diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7c433ac..f409319 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,13 @@
 All notable changes to Peptide Repo Core are documented here.
 Format: [Semantic Versioning](https://semver.org/).
 
+## [0.6.1] — 2026-06-13
+
+### Fixed
+- Fix: Yoast `wpseo_schema_webpage_type` passes `string|array` on singulars — accept `string|array` in `retype_to_medical_webpage()` signature; return type updated to match (PHP 8.1+ union). Eliminates `TypeError` → HTTP 500 on all singular pages (monographs + plain Pages) under `strict_types=1`.
+- Fix: `enrich_webpage_piece` compared `'WebPage' === $piece['@type']` but Yoast sets `@type` as an array on singular pages. Normalise `@type` to array before `in_array()` check; `lastReviewed`, `reviewedBy`, and `audience` now inject correctly.
+- Add regression test `tests/unit/test-jsonld-webpage.php`: exercises `retype_to_medical_webpage()` with array input for peptide and non-peptide contexts (no TypeError), and `enrich_webpage_piece` with `@type` as array (enrichment injects). Tests fail against v0.6.0, pass after fix.
+
 ## [0.6.0] — 2026-06-13
 
 ### Added
diff --git a/includes/frontend/class-pr-core-jsonld-webpage.php b/includes/frontend/class-pr-core-jsonld-webpage.php
index 4fd85d7..4b51e87 100644
--- a/includes/frontend/class-pr-core-jsonld-webpage.php
+++ b/includes/frontend/class-pr-core-jsonld-webpage.php
@@ -28,10 +28,17 @@ class PR_Core_Jsonld_Webpage {
 	 * Hooked on: wpseo_schema_webpage_type (Yoast SEO ≥ 19.x).
 	 * Passes through unchanged on non-peptide pages.
 	 *
-	 * @param string $type Existing Yoast page type (e.g., 'WebPage').
-	 * @return string 'MedicalWebPage' on peptide singles, $type otherwise.
+	 * Yoast contract: passes a plain string (e.g., 'CollectionPage') for
+	 * non-singular special types, but an ARRAY (e.g., ['WebPage']) for every
+	 * singular page — including plain Pages and monographs. Under strict_types=1
+	 * a string-only signature throws TypeError on all singulars. This method
+	 * therefore accepts string|array and returns the same union type so callers
+	 * (Yoast's filter chain) receive the value in the shape they passed it.
+	 *
+	 * @param string|array $type Existing Yoast page type — string or string[].
+	 * @return string|array 'MedicalWebPage' (string) on peptide singles; $type unchanged otherwise.
 	 */
-	public function retype_to_medical_webpage( string $type ): string {
+	public function retype_to_medical_webpage( string|array $type ): string|array {
 		if ( is_singular( PR_Core_Peptide_CPT::POST_TYPE ) ) {
 			return 'MedicalWebPage';
 		}
@@ -55,8 +62,8 @@ class PR_Core_Jsonld_Webpage {
 			return $graph;
 		}
 
-		$post_id      = get_the_ID();
-		$enrichments  = $this->build_webpage_enrichments( (int) $post_id );
+		$post_id     = get_the_ID();
+		$enrichments = $this->build_webpage_enrichments( (int) $post_id );
 
 		if ( empty( $enrichments ) ) {
 			return $graph;
@@ -67,8 +74,12 @@ class PR_Core_Jsonld_Webpage {
 				continue;
 			}
 
-			$type = $piece['@type'] ?? '';
-			if ( 'WebPage' === $type || 'MedicalWebPage' === $type ) {
+			// Yoast may set @type as a plain string OR an array of strings.
+			// Normalise to array so in_array() works regardless of shape.
+			$raw_type = $piece['@type'] ?? '';
+			$types    = is_array( $raw_type ) ? $raw_type : [ $raw_type ];
+
+			if ( in_array( 'WebPage', $types, true ) || in_array( 'MedicalWebPage', $types, true ) ) {
 				foreach ( $enrichments as $key => $value ) {
 					$piece[ $key ] = $value;
 				}
diff --git a/peptide-repo-core.php b/peptide-repo-core.php
index 1b95b45..40ba309 100644
--- a/peptide-repo-core.php
+++ b/peptide-repo-core.php
@@ -3,7 +3,7 @@
  * Plugin Name: Peptide Repo Core
  * Plugin URI:  https://peptiderepo.com
  * Description: Canonical peptide schema — shared data layer for the peptiderepo.com ecosystem. Provides the peptide CPT, dosing rows, legal status cells, AI candidate queue, disclaimer component, and JSON-LD output.
- * Version:     0.6.0
+ * Version:     0.6.1
  * Author:      peptiderepo
  * Author URI:  https://peptiderepo.com
  * License:     GPL-2.0-or-later
@@ -22,7 +22,7 @@ if ( ! defined( 'ABSPATH' ) ) {
 
