diff --git a/assets/src/components/reader-theme-carousel/index.js b/assets/src/components/reader-theme-carousel/index.js
index f5b23a5506f..de974d31a35 100644
--- a/assets/src/components/reader-theme-carousel/index.js
+++ b/assets/src/components/reader-theme-carousel/index.js
@@ -1,8 +1,3 @@
-/**
- * External dependencies
- */
-import { AMP_QUERY_VAR, DEFAULT_AMP_QUERY_VAR, AMP_QUERY_VAR_CUSTOMIZED_LATE } from 'amp-settings'; // From WP inline script.
-
 /**
  * WordPress dependencies
  */
@@ -155,25 +150,7 @@ export function ReaderThemeCarousel() {
 					hasUnavailableThemes && (
 						<AMPNotice type={ NOTICE_TYPE_INFO }>
 							<p>
-								{ AMP_QUERY_VAR_CUSTOMIZED_LATE
-								/* dangerouslySetInnerHTML reason: Injection of code tags. */
-									? (
-										<span
-											dangerouslySetInnerHTML={ {
-												__html: sprintf(
-													/* translators: 1: customized AMP query var, 2: default query var, 3: the AMP_QUERY_VAR constant name, 4: the amp_query_var filter, 5: the plugins_loaded action */
-													__( 'Some of the themes below are not available because your site (probably the active theme) has customized the AMP query var too late (it is set to %1$s as opposed to the default of %2$s). Please make sure that any customizations done by defining the %3$s constant or adding an %4$s filter are done before the %5$s action with priority 8.', 'amp' ),
-													`<code>${ AMP_QUERY_VAR }</code>`,
-													`<code>${ DEFAULT_AMP_QUERY_VAR }</code>`,
-													'<code>AMP_QUERY_VAR</code>',
-													'<code>amp_query_var</code>',
-													'<code>plugins_loaded</code>',
-												),
-											} }
-										/>
-									)
-									: __( 'Some supported themes cannot be installed automatically on this site. To use, please install them manually or contact your hosting provider.', 'amp' )
-								}
+								{ __( 'Some supported themes cannot be installed automatically on this site. To use, please install them manually or contact your hosting provider.', 'amp' ) }
 							</p>
 							<AMPSettingToggle
 								text={ __( 'Show unavailable themes', 'amp' ) }
diff --git a/assets/src/components/reader-theme-selection/index.js b/assets/src/components/reader-theme-selection/index.js
index 120dc2c471a..79cfc42d649 100644
--- a/assets/src/components/reader-theme-selection/index.js
+++ b/assets/src/components/reader-theme-selection/index.js
@@ -2,12 +2,11 @@
  * External dependencies
  */
 import PropTypes from 'prop-types';
-import { AMP_QUERY_VAR, DEFAULT_AMP_QUERY_VAR, AMP_QUERY_VAR_CUSTOMIZED_LATE } from 'amp-settings'; // From WP inline script.
 
 /**
  * WordPress dependencies
  */
-import { __, sprintf } from '@wordpress/i18n';
+import { __ } from '@wordpress/i18n';
 import { useContext } from '@wordpress/element';
 
 /**
@@ -56,25 +55,7 @@ export function ReaderThemeSelection() {
 							{ __( 'Unavailable themes', 'amp' ) }
 						</h3>
 						<p>
-							{ AMP_QUERY_VAR_CUSTOMIZED_LATE
-								/* dangerouslySetInnerHTML reason: Injection of code tags. */
-								? (
-									<span
-										dangerouslySetInnerHTML={ {
-											__html: sprintf(
-												/* translators: 1: customized AMP query var, 2: default query var, 3: the AMP_QUERY_VAR constant name, 4: the amp_query_var filter, 5: the plugins_loaded action */
-												__( 'The following themes are not available because your site (probably the active theme) has customized the AMP query var too late (it is set to %1$s as opposed to the default of %2$s). Please make sure that any customizations done by defining the %3$s constant or adding an %4$s filter are done before the %5$s action with priority 8.', 'amp' ),
-												`<code>${ AMP_QUERY_VAR }</code>`,
-												`<code>${ DEFAULT_AMP_QUERY_VAR }</code>`,
-												'<code>AMP_QUERY_VAR</code>',
-												'<code>amp_query_var</code>',
-												'<code>plugins_loaded</code>',
-											),
-										} }
-									/>
-								)
-								: __( 'The following themes are compatible but cannot be installed automatically. Please install them manually, or contact your host if you are not able to do so.', 'amp' )
-							}
+							{ __( 'The following themes are compatible but cannot be installed automatically. Please install them manually, or contact your host if you are not able to do so.', 'amp' ) }
 						</p>
 						<ul className="choose-reader-theme__grid">
 							{ unavailableThemes.map( ( theme ) => (
diff --git a/assets/src/components/reader-themes-context-provider/index.js b/assets/src/components/reader-themes-context-provider/index.js
index f8d7ace506e..55308fc5902 100644
--- a/assets/src/components/reader-themes-context-provider/index.js
+++ b/assets/src/components/reader-themes-context-provider/index.js
@@ -9,7 +9,7 @@ import { __ } from '@wordpress/i18n';
  * External dependencies
  */
 import PropTypes from 'prop-types';
-import { AMP_QUERY_VAR_CUSTOMIZED_LATE, USING_FALLBACK_READER_THEME, LEGACY_THEME_SLUG } from 'amp-settings';
+import { USING_FALLBACK_READER_THEME, LEGACY_THEME_SLUG } from 'amp-settings';
 
