diff --git a/tests/Block_HTML_Sync_FiltersTest.php b/tests/Block_HTML_Sync_FiltersTest.php
new file mode 100644
index 000000000..de0b62144
--- /dev/null
+++ b/tests/Block_HTML_Sync_FiltersTest.php
@@ -0,0 +1,375 @@
+<?php
+/**
+ * Tests for Block_HTML_Sync_Filters class.
+ *
+ * @package WP_CLI\Entity
+ */
+
+namespace WP_CLI\Entity\Tests;
+
+use PHPUnit\Framework\TestCase;
+use WP_CLI\Entity\Block_HTML_Sync_Filters;
+
+/**
+ * Test the Block_HTML_Sync_Filters class.
+ */
+class Block_HTML_Sync_FiltersTest extends TestCase {
+
+	/**
+	 * Reset global filter log before each test.
+	 */
+	protected function setUp(): void {
+		parent::setUp();
+		global $wpcli_entity_filter_test_log;
+		$wpcli_entity_filter_test_log = [];
+	}
+
+	// =========================================================================
+	// Tests for sync_heading_level()
+	// =========================================================================
+
+	/**
+	 * Test that sync_heading_level updates heading tags across levels.
+	 */
+	public function test_sync_heading_level_updates_tags() {
+		$transitions = [
+			'h2 to h3' => [ 2, 3, '<h2>My Heading</h2>', '<h3>My Heading</h3>' ],
+			'h3 to h1' => [ 3, 1, '<h3>Main Title</h3>', '<h1>Main Title</h1>' ],
+			'h4 to h6' => [ 4, 6, '<h4>Sub Sub Heading</h4>', '<h6>Sub Sub Heading</h6>' ],
+			'h6 to h2' => [ 6, 2, '<h6>Deep Section</h6>', '<h2>Deep Section</h2>' ],
+			'h1 to h5' => [ 1, 5, '<h1>Document Title</h1>', '<h5>Document Title</h5>' ],
+		];
+
+		foreach ( $transitions as $desc => $data ) {
+			list( $from_level, $to_level, $inner_html, $expected ) = $data;
+
+			$block = [
+				'blockName'    => 'core/heading',
+				'attrs'        => [ 'level' => $from_level ],
+				'innerHTML'    => $inner_html,
+				'innerContent' => [ $inner_html ],
+			];
+
+			$result = Block_HTML_Sync_Filters::sync_heading_level( $block, [ 'level' => $to_level ], 'core/heading' );
+
+			$this->assertSame( $expected, $result['innerHTML'], "Failed asserting innerHTML on {$desc}" );
+			$this->assertSame( [ $expected ], $result['innerContent'], "Failed asserting innerContent on {$desc}" );
+		}
+	}
+
+	/**
+	 * Test that sync_heading_level preserves HTML attributes on the tag.
+	 */
+	public function test_sync_heading_level_preserves_attributes() {
+		$inner_html = '<h2 class="has-text-align-center wp-block-heading" id="custom-anchor" style="color:#ff0000">Heading Text</h2>';
+		$expected   = '<h4 class="has-text-align-center wp-block-heading" id="custom-anchor" style="color:#ff0000">Heading Text</h4>';
+
+		$block = [
+			'blockName'    => 'core/heading',
+			'attrs'        => [ 'level' => 2 ],
+			'innerHTML'    => $inner_html,
+			'innerContent' => [ $inner_html ],
+		];
+
+		$result = Block_HTML_Sync_Filters::sync_heading_level( $block, [ 'level' => 4 ], 'core/heading' );
+
+		$this->assertSame( $expected, $result['innerHTML'] );
+		$this->assertSame( [ $expected ], $result['innerContent'] );
+	}
+
+	/**
+	 * Test that sync_heading_level preserves nested formatting tags and text.
+	 */
+	public function test_sync_heading_level_preserves_nested_content() {
+		$inner_html = '<h2>Hello <strong>World</strong> with <a href="https://wordpress.org">WordPress</a></h2>';
+		$expected   = '<h3>Hello <strong>World</strong> with <a href="https://wordpress.org">WordPress</a></h3>';
+
+		$block = [
+			'blockName'    => 'core/heading',
+			'attrs'        => [ 'level' => 2 ],
+			'innerHTML'    => $inner_html,
+			'innerContent' => [ $inner_html ],
+		];
+
+		$result = Block_HTML_Sync_Filters::sync_heading_level( $block, [ 'level' => 3 ], 'core/heading' );
+
+		$this->assertSame( $expected, $result['innerHTML'] );
+		$this->assertSame( [ $expected ], $result['innerContent'] );
+	}
+
+	/**
+	 * Test that sync_heading_level ignores non-heading blocks.
