diff --git a/api/app/Http/Controllers/Api/V1/FormBuilder/PublicFormSubmissionController.php b/api/app/Http/Controllers/Api/V1/FormBuilder/PublicFormSubmissionController.php
index e78807e3..07145024 100644
--- a/api/app/Http/Controllers/Api/V1/FormBuilder/PublicFormSubmissionController.php
+++ b/api/app/Http/Controllers/Api/V1/FormBuilder/PublicFormSubmissionController.php
@@ -86,12 +86,13 @@ final class PublicFormSubmissionController extends Controller
 
         $values = (array) ($request->validated('values') ?? []);
         if ($values !== []) {
-            // Service throws FieldValidationException (422) on validation
-            // failure; the handler renders the standardised envelope.
-            $this->valueService->upsertMany($submission, $values, null);
+            // saveDraft → upsertMany + auto_save_count++ + FormSubmissionDraftUpdated
+            // event. FieldValidationException bubbles to the D6 envelope.
+            $this->submissionService->saveDraft($submission, $values, null);
         }
 
         if ($request->filled('first_interacted_at')) {
+            $submission->refresh();
             $submission->first_interacted_at ??= $request->validated('first_interacted_at');
             $submission->save();
         }
diff --git a/api/app/Http/Resources/FormBuilder/PublicFormSubmissionResource.php b/api/app/Http/Resources/FormBuilder/PublicFormSubmissionResource.php
index f086bb48..b9ab699e 100644
--- a/api/app/Http/Resources/FormBuilder/PublicFormSubmissionResource.php
+++ b/api/app/Http/Resources/FormBuilder/PublicFormSubmissionResource.php
@@ -69,21 +69,30 @@ final class PublicFormSubmissionResource extends JsonResource
 
     private function computeSchemaDrift(): bool
     {
-        if ($this->schema_version_at_submit === null) {
-            // Draft phase: drift is "version advanced since the submission
-            // was opened". We use schema.version vs the version the portal
-            // saw via PublicFormSchemaResource at open time — which is
-            // only derivable after submit. For drafts, schema_drift stays
-            // false; the submit response is the authoritative check.
+        // The draft was opened against version_at_open; at submit time the
+        // schema's current version is frozen into version_at_submit. Drift
+        // means the organiser edited the schema between the portal loading
+        // the form and the user hitting submit.
+        //
+        // Drafts without a submit stamp keep the comparison live against
+        // the current schema.version so an organiser edit during an open
+        // draft also surfaces drift on subsequent PUT/GET calls.
+
+        $atOpen = $this->schema_version_at_open;
+        if ($atOpen === null) {
             return false;
         }
 
-        $schema = $this->schema;
-        if ($schema === null) {
-            return false;
+        $other = $this->schema_version_at_submit;
+        if ($other === null) {
+            $schema = $this->schema;
+            if ($schema === null) {
+                return false;
+            }
+            $other = (int) $schema->version;
         }
 
-        return (int) $schema->version !== (int) $this->schema_version_at_submit;
+        return (int) $atOpen !== (int) $other;
     }
 
     /**
diff --git a/api/app/Models/FormBuilder/FormSubmission.php b/api/app/Models/FormBuilder/FormSubmission.php
index 12a88dca..c2641ab5 100644
--- a/api/app/Models/FormBuilder/FormSubmission.php
+++ b/api/app/Models/FormBuilder/FormSubmission.php
@@ -40,6 +40,7 @@ final class FormSubmission extends Model
         'reviewed_at',
         'review_notes',
         'submitted_at',
+        'schema_version_at_open',
         'schema_version_at_submit',
         'schema_snapshot',
         'submission_duration_seconds',
@@ -65,6 +66,7 @@ final class FormSubmission extends Model
         'opened_at' => 'datetime',
         'first_interacted_at' => 'datetime',
         'public_submitter_ip_anonymised_at' => 'datetime',
+        'schema_version_at_open' => 'int',
         'schema_version_at_submit' => 'int',
         'submission_duration_seconds' => 'int',
         'auto_save_count' => 'int',
diff --git a/api/app/Services/FormBuilder/FormSubmissionService.php b/api/app/Services/FormBuilder/FormSubmissionService.php
index f0198b50..6eba14f2 100644
--- a/api/app/Services/FormBuilder/FormSubmissionService.php
+++ b/api/app/Services/FormBuilder/FormSubmissionService.php
@@ -61,6 +61,7 @@ final class FormSubmissionService
             $submission->status = FormSubmissionStatus::DRAFT->value;
             $submission->is_test = (bool) ($context['is_test'] ?? false);
             $submission->opened_at = $context['opened_at'] ?? now();
+            $submission->schema_version_at_open = (int) $schema->version;
             $submission->idempotency_key = $context['idempotency_key'] ?? null;
             $submission->public_submitter_name = $context['public_submitter_name'] ?? null;
             $submission->public_submitter_email = $context['public_submitter_email'] ?? null;
diff --git a/api/bootstrap/app.php b/api/bootstrap/app.php
index b1e3ceb4..3194ba61 100644
--- a/api/bootstrap/app.php
+++ b/api/bootstrap/app.php
@@ -52,6 +52,21 @@ return Application::configure(basePath: dirname(__DIR__))
             return response()->json($body, $e->status, $e->headers);
         });
 
+        // FormRequest validation on /api/v1/public/forms/* → rewrap into
+        // the D6 envelope so every public endpoint error looks identical
+        // regardless of which layer surfaced it.
+        $exceptions->render(function (ValidationException $e, Request $request) {
+            if (! $request->is('api/v1/public/forms/*')) {
+                return null;
+            }
+
+            return response()->json([
+                'message' => $e->getMessage(),
+                'code' => 'VALIDATION_FAILED',
+                'errors' => $e->errors(),
+            ], $e->status);
+        });
+
         // Database connection / query errors → 503
         $exceptions->render(function (QueryException|PDOException $e, Request $request) {
             if ($request->expectsJson() || $request->is('api/*')) {
diff --git a/api/database/migrations/2026_04_22_100002_add_schema_version_at_open_to_form_submissions.php b/api/database/migrations/2026_04_22_100002_add_schema_version_at_open_to_form_submissions.php
new file mode 100644
index 00000000..f7ac8f93
--- /dev/null
+++ b/api/database/migrations/2026_04_22_100002_add_schema_version_at_open_to_form_submissions.php
@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('form_submissions', function (Blueprint $table): void {
+            // Version the portal rendered at draft-open time. Compared to
+            // schema_version_at_submit by PublicFormSubmissionResource so
+            // the portal can warn about drift before showing the success
+            // screen (S2c D5).
+            $table->unsignedInteger('schema_version_at_open')->nullable()->after('opened_at');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('form_submissions', function (Blueprint $table): void {
+            $table->dropColumn('schema_version_at_open');
+        });
+    }
+};