diff --git a/api/app/Enums/FormBuilder/FormFieldBindingMergeStrategy.php b/api/app/Enums/FormBuilder/FormFieldBindingMergeStrategy.php
index f66c0708..8df5558d 100644
--- a/api/app/Enums/FormBuilder/FormFieldBindingMergeStrategy.php
+++ b/api/app/Enums/FormBuilder/FormFieldBindingMergeStrategy.php
@@ -37,4 +37,31 @@ enum FormFieldBindingMergeStrategy: string
     {
         return $this !== self::Append;
     }
+
+    /**
+     * Whether this strategy is structurally valid against the given target
+     * type.
+     *
+     * Per RFC-WS-6 §V1 + ARCH-BINDINGS §4.2 (strategy x target-type validity matrix).
+     *
+     *                  | SCALAR | COLLECTION | RELATION |
+     *  Overwrite       | valid  | valid*     | valid    |
+     *  Append          | INVALID| valid      | INVALID  |
+     *  Replace         | valid  | valid      | valid    |
+     *  FirstWriteWins  | valid  | valid      | valid    |
+     *
+     *  * unusual but valid (overwrites entire collection)
+     *
+     * The PublishGuard AppendStrategyRequiresCollectionTarget uses this
+     * method to validate at publish time. Append on scalars is rejected
+     * because it requires a fingerprint mechanism for retry-idempotency
+     * that would embed implementation detail in domain data.
+     */
+    public function validForTargetType(BindingTargetType $type): bool
+    {
+        return match ($this) {
+            self::Append => $type === BindingTargetType::COLLECTION,
+            default => true,
+        };
+    }
 }
diff --git a/api/app/Events/FormBuilder/FormSubmissionIdentityMatchResolved.php b/api/app/Events/FormBuilder/FormSubmissionIdentityMatchResolved.php
new file mode 100644
index 00000000..a3da5719
--- /dev/null
+++ b/api/app/Events/FormBuilder/FormSubmissionIdentityMatchResolved.php
@@ -0,0 +1,58 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Events\FormBuilder;
+
+use Illuminate\Broadcasting\InteractsWithSockets;
+use Illuminate\Broadcasting\PrivateChannel;
+use Illuminate\Contracts\Broadcasting\ShouldBroadcast;
+use Illuminate\Foundation\Events\Dispatchable;
+use Illuminate\Queue\SerializesModels;
+
+/**
+ * Broadcast when TriggerPersonIdentityMatchOnFormSubmit (D2) writes the
+ * final identity_match_status. Frontend portal IdentityMatchBanner
+ * subscribes to this channel and refetches the submission resource on
+ * receipt, so the banner copy transitions from "we're checking matches…"
+ * to the final state without a manual reload.
+ *
+ * Per RFC-WS-6 §Q1 v1.3 addition 2.
+ *
+ * Wiring (the dispatch call from TriggerPersonIdentityMatchOnFormSubmit::handle)
+ * lands in D2. This class exists in D1 so D2's wiring is a one-liner.
+ */
+final class FormSubmissionIdentityMatchResolved implements ShouldBroadcast
+{
+    use Dispatchable;
+    use InteractsWithSockets;
+    use SerializesModels;
+
+    public function __construct(
+        public readonly string $submissionId,
+        public readonly string $status,        // 'matched' | 'no_match' | 'multiple_candidates'
+        public readonly int $matchCount,
+    ) {}
+
+    /**
+     * Private channel keyed on submission ULID.
+     *
+     * Frontend subscribes via `Echo.private('submission.${submissionId}')`.
+     * Authorization (only the submitter / organisation admins can subscribe)
+     * is the responsibility of the channel-authorization callback in
+     * routes/channels.php — that wiring lands in D2 alongside the dispatch.
+     *
+     * @return array<int, PrivateChannel>
+     */
+    public function broadcastOn(): array
+    {
+        return [
+            new PrivateChannel("submission.{$this->submissionId}"),
+        ];
+    }
+
+    public function broadcastAs(): string
+    {
+        return 'identity-match.resolved';
+    }
+}
diff --git a/api/app/Exceptions/FormBuilder/FormBindingApplicatorException.php b/api/app/Exceptions/FormBuilder/FormBindingApplicatorException.php
index 8cfd9f42..ebf1f3fb 100644
--- a/api/app/Exceptions/FormBuilder/FormBindingApplicatorException.php
+++ b/api/app/Exceptions/FormBuilder/FormBindingApplicatorException.php
@@ -5,19 +5,48 @@ declare(strict_types=1);
 namespace App\Exceptions\FormBuilder;
 
 use RuntimeException;
+use Throwable;
 
