diff --git a/includes/engine/class-tmdo-hook-bus.php b/includes/engine/class-tmdo-hook-bus.php
index dbce53e..02de79a 100644
--- a/includes/engine/class-tmdo-hook-bus.php
+++ b/includes/engine/class-tmdo-hook-bus.php
@@ -44,6 +44,15 @@ final class TMDO_Hook_Bus {
 		// ── 刪除攔截 ──────────────────────────────────────────
 		add_filter( "delete_{$type}_metadata", array( self::class, 'intercept_delete' ), 10, 5 );
 
+		// ── dual_write / shadow_read：原生寫入完成後，flat 從原生同步 ──
+		// Native is the source of truth in these modes; flat follows what
+		// native actually became (WP may skip or refuse a write).
+		add_action( "added_{$type}_meta", array( self::class, 'on_meta_added' ), 1, 4 );
+		add_action( "updated_{$type}_meta", array( self::class, 'on_meta_updated' ), 1, 4 );
+		add_action( "delete_{$type}_meta", array( self::class, 'before_meta_deleted' ), 1, 4 );
+		add_action( "deleted_{$type}_meta", array( self::class, 'on_meta_deleted' ), 1, 4 );
+		add_filter( "update_{$type}_metadata_by_mid", array( self::class, 'before_update_by_mid' ), 1, 4 );
+
 		// ── 實體刪除時自動清理 ───────────────────────────────
 		add_action(
 			$adapter->get_delete_hook(),
@@ -70,13 +79,13 @@ final class TMDO_Hook_Bus {
 	 */
 	public static function intercept_update( $check, $object_id, $meta_key, $meta_value, $prev_value ) {
 
-		// 已在短路中 → 避免遞迴
-		if ( ! empty( self::$internal_ops[ $object_id . ':' . $meta_key ] ) ) {
+		$type = self::resolve_type_from_current_filter();
+		if ( ! $type ) {
 			return $check;
 		}
 
-		$type = self::resolve_type_from_current_filter();
-		if ( ! $type ) {
+		// 已在短路中 → 避免遞迴
+		if ( ! empty( self::$internal_ops[ $type . ':' . $object_id . ':' . $meta_key ] ) ) {
 			return $check;
 		}
 
@@ -86,10 +95,10 @@ final class TMDO_Hook_Bus {
 		}
 
 		// ── Mode-aware dispatch ──────────────────────────────
-		// disabled  : 完全放行給 WP 原生 meta(回 null / $check)
-		// dual_write: 寫 flat,然後 return null 讓 WP 繼續寫 EAV
-		// shadow_read: 寫 flat,然後 return null 讓 WP 繼續寫 EAV
-		// aeav_only : 寫 flat,return true 短路 WP(不寫 EAV)
+		// disabled   : 完全放行給 WP 原生 meta(回 null / $check)
+		// dual_write : 放行 WP 寫 EAV;updated_/added_ action 再從原生同步 flat
+		// shadow_read: 同 dual_write
+		// aeav_only  : 寫 flat,return true 短路 WP(不寫 EAV)
 		if ( ! TMDO_Mode_Manager::writes_to_flat( $type ) ) {
 			return $check;  // disabled
 		}
@@ -105,6 +114,13 @@ final class TMDO_Hook_Bus {
 			return true;
 		}
 
+		// dual_write / shadow_read: native first. Writing flat here, before WP
+		// decides whether native changes at all (same value, non-matching
+		// prev_value), let flat drift; sync_from_native() runs after the write.
+		if ( TMDO_Mode_Manager::writes_to_eav( $type ) ) {
+			return null;
+		}
+
 		// 截取 before value(v1.3.1):audit_logger 等訂閱者需要變更前的值。
 		// v1.3.2:透過 filter `wpdo_capture_before_value` 可關閉以省一次 DB read。
 		$before_value = self::maybe_read_before_value( $type, (int) $object_id, $meta_key, $field_def, 'update' );
@@ -113,23 +129,18 @@ final class TMDO_Hook_Bus {
 
 		do_action( 'wpdo_after_write', $type, (int) $object_id, $meta_key, $meta_value, $flat_result, 'update', $before_value );
 
