diff --git a/api/database/seeders/ArtistTimetableDevSeeder.php b/api/database/seeders/ArtistTimetableDevSeeder.php
index 0f167fc7..ccf01208 100644
--- a/api/database/seeders/ArtistTimetableDevSeeder.php
+++ b/api/database/seeders/ArtistTimetableDevSeeder.php
@@ -116,26 +116,29 @@ final class ArtistTimetableDevSeeder
 
         $hourSlots = [16, 17, 18, 19, 20, 21, 22, 23];
 
+        // Per RFC §D10/§D17 + SCHEMA.md:1285+1329:
+        //   engagement.event_id = festival (the parent — one per artist per festival)
+        //   performance.event_id = sub-event ("show host" day)
+        // Same artist on multiple sub-events shares ONE engagement (D17:
+        // "Friday + Saturday under one combined deal = 1 engagement, 2 performances").
         foreach ($subEvents as $dayIdx => $subEvent) {
-            // Each sub-event gets 10 scheduled engagements across the 5 stages
+            // Each sub-event gets 10 scheduled performances across the 5 stages
             for ($i = 0; $i < 10; $i++) {
                 $artist = $bucket[$bucketIdx++ % $bucket->count()];
                 $status = $statusMix[$statusIdx++ % count($statusMix)];
 
-                // unique(artist_id, event_id): only one engagement per
-                // (artist, sub-event). Since we wrap the pool, skip if
-                // we'd violate the constraint.
-                $existing = ArtistEngagement::query()
+                // unique(artist_id, event_id) at the festival — reuse the engagement
+                // if the artist was already booked for this festival, adding another
+                // performance under it (D17 multi-perf path).
+                $engagement = ArtistEngagement::query()
                     ->where('artist_id', $artist->id)
-                    ->where('event_id', $subEvent->id)
-                    ->exists();
-                if ($existing) {
-                    continue;
+                    ->where('event_id', $festival->id)
+                    ->first();
+                if ($engagement === null) {
+                    $engagement = self::createEngagement($artist, $festival, $status);
+                    $engagements++;
                 }
 
-                $engagement = self::createEngagement($artist, $subEvent, $status);
-                $engagements++;
-
                 $hour = $hourSlots[$i % count($hourSlots)];
                 $minute = ($i % 4) * 15;
                 $stage = $stages->values()[$i % $stages->count()];
@@ -160,27 +163,29 @@ final class ArtistTimetableDevSeeder
         $u = 0;
         for ($i = 0; $i < 12; $i++) {
             $artist = $bucket[$bucketIdx++ % $bucket->count()];
-            $subEvent = $subEvents[$i % count($subEvents)];
 
+            // Skip if this artist already has an engagement at the festival
+            // (e.g. via the scheduled loop above).
             $existing = ArtistEngagement::query()
                 ->where('artist_id', $artist->id)
-                ->where('event_id', $subEvent->id)
+                ->where('event_id', $festival->id)
                 ->exists();
             if ($existing) {
                 continue;
             }
 
-            self::createEngagement($artist, $subEvent, $unscheduledStatusMix[$u++ % count($unscheduledStatusMix)]);
+            self::createEngagement($artist, $festival, $unscheduledStatusMix[$u++ % count($unscheduledStatusMix)]);
             $engagements++;
             $unscheduled++;
         }
 
         // ── One parked performance (= scheduled but stage_id = null) ──
-        // Exercises the Session 4 "wachtrij" UI.
+        // Exercises the Session 4 "wachtrij" UI. Engagement at festival level;
+        // performance.event_id is the sub-event the parked slot belongs to.
         $parkedArtist = $bucket[$bucketIdx++ % $bucket->count()];
         $parkedSub = $subEvents[0];
-        if (! ArtistEngagement::query()->where('artist_id', $parkedArtist->id)->where('event_id', $parkedSub->id)->exists()) {
-            $parkedEngagement = self::createEngagement($parkedArtist, $parkedSub, ArtistEngagementStatus::Confirmed);
+        if (! ArtistEngagement::query()->where('artist_id', $parkedArtist->id)->where('event_id', $festival->id)->exists()) {
+            $parkedEngagement = self::createEngagement($parkedArtist, $festival, ArtistEngagementStatus::Confirmed);
             Performance::create([
                 'engagement_id' => $parkedEngagement->id,
                 'event_id' => $parkedSub->id,
@@ -195,17 +200,17 @@ final class ArtistTimetableDevSeeder
         }
 