 /**
- * RFC-WS-6 §3 (Q3) — catastrophic applicator failure that bubbles to
- * the caller. Per-binding failures are captured in BindingPassResult,
- * not thrown.
+ * Base for all FormBindingApplicator-pipeline exceptions.
+ *
+ * Subclasses provide a `reasonCode()` that maps to:
+ * - failure_response_code on form_submissions (response-shape driver)
+ * - HTTP status code (422 / 503 / 500)
+ * - user-facing copy class (rendered by frontend)
+ *
+ * Per RFC-WS-6 §Q3 v1.3 addition 2.
+ *
+ * Concrete subclasses:
+ * - FormBindingSchemaConfigException — schema misconfiguration (422, schema_config_error)
+ * - FormBindingInfraException — infra issue, retryable (503, temporary_error)
+ *   - FormBindingApplicatorTimeoutException — deadline-wrapper exceeded (extends Infra)
+ * - FormBindingDataIntegrityException — data shape violation (422, data_integrity_error)
+ *
+ * The classifier (FormBindingExceptionClassifier) maps unknown Throwables
+ * to 'unknown_error' — that is the fallback for anything not in this
+ * hierarchy.
+ *
+ * `submissionId` is preserved as a public readonly property so D2's
+ * outer-transaction handler can structurally read it when writing the
+ * `form_submission_action_failures.context` JSON, instead of regex-parsing
+ * the message string.
  */
-final class FormBindingApplicatorException extends RuntimeException
+abstract class FormBindingApplicatorException extends RuntimeException
 {
     public function __construct(
-        public readonly string $reasonCode,
         public readonly string $submissionId,
-        ?string $message = null,
+        string $message,
+        ?Throwable $previous = null,
     ) {
-        parent::__construct($message ?? "FormBindingApplicator failed: {$reasonCode} (submission {$submissionId})");
+        parent::__construct($message, 0, $previous);
     }
+
+    /**
+     * Response-shape classification token. One of:
+     * - 'schema_config_error'
+     * - 'temporary_error'
+     * - 'data_integrity_error'
+     */
+    abstract public function reasonCode(): string;
 }
diff --git a/api/app/Exceptions/FormBuilder/FormBindingApplicatorTimeoutException.php b/api/app/Exceptions/FormBuilder/FormBindingApplicatorTimeoutException.php
new file mode 100644
index 00000000..79847178
--- /dev/null
+++ b/api/app/Exceptions/FormBuilder/FormBindingApplicatorTimeoutException.php
@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+/**
+ * Thrown by the deadline-wrapper around FormBindingApplicator when the
+ * inner transaction takes longer than config('form_builder.apply_deadline_seconds')
+ * (default 5).
+ *
+ * Extends FormBindingInfraException because from the user's perspective a
+ * timeout is identical to any other temporary infra issue: retry should
+ * help. The response-shape classification 'temporary_error' is inherited.
+ *
+ * Per RFC-WS-6 §Q1 v1.3 addition 4.
+ *
+ * The deadline-wrapper itself lands in D2 (modification to
+ * ApplyBindingsOnFormSubmit's apply call site). This class exists in D1
+ * so D2's wiring is straightforward.
+ */
+final class FormBindingApplicatorTimeoutException extends FormBindingInfraException
+{
+    // No reasonCode override — inherits 'temporary_error' from FormBindingInfraException.
+    // No constructor override — inherits (submissionId, message, previous) from the base.
+}
diff --git a/api/app/Exceptions/FormBuilder/FormBindingDataIntegrityException.php b/api/app/Exceptions/FormBuilder/FormBindingDataIntegrityException.php
new file mode 100644
index 00000000..860a1de5
--- /dev/null
+++ b/api/app/Exceptions/FormBuilder/FormBindingDataIntegrityException.php
@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+/**
+ * Data shape violation during binding apply.
+ *
+ * Examples: type mismatch between form_value and target attribute;
+ * foreign-key violation when the target references a soft-deleted entity;
+ * attempt to write to a column that has a unique constraint already
+ * violated by another row outside this submission.
+ *
+ * User-perceptible the same as schema_config (organiser fix needed); admin
+ * sees the difference via the exception class on the action-failures row.
+ *
+ * Maps to HTTP 422, failure_response_code='data_integrity_error'.
+ *
+ * Per RFC-WS-6 §Q3 v1.3 addition 2.
+ */
+final class FormBindingDataIntegrityException extends FormBindingApplicatorException
+{
+    public function reasonCode(): string
+    {
+        return 'data_integrity_error';
+    }
+}
diff --git a/api/app/Exceptions/FormBuilder/FormBindingInfraException.php b/api/app/Exceptions/FormBuilder/FormBindingInfraException.php
new file mode 100644
index 00000000..180aa993
--- /dev/null
+++ b/api/app/Exceptions/FormBuilder/FormBindingInfraException.php
@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+/**
+ * Infrastructure issue during binding apply.
+ *
+ * Examples: database connection lost, lock-for-update wait exceeded,
+ * race condition surfaced as a constraint violation that retry would
+ * resolve, applicator invoked outside a DB transaction (developer-error
+ * surfacing as infra-triage workflow).
+ *
+ * Maps to HTTP 503, failure_response_code='temporary_error'.
+ * User-facing copy: "Temporary issue, please try again." with retry-after.
+ *
+ * NOT final — FormBindingApplicatorTimeoutException extends this class.
+ *
+ * Per RFC-WS-6 §Q3 v1.3 addition 2.
+ */
+class FormBindingInfraException extends FormBindingApplicatorException
+{
+    public function reasonCode(): string
+    {
+        return 'temporary_error';
+    }
+}
diff --git a/api/app/Exceptions/FormBuilder/FormBindingSchemaConfigException.php b/api/app/Exceptions/FormBuilder/FormBindingSchemaConfigException.php
new file mode 100644
index 00000000..87148217
--- /dev/null
+++ b/api/app/Exceptions/FormBuilder/FormBindingSchemaConfigException.php
@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+/**
+ * Schema misconfiguration that publish-guards missed.
+ *
+ * Examples: column renamed without schema invalidation; binding target
+ * attribute that no longer exists on the entity; merge_strategy that's
+ * structurally invalid for the target type but passed publish for some
+ * reason; unregistered purpose value; null schema relation.
+ *
+ * Maps to HTTP 422, failure_response_code='schema_config_error'.
+ * User-facing copy: "This form has a configuration issue. Please contact
+ * the organiser. Reference: F-{ulid}"
+ *
+ * Per RFC-WS-6 §Q3 v1.3 addition 2.
+ */
+final class FormBindingSchemaConfigException extends FormBindingApplicatorException
+{
+    public function reasonCode(): string
+    {
+        return 'schema_config_error';
+    }
+}
diff --git a/api/app/Exceptions/FormBuilder/IdentityMatchInvariantViolation.php b/api/app/Exceptions/FormBuilder/IdentityMatchInvariantViolation.php
new file mode 100644
index 00000000..5863622f
--- /dev/null
+++ b/api/app/Exceptions/FormBuilder/IdentityMatchInvariantViolation.php
@@ -0,0 +1,36 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+use DomainException;
+
+/**
+ * Thrown by TriggerPersonIdentityMatchOnFormSubmit when the post-ApplyBindings
+ * invariant breaks.
+ *
+ * Per RFC-WS-6 §Q2 + ARCH-BINDINGS §7.3:
+ *   Post ApplyBindingsOnFormSubmit::handle for event_registration purpose:
+ *   subject_type='person' AND subject_id IS NOT NULL,
+ *   OR apply_status=ApplyStatus::FAILED.
+ *   No third state exists. Violation is a structural defect.
+ *
+ * This is NOT a FormBindingApplicatorException — the listener that throws
+ * this does not run inside the FormBindingApplicator pipeline. It indicates
+ * the pipeline succeeded according to apply_status but produced incoherent
+ * subject state, which means a publish-guard gap or a race with manual
+ * data manipulation.
+ *
+ * Routed via Laravel queue worker → GlitchTip + form_submission_action_failures
+ * row (the catch + outer-transaction handler in TriggerPersonIdentityMatchOnFormSubmit
+ * mirrors ApplyBindingsOnFormSubmit's pattern). Wiring lands in D2.
+ */
+final class IdentityMatchInvariantViolation extends DomainException
+{
+    // Plain DomainException subclass. No reasonCode — when this fires,
+    // 'unknown_error' (the classifier's default fallback) is the right
+    // response-shape because users cannot meaningfully act on it; admins
+    // triage via GlitchTip. D2's invariant-throw path writes its own
+    // context JSON to form_submission_action_failures.
+}
diff --git a/api/app/FormBuilder/Bindings/FormBindingApplicator.php b/api/app/FormBuilder/Bindings/FormBindingApplicator.php
index d25391e7..8501cd10 100644
--- a/api/app/FormBuilder/Bindings/FormBindingApplicator.php
+++ b/api/app/FormBuilder/Bindings/FormBindingApplicator.php
@@ -7,6 +7,8 @@ namespace App\FormBuilder\Bindings;
 use App\Enums\FormBuilder\BindingTargetType;
 use App\Enums\FormBuilder\FormFieldBindingMergeStrategy;
 use App\Exceptions\FormBuilder\FormBindingApplicatorException;
+use App\Exceptions\FormBuilder\FormBindingInfraException;
+use App\Exceptions\FormBuilder\FormBindingSchemaConfigException;
 use App\FormBuilder\Purposes\PurposeRegistry;
 use App\Models\FormBuilder\FormSubmission;
 use Illuminate\Database\Eloquent\Model;
@@ -46,27 +48,25 @@ class FormBindingApplicator
     public function apply(FormSubmission $submission, ?string $sectionId = null): BindingPassResult
     {
         if (DB::transactionLevel() < 1) {
-            throw new FormBindingApplicatorException(
-                'no_transaction',
-                (string) $submission->id,
-                'FormBindingApplicator must be invoked inside DB::transaction',
+            throw new FormBindingInfraException(
+                submissionId: (string) $submission->id,
+                message: 'FormBindingApplicator must be invoked inside DB::transaction',
             );
         }
 
         /** @var \App\Models\FormBuilder\FormSchema|null $schema */
         $schema = $submission->schema;
         if ($schema === null) {
-            throw new FormBindingApplicatorException(
-                'no_schema',
-                (string) $submission->id,
+            throw new FormBindingSchemaConfigException(
+                submissionId: (string) $submission->id,
+                message: "schema null for submission {$submission->id}",
             );
         }
         $purposeValue = $schema->purpose->value;
         if (! $this->purposeRegistry->has($purposeValue)) {
-            throw new FormBindingApplicatorException(
-                'unknown_purpose',
-                (string) $submission->id,
-                "purpose '{$purposeValue}' not registered",
+            throw new FormBindingSchemaConfigException(
+                submissionId: (string) $submission->id,
+                message: "purpose '{$purposeValue}' not registered",
             );
         }
 