+	 */
+	public function test_sync_heading_level_ignores_non_heading_block() {
+		$block = [
+			'blockName'    => 'core/paragraph',
+			'attrs'        => [],
+			'innerHTML'    => '<p>Some paragraph text</p>',
+			'innerContent' => [ '<p>Some paragraph text</p>' ],
+		];
+
+		$result = Block_HTML_Sync_Filters::sync_heading_level( $block, [ 'level' => 3 ], 'core/paragraph' );
+
+		$this->assertSame( $block, $result );
+	}
+
+	/**
+	 * Test that sync_heading_level returns unmodified block when level attribute is missing.
+	 */
+	public function test_sync_heading_level_ignores_missing_level_attribute() {
+		$block = [
+			'blockName'    => 'core/heading',
+			'attrs'        => [ 'level' => 2 ],
+			'innerHTML'    => '<h2>Heading</h2>',
+			'innerContent' => [ '<h2>Heading</h2>' ],
+		];
+
+		$result = Block_HTML_Sync_Filters::sync_heading_level( $block, [ 'content' => 'Updated text' ], 'core/heading' );
+
+		$this->assertSame( $block, $result );
+	}
+
+	/**
+	 * Test that sync_heading_level ignores out-of-range heading levels (< 1 or > 6).
+	 */
+	public function test_sync_heading_level_ignores_invalid_levels() {
+		$invalid_levels = [ 0, -1, 7, 10 ];
+
+		foreach ( $invalid_levels as $invalid_level ) {
+			$block = [
+				'blockName'    => 'core/heading',
+				'attrs'        => [ 'level' => 2 ],
+				'innerHTML'    => '<h2>Heading</h2>',
+				'innerContent' => [ '<h2>Heading</h2>' ],
+			];
+
+			$result = Block_HTML_Sync_Filters::sync_heading_level( $block, [ 'level' => $invalid_level ], 'core/heading' );
+
+			$this->assertSame( $block, $result, "Failed on invalid level {$invalid_level}" );
+		}
+	}
+
+	/**
+	 * Test that sync_heading_level handles empty innerHTML gracefully.
+	 */
+	public function test_sync_heading_level_handles_empty_inner_html() {
+		$block_empty_string = [
+			'blockName'    => 'core/heading',
+			'attrs'        => [ 'level' => 2 ],
+			'innerHTML'    => '',
+			'innerContent' => [],
+		];
+
+		$result = Block_HTML_Sync_Filters::sync_heading_level( $block_empty_string, [ 'level' => 3 ], 'core/heading' );
+		$this->assertSame( $block_empty_string, $result );
+
+		$block_missing_key = [
+			'blockName' => 'core/heading',
+			'attrs'     => [ 'level' => 2 ],
+		];
+
+		$result_missing = Block_HTML_Sync_Filters::sync_heading_level( $block_missing_key, [ 'level' => 3 ], 'core/heading' );
+		$this->assertSame( $block_missing_key, $result_missing );
+	}
+
+	/**
+	 * Test that sync_heading_level leaves block intact when new level is identical to current.
+	 */
+	public function test_sync_heading_level_unchanged_when_same_level() {
+		$inner_html = '<h2>Current Level</h2>';
+		$block      = [
+			'blockName'    => 'core/heading',
+			'attrs'        => [ 'level' => 2 ],
+			'innerHTML'    => $inner_html,
+			'innerContent' => [ $inner_html ],
+		];
+
+		$result = Block_HTML_Sync_Filters::sync_heading_level( $block, [ 'level' => 2 ], 'core/heading' );
+
+		$this->assertSame( $inner_html, $result['innerHTML'] );
+		$this->assertSame( [ $inner_html ], $result['innerContent'] );
+	}
+
+	// =========================================================================
+	// Tests for sync_list_type()
+	// =========================================================================
+
+	/**
+	 * Test that sync_list_type converts unordered list to ordered list.
