diff --git a/api/app/Exceptions/FormBuilder/FieldValidationException.php b/api/app/Exceptions/FormBuilder/FieldValidationException.php
new file mode 100644
index 00000000..a0125506
--- /dev/null
+++ b/api/app/Exceptions/FormBuilder/FieldValidationException.php
@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+/**
+ * 422 for per-field validation failures raised by FormValueService when
+ * enforcing form_fields.validation_rules. FormRequests still throw
+ * Laravel's ValidationException directly; this class is for failures
+ * detected below the request layer.
+ */
+final class FieldValidationException extends PublicFormApiException
+{
+    /**
+     * @param  array<string, array<int, string>>  $fieldErrors
+     */
+    public function __construct(array $fieldErrors, string $message = 'The submitted values failed validation.')
+    {
+        parent::__construct('VALIDATION_FAILED', 422, $message);
+        $this->fieldErrors = $fieldErrors;
+    }
+}
diff --git a/api/app/Exceptions/FormBuilder/PublicFormApiException.php b/api/app/Exceptions/FormBuilder/PublicFormApiException.php
new file mode 100644
index 00000000..45e59605
--- /dev/null
+++ b/api/app/Exceptions/FormBuilder/PublicFormApiException.php
@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+use RuntimeException;
+
+/**
+ * Base for every public form-builder API error. Renders as the S2c D6
+ * standardised envelope:
+ *
+ *   { "message": "...", "code": "...", "errors"?: {...} }
+ *
+ * Concrete subclasses are tiny — they only set the machine code, HTTP
+ * status, and the user-facing message.
+ */
+abstract class PublicFormApiException extends RuntimeException
+{
+    /** @var array<string, array<int, string>>|null */
+    public ?array $fieldErrors = null;
+
+    /** @var array<string, string> */
+    public array $headers = [];
+
+    public function __construct(
+        public readonly string $publicCode,
+        public readonly int $status,
+        string $message,
+    ) {
+        parent::__construct($message);
+    }
+}
diff --git a/api/app/Exceptions/FormBuilder/RateLimitedException.php b/api/app/Exceptions/FormBuilder/RateLimitedException.php
new file mode 100644
index 00000000..9d0bf025
--- /dev/null
+++ b/api/app/Exceptions/FormBuilder/RateLimitedException.php
@@ -0,0 +1,14 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+final class RateLimitedException extends PublicFormApiException
+{
+    public function __construct(int $retryAfterSeconds)
+    {
+        parent::__construct('RATE_LIMITED', 429, 'Too many submissions. Try again later.');
+        $this->headers['Retry-After'] = (string) max($retryAfterSeconds, 0);
+    }
+}
diff --git a/api/app/Exceptions/FormBuilder/SchemaNotFoundException.php b/api/app/Exceptions/FormBuilder/SchemaNotFoundException.php
new file mode 100644
index 00000000..31e669be
--- /dev/null
+++ b/api/app/Exceptions/FormBuilder/SchemaNotFoundException.php
@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+final class SchemaNotFoundException extends PublicFormApiException
+{
+    public function __construct()
+    {
+        parent::__construct('SCHEMA_NOT_FOUND', 404, 'Form not found.');
+    }
+}
diff --git a/api/app/Exceptions/FormBuilder/SchemaUnpublishedException.php b/api/app/Exceptions/FormBuilder/SchemaUnpublishedException.php
new file mode 100644
index 00000000..f21a71ad
--- /dev/null
+++ b/api/app/Exceptions/FormBuilder/SchemaUnpublishedException.php
@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+final class SchemaUnpublishedException extends PublicFormApiException
+{
+    public function __construct()
+    {
+        parent::__construct('SCHEMA_UNPUBLISHED', 410, 'Form is not currently accepting submissions.');
+    }
+}
diff --git a/api/app/Exceptions/FormBuilder/SubmissionAlreadySubmittedException.php b/api/app/Exceptions/FormBuilder/SubmissionAlreadySubmittedException.php
new file mode 100644
index 00000000..4a83af32
--- /dev/null
+++ b/api/app/Exceptions/FormBuilder/SubmissionAlreadySubmittedException.php
@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+final class SubmissionAlreadySubmittedException extends PublicFormApiException
+{
+    public function __construct()
+    {
+        parent::__construct(
+            'SUBMISSION_ALREADY_SUBMITTED',
+            409,
+            'This submission has already been submitted and can no longer be edited.',
+        );
+    }
+}
diff --git a/api/app/Exceptions/FormBuilder/TokenExpiredException.php b/api/app/Exceptions/FormBuilder/TokenExpiredException.php
new file mode 100644
index 00000000..7082f126
--- /dev/null
+++ b/api/app/Exceptions/FormBuilder/TokenExpiredException.php
@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+final class TokenExpiredException extends PublicFormApiException
+{
+    public function __construct()
+    {
+        parent::__construct('TOKEN_EXPIRED', 410, 'This form link has expired.');
+    }
+}
diff --git a/api/app/Exceptions/FormBuilder/TokenRevokedException.php b/api/app/Exceptions/FormBuilder/TokenRevokedException.php
new file mode 100644
index 00000000..e3efe805
--- /dev/null
+++ b/api/app/Exceptions/FormBuilder/TokenRevokedException.php
@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+final class TokenRevokedException extends PublicFormApiException
+{
+    public function __construct()
+    {
+        parent::__construct('TOKEN_REVOKED', 410, 'This form link has been revoked by the organiser.');
+    }
+}
diff --git a/api/bootstrap/app.php b/api/bootstrap/app.php
index 74648dda..b1e3ceb4 100644
--- a/api/bootstrap/app.php
+++ b/api/bootstrap/app.php
@@ -39,6 +39,19 @@ return Application::configure(basePath: dirname(__DIR__))
         ]);
     })
     ->withExceptions(function (Exceptions $exceptions): void {
+        // Public Form Builder standardised error envelope (S2c D6).
+        $exceptions->render(function (\App\Exceptions\FormBuilder\PublicFormApiException $e, Request $request) {
+            $body = [
+                'message' => $e->getMessage(),
+                'code' => $e->publicCode,
+            ];
+            if ($e->fieldErrors !== null) {
+                $body['errors'] = $e->fieldErrors;
+            }
+
+            return response()->json($body, $e->status, $e->headers);
+        });
+
         // Database connection / query errors → 503
         $exceptions->render(function (QueryException|PDOException $e, Request $request) {
             if ($request->expectsJson() || $request->is('api/*')) {