@@ -168,6 +168,7 @@ class FormBindingApplicator
         // Per-strategy matrix. RFC §3 Q7.
         if ($newValue === null) {
             $behaviour = $strategy->nullWinnerBehaviour();
+
             return match ($behaviour) {
                 'write' => null,
                 'noop' => self::NO_OP,
diff --git a/api/app/FormBuilder/Bindings/FormBindingExceptionClassifier.php b/api/app/FormBuilder/Bindings/FormBindingExceptionClassifier.php
new file mode 100644
index 00000000..36d2f04f
--- /dev/null
+++ b/api/app/FormBuilder/Bindings/FormBindingExceptionClassifier.php
@@ -0,0 +1,39 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\FormBuilder\Bindings;
+
+use App\Exceptions\FormBuilder\FormBindingApplicatorException;
+use Throwable;
+
+/**
+ * Maps a Throwable to a failure_response_code string.
+ *
+ * Used by both ApplyBindingsOnFormSubmit::handle's catch block (D2) and
+ * FormFailureRetryService::recordFailure (D2 update). Centralised so the
+ * listener and the retry-service produce identical classifications and a
+ * single behaviour change requires a single edit.
+ *
+ * Resolution order:
+ * 1. If the Throwable is a FormBindingApplicatorException, return its reasonCode().
+ *    Subclass dispatch handles SchemaConfig / Infra / DataIntegrity / Timeout
+ *    (Timeout extends Infra so it inherits 'temporary_error').
+ * 2. Otherwise, return 'unknown_error' — anything outside the hierarchy
+ *    (database connection lost not surfaced as Infra, generic RuntimeException
+ *    from a non-applicator code path, IdentityMatchInvariantViolation if it
+ *    somehow leaks here) is unknown from the response-shape perspective.
+ *
+ * Per RFC-WS-6 §Q3 v1.3 addition 2.
+ */
+final class FormBindingExceptionClassifier
+{
+    public static function classify(Throwable $exception): string
+    {
+        if ($exception instanceof FormBindingApplicatorException) {
+            return $exception->reasonCode();
+        }
+
+        return 'unknown_error';
+    }
+}
diff --git a/api/app/Models/FormBuilder/FormSubmission.php b/api/app/Models/FormBuilder/FormSubmission.php
index d871a23a..b9be4492 100644
--- a/api/app/Models/FormBuilder/FormSubmission.php
+++ b/api/app/Models/FormBuilder/FormSubmission.php
@@ -11,9 +11,9 @@ use App\Models\Event;
 use App\Models\Organisation;
 use App\Models\Scopes\OrganisationScope;
 use App\Models\User;
+use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Database\Eloquent\Concerns\HasUlids;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
-use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
 use Illuminate\Database\Eloquent\Relations\HasMany;
@@ -32,7 +32,7 @@ final class FormSubmission extends Model
 
     protected static function booted(): void
     {
-        static::addGlobalScope(new OrganisationScope());
+        self::addGlobalScope(new OrganisationScope);
     }
 
     /** @return array{column: string} */
@@ -72,6 +72,7 @@ final class FormSubmission extends Model
         'first_interacted_at',
         'idempotency_key',
         'identity_match_status',
+        'failure_response_code',
     ];
 
     /** @var array<string, string> */
@@ -80,6 +81,11 @@ final class FormSubmission extends Model
         'review_status' => FormSubmissionReviewStatus::class,
         'apply_status' => ApplyStatus::class,
         'apply_completed_at' => 'datetime',
+        // Plain string (not enum). The exception subclass on
+        // form_submission_action_failures is the canonical source of
+        // classification truth; this column is a denormalised mirror for
+        // response-shape rendering. Per RFC-WS-6 §Q3 v1.3 addition 2.
+        'failure_response_code' => 'string',
         'schema_snapshot' => 'array',
         'is_test' => 'bool',
         'submitted_at' => 'datetime',
diff --git a/api/database/factories/FormBuilder/FormSubmissionFactory.php b/api/database/factories/FormBuilder/FormSubmissionFactory.php
index 4bc4b264..d4ca460a 100644
--- a/api/database/factories/FormBuilder/FormSubmissionFactory.php
+++ b/api/database/factories/FormBuilder/FormSubmissionFactory.php
@@ -64,4 +64,23 @@ final class FormSubmissionFactory extends Factory
             'organisation_id' => $event->organisation_id,
         ]);
     }
+
+    /**
+     * Set failure_response_code. Typically used in tests asserting the
+     * downstream rendering behaviour for failed submissions.
+     *
+     * Valid values per RFC-WS-6 §Q3 v1.3 addition 2:
+     * - 'schema_config_error'
+     * - 'temporary_error'
+     * - 'data_integrity_error'
+     * - 'unknown_error'
+     *
+     * Note: writing this value via factory does NOT itself set apply_status
+     * to FAILED. Pair with an explicit apply_status attribute in tests
+     * that need a coherent failed-submission fixture.
+     */
+    public function withFailureResponseCode(string $code): static
+    {
+        return $this->state(fn () => ['failure_response_code' => $code]);
+    }
 }
diff --git a/api/database/migrations/2026_05_08_000001_add_failure_response_code_to_form_submissions.php b/api/database/migrations/2026_05_08_000001_add_failure_response_code_to_form_submissions.php
new file mode 100644
index 00000000..239f3480
--- /dev/null
+++ b/api/database/migrations/2026_05_08_000001_add_failure_response_code_to_form_submissions.php
@@ -0,0 +1,47 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+/**
+ * Per RFC-WS-6 §Q3 v1.3 addition 2 + ARCH-BINDINGS §7.1 v1.2.
+ *
+ * Mirrors the FormBindingApplicatorException subclass classification onto
+ * a denormalised string column for response-shape rendering. Values:
+ *   - schema_config_error  (HTTP 422)
+ *   - temporary_error      (HTTP 503)
+ *   - data_integrity_error (HTTP 422)
+ *   - unknown_error        (HTTP 500)
+ *
+ * Read by the response renderer when apply_status='failed' to surface
+ * contextual user-facing error copy. The exception subclass on the
+ * action-failures row is the canonical source; this column is the
+ * response-shape driver. Both reference the same submission.id ULID.
+ *
+ * NULL when apply_status is not 'failed' (per the lifecycle:
+ * pending/completed/partial → no failure to classify).
+ */
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('form_submissions', function (Blueprint $table): void {
+            $table->string('failure_response_code', 40)
+                ->nullable()
+                ->after('apply_status');
+
+            $table->index('failure_response_code', 'fs_failure_response_code_idx');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('form_submissions', function (Blueprint $table): void {
+            $table->dropIndex('fs_failure_response_code_idx');
+            $table->dropColumn('failure_response_code');
+        });
+    }
+};
diff --git a/api/database/schema/mysql-schema.sql b/api/database/schema/mysql-schema.sql
index 190c5959..8daf9e65 100644
--- a/api/database/schema/mysql-schema.sql
+++ b/api/database/schema/mysql-schema.sql
@@ -761,6 +761,7 @@ CREATE TABLE `form_submissions` (
   `anonymised_at` timestamp NULL DEFAULT NULL,
   `identity_match_status` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci DEFAULT NULL,
   `apply_status` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci DEFAULT NULL,
+  `failure_response_code` varchar(40) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
   `apply_completed_at` timestamp NULL DEFAULT NULL,
   `search_index` mediumtext CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci,
   `created_at` timestamp NULL DEFAULT NULL,
@@ -778,6 +779,7 @@ CREATE TABLE `form_submissions` (
   KEY `fs_event_status_idx` (`event_id`,`status`),
   KEY `fs_schema_apply_status_idx` (`form_schema_id`,`apply_status`),
   KEY `fs_org_apply_status_idx` (`organisation_id`,`apply_status`),
+  KEY `fs_failure_response_code_idx` (`failure_response_code`),
   FULLTEXT KEY `fs_search_index_fulltext` (`search_index`),
   CONSTRAINT `form_submissions_event_id_foreign` FOREIGN KEY (`event_id`) REFERENCES `events` (`id`) ON DELETE SET NULL,
   CONSTRAINT `form_submissions_form_schema_id_foreign` FOREIGN KEY (`form_schema_id`) REFERENCES `form_schemas` (`id`) ON DELETE CASCADE,
@@ -1773,3 +1775,4 @@ INSERT INTO `migrations` (`id`, `migration`, `batch`) VALUES (123,'2026_04_28_10
 INSERT INTO `migrations` (`id`, `migration`, `batch`) VALUES (124,'2026_04_28_140000_add_kvk_number_to_companies_table',2);
 INSERT INTO `migrations` (`id`, `migration`, `batch`) VALUES (125,'2026_04_28_180000_create_form_submission_action_failure_retry_attempts_table',2);
 INSERT INTO `migrations` (`id`, `migration`, `batch`) VALUES (126,'2026_04_28_181000_add_exception_trace_to_form_submission_action_failures',2);
+INSERT INTO `migrations` (`id`, `migration`, `batch`) VALUES (127,'2026_05_08_000001_add_failure_response_code_to_form_submissions',3);
diff --git a/api/phpstan-baseline.neon b/api/phpstan-baseline.neon
index f883d7ce..0478310e 100644
--- a/api/phpstan-baseline.neon
+++ b/api/phpstan-baseline.neon
@@ -7404,6 +7404,54 @@ parameters:
 			count: 1
 			path: tests/Unit/ExampleTest.php
 