+	 */
+	public function test_sync_list_type_converts_unordered_to_ordered() {
+		$inner_html = '<ul><li>First</li><li>Second</li></ul>';
+		$expected   = '<ol><li>First</li><li>Second</li></ol>';
+
+		$block = [
+			'blockName'    => 'core/list',
+			'attrs'        => [ 'ordered' => false ],
+			'innerHTML'    => $inner_html,
+			'innerContent' => [ $inner_html ],
+		];
+
+		$result = Block_HTML_Sync_Filters::sync_list_type( $block, [ 'ordered' => true ], 'core/list' );
+
+		$this->assertSame( $expected, $result['innerHTML'] );
+		$this->assertSame( [ $expected ], $result['innerContent'] );
+	}
+
+	/**
+	 * Test that sync_list_type converts ordered list to unordered list.
+	 */
+	public function test_sync_list_type_converts_ordered_to_unordered() {
+		$inner_html = '<ol><li>First</li><li>Second</li></ol>';
+		$expected   = '<ul><li>First</li><li>Second</li></ul>';
+
+		$block = [
+			'blockName'    => 'core/list',
+			'attrs'        => [ 'ordered' => true ],
+			'innerHTML'    => $inner_html,
+			'innerContent' => [ $inner_html ],
+		];
+
+		$result = Block_HTML_Sync_Filters::sync_list_type( $block, [ 'ordered' => false ], 'core/list' );
+
+		$this->assertSame( $expected, $result['innerHTML'] );
+		$this->assertSame( [ $expected ], $result['innerContent'] );
+	}
+
+	/**
+	 * Test that sync_list_type preserves list attributes.
+	 */
+	public function test_sync_list_type_preserves_attributes() {
+		$inner_html = '<ul class="wp-block-list is-style-checkmark" id="my-list" data-role="checklist"><li>Item</li></ul>';
+		$expected   = '<ol class="wp-block-list is-style-checkmark" id="my-list" data-role="checklist"><li>Item</li></ol>';
+
+		$block = [
+			'blockName'    => 'core/list',
+			'attrs'        => [ 'ordered' => false ],
+			'innerHTML'    => $inner_html,
+			'innerContent' => [ $inner_html ],
+		];
+
+		$result = Block_HTML_Sync_Filters::sync_list_type( $block, [ 'ordered' => true ], 'core/list' );
+
+		$this->assertSame( $expected, $result['innerHTML'] );
+		$this->assertSame( [ $expected ], $result['innerContent'] );
+	}
+
+	/**
+	 * Test that sync_list_type preserves nested formatting and items.
+	 */
+	public function test_sync_list_type_preserves_nested_items() {
+		$inner_html = '<ul><li>Item 1 with <a href="#">link</a></li><li>Item 2 with <strong>bold</strong></li></ul>';
+		$expected   = '<ol><li>Item 1 with <a href="#">link</a></li><li>Item 2 with <strong>bold</strong></li></ol>';
+
+		$block = [
+			'blockName'    => 'core/list',
+			'attrs'        => [ 'ordered' => false ],
+			'innerHTML'    => $inner_html,
+			'innerContent' => [ $inner_html ],
+		];
+
+		$result = Block_HTML_Sync_Filters::sync_list_type( $block, [ 'ordered' => true ], 'core/list' );
+
+		$this->assertSame( $expected, $result['innerHTML'] );
+		$this->assertSame( [ $expected ], $result['innerContent'] );
+	}
+
+	/**
+	 * Test that sync_list_type ignores non-list blocks.
+	 */
+	public function test_sync_list_type_ignores_non_list_block() {
+		$block = [
+			'blockName'    => 'core/paragraph',
+			'attrs'        => [],
+			'innerHTML'    => '<p>Some text</p>',
+			'innerContent' => [ '<p>Some text</p>' ],
+		];
+
+		$result = Block_HTML_Sync_Filters::sync_list_type( $block, [ 'ordered' => true ], 'core/paragraph' );
+
+		$this->assertSame( $block, $result );
+	}
+
+	/**
+	 * Test that sync_list_type returns unmodified block when ordered attribute is missing.
+	 */
+	public function test_sync_list_type_ignores_missing_ordered_attribute() {
+		$block = [
+			'blockName'    => 'core/list',
+			'attrs'        => [ 'ordered' => false ],
+			'innerHTML'    => '<ul><li>Item</li></ul>',
+			'innerContent' => [ '<ul><li>Item</li></ul>' ],
+		];
+
+		$result = Block_HTML_Sync_Filters::sync_list_type( $block, [ 'reversed' => true ], 'core/list' );
+
+		$this->assertSame( $block, $result );
+	}
+
+	/**
+	 * Test that sync_list_type handles empty innerHTML gracefully.