         // ── B2B pair on Mainstage Saturday with 3-min offset (B2B detector seed) ──
-        // Two artists overlap on the same stage/lane within 3 minutes.
+        // Two artists with consecutive performances on the same stage within 3 minutes.
         $zaterdag = $subEvents[1];
         $b2bA = $bucket[$bucketIdx++ % $bucket->count()];
         $b2bB = $bucket[$bucketIdx++ % $bucket->count()];
         $mainstage = $stages['Mainstage'] ?? $stages->first();
 
         foreach ([['artist' => $b2bA, 'minute' => 30], ['artist' => $b2bB, 'minute' => 33]] as $pair) {
-            if (ArtistEngagement::query()->where('artist_id', $pair['artist']->id)->where('event_id', $zaterdag->id)->exists()) {
+            if (ArtistEngagement::query()->where('artist_id', $pair['artist']->id)->where('event_id', $festival->id)->exists()) {
                 continue;
             }
-            $eng = self::createEngagement($pair['artist'], $zaterdag, ArtistEngagementStatus::Contracted);
+            $eng = self::createEngagement($pair['artist'], $festival, ArtistEngagementStatus::Contracted);
             self::createPerformance($eng, $zaterdag, $mainstage, 23, $pair['minute'], 60);
             $engagements++;
             $performances++;
@@ -330,16 +335,22 @@ final class ArtistTimetableDevSeeder
         $performances = 0;
         $unscheduled = 0;
 
+        // Same model as the festival path: engagement at the parent series
+        // event, performance at the per-week sub-event. A resident artist
+        // playing multiple weeks gets ONE engagement with N performances.
         foreach ($subEvents as $weekIdx => $week) {
             for ($i = 0; $i < $perWeekScheduled; $i++) {
                 $artist = $bucket[$bucketIdx++ % $bucket->count()];
-                if (ArtistEngagement::query()->where('artist_id', $artist->id)->where('event_id', $week->id)->exists()) {
-                    continue;
-                }
-
                 $status = [ArtistEngagementStatus::Contracted, ArtistEngagementStatus::Confirmed, ArtistEngagementStatus::Option][$i % 3];
-                $engagement = self::createEngagement($artist, $week, $status);
-                $engagements++;
+
+                $engagement = ArtistEngagement::query()
+                    ->where('artist_id', $artist->id)
+                    ->where('event_id', $parent->id)
+                    ->first();
+                if ($engagement === null) {
+                    $engagement = self::createEngagement($artist, $parent, $status);
+                    $engagements++;
+                }
 
                 $stage = $stages->values()[$i % $stages->count()];
                 self::createPerformance($engagement, $week, $stage, 14 + $i * 2, 0, 60);
@@ -348,11 +359,11 @@ final class ArtistTimetableDevSeeder
 