 /**
  * Internal dependencies
@@ -282,7 +282,7 @@ export function ReaderThemesContextProvider( { wpAjaxUrl, children, currentTheme
 	const { availableThemes, unavailableThemes } = useMemo(
 		() => ( filteredThemes || [] ).reduce(
 			( collections, theme ) => {
-				if ( ( AMP_QUERY_VAR_CUSTOMIZED_LATE && theme.slug !== LEGACY_THEME_SLUG ) || theme.availability === 'non-installable' ) {
+				if ( theme.availability === 'non-installable' ) {
 					collections.unavailableThemes.push( theme );
 				} else {
 					collections.availableThemes.push( theme );
diff --git a/assets/src/onboarding-wizard/pages/choose-reader-theme/index.js b/assets/src/onboarding-wizard/pages/choose-reader-theme/index.js
index cf71ffb174e..53975f2e104 100644
--- a/assets/src/onboarding-wizard/pages/choose-reader-theme/index.js
+++ b/assets/src/onboarding-wizard/pages/choose-reader-theme/index.js
@@ -1,8 +1,3 @@
-/**
- * External dependencies
- */
-import { LEGACY_THEME_SLUG, AMP_QUERY_VAR_CUSTOMIZED_LATE } from 'amp-settings'; // From WP inline script.
-
 /**
  * WordPress dependencies
  */
@@ -35,9 +30,7 @@ export function ChooseReaderTheme() {
 			themes &&
 			readerTheme &&
 			canGoForward === false &&
-			! AMP_QUERY_VAR_CUSTOMIZED_LATE
-				? themes.map( ( { slug } ) => slug ).includes( readerTheme )
-				: readerTheme === LEGACY_THEME_SLUG
+			themes.map( ( { slug } ) => slug ).includes( readerTheme )
 		) {
 			setCanGoForward( true );
 		}
diff --git a/includes/amp-helper-functions.php b/includes/amp-helper-functions.php
index 8c33b5538c2..54e52e1ae09 100644
--- a/includes/amp-helper-functions.php
+++ b/includes/amp-helper-functions.php
@@ -6,6 +6,7 @@
  */
 
 use AmpProject\AmpWP\Admin\ReaderThemes;
+use AmpProject\AmpWP\AmpSlugCustomizationWatcher;
 use AmpProject\AmpWP\AmpWpPluginFactory;
 use AmpProject\AmpWP\Exception\InvalidService;
 use AmpProject\AmpWP\Icon;
@@ -531,10 +532,23 @@ function _amp_bootstrap_customizer() {
  * may be deprecated in the future. Normally the slug should be just 'amp'.
  *
  * @since 0.7
+ * @since 2.1 Added $ignore_late_defined_slug argument.
  *
+ * @param bool $ignore_late_defined_slug Whether to ignore the late defined slug.
  * @return string Slug used for query var, endpoint, and post type support.
  */
-function amp_get_slug() {
+function amp_get_slug( $ignore_late_defined_slug = false ) {
+
+	// When a slug was defined late according to AmpSlugCustomizationWatcher, the slug will be stored in the
+	// LATE_DEFINED_SLUG option by the PairedRouting service so that it can be used early. This is only needed until
+	// the after_setup_theme action fires, because at that time the late-defined slug will have been established.
+	if ( ! $ignore_late_defined_slug && ! did_action( AmpSlugCustomizationWatcher::LATE_DETERMINATION_ACTION ) ) {
+		$slug = AMP_Options_Manager::get_option( Option::LATE_DEFINED_SLUG );
+		if ( ! empty( $slug ) && is_string( $slug ) ) {
+			return $slug;
+		}
+	}
+
 	/**
 	 * Filter the AMP query variable.
 	 *
diff --git a/includes/class-amp-theme-support.php b/includes/class-amp-theme-support.php
index ddfc6ccde19..069511c8074 100644
--- a/includes/class-amp-theme-support.php
+++ b/includes/class-amp-theme-support.php
@@ -833,6 +833,7 @@ public static function add_hooks() {
 		if ( has_action( 'wp_head', 'print_emoji_detection_script' ) ) {
 			remove_action( 'wp_head', 'print_emoji_detection_script', 7 );
 			remove_action( 'wp_print_styles', 'print_emoji_styles' );
+			remove_action( 'wp_footer', 'gutenberg_the_skip_link' ); // Temporary workaround for <https://github.com/ampproject/amp-wp/issues/6115>.
 			add_action( 'wp_print_styles', [ __CLASS__, 'print_emoji_styles' ] );
 			add_filter( 'the_title', 'wp_staticize_emoji' );
 			add_filter( 'the_excerpt', 'wp_staticize_emoji' );
diff --git a/includes/options/class-amp-options-manager.php b/includes/options/class-amp-options-manager.php
index 21bd8eb7a40..cfb4b9d36b9 100644
--- a/includes/options/class-amp-options-manager.php
+++ b/includes/options/class-amp-options-manager.php
@@ -60,28 +60,6 @@ public static function register_settings() {
 				'sanitize_callback' => [ __CLASS__, 'validate_options' ],
 			]
 		);
-
-		add_action( 'update_option_' . self::OPTION_NAME, [ __CLASS__, 'maybe_flush_rewrite_rules' ], 10, 2 );
-	}
-
-	/**
-	 * Flush rewrite rules if the supported_post_types have changed.
-	 *
-	 * @since 0.6.2
-	 *
-	 * @param array $old_options Old options.
-	 * @param array $new_options New options.
-	 */
-	public static function maybe_flush_rewrite_rules( $old_options, $new_options ) {
-		$old_post_types = isset( $old_options[ Option::SUPPORTED_POST_TYPES ] ) ? $old_options[ Option::SUPPORTED_POST_TYPES ] : [];
-		$new_post_types = isset( $new_options[ Option::SUPPORTED_POST_TYPES ] ) ? $new_options[ Option::SUPPORTED_POST_TYPES ] : [];
-		sort( $old_post_types );
-		sort( $new_post_types );
-		if ( $old_post_types !== $new_post_types ) {
-			// Flush rewrite rules.
-			add_rewrite_endpoint( amp_get_slug(), EP_PERMALINK );
-			flush_rewrite_rules( false );
-		}
 	}
 