+	 */
+	public function test_sync_list_type_handles_empty_inner_html() {
+		$block_empty_string = [
+			'blockName'    => 'core/list',
+			'attrs'        => [ 'ordered' => false ],
+			'innerHTML'    => '',
+			'innerContent' => [],
+		];
+
+		$result = Block_HTML_Sync_Filters::sync_list_type( $block_empty_string, [ 'ordered' => true ], 'core/list' );
+		$this->assertSame( $block_empty_string, $result );
+
+		$block_missing_key = [
+			'blockName' => 'core/list',
+			'attrs'     => [ 'ordered' => false ],
+		];
+
+		$result_missing = Block_HTML_Sync_Filters::sync_list_type( $block_missing_key, [ 'ordered' => true ], 'core/list' );
+		$this->assertSame( $block_missing_key, $result_missing );
+	}
+
+	/**
+	 * Test that sync_list_type returns unchanged block when list type matches requested state.
+	 */
+	public function test_sync_list_type_unchanged_when_same_state() {
+		$inner_html = '<ul><li>Item</li></ul>';
+		$block      = [
+			'blockName'    => 'core/list',
+			'attrs'        => [ 'ordered' => false ],
+			'innerHTML'    => $inner_html,
+			'innerContent' => [ $inner_html ],
+		];
+
+		$result = Block_HTML_Sync_Filters::sync_list_type( $block, [ 'ordered' => false ], 'core/list' );
+
+		$this->assertSame( $inner_html, $result['innerHTML'] );
+		$this->assertSame( [ $inner_html ], $result['innerContent'] );
+	}
+
+	// =========================================================================
+	// Tests for register()
+	// =========================================================================
+
+	/**
+	 * Test that register() hooks both sync callbacks to wp_cli_post_block_update_html.
+	 */
+	public function test_register_hooks() {
+		global $wpcli_entity_filter_test_log;
+
+		Block_HTML_Sync_Filters::register();
+
+		$this->assertIsArray( $wpcli_entity_filter_test_log );
+		$this->assertCount( 2, $wpcli_entity_filter_test_log );
+
+		$this->assertSame( 'wp_cli_post_block_update_html', $wpcli_entity_filter_test_log[0]['hook_name'] );
+		$this->assertSame( [ Block_HTML_Sync_Filters::class, 'sync_heading_level' ], $wpcli_entity_filter_test_log[0]['callback'] );
+		$this->assertSame( 10, $wpcli_entity_filter_test_log[0]['priority'] );
+		$this->assertSame( 3, $wpcli_entity_filter_test_log[0]['accepted_args'] );
+
+		$this->assertSame( 'wp_cli_post_block_update_html', $wpcli_entity_filter_test_log[1]['hook_name'] );
+		$this->assertSame( [ Block_HTML_Sync_Filters::class, 'sync_list_type' ], $wpcli_entity_filter_test_log[1]['callback'] );
+		$this->assertSame( 10, $wpcli_entity_filter_test_log[1]['priority'] );
+		$this->assertSame( 3, $wpcli_entity_filter_test_log[1]['accepted_args'] );
+	}
+}
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index 8a813c5fa..afacbde1c 100644
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -14,6 +14,33 @@
 	require_once $autoloader;
 }
 
+// Global stub for add_filter when running unit tests outside WordPress.
+if ( ! function_exists( 'add_filter' ) ) {
+	/**
+	 * Stub add_filter for isolated unit testing.
+	 *
+	 * @param string   $hook_name     The filter hook name.
+	 * @param callable $callback      The callback function.
+	 * @param int      $priority      Optional. Priority. Default 10.
+	 * @param int      $accepted_args Optional. Accepted arguments. Default 1.
+	 * @return true
+	 */
+	// phpcs:ignore WordPress.NamingConventions.PrefixAllGlobals.NonPrefixedFunctionFound -- Mocking WordPress core function for isolated testing.
+	function add_filter( $hook_name, $callback, $priority = 10, $accepted_args = 1 ) {
+		global $wpcli_entity_filter_test_log;
+		if ( ! is_array( $wpcli_entity_filter_test_log ) ) {
+			$wpcli_entity_filter_test_log = [];
+		}
+		$wpcli_entity_filter_test_log[] = [
+			'hook_name'     => $hook_name,
+			'callback'      => $callback,
+			'priority'      => $priority,
+			'accepted_args' => $accepted_args,
+		];
+		return true;
+	}
+}
+
 // Load the polyfills - these are what we're testing.
 // Note: We load polyfills.php directly to ensure our polyfill functions are defined
 // even if the native PHP functions exist.