-		// 若 mode 也要寫 EAV → return null 讓 WP 繼續
-		if ( TMDO_Mode_Manager::writes_to_eav( $type ) ) {
-			return null;  // dual_write / shadow_read
-		}
-
 		return $flat_result;  // aeav_only
 	}
 
 	public static function intercept_add( $check, $object_id, $meta_key, $meta_value, $unique ) {
 
-		// 已在短路中 → 避免遞迴
-		if ( ! empty( self::$internal_ops[ $object_id . ':' . $meta_key ] ) ) {
+		$type = self::resolve_type_from_current_filter();
+		if ( ! $type ) {
 			return $check;
 		}
 
-		$type = self::resolve_type_from_current_filter();
-		if ( ! $type ) {
+		// 已在短路中 → 避免遞迴
+		if ( ! empty( self::$internal_ops[ $type . ':' . $object_id . ':' . $meta_key ] ) ) {
 			return $check;
 		}
 
@@ -150,6 +161,12 @@ final class TMDO_Hook_Bus {
 			return true;
 		}
 
+		// dual_write / shadow_read: native first (add($unique) may be refused);
+		// sync_from_native() runs on added_{type}_meta.
+		if ( TMDO_Mode_Manager::writes_to_eav( $type ) ) {
+			return null;
+		}
+
 		// UAE 的設計:每個 entity 只有 1 row,所以 add 與 update 等效(Upsert)。
 		// 截取 before value(v1.3.1):add 情境下多半為 null,但若 row 已存在而 user 呼叫 add 也能抓到舊值。
 		$before_value = self::maybe_read_before_value( $type, (int) $object_id, $meta_key, $field_def, 'add' );
@@ -158,11 +175,225 @@ final class TMDO_Hook_Bus {
 
 		do_action( 'wpdo_after_write', $type, (int) $object_id, $meta_key, $meta_value, $flat_result, 'add', $before_value );
 
-		if ( TMDO_Mode_Manager::writes_to_eav( $type ) ) {
-			return null;
+		return $flat_result;
+	}
+
+	// ─────────────────────────────────────────────────────────
+	// dual_write / shadow_read：原生寫入後同步 flat
+	// ─────────────────────────────────────────────────────────
+
+	/**
+	 * Entity IDs whose native rows a pending delete removes, keyed "type:key".
+	 *
+	 * @var array<string, int[]>
+	 */
+	private static array $pending_deletes = array();
+
+	/**
+	 * Old keys of rows renamed through update_metadata_by_mid, keyed by meta ID.
+	 *
+	 * @var array<string, array{type:string, id:int, key:string}>
+	 */
+	private static array $pending_renames = array();
+
+	/**
+	 * Copy the entity's current native value for a key into its flat column
+	 * (NULL when native has no row). Runs in dual_write / shadow_read only,
+	 * after WP wrote native, so flat always ends up equal to native — also
+	 * when WP skipped or refused the write, and when an earlier direct SQL
+	 * write had left flat stale.
+	 *
+	 * @param string $type       Entity type.
+	 * @param int    $object_id  Entity ID.
+	 * @param string $meta_key   Meta key.
+	 * @param mixed  $meta_value Value the caller wrote (for listeners).
+	 * @param string $op         'add' | 'update' | 'delete'.
+	 * @param bool   $delete_all Whether the delete was a delete_all.
+	 * @return void
+	 */
+	public static function sync_from_native( string $type, int $object_id, string $meta_key, $meta_value, string $op, bool $delete_all = false ): void {
+		if ( ! TMDO_Mode_Manager::writes_to_flat( $type ) || ! TMDO_Mode_Manager::writes_to_eav( $type ) ) {
+			return;
+		}
+		$field_def = self::field_for( $type, $object_id, $meta_key );
+		if ( ! $field_def || 'flat' !== self::decide_route( $op, $type, $object_id, $meta_key, $meta_value ) ) {
+			return;
 		}
 
