diff --git a/api/app/Observers/AdvanceSectionObserver.php b/api/app/Observers/AdvanceSectionObserver.php
new file mode 100644
index 00000000..9ef16028
--- /dev/null
+++ b/api/app/Observers/AdvanceSectionObserver.php
@@ -0,0 +1,96 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Observers;
+
+use App\Enums\Artist\AdvanceSectionSubmissionStatus;
+use App\Models\AdvanceSection;
+use App\Models\ArtistEngagement;
+use App\Models\Scopes\OrganisationScope;
+use Illuminate\Support\Facades\DB;
+
+/**
+ * Observer for AdvanceSection.
+ *
+ * Closes ART-OBSERVER-ADVANCE-AGGREGATE: keeps
+ * `artist_engagements.advancing_completed_count` and
+ * `advancing_total_count` in sync with the actual section state.
+ *
+ * Recomputes on every relevant lifecycle event:
+ *   - created  → totals always change
+ *   - updated  → only when submission_status changed (label/sort_order
+ *                edits do not affect counters)
+ *   - deleted  → totals always change (parent may already be soft-
+ *                deleted via ArtistEngagementObserver cascade; in that
+ *                case the recompute is a no-op since the engagement is
+ *                gone — guarded explicitly).
+ *
+ * Race-condition guarantee: two simultaneous section-status changes
+ * for the same engagement could read identical pre-state and write
+ * conflicting counter values. The recompute uses lockForUpdate inside
+ * a DB::transaction so the second writer waits for the first to
+ * commit, then re-reads from the locked rows. This is the same
+ * concurrency idiom documented in the runbook deel 5 for any
+ * aggregate-counter observer.
+ *
+ * The engagement update inside recompute() calls disableLogging() —
+ * counter sync is housekeeping, not an audit-worthy event. The
+ * section's own updated event is logged via LogsActivity on the
+ * AdvanceSection model.
+ */
+final class AdvanceSectionObserver
+{
+    public function created(AdvanceSection $section): void
+    {
+        $this->recompute($section);
+    }
+
+    public function updated(AdvanceSection $section): void
+    {
+        if (! $section->wasChanged('submission_status')) {
+            return;
+        }
+
+        $this->recompute($section);
+    }
+
+    public function deleted(AdvanceSection $section): void
+    {
+        $this->recompute($section);
+    }
+
+    private function recompute(AdvanceSection $section): void
+    {
+        $engagementId = $section->engagement_id;
+
+        DB::transaction(function () use ($engagementId): void {
+            $engagement = ArtistEngagement::query()
+                ->withoutGlobalScope(OrganisationScope::class)
+                ->whereKey($engagementId)
+                ->lockForUpdate()
+                ->first();
+
+            if ($engagement === null) {
+                return;
+            }
+
+            $rows = AdvanceSection::query()
+                ->withoutGlobalScope(OrganisationScope::class)
+                ->where('engagement_id', $engagementId)
+                ->lockForUpdate()
+                ->get(['submission_status']);
+
+            $total = $rows->count();
+            $completed = $rows
+                ->where('submission_status', AdvanceSectionSubmissionStatus::Approved)
+                ->count();
+
+            $engagement->disableLogging();
+            $engagement->update([
+                'advancing_completed_count' => $completed,
+                'advancing_total_count' => $total,
+            ]);
+        });
+    }
+}
diff --git a/api/app/Providers/AppServiceProvider.php b/api/app/Providers/AppServiceProvider.php
index 04217e5e..5389abdd 100644
--- a/api/app/Providers/AppServiceProvider.php
+++ b/api/app/Providers/AppServiceProvider.php
@@ -164,9 +164,10 @@ class AppServiceProvider extends ServiceProvider
         FormFieldLibrary::observe(FormFieldChildTablesCascadeObserver::class);
 
         // RFC-TIMETABLE v0.2 — engagement denorm + cross-tenant guard,
-        // performance optimistic-lock bump.
+        // performance optimistic-lock bump, advance-section counter sync.
         \App\Models\ArtistEngagement::observe(\App\Observers\ArtistEngagementObserver::class);
         \App\Models\Performance::observe(\App\Observers\PerformanceObserver::class);
+        \App\Models\AdvanceSection::observe(\App\Observers\AdvanceSectionObserver::class);
 
         // RFC-WS-6 v1.3 §Q1 — FormSubmissionSubmitted listener layout.
         //