-');
+    if ($tmp === false) {
+        fwrite(STDERR, "audit-closed-but-partial: tempnam failed for umbrella\n");
+        exit(1);
+    }
+    file_put_contents($tmp, $body);
+    ensure_label($repo, 'needs-respin');
+    $cmd = 'gh issue create --repo '.escapeshellarg($repo)
+        .' --title '.escapeshellarg($title)
+        .' --body-file '.escapeshellarg($tmp)
+        .' --label '.escapeshellarg('needs-respin')
+        .' 2>&1';
+    exec($cmd, $out, $code);
+    if ($code !== 0) {
+        $out = [];
+        $cmd = 'gh issue create --repo '.escapeshellarg($repo)
+            .' --title '.escapeshellarg($title)
+            .' --body-file '.escapeshellarg($tmp)
+            .' 2>&1';
+        exec($cmd, $out, $code);
+    }
+    @unlink($tmp);
+    $text = implode("\n", $out);
+    if ($code !== 0 || !preg_match('#/issues/(\d+)#', $text, $m)) {
+        fwrite(STDERR, "audit-closed-but-partial: failed to create umbrella issue: {$text}\n");
+        exit(1);
+    }
+
+    return (int) $m[1];
+}
+
+function ensure_label(string $repo, string $label): void
+{
+    $cmd = 'gh label list --repo '.escapeshellarg($repo).' --limit 200 --json name -q '."'.[].name' 2>/dev/null";
+    // simpler:
+    $cmd = 'gh label list --repo '.escapeshellarg($repo).' --limit 200 --json name 2>/dev/null';
+    $data = json_decode((string) shell_exec($cmd), true);
+    $names = [];
+    if (is_array($data)) {
+        foreach ($data as $row) {
+            $names[] = (string) ($row['name'] ?? '');
+        }
+    }
+    if (in_array($label, $names, true)) {
+        return;
+    }
+    $create = 'gh label create '.escapeshellarg($label)
+        .' --repo '.escapeshellarg($repo)
+        .' --description '.escapeshellarg('Closed before Done-when was complete; needs a respin child (#36400)')
+        .' --color '.escapeshellarg('B60205').' 2>&1';
+    exec($create, $out, $code);
+    if ($code !== 0) {
+        fwrite(STDERR, "audit-closed-but-partial: could not create label {$label}: ".implode("\n", $out)."\n");
+    }
+}
+
+function apply_label(string $repo, int $number, string $label): void
+{
+    $cmd = 'gh issue edit '.escapeshellarg((string) $number)
+        .' --repo '.escapeshellarg($repo)
+        .' --add-label '.escapeshellarg($label).' 2>&1';
+    exec($cmd, $out, $code);
+    if ($code !== 0) {
+        fwrite(STDERR, "audit-closed-but-partial: failed to label #{$number}: ".implode("\n", $out)."\n");
+    }
+}
+
+function post_tracker_comment(string $repo, int $tracker, string $body): void
+{
+    $tmp = tempnam(sys_get_temp_dir(), 'audit36400-');
+    if ($tmp === false) {
+        fwrite(STDERR, "audit-closed-but-partial: tempnam failed\n");
+        exit(1);
+    }
+    file_put_contents($tmp, $body);
+    $cmd = 'gh issue comment '.escapeshellarg((string) $tracker)
+        .' --repo '.escapeshellarg($repo)
+        .' --body-file '.escapeshellarg($tmp).' 2>&1';
+    exec($cmd, $out, $code);
+    @unlink($tmp);
+    if ($code !== 0) {
+        fwrite(STDERR, "audit-closed-but-partial: failed to comment on #{$tracker}: ".implode("\n", $out)."\n");
+        exit(1);
+    }
+}
diff --git a/script/check-issue-close-scope.php b/script/check-issue-close-scope.php
new file mode 100755
index 00000000000..104972a8184
--- /dev/null
+++ b/script/check-issue-close-scope.php
@@ -0,0 +1,270 @@
+#!/usr/bin/env php
+<?php
+
+declare(strict_types=1);
+
+/**
+ * Gate: a PR may say "Closes #N" only when every Done-when checkbox from issue N
+ * appears ticked in the PR body (#36400).
+ *
+ * Usage:
+ *   php script/check-issue-close-scope.php --pr-body FILE --issue-body FILE [--issue-number N]
+ *   php script/check-issue-close-scope.php --pr-body FILE [--repo OWNER/REPO]
+ *   php script/check-issue-close-scope.php --self-test
+ *
+ * Exit 0 when every Closes #N is backed by a full ticked Done-when copy.