-		return $flat_result;
+		$before = self::maybe_read_before_value( $type, $object_id, $meta_key, $field_def, $op );
+		$native = self::native_first_value( $type, $object_id, $meta_key );
+		$result = self::perform_upsert( $type, $object_id, $meta_key, null === $native ? null : maybe_unserialize( $native ), $field_def );
+
+		if ( 'delete' === $op ) {
+			do_action( 'wpdo_after_delete', $type, $object_id, $meta_key, $meta_value, $result, $delete_all, $before );
+		} else {
+			do_action( 'wpdo_after_write', $type, $object_id, $meta_key, $meta_value, $result, $op, $before );
+		}
+	}
+
+	/**
+	 * `added_{type}_meta` action.
+	 *
+	 * @param int    $meta_id    Meta ID.
+	 * @param int    $object_id  Entity ID.
+	 * @param string $meta_key   Meta key.
+	 * @param mixed  $meta_value Meta value.
+	 */
+	public static function on_meta_added( $meta_id, $object_id, $meta_key, $meta_value ): void {
+		$type = self::resolve_type_from_meta_hook();
+		if ( $type ) {
+			self::sync_from_native( $type, (int) $object_id, (string) $meta_key, $meta_value, 'add' );
+		}
+	}
+
+	/**
+	 * `updated_{type}_meta` action; also re-syncs the old key of a row renamed
+	 * through update_metadata_by_mid.
+	 *
+	 * @param int    $meta_id    Meta ID.
+	 * @param int    $object_id  Entity ID.
+	 * @param string $meta_key   Meta key.
+	 * @param mixed  $meta_value Meta value.
+	 */
+	public static function on_meta_updated( $meta_id, $object_id, $meta_key, $meta_value ): void {
+		$type = self::resolve_type_from_meta_hook();
+		if ( ! $type ) {
+			return;
+		}
+		self::sync_from_native( $type, (int) $object_id, (string) $meta_key, $meta_value, 'update' );
+
+		$rename_key = $type . ':' . (int) $meta_id;
+		if ( isset( self::$pending_renames[ $rename_key ] ) ) {
+			$old = self::$pending_renames[ $rename_key ];
+			unset( self::$pending_renames[ $rename_key ] );
+			self::sync_from_native( $type, $old['id'], $old['key'], null, 'delete' );
+		}
+	}
+
+	/**
+	 * `update_{type}_metadata_by_mid` filter: remember the old key when a row
+	 * is renamed, so its flat column can follow. Never changes $check.
+	 *
+	 * @param mixed        $check      Short-circuit value.
+	 * @param int          $meta_id    Meta ID.
+	 * @param mixed        $meta_value New value.
+	 * @param string|false $meta_key   New key, or false to keep it.
+	 * @return mixed
+	 */
+	public static function before_update_by_mid( $check, $meta_id, $meta_value, $meta_key ) {
+		$type = self::resolve_type_from_meta_hook();
+		if ( ! $type || false === $meta_key ) {
+			return $check;
+		}
+		$row = self::native_row_by_mid( $type, (int) $meta_id );
+		if ( $row && $row['key'] !== $meta_key && TMDO_Entity_Registry::get_field( $type, $row['key'] ) ) {
+			self::$pending_renames[ $type . ':' . (int) $meta_id ] = array(
+				'type' => $type,
+				'id'   => $row['id'],
+				'key'  => $row['key'],
+			);
+		}
+		return $check;
+	}
+
+	/**
+	 * `delete_{type}_meta` action (rows still exist): record which entities a
+	 * delete touches — a delete_all passes object ID 0.
+	 *
+	 * @param int[]  $meta_ids   Meta IDs to delete.
+	 * @param int    $object_id  Entity ID (0 for delete_all).
+	 * @param string $meta_key   Meta key.
+	 * @param mixed  $meta_value Meta value.
+	 */