 /* ── Constants ────────────────────────────────────────────────────────── */
 
-define( 'PR_CORE_VERSION', '0.6.0' );
+define( 'PR_CORE_VERSION', '0.6.1' );
 define( 'PR_CORE_PLUGIN_FILE', __FILE__ );
 define( 'PR_CORE_PLUGIN_DIR', plugin_dir_path( __FILE__ ) );
 define( 'PR_CORE_PLUGIN_URL', plugin_dir_url( __FILE__ ) );
diff --git a/tests/unit/test-jsonld-webpage.php b/tests/unit/test-jsonld-webpage.php
new file mode 100644
index 0000000..1539792
--- /dev/null
+++ b/tests/unit/test-jsonld-webpage.php
@@ -0,0 +1,165 @@
+<?php
+/**
+ * Regression tests for v0.6.1: Yoast wpseo_schema_webpage_type array contract.
+ *
+ * Root cause caught on staging: v0.6.0 retype_to_medical_webpage() was
+ * strict-typed string, but Yoast passes ['WebPage'] (array) on every singular
+ * page. strict_types=1 → TypeError → HTTP 500 on all monographs + plain Pages.
+ * Secondary bug: enrich_webpage_piece compared 'WebPage' === $piece['@type']
+ * but @type is an array, so enrichment (lastReviewed/reviewedBy/audience)
+ * never injected on any page.
+ *
+ * These tests FAIL against v0.6.0 and PASS after the v0.6.1 fix.
+ *
+ * Run: php tests/unit/test-jsonld-webpage.php
+ * Exit 0 = all pass, 1 = any failure.
+ *
+ * @package PeptideRepoCore
+ */
+
+declare(strict_types=1);
+
+require_once __DIR__ . '/../bootstrap.php';
+
+// ── Additional stubs ─────────────────────────────────────────────────────
+
+if ( ! defined( 'PR_CORE_TARGET_SCHEMA_VERSION' ) ) {
+	define( 'PR_CORE_TARGET_SCHEMA_VERSION', 4 );
+}
+
+$GLOBALS['pr_test_postmeta']      = [];
+$GLOBALS['pr_test_is_singular']   = false;
+$GLOBALS['pr_test_singular_type'] = '';
+$GLOBALS['pr_test_the_id']        = 0;
+
+function get_post_meta( $post_id, string $key, bool $single = false ) {
+	if ( $single ) {
+		return $GLOBALS['pr_test_postmeta'][ (int) $post_id ][ $key ] ?? '';
+	}
+	return array_values( $GLOBALS['pr_test_postmeta'][ (int) $post_id ] ?? [] );
+}
+
+function get_permalink( $post_id = null ): string {
+	return 'https://peptiderepo.com/peptides/bpc-157/';
+}
+
+function get_the_ID(): int {
+	return $GLOBALS['pr_test_the_id'];
+}
+
+function is_singular( $type = '' ): bool {
+	if ( '' === $type ) {
+		return $GLOBALS['pr_test_is_singular'];
+	}
+	return $GLOBALS['pr_test_is_singular'] && ( $GLOBALS['pr_test_singular_type'] === $type );
+}
+
+function get_post_modified_time( $format, $gmt, $post_id ) {
+	return '2026-04-25';
+}
+
+function get_userdata( int $id ) {
+	return false;
+}
+
+function get_author_posts_url( int $id ): string {
+	return 'https://peptiderepo.com/author/' . $id;
+}
+
+function home_url( string $path = '' ): string {
+	return 'https://peptiderepo.com' . $path;
+}
+
+function apply_filters( string $tag, $value ) {
+	return $value;
+}
+
+// Load class under test.
+require_once PR_CORE_PLUGIN_DIR . 'includes/frontend/class-pr-core-jsonld-webpage.php';
+
+echo "== JSON-LD WebPage v0.6.1 regression tests ==\n\n";
+
+$enricher = new PR_Core_Jsonld_Webpage();
+
+// ── retype_to_medical_webpage: array input, non-peptide singular ──────────
+
+echo "retype_to_medical_webpage — array input:\n";
+
