wp_register_ability returns NULL when called from plugin during `wp_abilities_api_init`

[gelakhubutia]

Description

When trying to register a custom ability from a WordPress plugin using the recommended wp_abilities_api_init hook, wp_register_ability consistently returns NULL even when called correctly during the action.

Environment

• WordPress 6.9.1 (Abilities API built into core)
• MCP Adapter plugin v0.4.1 (installed via Composer with Jetpack autoloader)
• WooCommerce 10.5.2
• PHP 8.x, shared hosting (cPanel)

Steps to Reproduce

1. Install wordpress/mcp-adapter via Composer in wp-content/plugins/
2. Create a plugin with the following code:

add_action( 'wp_abilities_api_init', function() {
    $result = wp_register_ability( 'my-plugin/my-ability', [
        'label'       => 'My Ability',
        'description' => 'Test ability',
        'input_schema' => [ 'type' => 'object', 'properties' => [] ],
        'execute_callback' => function( $input ) { return [ 'success' => true ]; },
        'permission_callback' => function() { return current_user_can( 'manage_options' ); },
        'meta' => [ 'show_in_rest' => true ],
    ]);
    error_log( 'Result: ' . ( $result ? get_class( $result ) : 'NULL' ) );
}, 5 );

add_action( 'plugins_loaded', function() {
    if ( class_exists( 'WP\MCP\Core\McpAdapter' ) ) {
        \WP\MCP\Core\McpAdapter::instance();
    }
}, 1 );

3. Run wp eval 'var_dump(array_keys(wp_get_abilities()));'

Expected Behavior

my-plugin/my-ability should appear in the abilities list.

Actual Behavior

wp_register_ability returns NULL. The hook fires correctly (doing_action('wp_abilities_api_init') returns true), but the registration silently fails.

Debug Findings

• doing_action('wp_abilities_api_init') returns YES inside the callback ✅
• wp_register_ability is loaded from wp-includes/abilities-api.php (core) ✅
• McpAdapter::instance() successfully initializes ✅
• WooCommerce is also hooked into wp_abilities_api_init and its abilities register fine
• The registry appears to already be initialized before our plugin's callback runs, possibly due to WooCommerce triggering WP_Abilities_Registry::get_instance() first

Question

Is there a timing conflict between WooCommerce's bundled abilities-api vendor copy and WordPress core's native implementation in 6.9? Is there a recommended way to register abilities from a plugin when WooCommerce is also present?

[flodufour]

Hey @gelakhubutia,

From the doc :

WordPress abilities are NOT accessible via default MCP server by default. To make an ability available through the default MCP server, you must explicitly add mcp.public: true to the ability's metadata.

'meta' => [
    'mcp' => [
        'public' => true,  // Required for MCP access
        'type'   => 'tool' // Optional: 'tool' (default), 'resource', or 'prompt'
    ],
    'annotations' => [...] // Optional MCP annotations
]

Could this be the solution to your problem ?