+	public static function before_meta_deleted( $meta_ids, $object_id, $meta_key, $meta_value ): void {
+		$type = self::resolve_type_from_meta_hook();
+		if ( ! $type || ! TMDO_Entity_Registry::get_field( $type, (string) $meta_key ) || ! TMDO_Mode_Manager::writes_to_eav( $type ) ) {
+			return;
+		}
+		$ids = array();
+		foreach ( (array) $meta_ids as $mid ) {
+			$row = self::native_row_by_mid( $type, (int) $mid );
+			if ( $row ) {
+				$ids[ $row['id'] ] = $row['id'];
+			}
+		}
+		self::$pending_deletes[ $type . ':' . $meta_key ] = array_values( $ids );
+	}
+
+	/**
+	 * `deleted_{type}_meta` action: re-sync every entity the delete touched.
+	 *
+	 * @param int[]  $meta_ids   Deleted meta IDs.
+	 * @param int    $object_id  Entity ID (0 for delete_all).
+	 * @param string $meta_key   Meta key.
+	 * @param mixed  $meta_value Meta value.
+	 */
+	public static function on_meta_deleted( $meta_ids, $object_id, $meta_key, $meta_value ): void {
+		$type = self::resolve_type_from_meta_hook();
+		if ( ! $type ) {
+			return;
+		}
+		$pending_key = $type . ':' . $meta_key;
+		$ids         = self::$pending_deletes[ $pending_key ] ?? array( (int) $object_id );
+		unset( self::$pending_deletes[ $pending_key ] );
+
+		foreach ( $ids as $id ) {
+			if ( $id > 0 ) {
+				self::sync_from_native( $type, (int) $id, (string) $meta_key, $meta_value, 'delete', 0 === (int) $object_id );
+			}
+		}
+	}
+
+	/**
+	 * First native value of a key (the one get_*_meta( $id, $key, true ) returns),
+	 * read with SQL so no meta filter runs. Null when the entity has no row.
+	 *
+	 * @param string $type      Entity type.
+	 * @param int    $object_id Entity ID.
+	 * @param string $meta_key  Meta key.
+	 * @return string|null
+	 */
+	private static function native_first_value( string $type, int $object_id, string $meta_key ): ?string {
+		global $wpdb;
+		$adapter = TMDO_Entity_Registry::get_adapter( $type );
+		if ( ! $adapter ) {
+			return null;
+		}
+		$table  = $adapter->get_native_meta_table();
+		$id_col = $adapter->get_entity_id_column();
+		$pk     = 'user' === $type ? 'umeta_id' : 'meta_id';
+		// get_results(): get_var() turns an empty string into null.
+		$rows = $wpdb->get_results(
+			$wpdb->prepare( "SELECT meta_value FROM `{$table}` WHERE `{$id_col}` = %d AND meta_key = %s ORDER BY `{$pk}` LIMIT 1", $object_id, $meta_key ),
+			ARRAY_A
+		);
+		return $rows ? (string) $rows[0]['meta_value'] : null;
+	}
+
+	/**
+	 * Entity ID and key of a native meta row.
+	 *
+	 * @param string $type    Entity type.
+	 * @param int    $meta_id Meta ID.
+	 * @return array{id:int, key:string}|null
+	 */
+	private static function native_row_by_mid( string $type, int $meta_id ): ?array {
+		global $wpdb;
+		$adapter = TMDO_Entity_Registry::get_adapter( $type );
+		if ( ! $adapter ) {
+			return null;
+		}
+		$table  = $adapter->get_native_meta_table();
+		$id_col = $adapter->get_entity_id_column();
+		$pk     = 'user' === $type ? 'umeta_id' : 'meta_id';
+		$row    = $wpdb->get_row( $wpdb->prepare( "SELECT `{$id_col}` AS id, meta_key AS mkey FROM `{$table}` WHERE `{$pk}` = %d", $meta_id ), ARRAY_A );
+		if ( ! $row ) {
+			return null;
+		}
+		return array(
+			'id'  => (int) $row['id'],
+			'key' => (string) $row['mkey'],
+		);
 	}
 