 	/**
diff --git a/src/Admin/OnboardingWizardSubmenuPage.php b/src/Admin/OnboardingWizardSubmenuPage.php
index 9a18557a908..5033bb291be 100644
--- a/src/Admin/OnboardingWizardSubmenuPage.php
+++ b/src/Admin/OnboardingWizardSubmenuPage.php
@@ -220,8 +220,6 @@ public function enqueue_assets( $hook_suffix ) {
 		$setup_wizard_data = [
 			'AMP_OPTIONS_KEY'                    => AMP_Options_Manager::OPTION_NAME,
 			'AMP_QUERY_VAR'                      => amp_get_slug(),
-			'DEFAULT_AMP_QUERY_VAR'              => QueryVar::AMP,
-			'AMP_QUERY_VAR_CUSTOMIZED_LATE'      => $amp_slug_customization_watcher->did_customize_late(),
 			'LEGACY_THEME_SLUG'                  => ReaderThemes::DEFAULT_READER_THEME,
 			'APP_ROOT_ID'                        => self::APP_ROOT_ID,
 			'CUSTOMIZER_LINK'                    => add_query_arg(
diff --git a/src/Admin/OptionsMenu.php b/src/Admin/OptionsMenu.php
index fb78c759b39..0883d541e5d 100644
--- a/src/Admin/OptionsMenu.php
+++ b/src/Admin/OptionsMenu.php
@@ -214,28 +214,26 @@ public function enqueue_assets( $hook_suffix ) {
 		$is_reader_theme = $this->reader_themes->theme_data_exists( get_stylesheet() );
 
 		$js_data = [
-			'AMP_QUERY_VAR'                 => amp_get_slug(),
-			'DEFAULT_AMP_QUERY_VAR'         => QueryVar::AMP,
-			'AMP_QUERY_VAR_CUSTOMIZED_LATE' => $amp_slug_customization_watcher->did_customize_late(),
-			'CURRENT_THEME'                 => [
+			'AMP_QUERY_VAR'               => amp_get_slug(),
+			'CURRENT_THEME'               => [
 				'name'            => $theme->get( 'Name' ),
 				'description'     => $theme->get( 'Description' ),
 				'is_reader_theme' => $is_reader_theme,
 				'screenshot'      => $theme->get_screenshot() ?: null,
 				'url'             => $theme->get( 'ThemeURI' ),
 			],
-			'OPTIONS_REST_PATH'             => '/amp/v1/options',
-			'READER_THEMES_REST_PATH'       => '/amp/v1/reader-themes',
-			'IS_CORE_THEME'                 => in_array(
+			'OPTIONS_REST_PATH'           => '/amp/v1/options',
+			'READER_THEMES_REST_PATH'     => '/amp/v1/reader-themes',
+			'IS_CORE_THEME'               => in_array(
 				get_stylesheet(),
 				AMP_Core_Theme_Sanitizer::get_supported_themes(),
 				true
 			),
-			'LEGACY_THEME_SLUG'             => ReaderThemes::DEFAULT_READER_THEME,
-			'USING_FALLBACK_READER_THEME'   => $this->reader_themes->using_fallback_theme(),
-			'THEME_SUPPORT_ARGS'            => AMP_Theme_Support::get_theme_support_args(),
-			'THEME_SUPPORTS_READER_MODE'    => AMP_Theme_Support::supports_reader_mode(),
-			'UPDATES_NONCE'                 => wp_create_nonce( 'updates' ),
+			'LEGACY_THEME_SLUG'           => ReaderThemes::DEFAULT_READER_THEME,
+			'USING_FALLBACK_READER_THEME' => $this->reader_themes->using_fallback_theme(),
+			'THEME_SUPPORT_ARGS'          => AMP_Theme_Support::get_theme_support_args(),
+			'THEME_SUPPORTS_READER_MODE'  => AMP_Theme_Support::supports_reader_mode(),
+			'UPDATES_NONCE'               => wp_create_nonce( 'updates' ),
 		];
 
 		wp_add_inline_script(
diff --git a/src/Admin/SiteHealth.php b/src/Admin/SiteHealth.php
index 7a4d6d44d8f..d4b3e4df694 100644
--- a/src/Admin/SiteHealth.php
+++ b/src/Admin/SiteHealth.php
@@ -10,12 +10,14 @@
 use AMP_Options_Manager;
 use AMP_Theme_Support;
 use AMP_Post_Type_Support;
+use AmpProject\AmpWP\AmpSlugCustomizationWatcher;
 use AmpProject\AmpWP\BackgroundTask\MonitorCssTransientCaching;
 use AmpProject\AmpWP\Infrastructure\Conditional;
 use AmpProject\AmpWP\Infrastructure\Delayed;
 use AmpProject\AmpWP\Infrastructure\Registerable;
 use AmpProject\AmpWP\Infrastructure\Service;
 use AmpProject\AmpWP\Option;
+use AmpProject\AmpWP\QueryVar;
 