+		-
+			message: '#^Call to function is_subclass_of\(\) with ''App\\\\Exceptions\\\\FormBuilder\\\\FormBindingApplicatorException'' and ''RuntimeException'' will always evaluate to true\.$#'
+			identifier: function.alreadyNarrowedType
+			count: 1
+			path: tests/Unit/Exceptions/FormBuilder/FormBindingApplicatorExceptionHierarchyTest.php
+
+		-
+			message: '#^Call to function is_subclass_of\(\) with ''App\\\\Exceptions\\\\FormBuilder\\\\FormBindingApplicatorTimeoutException'' and ''App\\\\Exceptions\\\\FormBuilder\\\\FormBindingInfraException'' will always evaluate to true\.$#'
+			identifier: function.alreadyNarrowedType
+			count: 1
+			path: tests/Unit/Exceptions/FormBuilder/FormBindingApplicatorExceptionHierarchyTest.php
+
+		-
+			message: '#^Call to function is_subclass_of\(\) with ''App\\\\Exceptions\\\\FormBuilder\\\\FormBindingApplicatorTimeoutException''\|''App\\\\Exceptions\\\\FormBuilder\\\\FormBindingDataIntegrityException''\|''App\\\\Exceptions\\\\FormBuilder\\\\FormBindingInfraException''\|''App\\\\Exceptions\\\\FormBuilder\\\\FormBindingSchemaConfigException'' and ''App\\\\Exceptions\\\\FormBuilder\\\\FormBindingApplicatorException'' will always evaluate to true\.$#'
+			identifier: function.alreadyNarrowedType
+			count: 1
+			path: tests/Unit/Exceptions/FormBuilder/FormBindingApplicatorExceptionHierarchyTest.php
+
+		-
+			message: '#^Call to function is_subclass_of\(\) with ''App\\\\Exceptions\\\\FormBuilder\\\\IdentityMatchInvariantViolation'' and ''App\\\\Exceptions\\\\FormBuilder\\\\FormBindingApplicatorException'' will always evaluate to false\.$#'
+			identifier: function.impossibleType
+			count: 1
+			path: tests/Unit/Exceptions/FormBuilder/FormBindingApplicatorExceptionHierarchyTest.php
+
+		-
+			message: '#^Call to function is_subclass_of\(\) with ''App\\\\Exceptions\\\\FormBuilder\\\\IdentityMatchInvariantViolation'' and ''DomainException'' will always evaluate to true\.$#'
+			identifier: function.alreadyNarrowedType
+			count: 1
+			path: tests/Unit/Exceptions/FormBuilder/FormBindingApplicatorExceptionHierarchyTest.php
+
+		-
+			message: '#^Call to method PHPUnit\\Framework\\Assert\:\:assertFalse\(\) with false will always evaluate to true\.$#'
+			identifier: method.alreadyNarrowedType
+			count: 1
+			path: tests/Unit/Exceptions/FormBuilder/FormBindingApplicatorExceptionHierarchyTest.php
+
+		-
+			message: '#^Call to method PHPUnit\\Framework\\Assert\:\:assertTrue\(\) with true and ''Class App…'' will always evaluate to true\.$#'
+			identifier: method.alreadyNarrowedType
+			count: 1
+			path: tests/Unit/Exceptions/FormBuilder/FormBindingApplicatorExceptionHierarchyTest.php
+
+		-
+			message: '#^Call to method PHPUnit\\Framework\\Assert\:\:assertTrue\(\) with true will always evaluate to true\.$#'
+			identifier: method.alreadyNarrowedType
+			count: 3
+			path: tests/Unit/Exceptions/FormBuilder/FormBindingApplicatorExceptionHierarchyTest.php
+
 		-
 			message: '#^Access to an undefined property Illuminate\\Database\\Eloquent\\Model\:\:\$id\.$#'
 			identifier: property.notFound
diff --git a/api/tests/Feature/FormBuilder/Bindings/FormBindingApplicatorIntegrationTest.php b/api/tests/Feature/FormBuilder/Bindings/FormBindingApplicatorIntegrationTest.php
index cb7aa846..3e70b83a 100644
--- a/api/tests/Feature/FormBuilder/Bindings/FormBindingApplicatorIntegrationTest.php
+++ b/api/tests/Feature/FormBuilder/Bindings/FormBindingApplicatorIntegrationTest.php
@@ -65,11 +65,15 @@ final class FormBindingApplicatorIntegrationTest extends TestCase
         // RefreshDatabase wraps every PHPUnit test in a transaction; the
         // guard is exercised via the listener path (ApplyBindingsOnFormSubmit)
         // which opens its own transaction explicitly. Verify the guard
-        // exists in the source.
+        // exists in the source by checking for the throw of
+        // FormBindingInfraException (per RFC-WS-6 §Q3 v1.3 addition 2 — the
+        // 'no_transaction' developer-error maps onto temporary_error so the
+        // GlitchTip alert + retry-after workflow fires).
         $reflection = new \ReflectionClass(FormBindingApplicator::class);
         $source = file_get_contents($reflection->getFileName());
-        $this->assertStringContainsString('no_transaction', $source);
+        $this->assertStringContainsString('FormBindingInfraException', $source);
         $this->assertStringContainsString('DB::transactionLevel()', $source);
+        $this->assertStringContainsString('must be invoked inside DB::transaction', $source);
     }
 
     private function applicator(): FormBindingApplicator
@@ -151,7 +155,7 @@ final class FormBindingApplicatorIntegrationTest extends TestCase
 
     private function writeValue(string $submissionId, string $fieldId, mixed $value): void
     {
-        $row = new FormValue();
+        $row = new FormValue;
         $row->form_submission_id = $submissionId;
         $row->form_field_id = $fieldId;
         $row->setAttribute('value', $value);
diff --git a/api/tests/Feature/FormBuilder/Bindings/FormFieldBindingMigrationTest.php b/api/tests/Feature/FormBuilder/Bindings/FormFieldBindingMigrationTest.php
index 97e06d67..f797018c 100644
--- a/api/tests/Feature/FormBuilder/Bindings/FormFieldBindingMigrationTest.php
+++ b/api/tests/Feature/FormBuilder/Bindings/FormFieldBindingMigrationTest.php
@@ -55,9 +55,10 @@ final class FormFieldBindingMigrationTest extends TestCase
         // create-conditional-logic-groups) + 5 WS-5b migrations
         // (drop-validation-cols, configs-backfill, create-configs,
         // validation-rules-backfill, create-validation-rules) +
-        // 2 WS-6 migrations (action-failures, apply-status) +
-        // 2 WS-5a migrations (drop-binding-cols, create-bindings) = 16.
-        $this->artisan('migrate:rollback', ['--step' => 21])->assertSuccessful();
+        // WS-6 migrations (action-failures, apply-status, retry-attempts,
+        // exception-trace, failure-response-code [v1.3-delta D1]) +
+        // 2 WS-5a migrations (drop-binding-cols, create-bindings).
+        $this->artisan('migrate:rollback', ['--step' => 22])->assertSuccessful();
         $this->assertFalse(Schema::hasTable('form_field_bindings'));
         $this->assertTrue(Schema::hasColumn('form_fields', 'binding'));
         $this->assertTrue(Schema::hasColumn('form_field_library', 'default_binding'));
@@ -118,8 +119,9 @@ final class FormFieldBindingMigrationTest extends TestCase
 
     public function test_rollback_reconstructs_json_and_drops_table(): void
     {
-        // Walk back the full WS-5d + WS-5c + WS-6 + WS-5b + WS-5a stack (16 migrations).
-        $this->artisan('migrate:rollback', ['--step' => 21])->assertSuccessful();
+        // Walk back the full WS-5d + WS-5c + WS-6 (incl. v1.3-delta D1
+        // failure_response_code) + WS-5b + WS-5a stack.
+        $this->artisan('migrate:rollback', ['--step' => 22])->assertSuccessful();
         [$fieldAId] = $this->seedFieldsWithBindingJson();
         [$libAId] = $this->seedLibraryWithBindingJson();
 