 	/**
@@ -322,8 +553,8 @@ final class TMDO_Hook_Bus {
 		$db_value = TMDO_Type_Caster::to_db( $meta_value, $field_def );
 		$format   = TMDO_Type_Caster::get_wpdb_format( $field_def['type'] );
 
-		// 鎖防遞迴
-		$lock_key                        = $entity_id . ':' . $meta_key;
+		// 鎖防遞迴（含 entity type：user 5 與 post 5 不可互鎖）
+		$lock_key                        = $type . ':' . $entity_id . ':' . $meta_key;
 		self::$internal_ops[ $lock_key ] = true;
 
 		try {
@@ -517,6 +748,12 @@ final class TMDO_Hook_Bus {
 			return true;
 		}
 
+		// dual_write / shadow_read: WP deletes native (also delete_all);
+		// deleted_{type}_meta re-syncs every touched entity from native.
+		if ( TMDO_Mode_Manager::writes_to_eav( $type ) ) {
+			return null;
+		}
+
 		// 截取 before value(v1.3.1)
 		$before_value = self::maybe_read_before_value( $type, (int) $object_id, $meta_key, $field_def, 'delete' );
 
@@ -601,12 +838,7 @@ final class TMDO_Hook_Bus {
 
 		do_action( 'wpdo_after_delete', $type, (int) $object_id, $meta_key, $meta_value, $result, (bool) $delete_all, $before_value );
 
-		// 若還要寫 EAV → return null 讓 WP 繼續刪除原生 meta
-		if ( TMDO_Mode_Manager::writes_to_eav( $type ) ) {
-			return null;
-		}
-
-		return $result !== false;
+		return $result !== false;  // aeav_only
 	}
 
 	// ─────────────────────────────────────────────────────────
@@ -666,6 +898,17 @@ final class TMDO_Hook_Bus {
 		return null;
 	}
 
+	/**
+	 * Entity type from the meta action / by-mid filter being run
+	 * (added_user_meta, deleted_post_meta, update_term_metadata_by_mid, …).
+	 */
+	private static function resolve_type_from_meta_hook(): ?string {
+		if ( preg_match( '/^(?:(?:added|updated|delete|deleted)_(post|user|term|comment)_meta|update_(post|user|term|comment)_metadata_by_mid)$/', current_filter(), $matches ) ) {
+			return '' !== $matches[1] ? $matches[1] : $matches[2];
+		}
+		return null;
+	}
+
 	/**
 	 * 欄位定義查詢；post meta 依該 post 本身的 post_type 解析（見 TMDO_Entity_Registry::get_field()）。
 	 *
diff --git a/tests/integration/HookBusIntegrationTest.php b/tests/integration/HookBusIntegrationTest.php
index 7704bf5..fd66c4a 100644
--- a/tests/integration/HookBusIntegrationTest.php
+++ b/tests/integration/HookBusIntegrationTest.php
@@ -202,14 +202,71 @@ class HookBusIntegrationTest extends TestCase {
 		$this->assertTrue( $result, 'aeav_only must return true to short-circuit EAV write' );
 	}
 
-	public function test_update_writes_flat_and_returns_null_in_dual_write_mode(): void {
+	/**
+	 * dual_write: the pre-filter only lets WP write native; flat is synced
+	 * from native afterwards (updated_/added_ actions → sync_from_native).
+	 */
+	public function test_update_leaves_flat_to_the_native_sync_in_dual_write_mode(): void {
+		global $wpdb;
 		self::set_user_mode( WPDO_Mode_Manager::MODE_DUAL_WRITE );
 		$GLOBALS['_wp_current_filter'] = 'update_user_metadata';
 
 		$result = WPDO_Hook_Bus::intercept_update( null, 3, 'first_name', 'Carol', '' );
 