+ * Exit 1 when a Closes #N lacks matching `- [x]` lines (or the issue has no Done when).
+ * "Part of #N" alone does not trigger the gate.
+ */
+
+$root = dirname(__DIR__);
+$selfTest = in_array('--self-test', $argv, true);
+
+if ($selfTest) {
+    exit(run_self_test($root));
+}
+
+$prBodyPath = null;
+$issueBodyPath = null;
+$issueNumber = null;
+$repo = 'PurHur/php-compiler';
+
+for ($i = 1; $i < $argc; $i++) {
+    $arg = $argv[$i];
+    if ($arg === '--pr-body' && isset($argv[$i + 1])) {
+        $prBodyPath = $argv[++$i];
+        continue;
+    }
+    if ($arg === '--issue-body' && isset($argv[$i + 1])) {
+        $issueBodyPath = $argv[++$i];
+        continue;
+    }
+    if ($arg === '--issue-number' && isset($argv[$i + 1])) {
+        $issueNumber = (int) $argv[++$i];
+        continue;
+    }
+    if ($arg === '--repo' && isset($argv[$i + 1])) {
+        $repo = $argv[++$i];
+        continue;
+    }
+    if ($arg === '--help' || $arg === '-h') {
+        fwrite(STDOUT, "Usage: check-issue-close-scope.php --pr-body FILE [--issue-body FILE|--repo OWNER/REPO] [--self-test]\n");
+        exit(0);
+    }
+}
+
+if ($prBodyPath === null || !is_readable($prBodyPath)) {
+    fwrite(STDERR, "check-issue-close-scope: --pr-body FILE is required and must be readable\n");
+    exit(1);
+}
+
+$prBody = (string) file_get_contents($prBodyPath);
+$closes = extract_closes_numbers($prBody);
+if ($closes === []) {
+    fwrite(STDOUT, "check-issue-close-scope: OK (no Closes #N in PR body)\n");
+    exit(0);
+}
+
+$errors = [];
+if ($issueBodyPath !== null) {
+    if (!is_readable($issueBodyPath)) {
+        fwrite(STDERR, "check-issue-close-scope: --issue-body not readable: {$issueBodyPath}\n");
+        exit(1);
+    }
+    $issueBody = (string) file_get_contents($issueBodyPath);
+    $n = $issueNumber ?? ($closes[0] ?? 0);
+    $errors = array_merge($errors, validate_closes_against_issue($n, $issueBody, $prBody));
+    foreach ($closes as $other) {
+        if ($other !== $n) {
+            $errors[] = "Closes #{$other} present but only --issue-body for #{$n} was supplied; fetch live or pass one issue at a time";
+        }
+    }
+} else {
+    foreach ($closes as $n) {
+        $issueBody = fetch_issue_body($repo, $n);
+        if ($issueBody === null) {
+            $errors[] = "Closes #{$n}: could not fetch issue body via gh (repo {$repo})";
+            continue;
+        }
+        $errors = array_merge($errors, validate_closes_against_issue($n, $issueBody, $prBody));
+    }
+}
+
+if ($errors !== []) {
+    foreach ($errors as $err) {
+        fwrite(STDERR, "check-issue-close-scope: {$err}\n");
+    }
+    fwrite(STDERR, "check-issue-close-scope: use \"Part of #N\" for partial work, or tick every Done-when box in the PR body (#36400)\n");
+    exit(1);
+}
+
+fwrite(STDOUT, 'check-issue-close-scope: OK ('.count($closes)." Closes # covered)\n");
+exit(0);
+
+/**
+ * @return list<int>
+ */
+function extract_closes_numbers(string $body): array
+{
+    // Do not treat "Part of #N" as a close. Match Closes/Fixes/Resolves variants.
