feat: add typed message tags

README.md

@@ -135,6 +135,26 @@ same request with the same idempotency key, the API will return the same respons
 
 For more information, refer to the [documentation](https://docs.lettermint.co/platform/emails/idempotency).
 
+### Message tags
+
+```php
+use Lettermint\Objects\MessageTag;
+
+$response = $lettermint->email
+    ->from('sender@example.com')
+    ->to('recipient@example.com')
+    ->subject('Welcome')
+    ->tag('legacy-tag')
+    ->tags([
+        new MessageTag('campaign', 'welcome'),
+        new MessageTag('customer', 'new'),
+    ])
+    ->send();
+```
+
+`tag()` remains available for the legacy single tag. `tags()` accepts typed
+`MessageTag` values and the previous name/value array form.
+
 ### API Client
 
 Use the API client for team-scoped resources such as projects, domains, routes, suppressions, stats, messages, and webhooks:

src/Endpoints/EmailEndpoint.php

@@ -2,6 +2,8 @@
 
 namespace Lettermint\Endpoints;
 
+use InvalidArgumentException;
+use Lettermint\Objects\MessageTag;
 use Lettermint\Responses\SendBatchMailResponse;
 use Lettermint\Responses\SendMailResponse;
 
@@ -247,6 +249,10 @@ public function metadata(array $metadata): self
      */
     public function tag(?string $tag): self
     {
+        if ($tag !== null && count($this->payload['tags'] ?? []) >= 20) {
+            throw new InvalidArgumentException('A legacy tag and no more than 19 message tags are permitted.');
+        }
+
         $this->payload['tag'] = $tag;
 
         return $this;
@@ -255,11 +261,32 @@ public function tag(?string $tag): self
     /**
      * Set reusable name-value tags for the email.
      *
-     * @param  list<array{name: string, value: string}>  $tags
+     * Existing name/value arrays remain supported for backward compatibility.
+     *
+     * @param  list<MessageTag|array{name: string, value: string}>  $tags
      */
     public function tags(array $tags): self
     {
-        $this->payload['tags'] = $tags;
+        $maximum = isset($this->payload['tag']) ? 19 : 20;
+        if (count($tags) > $maximum) {
+            throw new InvalidArgumentException("No more than {$maximum} message tags are permitted.");
+        }
+
+        $normalized = array_map(
+            static fn (MessageTag|array $tag): MessageTag => $tag instanceof MessageTag
+                ? $tag
+                : new MessageTag($tag['name'], $tag['value']),
+            $tags,
+        );
+        $names = array_map(static fn (MessageTag $tag): string => $tag->name, $normalized);
+        if (count($names) !== count(array_unique($names, SORT_STRING))) {
+            throw new InvalidArgumentException('Message tag names must be unique and case-sensitive.');
+        }
+
+        $this->payload['tags'] = array_map(
+            static fn (MessageTag $tag): array => $tag->toArray(),
+            $normalized,
+        );
 
         return $this;
     }

src/Objects/MessageTag.php

@@ -0,0 +1,32 @@
+<?php
+
+namespace Lettermint\Objects;
+
+use InvalidArgumentException;
+
+/** A reusable exact-match message tag. */
+final readonly class MessageTag
+{
+    public function __construct(
+        public string $name,
+        public string $value,
+    ) {
+        if (! preg_match('/^[A-Za-z0-9_-]{1,32}$/D', $name)) {
+            throw new InvalidArgumentException('Message tag names must match ^[A-Za-z0-9_-]{1,32}$');
+        }
+
+        if (str_starts_with(strtolower($name), '__lettermint')) {
+            throw new InvalidArgumentException('Message tag names must not start with __lettermint');
+        }
+
+        if (! preg_match('/^[A-Za-z0-9_-]{1,64}$/D', $value)) {
+            throw new InvalidArgumentException('Message tag values must match ^[A-Za-z0-9_-]{1,64}$');
+        }
+    }
+
+    /** @return array{name: string, value: string} */
+    public function toArray(): array
+    {
+        return ['name' => $this->name, 'value' => $this->value];
+    }
+}

tests/Endpoints/EmailEndpointTest.php

@@ -2,6 +2,7 @@
 
 use Lettermint\Client\HttpClient;
 use Lettermint\Endpoints\EmailEndpoint;
+use Lettermint\Objects\MessageTag;
 use Lettermint\Responses\SendBatchMailResponse;
 use Lettermint\Responses\SendMailResponse;
 
@@ -517,3 +518,37 @@
         ->tags($tags)
         ->send();
 });
+
+test('it handles typed message tags and legacy arrays', function () {
+    $this->httpClient
+        ->shouldReceive('post')
+        ->once()
+        ->with('/v1/send', Mockery::on(fn (array $payload): bool => $payload['tags'] === [
+            ['name' => 'campaign', 'value' => 'welcome'],
+            ['name' => 'customer', 'value' => 'new'],
+        ]), [])
+        ->andReturn(['message_id' => '123', 'status' => 'pending']);
+
+    $this->endpoint
+        ->from('sender@example.com')
+        ->to('recipient@example.com')
+        ->subject('Test Subject')
+        ->tags([
+            new MessageTag('campaign', 'welcome'),
+            ['name' => 'customer', 'value' => 'new'],
+        ])
+        ->send();
+});
+
+test('it rejects invalid reusable tags', function (array $tags) {
+    $this->endpoint->tags($tags);
+})->with([
+    'duplicate names' => [[['name' => 'same', 'value' => 'one'], ['name' => 'same', 'value' => 'two']]],
+    'reserved name' => [[['name' => '__LETTERMINT_internal', 'value' => 'one']]],
+    'invalid value' => [[['name' => 'valid', 'value' => 'invalid value']]],
+])->throws(InvalidArgumentException::class);
+
+test('it counts the legacy tag in the reusable tag limit', function () {
+    $tags = array_map(fn (int $index): array => ['name' => "tag_{$index}", 'value' => 'one'], range(1, 20));
+    $this->endpoint->tag('legacy')->tags($tags);
+})->throws(InvalidArgumentException::class);