-		$this->assertSame( 'Carol', $this->read_flat_first_name( 3 ) );
 		$this->assertNull( $result, 'dual_write must return null so WP continues to write EAV' );
+		$this->assertNull( $this->read_flat_first_name( 3 ), 'no flat write before native is written' );
+
+		$this->with_usermeta(
+			function () use ( $wpdb ) {
+				$wpdb->insert( $wpdb->usermeta, array( 'user_id' => 3, 'meta_key' => 'first_name', 'meta_value' => 'Carol' ) );
+
+				WPDO_Hook_Bus::sync_from_native( 'user', 3, 'first_name', 'Carol', 'update' );
+
+				$this->assertSame( 'Carol', $this->read_flat_first_name( 3 ) );
+			}
+		);
+	}
+
+	/**
+	 * delete_all passes object ID 0: the delete_ action records the touched
+	 * users from the meta IDs, the deleted_ action re-syncs each to NULL.
+	 */
+	public function test_delete_all_resyncs_every_touched_entity_in_dual_write_mode(): void {
+		global $wpdb;
+		self::set_user_mode( WPDO_Mode_Manager::MODE_DUAL_WRITE );
+		$wpdb->insert( self::FLAT_TABLE, array( 'user_id' => 21, 'first_name' => 'A' ) );
+		$wpdb->insert( self::FLAT_TABLE, array( 'user_id' => 22, 'first_name' => 'B' ) );
+
+		$this->with_usermeta(
+			function () use ( $wpdb ) {
+				$wpdb->insert( $wpdb->usermeta, array( 'user_id' => 21, 'meta_key' => 'first_name', 'meta_value' => 'A' ) );
+				$mid_a = (int) $wpdb->insert_id;
+				$wpdb->insert( $wpdb->usermeta, array( 'user_id' => 22, 'meta_key' => 'first_name', 'meta_value' => 'B' ) );
+				$mid_b = (int) $wpdb->insert_id;
+
+				$GLOBALS['_wp_current_filter'] = 'delete_user_meta';
+				WPDO_Hook_Bus::before_meta_deleted( array( $mid_a, $mid_b ), 0, 'first_name', '' );
+				$wpdb->query( "DELETE FROM `{$wpdb->usermeta}` WHERE meta_key = 'first_name'" );
+				$GLOBALS['_wp_current_filter'] = 'deleted_user_meta';
+				WPDO_Hook_Bus::on_meta_deleted( array( $mid_a, $mid_b ), 0, 'first_name', '' );
+
+				$this->assertNull( $this->read_flat_first_name( 21 ) );
+				$this->assertNull( $this->read_flat_first_name( 22 ) );
+			}
+		);
+	}
+
+	/**
+	 * Run $fn with a native usermeta table, dropped afterwards.
+	 */
+	private function with_usermeta( callable $fn ): void {
+		global $wpdb;
+		$wpdb->query( "CREATE TABLE IF NOT EXISTS `{$wpdb->usermeta}` ( umeta_id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT, user_id BIGINT UNSIGNED NOT NULL DEFAULT 0, meta_key VARCHAR(255) DEFAULT NULL, meta_value LONGTEXT, PRIMARY KEY (umeta_id) ) ENGINE=InnoDB" );
+		try {
+			$fn();
+		} finally {
+			$wpdb->query( "DROP TABLE IF EXISTS `{$wpdb->usermeta}`" );
+		}
 	}
 
 	public function test_update_passthrough_for_unregistered_key(): void {
diff --git a/tests/integration/PostStatusSyncTest.php b/tests/integration/PostStatusSyncTest.php
index f4f08f7..b4b4310 100644
--- a/tests/integration/PostStatusSyncTest.php
+++ b/tests/integration/PostStatusSyncTest.php
@@ -160,7 +160,8 @@ class PostStatusSyncTest extends TestCase {
 		TMDO_Post_Status_Sync::run();
 		$mode = new ReflectionProperty( WPDO_Mode_Manager::class, 'cache' );
 		$mode->setAccessible( true );
-		$mode->setValue( null, array( 'post' => 'dual_write', 'user' => 'disabled', 'term' => 'disabled', 'comment' => 'disabled' ) );
+		// aeav_only: the pre-filter itself writes flat (dual_write syncs from native after the write).
+		$mode->setValue( null, array( 'post' => 'aeav_only', 'user' => 'disabled', 'term' => 'disabled', 'comment' => 'disabled' ) );
 		$GLOBALS['_wp_posts'][3]       = (object) array( 'ID' => 3, 'post_type' => 'hp_listing', 'post_status' => 'pending', 'post_date' => '2026-03-01 10:00:00' );
 		$GLOBALS['_wp_current_filter'] = 'update_post_metadata';
 
diff --git a/tests/live/README.md b/tests/live/README.md
index a98aa7c..0025e07 100644
--- a/tests/live/README.md
+++ b/tests/live/README.md
@@ -38,6 +38,7 @@ Exit code 0 when every test passed, 1 otherwise. Output per test:
 |---|---|
 | `shadow_write` | writes in shadow_read reach native meta (user, term, HivePress listing) |
 | `serialized` | array meta values are stored serialized in flat tables, never "Array" |
+| `resync` | dual_write / shadow_read: flat follows native after every write (4 entities × 2 modes: update, same value, delete, refused writes, stale flat, key rename by meta_id) |
 | `zero_side_effect` | zone tables that were never created are never queried on save / delete |
 