 /**
  * Class SiteHealth
@@ -34,6 +36,13 @@ final class SiteHealth implements Service, Registerable, Delayed, Conditional {
 	 */
 	private $css_transient_caching;
 
+	/**
+	 * Service that checks when the AMP slug was defined.
+	 *
+	 * @var AmpSlugCustomizationWatcher
+	 */
+	private $amp_slug_customization_watcher;
+
 	/**
 	 * Check whether the conditional object is currently needed.
 	 *
@@ -55,10 +64,12 @@ public static function get_registration_action() {
 	/**
 	 * SiteHealth constructor.
 	 *
-	 * @param MonitorCssTransientCaching $css_transient_caching CSS transient caching monitoring service.
+	 * @param MonitorCssTransientCaching  $css_transient_caching          CSS transient caching monitoring service.
+	 * @param AmpSlugCustomizationWatcher $amp_slug_customization_watcher AMP slug customization watcher.
 	 */
-	public function __construct( MonitorCssTransientCaching $css_transient_caching ) {
-		$this->css_transient_caching = $css_transient_caching;
+	public function __construct( MonitorCssTransientCaching $css_transient_caching, AmpSlugCustomizationWatcher $amp_slug_customization_watcher ) {
+		$this->css_transient_caching          = $css_transient_caching;
+		$this->amp_slug_customization_watcher = $amp_slug_customization_watcher;
 	}
 
 	/**
@@ -72,6 +83,15 @@ public function register() {
 		add_action( 'admin_print_styles-site-health.php', [ $this, 'add_styles' ] );
 	}
 
+	/**
+	 * Get badge label.
+	 *
+	 * @return string AMP.
+	 */
+	private function get_badge_label() {
+		return esc_html__( 'AMP', 'amp' );
+	}
+
 	/**
 	 * Adds Site Health tests related to this plugin.
 	 *
@@ -83,7 +103,13 @@ public function add_tests( $tests ) {
 			'label' => esc_html__( 'Persistent object cache', 'amp' ),
 			'test'  => [ $this, 'persistent_object_cache' ],
 		];
-		$tests['direct']['amp_curl_multi_functions']    = [
+		if ( ! amp_is_canonical() && QueryVar::AMP !== amp_get_slug() ) {
+			$tests['direct']['amp_slug_definition_timing'] = [
+				'label' => esc_html__( 'AMP slug (query var) definition timing', 'amp' ),
+				'test'  => [ $this, 'slug_definition_timing' ],
+			];
+		}
+		$tests['direct']['amp_curl_multi_functions'] = [
 			'label' => esc_html__( 'cURL multi functions', 'amp' ),
 			'test'  => [ $this, 'curl_multi_functions' ],
 		];
@@ -129,28 +155,69 @@ private function get_persistent_object_cache_learn_more_action() {
 	 */
 	public function persistent_object_cache() {
 		$is_using_object_cache = wp_using_ext_object_cache();
-		$data                  = [
+
+		return [
 			'badge'       => [
-				'label' => esc_html__( 'AMP', 'amp' ),
+				'label' => $this->get_badge_label(),
 				'color' => $is_using_object_cache ? 'green' : 'orange',
 			],
 			'description' => esc_html__( 'The AMP plugin performs at its best when persistent object cache is enabled. Object caching is used to more effectively store image dimensions and parsed CSS.', 'amp' ),
 			'actions'     => $this->get_persistent_object_cache_learn_more_action(),
 			'test'        => 'amp_persistent_object_cache',
+			'status'      => $is_using_object_cache ? 'good' : 'recommended',
+			'label'       => $is_using_object_cache
+				? esc_html__( 'Persistent object caching is enabled', 'amp' )
+				: esc_html__( 'Persistent object caching is not enabled', 'amp' ),
 		];
+	}
 
-		$status = $is_using_object_cache ? 'good' : 'recommended';
-		$label  = $is_using_object_cache
-			? __( 'Persistent object caching is enabled', 'amp' )
-			: __( 'Persistent object caching is not enabled', 'amp' );
+	/**
+	 * Gets the test result data for whether the AMP slug (query var) was defined late.
+	 *
+	 * @return array The test data.
+	 */
+	public function slug_definition_timing() {
+		$is_defined_late = $this->amp_slug_customization_watcher->did_customize_late();
 
-		return array_merge(
-			$data,
-			[
-				'status' => $status,
-				'label'  => esc_html( $label ),
-			]
+		$data = [];
+
+		$data['test']  = 'amp_slug_definition_timing';
+		$data['badge'] = [
+			'label' => $this->get_badge_label(),
+			'color' => $is_defined_late ? 'orange' : 'green',
+		];
+
+		$data['status'] = $is_defined_late ? 'recommended' : 'good';
+
+		$data['label'] = $is_defined_late
+			? esc_html__( 'The AMP slug (query var) was defined late', 'amp' )
+			: esc_html__( 'The AMP slug (query var) was defined early', 'amp' );
+
+		$data['description'] = sprintf(
+			/* translators: %s is 'plugins_loaded'  */
+			esc_html__( 'For best results, the AMP slug (query var) should be available early in WordPress\'s execution flow, specifically before the %s action occurs (at priority 4). This slug is used to construct the paired AMP URLs.', 'amp' ),
+			'<code>plugins_loaded</code>'
 		);
+		$data['description'] .= ' ';
+
+		if ( $is_defined_late ) {
+			$data['description'] .= sprintf(
+				/* translators: %1$s is the value of the slug, %2$s is the constant name, %3$s is the filter name */
+				esc_html__( 'Your AMP slug (%1$s) is defined late, most likely in the theme via the %2$s filter or %3$s constant. Make sure this is being defined in a plugin at the top level so it happens before %4$s. While a late-defined AMP slug will still work, it requires extra work to make the slug available earlier in WordPress execution by storing it in an option.', 'amp' ),
+				sprintf( '<code>%s</code>', esc_html( amp_get_slug() ) ),
+				'<code>amp_query_var</code>',
+				'<code>AMP_QUERY_VAR</code>',
+				'<code>plugins_loaded</code>'
+			);
+		} else {
+			$data['description'] .= sprintf(
+				/* translators: %1$s is the value of the slug */
+				esc_html__( 'Your AMP slug (%s) is defined early so you\'re good to go!', 'amp' ),
+				sprintf( '<code>%s</code>', esc_html( amp_get_slug() ) )
+			);
+		}
+
+		return $data;
 	}
 