@@ -130,11 +132,12 @@ final class FormFieldBindingMigrationTest extends TestCase
         $this->assertSame(5, DB::table('form_field_bindings')->count());
 
         // Step back over WS-5d (3 migrations) + WS-5c (4 migrations) +
-        // WS-6 (2 migrations) + WS-5b (5 migrations) in one go → restores
-        // the pre-WS-5b state (conditional-logic, validation-rules, configs
-        // and options tables gone, validation_rules + options JSON columns
-        // reappear on source tables; binding contract intact).
-        $this->artisan('migrate:rollback', ['--step' => 19])->assertSuccessful();
+        // WS-6 (action-failures, apply-status, retry-attempts, exception-trace,
+        // failure-response-code [v1.3-delta D1]) + WS-5b (5 migrations) in one
+        // go → restores the pre-WS-5b state (conditional-logic, validation-rules,
+        // configs and options tables gone, validation_rules + options JSON
+        // columns reappear on source tables; binding contract intact).
+        $this->artisan('migrate:rollback', ['--step' => 20])->assertSuccessful();
         $this->assertFalse(Schema::hasTable('form_field_options'));
         $this->assertFalse(Schema::hasTable('form_field_conditional_logic_groups'));
         $this->assertFalse(Schema::hasTable('form_field_conditional_logic_conditions'));
diff --git a/api/tests/Feature/FormBuilder/ConditionalLogic/ConditionalLogicBackfillTest.php b/api/tests/Feature/FormBuilder/ConditionalLogic/ConditionalLogicBackfillTest.php
index 044d1655..d6db383a 100644
--- a/api/tests/Feature/FormBuilder/ConditionalLogic/ConditionalLogicBackfillTest.php
+++ b/api/tests/Feature/FormBuilder/ConditionalLogic/ConditionalLogicBackfillTest.php
@@ -49,7 +49,7 @@ final class ConditionalLogicBackfillTest extends TestCase
         // create-options + WS-5c drop-cl-col + WS-5c backfill-cl
         // migrations to land in the conditional-logic JSON-era state with
         // no relational form_field_options table yet.
-        $this->artisan('migrate:rollback', ['--step' => 10])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 11])->assertSuccessful();
         $this->assertTrue(Schema::hasColumn('form_fields', 'conditional_logic'));
 
         $fieldId = $this->seedFieldWithJson([
@@ -170,7 +170,7 @@ final class ConditionalLogicBackfillTest extends TestCase
         ]);
 
         // Roll back only the backfill migration — writes the JSON back.
-        $this->artisan('migrate:rollback', ['--step' => 10])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 11])->assertSuccessful();
 
         $reconstructed = DB::table('form_fields')
             ->where('id', $fieldId)
@@ -203,7 +203,7 @@ final class ConditionalLogicBackfillTest extends TestCase
 
     public function test_unknown_top_level_key_fails_migration(): void
     {
-        $this->artisan('migrate:rollback', ['--step' => 10])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 11])->assertSuccessful();
 
         $this->seedFieldWithJson([
             'hide_when' => ['all' => [['field_slug' => 'x', 'operator' => 'equals', 'value' => 1]]],
@@ -216,7 +216,7 @@ final class ConditionalLogicBackfillTest extends TestCase
 
     public function test_unknown_comparison_operator_fails_migration(): void
     {
-        $this->artisan('migrate:rollback', ['--step' => 10])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 11])->assertSuccessful();
 
         $this->seedFieldWithJson([
             'show_when' => ['all' => [['field_slug' => 'x', 'operator' => 'matches_regex', 'value' => 'y']]],
diff --git a/api/tests/Feature/FormBuilder/Configs/FormFieldConfigBackfillAndDropTest.php b/api/tests/Feature/FormBuilder/Configs/FormFieldConfigBackfillAndDropTest.php
index a8b35ede..0490105a 100644
--- a/api/tests/Feature/FormBuilder/Configs/FormFieldConfigBackfillAndDropTest.php
+++ b/api/tests/Feature/FormBuilder/Configs/FormFieldConfigBackfillAndDropTest.php
@@ -30,7 +30,7 @@ final class FormFieldConfigBackfillAndDropTest extends TestCase
         // Roll back 4 WS-5c migrations + 2 WS-6 migrations + 5 WS-5b
         // migrations = 11, to get the pre-WS-5b state where the JSON column
         // still exists on form_fields / form_field_library.
-        $this->artisan('migrate:rollback', ['--step' => 16])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 17])->assertSuccessful();
         $this->assertTrue(Schema::hasColumn('form_fields', 'validation_rules'));
 
         $fieldId = $this->seedField([
diff --git a/api/tests/Feature/FormBuilder/FormSubmissionFailureResponseCodeTest.php b/api/tests/Feature/FormBuilder/FormSubmissionFailureResponseCodeTest.php
new file mode 100644
index 00000000..9f4f6492
--- /dev/null
+++ b/api/tests/Feature/FormBuilder/FormSubmissionFailureResponseCodeTest.php
@@ -0,0 +1,61 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Feature\FormBuilder;
+
+use App\Enums\FormBuilder\ApplyStatus;
+use App\Models\FormBuilder\FormSubmission;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Tests\TestCase;
+
+/**
+ * Per RFC-WS-6 §Q3 v1.3 addition 2.
+ *
+ * Verifies the cast (plain string), default-NULL behaviour, and that
+ * the factory state method composes with apply_status attribute writes
+ * for coherent failed-submission fixtures.
+ */
+final class FormSubmissionFailureResponseCodeTest extends TestCase
+{
+    use RefreshDatabase;
+
+    public function test_failure_response_code_persists_as_string(): void
+    {
+        $submission = FormSubmission::factory()
+            ->withFailureResponseCode('schema_config_error')
+            ->create();
+
+        $reloaded = $submission->fresh();
+        $this->assertSame('schema_config_error', $reloaded->failure_response_code);
+        $this->assertIsString($reloaded->failure_response_code);
+    }
+
+    public function test_failure_response_code_null_by_default(): void
+    {
+        $submission = FormSubmission::factory()->create();
+        $this->assertNull($submission->fresh()->failure_response_code);
+    }
+
+    public function test_factory_state_composes_with_apply_status(): void
+    {
+        $submission = FormSubmission::factory()
+            ->withFailureResponseCode('temporary_error')
+            ->create(['apply_status' => ApplyStatus::FAILED]);
+
+        $reloaded = $submission->fresh();
+        $this->assertSame(ApplyStatus::FAILED, $reloaded->apply_status);
+        $this->assertSame('temporary_error', $reloaded->failure_response_code);
+    }
+
+    public function test_failure_response_code_round_trips_for_each_canonical_value(): void
+    {
+        foreach (['schema_config_error', 'temporary_error', 'data_integrity_error', 'unknown_error'] as $code) {
+            $submission = FormSubmission::factory()
+                ->withFailureResponseCode($code)
+                ->create();
+
+            $this->assertSame($code, $submission->fresh()->failure_response_code);
+        }
+    }
+}
diff --git a/api/tests/Feature/FormBuilder/Options/FormFieldOptionsBackfillTest.php b/api/tests/Feature/FormBuilder/Options/FormFieldOptionsBackfillTest.php
index 4fc7b045..173c685c 100644
--- a/api/tests/Feature/FormBuilder/Options/FormFieldOptionsBackfillTest.php
+++ b/api/tests/Feature/FormBuilder/Options/FormFieldOptionsBackfillTest.php
@@ -47,7 +47,7 @@ final class FormFieldOptionsBackfillTest extends TestCase
         // Roll back only the backfill migration (latest WS-5d step).
         // Leaves the form_field_options table in place, JSON columns
         // present on the source tables, and snapshots in pre-WS-5d shape.
-        $this->artisan('migrate:rollback', ['--step' => 6])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 7])->assertSuccessful();
         $this->assertTrue(Schema::hasTable('form_field_options'));
         $this->assertTrue(Schema::hasColumn('form_fields', 'options'));
 