 ## Tools (`tools/`)
diff --git a/tests/live/lib/fixtures.php b/tests/live/lib/fixtures.php
index 1c0fe4c..985b0bd 100644
--- a/tests/live/lib/fixtures.php
+++ b/tests/live/lib/fixtures.php
@@ -113,6 +113,29 @@ final class TMDO_Live_Fixtures {
 		return (int) $id;
 	}
 
+	/**
+	 * Create a throwaway comment (on a throwaway post of $post_type).
+	 *
+	 * @param string $post_type    Post type of the parent post.
+	 * @param string $comment_type Comment type.
+	 */
+	public static function comment( string $post_type, string $comment_type = 'comment' ): int {
+		$post_id = self::post( $post_type );
+		$id      = wp_insert_comment(
+			array(
+				'comment_post_ID'  => $post_id,
+				'comment_content'  => 'tmdo-live',
+				'comment_type'     => $comment_type,
+				'comment_approved' => 1,
+			)
+		);
+		if ( ! $id ) {
+			throw new TMDO_Live_Failure( 'fixture comment could not be created' );
+		}
+		self::$created[] = array( 'comment', (int) $id );
+		return (int) $id;
+	}
+
 	/**
 	 * Native meta values for a key, read with SQL (no filters, no cache).
 	 *
@@ -191,6 +214,8 @@ final class TMDO_Live_Fixtures {
 				wp_delete_term( $c[1], $c[2] );
 			} elseif ( 'post' === $c[0] ) {
 				wp_delete_post( $c[1], true );
+			} elseif ( 'comment' === $c[0] ) {
+				wp_delete_comment( $c[1], true );
 			}
 		}
 		self::$created = array();
diff --git a/tests/live/suites/resync.php b/tests/live/suites/resync.php
new file mode 100644
index 0000000..649b815
--- /dev/null
+++ b/tests/live/suites/resync.php
@@ -0,0 +1,112 @@
+<?php
+/**
+ * Suite: in dual_write / shadow_read the flat value always follows native meta.
+ *
+ * Before PR-057 the Hook Bus wrote flat in the pre-filter, before WP decided
+ * whether native changed at all: a non-matching prev_value, add($unique) on an
+ * existing key or a key change via update_metadata_by_mid left flat and native
+ * apart, and nothing healed a flat value made stale by a direct SQL write.
+ *
+ * @package WP_Data_Optimizer
+ */
+
+declare(strict_types=1);
+
+/**
+ * Entity cases: [entity type, key, fixture factory, post type for group lookup].
+ *
+ * @return array<string, array{0:string, 1:string, 2:callable, 3:string}>
+ */
+function tmdo_live_resync_cases(): array {
+	return array(
+		'user first_name'  => array( 'user', 'first_name', array( 'TMDO_Live_Fixtures', 'user' ), '' ),
+		'hp_listing price' => array( 'post', 'hp_price', static fn() => TMDO_Live_Fixtures::post( 'hp_listing' ), 'hp_listing' ),
+		'hp term icon'     => array( 'term', 'hp_icon', static fn() => TMDO_Live_Fixtures::term( 'hp_listing_category' ), '' ),
+		'hp review rating' => array( 'comment', 'hp_rating', static fn() => TMDO_Live_Fixtures::comment( 'hp_listing', 'hp_review' ), '' ),
+	);
+}
+
+/**
+ * Assert flat equals the first native value (numeric-aware; missing native = NULL flat).
+ */
+function tmdo_live_assert_in_step( string $type, int $id, string $key, string $post_type, string $context ): void {
+	$native = TMDO_Live_Fixtures::native( $type, $id, $key );
+	$flat   = TMDO_Live_Fixtures::flat( $type, $id, $key, $post_type );
+	$expect = $native ? $native[0] : null;
+	if ( 'NO-ROW' === $flat ) {
+		$flat = null;
+	}
+	$same = ( null === $expect || null === $flat )
+		? $expect === $flat
+		: ( is_numeric( $expect ) && is_numeric( $flat ) ? (float) $expect === (float) $flat : $expect === $flat );