 	/**
@@ -172,7 +239,7 @@ static function( $function_name ) {
 
 		$data = [
 			'badge'       => [
-				'label' => esc_html__( 'AMP', 'amp' ),
+				'label' => $this->get_badge_label(),
 				'color' => $undefined_curl_functions ? 'orange' : 'green',
 			],
 			'description' => esc_html__( 'The AMP plugin is able to more efficiently determine the dimensions of images lacking width or height by making parallel requests via cURL multi.', 'amp' ),
@@ -245,7 +312,7 @@ public function icu_version() {
 
 		$data = [
 			'badge'       => [
-				'label' => esc_html__( 'AMP', 'amp' ),
+				'label' => $this->get_badge_label(),
 				'color' => $is_proper_version ? 'green' : 'orange',
 			],
 			'description' => esc_html(
@@ -312,7 +379,7 @@ public function css_transient_caching() {
 
 		$data = [
 			'badge'       => [
-				'label' => esc_html__( 'AMP', 'amp' ),
+				'label' => $this->get_badge_label(),
 				'color' => $color,
 			],
 			'description' => wp_kses(
@@ -377,7 +444,7 @@ public function xdebug_extension() {
 			compact( 'status', 'label', 'description' ),
 			[
 				'badge' => [
-					'label' => esc_html__( 'AMP', 'amp' ),
+					'label' => $this->get_badge_label(),
 					'color' => $color,
 				],
 				'test'  => 'amp_xdebug_extension',
@@ -396,9 +463,19 @@ public function add_debug_information( $debugging_information ) {
 			$debugging_information,
 			[
 				'amp_wp' => [
-					'label'       => esc_html__( 'AMP', 'amp' ),
+					'label'       => $this->get_badge_label(),
 					'description' => esc_html__( 'Debugging information for the Official AMP Plugin for WordPress.', 'amp' ),
 					'fields'      => [
+						'amp_slug_query_var'      => [
+							'label'   => 'AMP slug query var',
+							'value'   => amp_get_slug(),
+							'private' => false,
+						],
+						'amp_slug_defined_late'   => [
+							'label'   => 'AMP slug defined late',
+							'value'   => $this->amp_slug_customization_watcher->did_customize_late() ? 'true' : 'false',
+							'private' => false,
+						],
 						'amp_mode_enabled'        => [
 							'label'   => 'AMP mode enabled',
 							'value'   => AMP_Options_Manager::get_option( Option::THEME_SUPPORT ),
diff --git a/src/AmpSlugCustomizationWatcher.php b/src/AmpSlugCustomizationWatcher.php
index 342b0b88771..2b3d1542aa1 100644
--- a/src/AmpSlugCustomizationWatcher.php
+++ b/src/AmpSlugCustomizationWatcher.php
@@ -18,6 +18,13 @@
  */
 final class AmpSlugCustomizationWatcher implements Service, Registerable {
 
+	/**
+	 * Action at which a slug can be considered late-defined, which is the ideal case.
+	 *
+	 * @var string
+	 */
+	const LATE_DETERMINATION_ACTION = 'after_setup_theme';
+
 	/**
 	 * Whether the slug was customized early (at plugins_loaded action, priority 8).
 	 *
@@ -68,10 +75,10 @@ public function did_customize_late() {
 	 * determined, so for Reader themes to apply the logic in `ReaderThemeLoader` must run beforehand.
 	 */
 	public function determine_early_customization() {
-		if ( QueryVar::AMP !== amp_get_slug() ) {
+		if ( QueryVar::AMP !== amp_get_slug( true ) ) {
 			$this->is_customized_early = true;
 		} else {
-			add_action( 'after_setup_theme', [ $this, 'determine_late_customization' ], 4 );
+			add_action( self::LATE_DETERMINATION_ACTION, [ $this, 'determine_late_customization' ], 4 );
 		}
 	}
 
@@ -90,7 +97,7 @@ public function determine_early_customization() {
 	 * @see amp_after_setup_theme()
 	 */
 	public function determine_late_customization() {
-		if ( QueryVar::AMP !== amp_get_slug() ) {
+		if ( QueryVar::AMP !== amp_get_slug( true ) ) {
 			$this->is_customized_late = true;
 		}
 	}
diff --git a/src/AmpWpPlugin.php b/src/AmpWpPlugin.php
index d131611ac28..88952cca52e 100644
--- a/src/AmpWpPlugin.php
+++ b/src/AmpWpPlugin.php
@@ -189,6 +189,7 @@ protected function get_arguments() {
 	 */
 	protected function get_shared_instances() {
 		return [
+			AmpSlugCustomizationWatcher::class,
 			PluginRegistry::class,
 			Instrumentation\StopWatch::class,
 			DevTools\CallbackReflection::class,
diff --git a/src/Option.php b/src/Option.php
index d9757734c63..a8371fcfcf1 100644
--- a/src/Option.php
+++ b/src/Option.php
@@ -155,10 +155,17 @@ interface Option {
 	/**
 	 * The key of the option storing whether the setup wizard has been completed.
 	 *
-	 * @var boolean
+	 * @var string
 	 */
 	const PLUGIN_CONFIGURED = 'plugin_configured';
 