+    if (!preg_match_all(
+        '/\b(?:close[sd]?|fix(?:e[sd])?|resolve[sd]?)\s+#(\d+)\b/i',
+        $body,
+        $m
+    )) {
+        return [];
+    }
+    $nums = array_map('intval', $m[1]);
+    $nums = array_values(array_unique($nums));
+    sort($nums);
+
+    return $nums;
+}
+
+/**
+ * @return list<string> normalized checkbox texts (no leading - [x]/ empty if none
+ */
+function extract_done_when_items(string $issueBody): array
+{
+    if (!preg_match('/^##\s+Done when\s*$/mi', $issueBody, $m, PREG_OFFSET_CAPTURE)) {
+        return [];
+    }
+    $start = (int) $m[0][1] + strlen($m[0][0]);
+    $rest = substr($issueBody, $start);
+    // Stop at next ## heading or EOF
+    if (preg_match('/^##\s+/m', $rest, $next, PREG_OFFSET_CAPTURE)) {
+        $rest = substr($rest, 0, (int) $next[0][1]);
+    }
+    $items = [];
+    if (preg_match_all('/^\s*-\s*\[([ xX])\]\s*(.+?)\s*$/m', $rest, $boxes, PREG_SET_ORDER)) {
+        foreach ($boxes as $box) {
+            $text = normalize_checkbox_text($box[2]);
+            if ($text !== '') {
+                $items[] = $text;
+            }
+        }
+    }
+
+    return $items;
+}
+
+function normalize_checkbox_text(string $text): string
+{
+    $text = html_entity_decode($text, ENT_QUOTES | ENT_HTML5, 'UTF-8');
+    $text = preg_replace('/\s+/u', ' ', trim($text)) ?? trim($text);
+
+    return $text;
+}
+
+/**
+ * @return list<string> ticked checkbox texts in PR body
+ */
+function extract_ticked_items(string $prBody): array
+{
+    $items = [];
+    if (preg_match_all('/^\s*-\s*\[[xX]\]\s*(.+?)\s*$/m', $prBody, $boxes, PREG_SET_ORDER)) {
+        foreach ($boxes as $box) {
+            $text = normalize_checkbox_text($box[1]);
+            if ($text !== '') {
+                $items[] = $text;
+            }
+        }
+    }
+
+    return $items;
+}
+
+/**
+ * @return list<string> error messages
+ */
+function validate_closes_against_issue(int $issueNumber, string $issueBody, string $prBody): array
+{
+    $errors = [];
+    $required = extract_done_when_items($issueBody);
+    if ($required === []) {
+        $errors[] = "Closes #{$issueNumber}: issue has no ## Done when checklist — add one before closing, or use Part of #{$issueNumber}";
+
+        return $errors;
+    }
+    $ticked = extract_ticked_items($prBody);
+    $tickedSet = array_fill_keys($ticked, true);
+    foreach ($required as $item) {
+        if (!isset($tickedSet[$item])) {
+            $errors[] = "Closes #{$issueNumber}: missing ticked Done-when item: `- [x] {$item}`";
+        }
+    }
+
+    return $errors;
+}
+
+function fetch_issue_body(string $repo, int $number): ?string
+{
+    $cmd = 'gh issue view '.escapeshellarg((string) $number)
+        .' --repo '.escapeshellarg($repo)
+        .' --json body -q .body 2>/dev/null';
+    $out = shell_exec($cmd);
+    if ($out === null || $out === '') {
+        return null;
+    }
+
+    return $out;
+}
+
+function run_self_test(string $root): int
+{
+    $fixtureDir = $root.'/test/fixtures/issue-close-scope';
+    $issue = $fixtureDir.'/sample-issue.md';
+    $bad = $fixtureDir.'/pr-closes-without-ticks.md';
+    $good = $fixtureDir.'/pr-closes-with-ticks.md';
+    $part = $fixtureDir.'/pr-part-of-only.md';
+    foreach ([$issue, $bad, $good, $part] as $path) {
+        if (!is_readable($path)) {
+            fwrite(STDERR, "check-issue-close-scope: missing fixture {$path}\n");
+
+            return 1;
+        }
+    }
+
+    $php = escapeshellarg(PHP_BINARY);
+    $script = escapeshellarg($root.'/script/check-issue-close-scope.php');
+
+    $badCmd = "{$php} {$script} --pr-body ".escapeshellarg($bad)
+        .' --issue-body '.escapeshellarg($issue).' --issue-number 36400';
+    exec($badCmd.' 2>&1', $badOut, $badCode);
+    if ($badCode === 0) {
+        fwrite(STDERR, "check-issue-close-scope: self-test FAIL — expected reject for Closes without ticks\n");
