diff --git a/app/Actions/CreateMessageReaction.php b/app/Actions/CreateMessageReaction.php
index 68eb21734..9491920b5 100644
--- a/app/Actions/CreateMessageReaction.php
+++ b/app/Actions/CreateMessageReaction.php
@@ -4,6 +4,7 @@
 
 namespace App\Actions;
 
+use App\Models\Conversation;
 use App\Models\Message;
 use App\Models\MessageReaction;
 use App\Models\User;
@@ -18,10 +19,26 @@ public function __construct(private UpdateMessageReactionsVersion $updateMessage
 
     /**
      * Inserted unless already there, so reacting twice with the same emoji leaves one reaction.
+     *
+     * The request checks the cap before this runs, but reactions sent at the same moment each see the others missing.
+     * So the cap is counted again here, under the conversation's lock: a reaction past it changes nothing. The
+     * conversation is the row locked first, as pins and removed reactions lock it before the message, so none of them
+     * can deadlock with this.
      */
     public function handle(User $user, Message $message, string $emoji): void
     {
         DB::transaction(function () use ($user, $message, $emoji): void {
+            Conversation::query()->whereKey($message->conversation_id)->lockForUpdate()->first();
+
+            $reacted = MessageReaction::query()
+                ->where('message_id', $message->id)
+                ->where('user_id', $user->id)
+                ->pluck('emoji');
+
+            if ($reacted->count() >= MessageReaction::MAX_PER_MESSAGE && $reacted->doesntContain($emoji)) {
+                return;
+            }
+
             $inserted = MessageReaction::query()->insertOrIgnore([
                 'message_id' => $message->id,
                 'user_id' => $user->id,
diff --git a/tests/Feature/Controllers/MessageReactionControllerTest.php b/tests/Feature/Controllers/MessageReactionControllerTest.php
index efc91fc2d..eec19a286 100644
--- a/tests/Feature/Controllers/MessageReactionControllerTest.php
+++ b/tests/Feature/Controllers/MessageReactionControllerTest.php
@@ -2,6 +2,7 @@
 
 declare(strict_types=1);
 
+use App\Actions\CreateMessageReaction;
 use App\Enums\RemotesFrom;
 use App\Models\Message;
 use App\Models\MessageReaction;
@@ -67,6 +68,27 @@
     $this->actingAs($mark)->postJson(route('message-reaction.store', $message), ['emoji' => '💀'])->assertOk();
 });
 
+it('keeps the cap when reactions sent at the same moment all passed the check', function (): void {
+    $mark = User::factory()->withRemotes()->create();
+    $nuno = User::factory()->withRemotes()->create();
+    $message = sendChatMessage($mark, $nuno);
+    $emoji = ['😂', '✅', '❤️', '🚀', '👀', '🔥', '🎉', '👍', '💯', '🙏', '🤔', '👏'];
+
+    foreach ($emoji as $each) {
+        resolve(CreateMessageReaction::class)->handle($nuno, $message, $each);
+    }
+
+    // Sent at the same moment as the twelfth, so the check let them through.
+    resolve(CreateMessageReaction::class)->handle($nuno, $message, '💀');
+    resolve(CreateMessageReaction::class)->handle($nuno, $message, '😂');
+    resolve(CreateMessageReaction::class)->handle($mark, $message, '💀');
+
+    expect(MessageReaction::query()->where('message_id', $message->id)->where('user_id', $nuno->id)->pluck('emoji')->all())
+        ->toEqualCanonicalizing($emoji)
+        ->and(MessageReaction::query()->where('message_id', $message->id)->where('user_id', $mark->id)->pluck('emoji')->all())
+        ->toBe(['💀']);
+});
+
 it('keeps reactions to the people who can see the message', function (): void {
     $mark = User::factory()->withRemotes()->create();
     $nuno = User::factory()->withRemotes()->create();