+	/**
+	 * Cached slug when it is defined late.
+	 *
+	 * @var string
+	 */
+	const LATE_DEFINED_SLUG = 'late_defined_slug';
+
 	/**
 	 * Suppressed plugins
 	 *
diff --git a/src/PairedRouting.php b/src/PairedRouting.php
index 1a2377dabd9..88fd5a1dae2 100644
--- a/src/PairedRouting.php
+++ b/src/PairedRouting.php
@@ -96,6 +96,13 @@ final class PairedRouting implements Service, Registerable {
 	 */
 	const CUSTOM_PAIRED_ENDPOINT_SOURCES = 'custom_paired_endpoint_sources';
 
+	/**
+	 * Action which is triggered when the late-defined slug needs to be updated in options.
+	 *
+	 * @var string
+	 */
+	const ACTION_UPDATE_LATE_DEFINED_SLUG_OPTION = 'amp_update_late_defined_slug_option';
+
 	/**
 	 * Paired URL service.
 	 *
@@ -117,6 +124,13 @@ final class PairedRouting implements Service, Registerable {
 	 */
 	private $callback_reflection;
 
+	/**
+	 * AMP slug customization watcher.
+	 *
+	 * @var AmpSlugCustomizationWatcher
+	 */
+	private $amp_slug_customization_watcher;
+
 	/**
 	 * Plugin registry.
 	 *
@@ -150,16 +164,18 @@ final class PairedRouting implements Service, Registerable {
 	/**
 	 * PairedRouting constructor.
 	 *
-	 * @param Injector           $injector            Injector.
-	 * @param CallbackReflection $callback_reflection Callback reflection.
-	 * @param PluginRegistry     $plugin_registry     Plugin registry.
-	 * @param PairedUrl          $paired_url          Paired URL service.
+	 * @param Injector                    $injector                       Injector.
+	 * @param CallbackReflection          $callback_reflection            Callback reflection.
+	 * @param PluginRegistry              $plugin_registry                Plugin registry.
+	 * @param PairedUrl                   $paired_url                     Paired URL service.
+	 * @param AmpSlugCustomizationWatcher $amp_slug_customization_watcher AMP slug customization watcher.
 	 */
-	public function __construct( Injector $injector, CallbackReflection $callback_reflection, PluginRegistry $plugin_registry, PairedUrl $paired_url ) {
-		$this->injector            = $injector;
-		$this->callback_reflection = $callback_reflection;
-		$this->plugin_registry     = $plugin_registry;
-		$this->paired_url          = $paired_url;
+	public function __construct( Injector $injector, CallbackReflection $callback_reflection, PluginRegistry $plugin_registry, PairedUrl $paired_url, AmpSlugCustomizationWatcher $amp_slug_customization_watcher ) {
+		$this->injector                       = $injector;
+		$this->callback_reflection            = $callback_reflection;
+		$this->plugin_registry                = $plugin_registry;
+		$this->paired_url                     = $paired_url;
+		$this->amp_slug_customization_watcher = $amp_slug_customization_watcher;
 	}
 
 	/**
@@ -172,12 +188,41 @@ public function register() {
 		add_filter( 'amp_default_options', [ $this, 'filter_default_options' ], 10, 2 );
 		add_filter( 'amp_options_updating', [ $this, 'sanitize_options' ], 10, 2 );
 
+		add_action( self::ACTION_UPDATE_LATE_DEFINED_SLUG_OPTION, [ $this, 'update_late_defined_slug_option' ] );
+		add_action( AmpSlugCustomizationWatcher::LATE_DETERMINATION_ACTION, [ $this, 'check_stale_late_defined_slug_option' ] );
+
 		add_action( 'template_redirect', [ $this, 'redirect_extraneous_paired_endpoint' ], 9 );
 
 		// Priority 7 needed to run before PluginSuppression::initialize() at priority 8.
 		add_action( 'plugins_loaded', [ $this, 'initialize_paired_request' ], 7 );
 	}
 
+	/**
+	 * Get the late defined slug, or null if it was not defined late.
+	 *
+	 * @return string|null Slug or null.
+	 */
+	public function get_late_defined_slug() {
+		return $this->amp_slug_customization_watcher->did_customize_late() ? amp_get_slug() : null;
+	}
+
+	/**
+	 * Update late-defined slug option.
+	 */
+	public function update_late_defined_slug_option() {
+		AMP_Options_Manager::update_option( Option::LATE_DEFINED_SLUG, $this->get_late_defined_slug() );
+	}
+
+	/**
+	 * Check whether the late-defined slug option is stale and a single event needs to be scheduled to update it.
+	 */
+	public function check_stale_late_defined_slug_option() {
+		$late_defined_slug = $this->get_late_defined_slug();
+		if ( AMP_Options_Manager::get_option( Option::LATE_DEFINED_SLUG ) !== $late_defined_slug ) {
+			wp_schedule_single_event( time(), self::ACTION_UPDATE_LATE_DEFINED_SLUG_OPTION );
+		}
+	}
+
 	/**
 	 * Get the paired URL structure.
 	 *
@@ -651,6 +696,7 @@ public function filter_default_options( $defaults, $options ) {
 		}
 