+// Non-peptide singular: Yoast passes ['WebPage']; must return unchanged, no TypeError.
+$GLOBALS['pr_test_is_singular']   = true;
+$GLOBALS['pr_test_singular_type'] = 'page';  // plain WP Page, not peptide CPT.
+
+$result = $enricher->retype_to_medical_webpage( [ 'WebPage' ] );
+pr_assert_equals( [ 'WebPage' ], $result, 'non-peptide singular: array returned unchanged' );
+
+// Peptide singular: Yoast passes ['WebPage']; must return 'MedicalWebPage' (string).
+$GLOBALS['pr_test_is_singular']   = true;
+$GLOBALS['pr_test_singular_type'] = 'peptide';
+
+$result = $enricher->retype_to_medical_webpage( [ 'WebPage' ] );
+pr_assert_equals( 'MedicalWebPage', $result, 'peptide singular: returns MedicalWebPage (string)' );
+
+// String input still works (non-singular special types like CollectionPage).
+$GLOBALS['pr_test_is_singular']   = false;
+$GLOBALS['pr_test_singular_type'] = '';
+
+$result = $enricher->retype_to_medical_webpage( 'CollectionPage' );
+pr_assert_equals( 'CollectionPage', $result, 'non-singular string: passes through unchanged' );
+
+// ── enrich_webpage_piece: @type as array injects enrichments ─────────────
+
+echo "\nenrich_webpage_piece — @type as array:\n";
+
+$GLOBALS['pr_test_is_singular']   = true;
+$GLOBALS['pr_test_singular_type'] = 'peptide';
+$GLOBALS['pr_test_the_id']        = 42;
+$GLOBALS['pr_test_postmeta'][42]  = [
+	'_pr_last_reviewed' => '2026-06-01',
+];
+
+// Graph piece with @type as an array (real Yoast shape on singulars).
+$graph = [
+	[
+		'@type' => [ 'WebPage' ],
+		'@id'   => 'https://peptiderepo.com/peptides/bpc-157/#webpage',
+		'name'  => 'BPC-157',
+	],
+];
+
+$result_graph = $enricher->enrich_webpage_piece( $graph, new stdClass() );
+$piece        = $result_graph[0] ?? [];
+
+pr_assert( isset( $piece['lastReviewed'] ), '@type=array: lastReviewed injected' );
+pr_assert_equals( '2026-06-01', $piece['lastReviewed'], '@type=array: lastReviewed value correct' );
+pr_assert( isset( $piece['reviewedBy'] ), '@type=array: reviewedBy injected' );
+pr_assert( isset( $piece['audience'] ), '@type=array: audience injected' );
+
+// Also works when @type is 'MedicalWebPage' (string — after our retype fires first).
+$graph_retyed = [
+	[
+		'@type' => 'MedicalWebPage',
+		'@id'   => 'https://peptiderepo.com/peptides/bpc-157/#webpage',
+		'name'  => 'BPC-157',
+	],
+];
+
+$result_retyped = $enricher->enrich_webpage_piece( $graph_retyed, new stdClass() );
+pr_assert( isset( $result_retyped[0]['lastReviewed'] ), 'MedicalWebPage string @type: enrichment injects' );
+
+// Non-peptide page: enrichment must NOT fire even with array @type.
+$GLOBALS['pr_test_is_singular']   = true;
+$GLOBALS['pr_test_singular_type'] = 'page';
+
+$graph_non_peptide = [
+	[
+		'@type' => [ 'WebPage' ],
+		'@id'   => 'https://peptiderepo.com/about/#webpage',
+	],
+];
+
+$result_non_peptide = $enricher->enrich_webpage_piece( $graph_non_peptide, new stdClass() );
+pr_assert( ! isset( $result_non_peptide[0]['lastReviewed'] ), 'non-peptide: lastReviewed NOT injected' );
+
+// ── Summary ──────────────────────────────────────────────────────────────
+
+exit( pr_test_summary() );