diff --git a/includes/core/class-pr-core-related-posts-provider.php b/includes/core/class-pr-core-related-posts-provider.php
new file mode 100644
index 0000000..ef59707
--- /dev/null
+++ b/includes/core/class-pr-core-related-posts-provider.php
@@ -0,0 +1,24 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * Contract for any provider that retrieves posts related to a peptide.
+ *
+ * What: Interface defining the get_posts() contract for related-post providers.
+ * Who calls it: PR_Core_Related_Posts_Section accepts any implementation.
+ * Dependencies: None.
+ *
+ * @see includes/core/class-pr-core-internal-posts-provider.php — default implementation.
+ * @see includes/core/class-pr-core-related-posts-section.php   — consumer.
+ */
+interface PR_Core_Related_Posts_Provider {
+
+	/**
+	 * Get posts related to a peptide.
+	 *
+	 * @param int $peptide_id The peptide post ID.
+	 * @param int $limit      Maximum number of posts to return.
+	 * @return array<int, \WP_Post> Array of WP_Post objects (may be empty).
+	 */
+	public function get_posts( int $peptide_id, int $limit ): array;
+}