+        fwrite(STDERR, implode("\n", $badOut)."\n");
+
+        return 1;
+    }
+    $badText = implode("\n", $badOut);
+    if (!str_contains($badText, 'missing ticked Done-when item')) {
+        fwrite(STDERR, "check-issue-close-scope: self-test FAIL — expected missing-tick message\n{$badText}\n");
+
+        return 1;
+    }
+
+    $goodCmd = "{$php} {$script} --pr-body ".escapeshellarg($good)
+        .' --issue-body '.escapeshellarg($issue).' --issue-number 36400';
+    exec($goodCmd.' 2>&1', $goodOut, $goodCode);
+    if ($goodCode !== 0) {
+        fwrite(STDERR, "check-issue-close-scope: self-test FAIL — expected OK for fully ticked Closes\n");
+        fwrite(STDERR, implode("\n", $goodOut)."\n");
+
+        return 1;
+    }
+
+    $partCmd = "{$php} {$script} --pr-body ".escapeshellarg($part)
+        .' --issue-body '.escapeshellarg($issue).' --issue-number 36400';
+    exec($partCmd.' 2>&1', $partOut, $partCode);
+    if ($partCode !== 0) {
+        fwrite(STDERR, "check-issue-close-scope: self-test FAIL — Part of #N must pass without ticks\n");
+        fwrite(STDERR, implode("\n", $partOut)."\n");
+
+        return 1;
+    }
+
+    fwrite(STDOUT, "check-issue-close-scope: self-test OK (reject bare Closes; accept ticked Closes; accept Part of)\n");
+
+    return 0;
+}
diff --git a/script/ci-common.sh b/script/ci-common.sh
index 323a902b4d1..ba1839396d1 100644
--- a/script/ci-common.sh
+++ b/script/ci-common.sh
@@ -123,6 +123,14 @@ ci_run_m2_spine_issue_hygiene_check() {
   "$PHP_BIN" "${PHP_OPTS[@]}" script/check-m2-spine-issue-hygiene.php
 }
 
+ci_run_issue_close_scope_check() {
+  if [[ "${ISSUE_CLOSE_SCOPE_GATE:-1}" != "1" ]]; then
+    return 0
+  fi
+  echo "Issue close-scope self-test (ISSUE_CLOSE_SCOPE_GATE=1, issue #36400)..."
+  "$PHP_BIN" "${PHP_OPTS[@]}" script/check-issue-close-scope.php --self-test
+}
+
 ci_run_examples_readme_sync_check() {
   if [[ "${EXAMPLES_README_SYNC_GATE:-1}" != "1" ]]; then
     return 0
@@ -558,6 +566,7 @@ ci_run_inventory_checks() {
   ci_ensure_generated_doc script/bootstrap-profile.php docs/bootstrap-profile.json
   ci_run_wave3_roadmap_sync_check
   ci_run_m2_spine_issue_hygiene_check
+  ci_run_issue_close_scope_check
   ci_run_examples_readme_sync_check
   ci_run_examples_ladder_discovery_check
   ci_run_rebuild_examples_005_sync_check
diff --git a/script/ci-defaults.env b/script/ci-defaults.env
index 9ece6b55289..7df9019ed5b 100755
--- a/script/ci-defaults.env
+++ b/script/ci-defaults.env
@@ -75,6 +75,7 @@ export BOOTSTRAP_PHPTYPES_UNIT_PROBE_GATE="${BOOTSTRAP_PHPTYPES_UNIT_PROBE_GATE:
 export BOOTSTRAP_M3_EMIT_TU_EXECUTE_GATE="${BOOTSTRAP_M3_EMIT_TU_EXECUTE_GATE:-0}"  # opt-in (#2444); PHPUnit @group selfhost-m3-emit after unit probes; default-on after #2442 green
 export CAPABILITY_SYNTAX_CHECK="${CAPABILITY_SYNTAX_CHECK:-1}"
 export M2_SPINE_ISSUE_HYGIENE_GATE="${M2_SPINE_ISSUE_HYGIENE_GATE:-1}"  # default on (#1819); opt-out for bulk spine PRs
+export ISSUE_CLOSE_SCOPE_GATE="${ISSUE_CLOSE_SCOPE_GATE:-1}"  # default on (#36400); self-test that bare Closes #N is rejected without ticked Done-when
 export WAVE3_ROADMAP_SYNC_GATE="${WAVE3_ROADMAP_SYNC_GATE:-1}"  # default on (#1814); opt-out WAVE3_ROADMAP_SYNC_GATE=0 for doc-only iteration
 export EXAMPLES_README_SYNC_GATE="${EXAMPLES_README_SYNC_GATE:-1}"  # default on (#1822); opt-out EXAMPLES_README_SYNC_GATE=0 for doc-only iteration
 export EXAMPLES_LADDER_DISCOVERY_GATE="${EXAMPLES_LADDER_DISCOVERY_GATE:-1}"  # default on (#1913); opt-out EXAMPLES_LADDER_DISCOVERY_GATE=0 for doc-only iteration
diff --git a/test/fixtures/issue-close-scope/pr-closes-with-ticks.md b/test/fixtures/issue-close-scope/pr-closes-with-ticks.md
new file mode 100644
index 00000000000..f89c5c58c56
--- /dev/null
+++ b/test/fixtures/issue-close-scope/pr-closes-with-ticks.md
@@ -0,0 +1,12 @@
+## Summary
+
+Closes #36400 with every Done-when box ticked.
