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/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/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/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);
+    }
+}