 		$defaults[ Option::PAIRED_URL_STRUCTURE ] = $value;
+		$defaults[ Option::LATE_DEFINED_SLUG ]    = null;
 
 		return $defaults;
 	}
@@ -665,6 +711,9 @@ public function filter_default_options( $defaults, $options ) {
 	 * @return array Sanitized options.
 	 */
 	public function sanitize_options( $options, $new_options ) {
+		// Cache the AMP slug in options when it is defined late so that it can be referred to early at plugins_loaded.
+		$options[ Option::LATE_DEFINED_SLUG ] = $this->get_late_defined_slug();
+
 		if (
 			isset( $new_options[ Option::PAIRED_URL_STRUCTURE ] )
 			&&
diff --git a/tests/php/src/PairedRoutingTest.php b/tests/php/src/PairedRoutingTest.php
index 1cceb3fc4b5..d8b7aacde0d 100644
--- a/tests/php/src/PairedRoutingTest.php
+++ b/tests/php/src/PairedRoutingTest.php
@@ -3,6 +3,7 @@
 namespace AmpProject\AmpWP\Tests;
 
 use AmpProject\AmpWP\Admin\ReaderThemes;
+use AmpProject\AmpWP\AmpSlugCustomizationWatcher;
 use AmpProject\AmpWP\Option;
 use AmpProject\AmpWP\PairedRouting;
 use AmpProject\AmpWP\Infrastructure\Service;
@@ -55,14 +56,110 @@ public function test__construct() {
 	public function test_register() {
 		remove_all_actions( 'plugins_loaded' ); // @todo This is needed because the instance already got registered.
 		$this->instance->register();
+
 		$this->assertEquals( 10, has_filter( 'amp_rest_options_schema', [ $this->instance, 'filter_rest_options_schema' ] ) );
 		$this->assertEquals( 10, has_filter( 'amp_rest_options', [ $this->instance, 'filter_rest_options' ] ) );
+
 		$this->assertEquals( 10, has_filter( 'amp_default_options', [ $this->instance, 'filter_default_options' ] ) );
 		$this->assertEquals( 10, has_filter( 'amp_options_updating', [ $this->instance, 'sanitize_options' ] ) );
+
+		$this->assertEquals( 10, has_action( PairedRouting::ACTION_UPDATE_LATE_DEFINED_SLUG_OPTION, [ $this->instance, 'update_late_defined_slug_option' ] ) );
+		$this->assertEquals( 10, has_action( 'after_setup_theme', [ $this->instance, 'check_stale_late_defined_slug_option' ] ) );
+
 		$this->assertEquals( 9, has_action( 'template_redirect', [ $this->instance, 'redirect_extraneous_paired_endpoint' ] ) );
 		$this->assertEquals( 7, has_action( 'plugins_loaded', [ $this->instance, 'initialize_paired_request' ] ) );
 	}
 
+	/** @return array */
+	public function get_data_for_test_get_late_defined_slug() {
+		return [
+			'not_customized'   => [
+				null,
+				false,
+			],
+			'customized_early' => [
+				'lite',
+				false,
+			],
+			'customized_late'  => [
+				'mobile',
+				true,
+			],
+		];
+	}
+
+	/**
+	 * @covers ::get_late_defined_slug()
+	 *
+	 * @dataProvider get_data_for_test_get_late_defined_slug
+	 * @param string|null $slug            Slug.
+	 * @param bool        $customized_late Customized late.
+	 */
+	public function test_get_late_defined_slug( $slug, $customized_late ) {
+		if ( $slug ) {
+			add_filter(
+				'amp_query_var',
+				static function () use ( $slug ) {
+					return $slug;
+				}
+			);
+			$this->assertEquals( amp_get_slug(), $slug );
+		}
+
+		/** @var AmpSlugCustomizationWatcher $amp_slug_customization_watcher */
+		$amp_slug_customization_watcher = $this->get_private_property( $this->instance, 'amp_slug_customization_watcher' );
+
+		$this->set_private_property( $amp_slug_customization_watcher, 'is_customized_early', ! $customized_late );
+		$this->set_private_property( $amp_slug_customization_watcher, 'is_customized_late', $customized_late );
+
+		if ( $customized_late ) {
+			$this->assertEquals( amp_get_slug(), $this->instance->get_late_defined_slug() );
+		} else {
+			$this->assertNull( $this->instance->get_late_defined_slug() );
+		}
+	}
+
+	/** @covers ::update_late_defined_slug_option() */
+	public function test_update_late_defined_slug_option() {
+		/** @var AmpSlugCustomizationWatcher $amp_slug_customization_watcher */
+		$amp_slug_customization_watcher = $this->get_private_property( $this->instance, 'amp_slug_customization_watcher' );
+
+		$this->assertFalse( $amp_slug_customization_watcher->did_customize_late() );
+		$this->instance->update_late_defined_slug_option();
+		$this->assertNull( AMP_Options_Manager::get_option( Option::LATE_DEFINED_SLUG ) );
+
+		add_filter(
+			'amp_query_var',
+			static function () {
+				return 'lite';
+			}
+		);
+		$this->set_private_property( $amp_slug_customization_watcher, 'is_customized_late', true );
+		$this->assertTrue( $amp_slug_customization_watcher->did_customize_late() );
+		$this->instance->update_late_defined_slug_option();
+		$this->assertEquals( 'lite', AMP_Options_Manager::get_option( Option::LATE_DEFINED_SLUG ) );
+	}
+
+	/** @covers ::check_stale_late_defined_slug_option() */
+	public function test_check_stale_late_defined_slug_option() {
+		/** @var AmpSlugCustomizationWatcher $amp_slug_customization_watcher */
+		$amp_slug_customization_watcher = $this->get_private_property( $this->instance, 'amp_slug_customization_watcher' );
+
+		$this->instance->check_stale_late_defined_slug_option();
+		$this->assertFalse( wp_next_scheduled( PairedRouting::ACTION_UPDATE_LATE_DEFINED_SLUG_OPTION ) );
+
+		add_filter(
+			'amp_query_var',
+			static function () {
+				return 'lite';
+			}
+		);
+		$this->set_private_property( $amp_slug_customization_watcher, 'is_customized_late', true );
+		$this->assertTrue( $amp_slug_customization_watcher->did_customize_late() );
+		$this->instance->check_stale_late_defined_slug_option();
+		$this->assertNotFalse( wp_next_scheduled( PairedRouting::ACTION_UPDATE_LATE_DEFINED_SLUG_OPTION ) );
+	}
+
 	/** @return array */
 	public function get_data_for_test_get_paired_url_structure() {
 		return [
@@ -628,16 +725,47 @@ public function test_filter_default_options( $options, $expected_structure ) {
 