@@ -136,7 +136,7 @@ final class FormFieldOptionsBackfillTest extends TestCase
 
     public function test_rollback_reconstructs_json_columns_and_snapshots(): void
     {
-        $this->artisan('migrate:rollback', ['--step' => 6])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 7])->assertSuccessful();
         [$selectId, $multiId, $libraryId] = $this->seedFieldsAndLibraryWithJson();
         $submissionId = $this->seedSubmissionWithSnapshot($selectId);
 
@@ -149,7 +149,7 @@ final class FormFieldOptionsBackfillTest extends TestCase
 
         // Step back over only the backfill migration → JSON columns repopulate
         // and snapshots revert to flat-string-array shape.
-        $this->artisan('migrate:rollback', ['--step' => 6])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 7])->assertSuccessful();
         $this->assertSame(0, DB::table('form_field_options')->count());
 
         $select = DB::table('form_fields')->where('id', $selectId)->first();
@@ -168,7 +168,7 @@ final class FormFieldOptionsBackfillTest extends TestCase
 
     public function test_fails_when_options_present_on_non_option_field_type(): void
     {
-        $this->artisan('migrate:rollback', ['--step' => 6])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 7])->assertSuccessful();
         $this->seedFieldWithOptions('TAG_PICKER', ['Veiligheid', 'Horeca']);
 
         $this->expectException(\RuntimeException::class);
@@ -178,7 +178,7 @@ final class FormFieldOptionsBackfillTest extends TestCase
 
     public function test_fails_when_options_contains_non_string_entry(): void
     {
-        $this->artisan('migrate:rollback', ['--step' => 6])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 7])->assertSuccessful();
 
         $this->seedFieldWithOptionsRaw('SELECT', json_encode([
             ['label' => 'A'],
@@ -192,7 +192,7 @@ final class FormFieldOptionsBackfillTest extends TestCase
 
     public function test_fails_when_options_is_object_shape(): void
     {
-        $this->artisan('migrate:rollback', ['--step' => 6])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 7])->assertSuccessful();
 
         $this->seedFieldWithOptionsRaw('SELECT', json_encode([
             'XS' => 'Extra small',
@@ -206,7 +206,7 @@ final class FormFieldOptionsBackfillTest extends TestCase
 
     public function test_fails_on_translations_length_mismatch(): void
     {
-        $this->artisan('migrate:rollback', ['--step' => 6])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 7])->assertSuccessful();
         $this->seedFieldWithOptionsRaw('SELECT', json_encode(['XS', 'S', 'M']), json_encode([
             'de' => ['options' => ['Klein', 'Mittel']], // 2 vs 3
         ]));
@@ -218,7 +218,7 @@ final class FormFieldOptionsBackfillTest extends TestCase
 
     public function test_fails_on_non_string_translation(): void
     {
-        $this->artisan('migrate:rollback', ['--step' => 6])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 7])->assertSuccessful();
         $this->seedFieldWithOptionsRaw('SELECT', json_encode(['XS', 'S']), json_encode([
             'de' => ['options' => ['Klein', 42]],
         ]));
@@ -230,7 +230,7 @@ final class FormFieldOptionsBackfillTest extends TestCase
 
     public function test_fails_on_oversized_translation(): void
     {
-        $this->artisan('migrate:rollback', ['--step' => 6])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 7])->assertSuccessful();
         $this->seedFieldWithOptionsRaw('SELECT', json_encode(['XS']), json_encode([
             'de' => ['options' => [str_repeat('x', 256)]],
         ]));
@@ -242,7 +242,7 @@ final class FormFieldOptionsBackfillTest extends TestCase
 
     public function test_fails_when_snapshot_options_present_on_non_option_field_type(): void
     {
-        $this->artisan('migrate:rollback', ['--step' => 6])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 7])->assertSuccessful();
         $this->seedTemplateWithSnapshotRaw([
             'fields' => [[
                 'id' => (string) Str::ulid(),
diff --git a/api/tests/Feature/FormBuilder/Schema/Ws6V13DeltaD1MigrationTest.php b/api/tests/Feature/FormBuilder/Schema/Ws6V13DeltaD1MigrationTest.php
new file mode 100644
index 00000000..0c3c18ec
--- /dev/null
+++ b/api/tests/Feature/FormBuilder/Schema/Ws6V13DeltaD1MigrationTest.php
@@ -0,0 +1,59 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Feature\FormBuilder\Schema;
+
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Support\Facades\Schema;
+use Tests\TestCase;
+
+/**
+ * Migration rehearsal for WS-6 v1.3-delta D1:
+ *   - 2026_05_08_000001_add_failure_response_code_to_form_submissions
+ *
+ * Verifies the column + index land per RFC-WS-6 §Q3 v1.3 addition 2 and
+ * ARCH-BINDINGS §7.1 v1.2.
+ */
+final class Ws6V13DeltaD1MigrationTest extends TestCase
+{
+    use RefreshDatabase;
+
+    public function test_form_submissions_has_failure_response_code_column(): void
+    {
+        $this->assertTrue(Schema::hasColumn('form_submissions', 'failure_response_code'));
+    }
+
+    public function test_failure_response_code_is_nullable_string_40(): void
+    {
+        $row = DB::selectOne(
+            'SELECT DATA_TYPE, CHARACTER_MAXIMUM_LENGTH, IS_NULLABLE, COLUMN_DEFAULT
+             FROM information_schema.COLUMNS
+             WHERE TABLE_SCHEMA = DATABASE()
+               AND TABLE_NAME = ?
+               AND COLUMN_NAME = ?',
+            ['form_submissions', 'failure_response_code'],
+        );
+
+        $this->assertNotNull($row, 'failure_response_code column missing');
+        $this->assertSame('varchar', strtolower((string) $row->DATA_TYPE));
+        $this->assertSame(40, (int) $row->CHARACTER_MAXIMUM_LENGTH);
+        $this->assertSame('YES', $row->IS_NULLABLE);
+        $this->assertNull($row->COLUMN_DEFAULT);
+    }
+
+    public function test_failure_response_code_index_present(): void
+    {
+        $row = DB::selectOne(
+            'SELECT INDEX_NAME
+             FROM information_schema.STATISTICS
+             WHERE TABLE_SCHEMA = DATABASE()
+               AND TABLE_NAME = ?
+               AND INDEX_NAME = ?',
+            ['form_submissions', 'fs_failure_response_code_idx'],
+        );
+
+        $this->assertNotNull($row, 'fs_failure_response_code_idx missing');
+    }
+}
diff --git a/api/tests/Feature/FormBuilder/ValidationRules/FormFieldValidationRuleBackfillTest.php b/api/tests/Feature/FormBuilder/ValidationRules/FormFieldValidationRuleBackfillTest.php
index 295c5455..29b86f6a 100644
--- a/api/tests/Feature/FormBuilder/ValidationRules/FormFieldValidationRuleBackfillTest.php
+++ b/api/tests/Feature/FormBuilder/ValidationRules/FormFieldValidationRuleBackfillTest.php
@@ -53,7 +53,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
         // validation-rules-backfill + create-validation-rules) = 14.
         // Brings us to the pre-WS-5b state: validation_rules JSON column
         // present, no relational tables for WS-5b/c/d.
-        $this->artisan('migrate:rollback', ['--step' => 19])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 20])->assertSuccessful();
         $this->assertFalse(Schema::hasTable('form_field_validation_rules'));
         $this->assertTrue(Schema::hasColumn('form_fields', 'validation_rules'));
 
@@ -114,7 +114,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
         // (validation_rules JSON column present; no relational tables for
         // WS-5b). Step count: drop-cols + configs-backfill + create-configs
         // + validation-rules-backfill + create-validation-rules = 5.
-        $this->artisan('migrate:rollback', ['--step' => 19])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 20])->assertSuccessful();
 
         $fieldId = $this->seedFieldWithJson([
             'field_type' => 'TAG_PICKER',
@@ -138,7 +138,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
         // (validation_rules JSON column present; no relational tables for
         // WS-5b). Step count: drop-cols + configs-backfill + create-configs
         // + validation-rules-backfill + create-validation-rules = 5.
-        $this->artisan('migrate:rollback', ['--step' => 19])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 20])->assertSuccessful();
 
