diff --git a/migrations/Version20260923101400.php b/migrations/Version20260923101400.php
new file mode 100644
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--- /dev/null
+++ b/migrations/Version20260923101400.php
@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * The unread count every folder sync ends with, without scanning the folder.
+ *
+ * MessageSyncer recounts `mailbox_id = ? AND seen_at IS NULL` after every
+ * sync of every folder. The unique (mailbox_id, imap_uid) index finds the
+ * folder's rows but then has to visit each one to test seen_at; a partial
+ * index holds only the unread rows, which in a well-kept mailbox is almost
+ * none of them. The total count already has an index to use.
+ *
+ * Migration-only, like the other partial indexes on this table: the ORM
+ * mapping cannot express the WHERE clause.
+ */
+final class Version20260923101400 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Partial index on message(mailbox_id) for unread rows.';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('CREATE INDEX idx_message_unseen_mailbox ON message (mailbox_id) WHERE seen_at IS NULL');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('DROP INDEX idx_message_unseen_mailbox');
+    }
+}