             for ($i = 0; $i < $perWeekUnscheduled; $i++) {
                 $artist = $bucket[$bucketIdx++ % $bucket->count()];
-                if (ArtistEngagement::query()->where('artist_id', $artist->id)->where('event_id', $week->id)->exists()) {
+                if (ArtistEngagement::query()->where('artist_id', $artist->id)->where('event_id', $parent->id)->exists()) {
                     continue;
                 }
                 $status = [ArtistEngagementStatus::Draft, ArtistEngagementStatus::Requested, ArtistEngagementStatus::Option][$i % 3];
-                self::createEngagement($artist, $week, $status);
+                self::createEngagement($artist, $parent, $status);
                 $engagements++;
                 $unscheduled++;
             }
diff --git a/api/tests/Feature/Artist/TimetableSeederControllerIntegrationTest.php b/api/tests/Feature/Artist/TimetableSeederControllerIntegrationTest.php
new file mode 100644
index 00000000..f7dc74f7
--- /dev/null
+++ b/api/tests/Feature/Artist/TimetableSeederControllerIntegrationTest.php
@@ -0,0 +1,200 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Feature\Artist;
+
+use App\Models\Event;
+use App\Models\Organisation;
+use App\Models\User;
+use Database\Seeders\ArtistTimetableDevSeeder;
+use Database\Seeders\RoleSeeder;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Laravel\Sanctum\Sanctum;
+use Tests\TestCase;
+
+/**
+ * Locks the model contract (RFC §D10/§D17, SCHEMA.md:1285+1329):
+ *
+ *   engagement.event_id  = festival or flat event
+ *   performance.event_id = sub-event or flat event ("show host")
+ *
+ * Without this test, the Session-4 visible-symptom regression ("API returns
+ * empty for a festival even when seeded") would slip through again because
+ * the existing controller tests build fixtures with factories that put
+ * engagement.event_id at the same level as performance.event_id, hiding
+ * the festival-vs-sub-event scoping.
+ *
+ * What we exercise: the actual `ArtistTimetableDevSeeder` against the actual
+ * `PerformanceController::index`, `ArtistEngagementController::index`, and
+ * `StageController::index`. Both `?day=` and unfiltered festival-level
+ * requests, so the controller's whereHas('engagement', …) at festival level
+ * AND the ?day filter on performance.event_id are both proved.
+ */
+final class TimetableSeederControllerIntegrationTest extends TestCase
+{
+    use RefreshDatabase;
+
+    private Organisation $org;
+
+    private User $orgAdmin;
+
+    private Event $festival;
+
+    /** @var array<int, Event> */
+    private array $subEvents;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+        $this->seed(RoleSeeder::class);
+
+        $this->org = Organisation::factory()->create();
+        $this->orgAdmin = User::factory()->create();
+        $this->org->users()->attach($this->orgAdmin, ['role' => 'org_admin']);
+
+        $this->festival = Event::factory()->for($this->org)->festival()->create([
+            'start_date' => '2026-07-10',
+            'end_date' => '2026-07-12',
+        ]);
+        $vrijdag = Event::factory()->for($this->org)->subEvent($this->festival)->create([
+            'start_date' => '2026-07-10',
+            'end_date' => '2026-07-10',
+        ]);
+        $zaterdag = Event::factory()->for($this->org)->subEvent($this->festival)->create([
+            'start_date' => '2026-07-11',
+            'end_date' => '2026-07-11',
+        ]);
+        $zondag = Event::factory()->for($this->org)->subEvent($this->festival)->create([
+            'start_date' => '2026-07-12',
+            'end_date' => '2026-07-12',
+        ]);
+        $this->subEvents = [$vrijdag, $zaterdag, $zondag];
+
+        $pool = ArtistTimetableDevSeeder::seedOrganisationPool($this->org, 125);
+        ArtistTimetableDevSeeder::seedForFestival($this->org, $this->festival, $this->subEvents, $pool);
+
+        Sanctum::actingAs($this->orgAdmin);
+    }
+
+    public function test_engagement_event_id_is_at_festival_level(): void
+    {
+        // Direct DB invariant — engagement.event_id == festival.id for the
+        // entire seeded fixture. This is the canonical model from RFC §D10/§D17.
+        $count = \App\Models\ArtistEngagement::query()
+            ->where('event_id', $this->festival->id)
+            ->count();
+        $this->assertGreaterThan(0, $count, 'No engagements found at festival level — seeder regressed to per-sub-event scoping');
+