         $fieldId = $this->seedFieldWithJson([
             'field_type' => 'TEXT',
@@ -165,7 +165,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
         // (validation_rules JSON column present; no relational tables for
         // WS-5b). Step count: drop-cols + configs-backfill + create-configs
         // + validation-rules-backfill + create-validation-rules = 5.
-        $this->artisan('migrate:rollback', ['--step' => 19])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 20])->assertSuccessful();
 
         $this->seedFieldWithJson([
             'field_type' => 'TEXT',
@@ -182,7 +182,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
         // (validation_rules JSON column present; no relational tables for
         // WS-5b). Step count: drop-cols + configs-backfill + create-configs
         // + validation-rules-backfill + create-validation-rules = 5.
-        $this->artisan('migrate:rollback', ['--step' => 19])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 20])->assertSuccessful();
 
         $this->seedFieldWithJson([
             'field_type' => 'BOOLEAN',
@@ -201,7 +201,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
         // full-back-then-full-forward cycle — rolling back all WS-5b
         // migrations restores the pre-WS-5b state (columns present on
         // source tables; validation rules relational table gone).
-        $this->artisan('migrate:rollback', ['--step' => 19])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 20])->assertSuccessful();
         [$numberId] = $this->seedFields();
 
         $this->artisan('migrate')->assertSuccessful();
@@ -216,7 +216,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
 
         // Roll back WS-5b fully → column reappears and carries canonical JSON
         // reconstructed from the relational rows.
-        $this->artisan('migrate:rollback', ['--step' => 19])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 20])->assertSuccessful();
         $this->assertTrue(Schema::hasColumn('form_fields', 'validation_rules'));
 