+	tmdo_live_assert_true( $same, sprintf( '%s — native %s, flat %s', $context, var_export( $expect, true ), var_export( $flat, true ) ) ); // phpcs:ignore WordPress.PHP.DevelopmentFunctions.error_log_var_export
+}
+
+foreach ( array( 'dual_write', 'shadow_read' ) as $tmdo_live_mode ) {
+	foreach ( tmdo_live_resync_cases() as $tmdo_live_label => $tmdo_live_case ) {
+		list( $type, $key, $make, $pt ) = $tmdo_live_case;
+
+		TMDO_Live::test(
+			"{$tmdo_live_mode} {$tmdo_live_label}: update, same value, delete",
+			static function () use ( $tmdo_live_mode, $type, $key, $make, $pt ) {
+				TMDO_Live_Fixtures::field( $type, $key, $pt );
+				TMDO_Live_Fixtures::use_modes( array( $type => $tmdo_live_mode ) );
+				$id = $make();
+
+				update_metadata( $type, $id, $key, '11' );
+				tmdo_live_assert_in_step( $type, $id, $key, $pt, 'new key' );
+				update_metadata( $type, $id, $key, '12' );
+				tmdo_live_assert_in_step( $type, $id, $key, $pt, 'update' );
+				update_metadata( $type, $id, $key, '12' );
+				tmdo_live_assert_in_step( $type, $id, $key, $pt, 'same value' );
+				delete_metadata( $type, $id, $key, '12' );
+				tmdo_live_assert_in_step( $type, $id, $key, $pt, 'delete with value' );
+			}
+		);
+
+		TMDO_Live::test(
+			"{$tmdo_live_mode} {$tmdo_live_label}: writes WP refuses leave flat alone",
+			static function () use ( $tmdo_live_mode, $type, $key, $make, $pt ) {
+				TMDO_Live_Fixtures::field( $type, $key, $pt );
+				TMDO_Live_Fixtures::use_modes( array( $type => $tmdo_live_mode ) );
+				$id = $make();
+				update_metadata( $type, $id, $key, '21' );
+
+				update_metadata( $type, $id, $key, '22', 'no-such-previous-value' );
+				tmdo_live_assert_in_step( $type, $id, $key, $pt, 'non-matching prev_value' );
+				add_metadata( $type, $id, $key, '23', true );
+				tmdo_live_assert_in_step( $type, $id, $key, $pt, 'add unique on existing key' );
+				update_metadata( $type, $id, $key, '24', '21' );
+				tmdo_live_assert_in_step( $type, $id, $key, $pt, 'matching prev_value' );
+			}
+		);
+
+		TMDO_Live::test(
+			"{$tmdo_live_mode} {$tmdo_live_label}: stale flat heals on the next write, key change by meta_id",
+			static function () use ( $tmdo_live_mode, $type, $key, $make, $pt ) {
+				global $wpdb;
+				TMDO_Live_Fixtures::field( $type, $key, $pt );
+				TMDO_Live_Fixtures::use_modes( array( $type => $tmdo_live_mode ) );
+				$id = $make();
+				update_metadata( $type, $id, $key, '31' );
+
+				$table  = _get_meta_table( $type );
+				$id_col = $type . '_id';
+				$pk     = 'user' === $type ? 'umeta_id' : 'meta_id';
+
+				// A direct SQL write (as WooCommerce does for stock) leaves flat stale.
+				// phpcs:ignore WordPress.DB.PreparedSQL.InterpolatedNotPrepared -- WP meta table names.
+				$wpdb->query( $wpdb->prepare( "UPDATE {$table} SET meta_value = '32' WHERE {$id_col} = %d AND meta_key = %s", $id, $key ) );
+				wp_cache_delete( $id, $type . '_meta' );
+				update_metadata( $type, $id, $key, '33' );
+				tmdo_live_assert_in_step( $type, $id, $key, $pt, 'next write after a direct SQL change' );
+
+				// Renaming the key by meta_id removes it from native: flat must follow.
+				// phpcs:ignore WordPress.DB.PreparedSQL.InterpolatedNotPrepared -- WP meta table names.
+				$mid = (int) $wpdb->get_var( $wpdb->prepare( "SELECT {$pk} FROM {$table} WHERE {$id_col} = %d AND meta_key = %s", $id, $key ) );
+				update_metadata_by_mid( $type, $mid, '34', 'tmdo_live_renamed' );
+				tmdo_live_assert_in_step( $type, $id, $key, $pt, 'key renamed by meta_id' );
+			}
+		);
+	}
+}