diff --git a/api/app/Exceptions/Artist/ArtistDeletedException.php b/api/app/Exceptions/Artist/ArtistDeletedException.php
new file mode 100644
index 00000000..49a76057
--- /dev/null
+++ b/api/app/Exceptions/Artist/ArtistDeletedException.php
@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\Artist;
+
+use DomainException;
+
+/**
+ * Raised by ArtistResolver::fromPortalToken when the engagement's
+ * portal_token matches but the master Artist has been soft-deleted.
+ * Per RFC v0.2 D27 the engagement itself remains usable; the portal
+ * flow surfaces a clear 410 Gone rather than crashing on a null
+ * subject downstream.
+ */
+final class ArtistDeletedException extends DomainException
+{
+    public function __construct(public readonly string $engagementId)
+    {
+        parent::__construct(sprintf(
+            'Master Artist for engagement %s has been deleted; portal flow is not available.',
+            $engagementId,
+        ));
+    }
+}
diff --git a/api/app/Exceptions/Artist/InvalidPortalTokenException.php b/api/app/Exceptions/Artist/InvalidPortalTokenException.php
new file mode 100644
index 00000000..ebc79857
--- /dev/null
+++ b/api/app/Exceptions/Artist/InvalidPortalTokenException.php
@@ -0,0 +1,21 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\Artist;
+
+use DomainException;
+
+/**
+ * Raised by ArtistResolver::fromPortalToken when the supplied portal
+ * token does not match any active artist_engagements row. Maps to a
+ * 404 at the HTTP boundary — distinguishes from ArtistDeletedException
+ * (engagement exists but master Artist is soft-deleted, → 410 Gone).
+ */
+final class InvalidPortalTokenException extends DomainException
+{
+    public static function create(): self
+    {
+        return new self('Portal token does not resolve to an active engagement.');
+    }
+}
diff --git a/api/app/FormBuilder/Resolvers/ArtistResolver.php b/api/app/FormBuilder/Resolvers/ArtistResolver.php
new file mode 100644
index 00000000..121d1c19
--- /dev/null
+++ b/api/app/FormBuilder/Resolvers/ArtistResolver.php
@@ -0,0 +1,59 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\FormBuilder\Resolvers;
+
+use App\Exceptions\Artist\ArtistDeletedException;
+use App\Exceptions\Artist\InvalidPortalTokenException;
+use App\Models\Artist;
+use App\Models\ArtistEngagement;
+use App\Models\Scopes\OrganisationScope;
+
+/**
+ * Engagement-scoped subject resolution for the artist_advance portal
+ * flow. Per ARCH-FORM-BUILDER §17.3 footnote and RFC-TIMETABLE v0.2
+ * D15: the master Artist is the FormSubmission subject (subject_type
+ * = 'artist'), but the engagement provides the event_id (denormalised
+ * onto form_submissions per WS-4) and any advance_section context.
+ *
+ * The portal token itself is stored on artist_engagements.portal_token
+ * as a SHA-256 hex digest (Session 1 commit eb6d396). Callers pass
+ * the plaintext token; we hash and look up.
+ *
+ * This resolver is the single shared helper for portal-token →
+ * engagement resolution. PortalTokenMiddleware delegates to it; the
+ * EngagementPortalController calls it directly to produce the value
+ * object the FormSubmissionService needs.
+ */
+final class ArtistResolver
+{
+    public function fromPortalToken(string $portalToken): ArtistResolverResult
+    {
+        $digest = hash('sha256', $portalToken);
+
+        $engagement = ArtistEngagement::query()
+            ->withoutGlobalScope(OrganisationScope::class)
+            ->where('portal_token', $digest)
+            ->first();
+
+        if ($engagement === null) {
+            throw InvalidPortalTokenException::create();
+        }
+
+        $artist = Artist::query()
+            ->withoutGlobalScope(OrganisationScope::class)
+            ->whereKey($engagement->artist_id)
+            ->first();
+
+        if (! $artist instanceof Artist) {
+            throw new ArtistDeletedException((string) $engagement->id);
+        }
+
+        return new ArtistResolverResult(
+            subject: $artist,
+            eventId: (string) $engagement->event_id,
+            engagement: $engagement,
+        );
+    }
+}
diff --git a/api/app/FormBuilder/Resolvers/ArtistResolverResult.php b/api/app/FormBuilder/Resolvers/ArtistResolverResult.php
new file mode 100644
index 00000000..2acbb0b9
--- /dev/null
+++ b/api/app/FormBuilder/Resolvers/ArtistResolverResult.php
@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\FormBuilder\Resolvers;
+
+use App\Models\Artist;
+use App\Models\ArtistEngagement;
+
+/**
+ * Value object returned by ArtistResolver::fromPortalToken.
+ *
+ * Per ARCH-FORM-BUILDER §17.3 footnote: artist_advance submissions use
+ * the master Artist as `subject` (preserves form_submissions.subject_type
+ * = 'artist'); `eventId` populates form_submissions.event_id per WS-4
+ * denormalisation; the engagement itself is returned so callers
+ * (controllers, listeners) can resolve advance_section context without
+ * a second query.
+ */
+final readonly class ArtistResolverResult
+{
+    public function __construct(
+        public Artist $subject,
+        public string $eventId,
+        public ArtistEngagement $engagement,
+    ) {}
+}