         $field = DB::table('form_fields')->where('id', $numberId)->first();
diff --git a/api/tests/Unit/Enums/FormBuilder/FormFieldBindingMergeStrategyValidForTargetTypeTest.php b/api/tests/Unit/Enums/FormBuilder/FormFieldBindingMergeStrategyValidForTargetTypeTest.php
new file mode 100644
index 00000000..7ee41e3c
--- /dev/null
+++ b/api/tests/Unit/Enums/FormBuilder/FormFieldBindingMergeStrategyValidForTargetTypeTest.php
@@ -0,0 +1,47 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Unit\Enums\FormBuilder;
+
+use App\Enums\FormBuilder\BindingTargetType;
+use App\Enums\FormBuilder\FormFieldBindingMergeStrategy;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Per RFC-WS-6 §V1 + ARCH-BINDINGS §4.2.
+ *
+ * Locks the strategy x target-type validity matrix. Append is the only
+ * non-trivial case (collection-only). Future contributors who alter the
+ * matrix without RFC follow-through will see a structural test failure.
+ */
+final class FormFieldBindingMergeStrategyValidForTargetTypeTest extends TestCase
+{
+    public function test_overwrite_valid_for_all_target_types(): void
+    {
+        $this->assertTrue(FormFieldBindingMergeStrategy::Overwrite->validForTargetType(BindingTargetType::SCALAR));
+        $this->assertTrue(FormFieldBindingMergeStrategy::Overwrite->validForTargetType(BindingTargetType::COLLECTION));
+        $this->assertTrue(FormFieldBindingMergeStrategy::Overwrite->validForTargetType(BindingTargetType::RELATION));
+    }
+
+    public function test_replace_valid_for_all_target_types(): void
+    {
+        $this->assertTrue(FormFieldBindingMergeStrategy::Replace->validForTargetType(BindingTargetType::SCALAR));
+        $this->assertTrue(FormFieldBindingMergeStrategy::Replace->validForTargetType(BindingTargetType::COLLECTION));
+        $this->assertTrue(FormFieldBindingMergeStrategy::Replace->validForTargetType(BindingTargetType::RELATION));
+    }
+
+    public function test_first_write_wins_valid_for_all_target_types(): void
+    {
+        $this->assertTrue(FormFieldBindingMergeStrategy::FirstWriteWins->validForTargetType(BindingTargetType::SCALAR));
+        $this->assertTrue(FormFieldBindingMergeStrategy::FirstWriteWins->validForTargetType(BindingTargetType::COLLECTION));
+        $this->assertTrue(FormFieldBindingMergeStrategy::FirstWriteWins->validForTargetType(BindingTargetType::RELATION));
+    }
+
+    public function test_append_valid_only_for_collection(): void
+    {
+        $this->assertFalse(FormFieldBindingMergeStrategy::Append->validForTargetType(BindingTargetType::SCALAR));
+        $this->assertTrue(FormFieldBindingMergeStrategy::Append->validForTargetType(BindingTargetType::COLLECTION));
+        $this->assertFalse(FormFieldBindingMergeStrategy::Append->validForTargetType(BindingTargetType::RELATION));
+    }
+}
diff --git a/api/tests/Unit/Events/FormBuilder/FormSubmissionIdentityMatchResolvedTest.php b/api/tests/Unit/Events/FormBuilder/FormSubmissionIdentityMatchResolvedTest.php
new file mode 100644
index 00000000..8ca38e62
--- /dev/null
+++ b/api/tests/Unit/Events/FormBuilder/FormSubmissionIdentityMatchResolvedTest.php
@@ -0,0 +1,54 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Unit\Events\FormBuilder;
+
+use App\Events\FormBuilder\FormSubmissionIdentityMatchResolved;
+use Illuminate\Broadcasting\PrivateChannel;
+use Illuminate\Contracts\Broadcasting\ShouldBroadcast;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Per RFC-WS-6 §Q1 v1.3 addition 2.
+ *
+ * Class-only tests — D2 wires the dispatch from
+ * TriggerPersonIdentityMatchOnFormSubmit::handle, and tests covering
+ * that wiring will live alongside the listener tests.
+ */
+final class FormSubmissionIdentityMatchResolvedTest extends TestCase
+{
+    public function test_implements_should_broadcast(): void
+    {
+        $event = new FormSubmissionIdentityMatchResolved('01HX', 'matched', 1);
+        $this->assertInstanceOf(ShouldBroadcast::class, $event);
+    }
+
+    public function test_broadcasts_on_private_submission_channel(): void
+    {
+        $event = new FormSubmissionIdentityMatchResolved('01HX1234567890', 'matched', 2);
+        $channels = $event->broadcastOn();
+
+        $this->assertCount(1, $channels);
+        $this->assertInstanceOf(PrivateChannel::class, $channels[0]);
+        // PrivateChannel prepends 'private-' to the name passed to its
+        // constructor; that's the wire-format the frontend Echo client
+        // subscribes to.
+        $this->assertSame('private-submission.01HX1234567890', $channels[0]->name);
+    }
+
+    public function test_broadcast_as(): void
+    {
+        $event = new FormSubmissionIdentityMatchResolved('01HX', 'matched', 1);
+        $this->assertSame('identity-match.resolved', $event->broadcastAs());
+    }
+
+    public function test_constructor_assigns_payload_readonly(): void
+    {
+        $event = new FormSubmissionIdentityMatchResolved('01HX', 'no_match', 0);
+
+        $this->assertSame('01HX', $event->submissionId);
+        $this->assertSame('no_match', $event->status);
+        $this->assertSame(0, $event->matchCount);
+    }
+}
diff --git a/api/tests/Unit/Exceptions/FormBuilder/FormBindingApplicatorExceptionHierarchyTest.php b/api/tests/Unit/Exceptions/FormBuilder/FormBindingApplicatorExceptionHierarchyTest.php
new file mode 100644
index 00000000..fa77afc3
--- /dev/null
+++ b/api/tests/Unit/Exceptions/FormBuilder/FormBindingApplicatorExceptionHierarchyTest.php
@@ -0,0 +1,139 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Unit\Exceptions\FormBuilder;
+
+use App\Exceptions\FormBuilder\FormBindingApplicatorException;
+use App\Exceptions\FormBuilder\FormBindingApplicatorTimeoutException;
+use App\Exceptions\FormBuilder\FormBindingDataIntegrityException;
+use App\Exceptions\FormBuilder\FormBindingInfraException;
+use App\Exceptions\FormBuilder\FormBindingSchemaConfigException;
+use App\Exceptions\FormBuilder\IdentityMatchInvariantViolation;
+use DomainException;
+use PHPUnit\Framework\TestCase;
+use ReflectionClass;
+use RuntimeException;
+
+/**
+ * Per RFC-WS-6 §Q3 v1.3 addition 2.
+ *
+ * Locks the exception hierarchy shape so future contributors cannot
+ * accidentally instantiate the abstract base, drop the reasonCode
+ * contract, or reparent a subclass.
+ */
+final class FormBindingApplicatorExceptionHierarchyTest extends TestCase
+{
+    public function test_base_class_is_abstract(): void
+    {
+        $reflection = new ReflectionClass(FormBindingApplicatorException::class);
+        $this->assertTrue($reflection->isAbstract());
+    }
+
+    public function test_base_extends_runtime_exception(): void
+    {
+        $this->assertTrue(is_subclass_of(FormBindingApplicatorException::class, RuntimeException::class));
+    }
+
+    public function test_schema_config_exception_constructor_and_reason_code(): void
+    {
+        $e = new FormBindingSchemaConfigException(
+            submissionId: '01HX1234567890ABCDEFGHJKMN',
+            message: 'schema null',
+        );
+
+        $this->assertSame('01HX1234567890ABCDEFGHJKMN', $e->submissionId);
+        $this->assertSame('schema null', $e->getMessage());
+        $this->assertSame('schema_config_error', $e->reasonCode());
+    }
+
+    public function test_infra_exception_constructor_and_reason_code(): void
+    {
+        $e = new FormBindingInfraException(
+            submissionId: '01HX1234567890ABCDEFGHJKMN',
+            message: 'no transaction',
+        );
+
+        $this->assertSame('01HX1234567890ABCDEFGHJKMN', $e->submissionId);
+        $this->assertSame('no transaction', $e->getMessage());
+        $this->assertSame('temporary_error', $e->reasonCode());
+    }
+
+    public function test_data_integrity_exception_constructor_and_reason_code(): void
+    {
+        $e = new FormBindingDataIntegrityException(
+            submissionId: '01HX1234567890ABCDEFGHJKMN',
+            message: 'fk violation',
+        );
+
+        $this->assertSame('01HX1234567890ABCDEFGHJKMN', $e->submissionId);
+        $this->assertSame('fk violation', $e->getMessage());
+        $this->assertSame('data_integrity_error', $e->reasonCode());
+    }
+
+    public function test_timeout_exception_constructor_and_inherited_reason_code(): void
+    {
+        $e = new FormBindingApplicatorTimeoutException(
+            submissionId: '01HX1234567890ABCDEFGHJKMN',
+            message: 'deadline exceeded after 5s',
+        );
+
+        $this->assertSame('01HX1234567890ABCDEFGHJKMN', $e->submissionId);
+        $this->assertSame('deadline exceeded after 5s', $e->getMessage());
+        // Inherited from FormBindingInfraException — no override.
+        $this->assertSame('temporary_error', $e->reasonCode());
+    }
+
+    public function test_timeout_extends_infra(): void
+    {
+        $this->assertTrue(is_subclass_of(
+            FormBindingApplicatorTimeoutException::class,
+            FormBindingInfraException::class,
+        ));
+    }
+
+    public function test_all_concrete_subclasses_extend_base(): void
+    {
+        $concreteSubclasses = [
+            FormBindingSchemaConfigException::class,
+            FormBindingInfraException::class,
+            FormBindingDataIntegrityException::class,
+            FormBindingApplicatorTimeoutException::class,
+        ];
+
+        foreach ($concreteSubclasses as $class) {
+            $this->assertTrue(
+                is_subclass_of($class, FormBindingApplicatorException::class),
+                "Class {$class} must extend FormBindingApplicatorException",
+            );
+        }
+    }
+
+    public function test_constructor_accepts_previous_throwable(): void
+    {
+        $cause = new RuntimeException('original');
+        $e = new FormBindingInfraException(
+            submissionId: '01HX',
+            message: 'wrapper',
+            previous: $cause,
+        );
+
+        $this->assertSame($cause, $e->getPrevious());
+    }
+
+    public function test_identity_match_invariant_violation_is_not_in_hierarchy(): void
+    {
+        $this->assertFalse(is_subclass_of(
+            IdentityMatchInvariantViolation::class,
+            FormBindingApplicatorException::class,
+        ));
+    }
+
+    public function test_identity_match_invariant_violation_is_domain_exception(): void
+    {
+        $this->assertTrue(is_subclass_of(
+            IdentityMatchInvariantViolation::class,
+            DomainException::class,
+        ));
+    }
+}
diff --git a/api/tests/Unit/FormBuilder/Bindings/FormBindingExceptionClassifierTest.php b/api/tests/Unit/FormBuilder/Bindings/FormBindingExceptionClassifierTest.php
new file mode 100644
index 00000000..d4b98a5c
--- /dev/null
+++ b/api/tests/Unit/FormBuilder/Bindings/FormBindingExceptionClassifierTest.php
@@ -0,0 +1,76 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Unit\FormBuilder\Bindings;
+
+use App\Exceptions\FormBuilder\FormBindingApplicatorTimeoutException;
+use App\Exceptions\FormBuilder\FormBindingDataIntegrityException;
+use App\Exceptions\FormBuilder\FormBindingInfraException;
+use App\Exceptions\FormBuilder\FormBindingSchemaConfigException;
+use App\Exceptions\FormBuilder\IdentityMatchInvariantViolation;
+use App\FormBuilder\Bindings\FormBindingExceptionClassifier;
+use PHPUnit\Framework\TestCase;
+use RuntimeException;
+
+/**
+ * Per RFC-WS-6 §Q3 v1.3 addition 2.
+ *
+ * The classifier is a single-responsibility helper: Throwable in,
+ * failure_response_code string out. Centralised so the listener catch
+ * block (D2) and the retry-service recordFailure (D2) produce identical
+ * classifications.
+ */
+final class FormBindingExceptionClassifierTest extends TestCase
+{
+    public function test_classifies_schema_config_exception(): void
+    {
+        $code = FormBindingExceptionClassifier::classify(
+            new FormBindingSchemaConfigException(submissionId: '01HX', message: 'x'),
+        );
+        $this->assertSame('schema_config_error', $code);
+    }
+
+    public function test_classifies_infra_exception(): void
+    {
+        $code = FormBindingExceptionClassifier::classify(
+            new FormBindingInfraException(submissionId: '01HX', message: 'x'),
+        );
+        $this->assertSame('temporary_error', $code);
+    }
+
+    public function test_classifies_timeout_as_temporary_error(): void
+    {
+        // Subclass dispatch — Timeout extends Infra, inherits reasonCode.
+        $code = FormBindingExceptionClassifier::classify(
+            new FormBindingApplicatorTimeoutException(submissionId: '01HX', message: 'deadline'),
+        );
+        $this->assertSame('temporary_error', $code);
+    }
+
+    public function test_classifies_data_integrity_exception(): void
+    {
+        $code = FormBindingExceptionClassifier::classify(
+            new FormBindingDataIntegrityException(submissionId: '01HX', message: 'fk'),
+        );
+        $this->assertSame('data_integrity_error', $code);
+    }
+
+    public function test_classifies_arbitrary_runtime_exception_as_unknown(): void
+    {
+        $code = FormBindingExceptionClassifier::classify(new RuntimeException('boom'));
+        $this->assertSame('unknown_error', $code);
+    }
+
+    public function test_classifies_identity_match_invariant_violation_as_unknown(): void
+    {
+        // IdentityMatchInvariantViolation is intentionally NOT in the
+        // FormBindingApplicatorException hierarchy (per RFC-WS-6 §Q2 — the
+        // listener that throws it runs outside the binding-applicator
+        // pipeline). It falls through to 'unknown_error' here, which is
+        // the right response-shape because users cannot meaningfully act
+        // on it; admins triage via GlitchTip.
+        $code = FormBindingExceptionClassifier::classify(new IdentityMatchInvariantViolation('x'));
+        $this->assertSame('unknown_error', $code);
+    }
+}