+        $offModelCount = \App\Models\ArtistEngagement::query()
+            ->whereIn('event_id', collect($this->subEvents)->pluck('id'))
+            ->count();
+        $this->assertSame(0, $offModelCount, 'Engagements seeded at sub-event level — Model A violated');
+    }
+
+    public function test_performance_event_id_is_at_sub_event_level(): void
+    {
+        $atFestival = \App\Models\Performance::query()
+            ->where('event_id', $this->festival->id)
+            ->count();
+        $this->assertSame(0, $atFestival, 'Performances seeded at festival level — Model A violated');
+
+        $atSubEvents = \App\Models\Performance::query()
+            ->whereIn('event_id', collect($this->subEvents)->pluck('id'))
+            ->count();
+        $this->assertGreaterThan(0, $atSubEvents);
+    }
+
+    public function test_performances_index_returns_data_for_festival_with_day_filter(): void
+    {
+        // The exact request the SPA makes: GET /events/{festival}/performances?day={subevent}
+        foreach ($this->subEvents as $subEvent) {
+            $response = $this->getJson(
+                "/api/v1/organisations/{$this->org->id}/events/{$this->festival->id}/performances?day={$subEvent->id}",
+            );
+
+            $response->assertOk();
+            $payload = $response->json('data');
+            $this->assertIsArray($payload);
+            $this->assertNotEmpty(
+                $payload,
+                "Performances index returned empty for festival={$this->festival->id} day={$subEvent->id} — controller engagement filter regressed",
+            );
+
+            // Every returned perf should carry event_id matching the requested ?day value
+            foreach ($payload as $perf) {
+                $this->assertSame($subEvent->id, $perf['event_id']);
+            }
+        }
+    }
+
+    public function test_performances_index_unfiltered_returns_all_festival_performances(): void
+    {
+        // No ?day — should return every performance across all sub-events of the festival.
+        $response = $this->getJson(
+            "/api/v1/organisations/{$this->org->id}/events/{$this->festival->id}/performances",
+        );
+
+        $response->assertOk();
+        $payload = $response->json('data');
+        $this->assertIsArray($payload);
+        $this->assertNotEmpty($payload);
+
+        $subEventIds = collect($this->subEvents)->pluck('id')->all();
+        foreach ($payload as $perf) {
+            $this->assertContains(
+                $perf['event_id'],
+                $subEventIds,
+                'Returned performance has an event_id outside the festival sub-event set',
+            );
+        }
+    }
+
+    public function test_performances_index_includes_parked_when_stage_id_null_filter(): void
+    {
+        $response = $this->getJson(
+            "/api/v1/organisations/{$this->org->id}/events/{$this->festival->id}/performances?stage_id=null",
+        );
+
+        $response->assertOk();
+        $payload = $response->json('data');
+        $this->assertNotEmpty($payload, 'Wachtrij should contain at least the seeded parked performance');
+        foreach ($payload as $perf) {
+            $this->assertNull($perf['stage_id']);
+        }
+    }
+
+    public function test_engagements_index_returns_seeded_engagements_for_festival(): void
+    {
+        $response = $this->getJson(
+            "/api/v1/organisations/{$this->org->id}/events/{$this->festival->id}/engagements",
+        );
+
+        $response->assertOk();
+        $payload = $response->json('data');
+        $this->assertIsArray($payload);
+        $this->assertNotEmpty($payload);
+
+        // Every returned engagement carries event_id = festival
+        foreach ($payload as $engagement) {
+            $this->assertSame($this->festival->id, $engagement['event_id']);
+        }
+    }
+
+    public function test_stages_index_returns_seeded_stages_for_festival(): void
+    {
+        $response = $this->getJson(
+            "/api/v1/organisations/{$this->org->id}/events/{$this->festival->id}/stages",
+        );
+
+        $response->assertOk();
+        $payload = $response->json('data');
+        $this->assertIsArray($payload);
+        $this->assertCount(5, $payload, 'Festival fixture seeds 5 stages');
+
+        // Every stage carries event_id = festival
+        foreach ($payload as $stage) {
+            $this->assertSame($this->festival->id, $stage['event_id']);
+        }
+    }
+}