+
+## Done when (from #36400)
+
+- [x] Template + gate live; a test PR that says "Closes #N" without the ticked list is rejected by the gate
+- [x] First weekly audit posted; every `needs-respin` from it has a fresh child issue
+
+## Test plan
+
+- [x] `php script/check-issue-close-scope.php --self-test`
diff --git a/test/fixtures/issue-close-scope/pr-closes-without-ticks.md b/test/fixtures/issue-close-scope/pr-closes-without-ticks.md
new file mode 100644
index 00000000000..e04f38876fa
--- /dev/null
+++ b/test/fixtures/issue-close-scope/pr-closes-without-ticks.md
@@ -0,0 +1,7 @@
+## Summary
+
+Closes #36400 without ticking Done-when — must be rejected by the gate.
+
+## Test plan
+
+- [ ] something unrelated
diff --git a/test/fixtures/issue-close-scope/pr-part-of-only.md b/test/fixtures/issue-close-scope/pr-part-of-only.md
new file mode 100644
index 00000000000..fae7e0a1b2b
--- /dev/null
+++ b/test/fixtures/issue-close-scope/pr-part-of-only.md
@@ -0,0 +1,9 @@
+## Summary
+
+Partial slice only — does not close the issue.
+
+Part of #36400
+
+## Remaining
+
+- Weekly audit automation still needs a scheduled job
diff --git a/test/fixtures/issue-close-scope/sample-issue.md b/test/fixtures/issue-close-scope/sample-issue.md
new file mode 100644
index 00000000000..aa6f7aa1a76
--- /dev/null
+++ b/test/fixtures/issue-close-scope/sample-issue.md
@@ -0,0 +1,8 @@
+## Done when
+
+- [ ] Template + gate live; a test PR that says "Closes #N" without the ticked list is rejected by the gate
+- [ ] First weekly audit posted; every `needs-respin` from it has a fresh child issue
+
+## Notes
+
+Fixture issue body for `script/check-issue-close-scope.php --self-test` (#36400).
diff --git a/test/unit/IssueCloseScopeTest.php b/test/unit/IssueCloseScopeTest.php
new file mode 100644
index 00000000000..e764a803240
--- /dev/null
+++ b/test/unit/IssueCloseScopeTest.php
@@ -0,0 +1,50 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPCompiler;
+
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Definition of Done close-scope gate (#36400).
+ */
+final class IssueCloseScopeTest extends TestCase
+{
+    public function testSelfTestRejectsBareClosesAndAcceptsTicks(): void
+    {
+        $root = dirname(__DIR__, 2);
+        $cmd = escapeshellarg(PHP_BINARY).' '
+            .escapeshellarg($root.'/script/check-issue-close-scope.php')
+            .' --self-test 2>&1';
+        exec($cmd, $out, $code);
+        $this->assertSame(0, $code, implode("\n", $out));
+        $this->assertStringContainsString('self-test OK', implode("\n", $out));
+    }
+
+    public function testContributingDocumentsDefinitionOfDone(): void
+    {
+        $body = (string) file_get_contents(dirname(__DIR__, 2).'/CONTRIBUTING.md');
+        $this->assertStringContainsString('Definition of Done', $body);
+        $this->assertStringContainsString('check-issue-close-scope.php', $body);
+        $this->assertStringContainsString('Part of #N', $body);
+    }
+
+    public function testPullRequestTemplateExists(): void
+    {
+        $path = dirname(__DIR__, 2).'/.github/PULL_REQUEST_TEMPLATE.md';
+        $this->assertFileExists($path);
+        $body = (string) file_get_contents($path);
+        $this->assertStringContainsString('Done when', $body);
+        $this->assertStringContainsString('Part of', $body);
+        $this->assertStringContainsString('#36400', $body);
+    }
+
+    public function testCiRunsCloseScopeSelfTest(): void
+    {
+        $common = (string) file_get_contents(dirname(__DIR__, 2).'/script/ci-common.sh');
+        $this->assertStringContainsString('check-issue-close-scope.php', $common);
+        $defaults = (string) file_get_contents(dirname(__DIR__, 2).'/script/ci-defaults.env');
+        $this->assertStringContainsString('ISSUE_CLOSE_SCOPE_GATE', $defaults);
+    }
+}