 	/** @covers ::sanitize_options() */
 	public function test_sanitize_options() {
-		$this->assertEmpty(
+		$this->assertEquals(
+			[
+				Option::LATE_DEFINED_SLUG => null,
+			],
 			$this->instance->sanitize_options(
 				[],
 				[ Option::PAIRED_URL_STRUCTURE => 'bogus' ]
 			)
 		);
 
+		/** @var AmpSlugCustomizationWatcher $amp_slug_customization_watcher */
+		$amp_slug_customization_watcher = $this->get_private_property( $this->instance, 'amp_slug_customization_watcher' );
+
+		$this->assertFalse( $amp_slug_customization_watcher->did_customize_late() );
 		foreach ( array_keys( PairedRouting::PAIRED_URL_STRUCTURES ) as $paired_url_structure ) {
 			$this->assertEquals(
-				[ Option::PAIRED_URL_STRUCTURE => $paired_url_structure ],
+				[
+					Option::LATE_DEFINED_SLUG    => null,
+					Option::PAIRED_URL_STRUCTURE => $paired_url_structure,
+				],
+				$this->instance->sanitize_options(
+					[],
+					[ Option::PAIRED_URL_STRUCTURE => $paired_url_structure ]
+				)
+			);
+		}
+
+		add_filter(
+			'amp_query_var',
+			static function () {
+				return 'lite';
+			}
+		);
+		$amp_slug_customization_watcher->determine_late_customization();
+		$this->assertTrue( $amp_slug_customization_watcher->did_customize_late() );
+		foreach ( array_keys( PairedRouting::PAIRED_URL_STRUCTURES ) as $paired_url_structure ) {
+			$this->assertEquals(
+				[
+					Option::LATE_DEFINED_SLUG    => 'lite',
+					Option::PAIRED_URL_STRUCTURE => $paired_url_structure,
+				],
 				$this->instance->sanitize_options(
 					[],
 					[ Option::PAIRED_URL_STRUCTURE => $paired_url_structure ]
diff --git a/tests/php/test-amp-helper-functions.php b/tests/php/test-amp-helper-functions.php
index 4c133aa893f..2afa6c540c2 100644
--- a/tests/php/test-amp-helper-functions.php
+++ b/tests/php/test-amp-helper-functions.php
@@ -11,6 +11,7 @@
 use AmpProject\AmpWP\Tests\Helpers\HandleValidation;
 use AmpProject\AmpWP\Tests\Helpers\LoadsCoreThemes;
 use AmpProject\AmpWP\Tests\DependencyInjectedTestCase;
+use AmpProject\AmpWP\AmpSlugCustomizationWatcher;
 
 /**
  * Class Test_AMP_Helper_Functions
@@ -270,6 +271,36 @@ static function () {
 		$this->assertSame( 'lite', amp_get_slug() );
 	}
 
+	/**
+	 * Test amp_get_slug() when late-defined slugs are involved.
+	 *
+	 * @covers ::amp_get_slug()
+	 */
+	public function test_amp_get_slug_late() {
+		$this->assertSame( 'amp', amp_get_slug() );
+
+		unset( $GLOBALS['wp_actions'][ AmpSlugCustomizationWatcher::LATE_DETERMINATION_ACTION ] );
+		$this->assertEquals( 0, did_action( AmpSlugCustomizationWatcher::LATE_DETERMINATION_ACTION ) );
+		AMP_Options_Manager::update_option( Option::LATE_DEFINED_SLUG, 'mobile' );
+
+		add_filter(
+			'amp_query_var',
+			static function () {
+				return 'lite';
+			}
+		);
+
+		$this->assertEquals( 'mobile', amp_get_slug() );
+		$this->assertEquals( 'mobile', amp_get_slug( false ) );
+		$this->assertEquals( 'lite', amp_get_slug( true ) );
+
+		do_action( AmpSlugCustomizationWatcher::LATE_DETERMINATION_ACTION );
+
+		$this->assertEquals( 'lite', amp_get_slug() );
+		$this->assertEquals( 'lite', amp_get_slug( false ) );
+		$this->assertEquals( 'lite', amp_get_slug( true ) );
+	}
+
 	/** @covers ::amp_is_canonical() */
 	public function test_amp_is_canonical() {
 		AMP_Options_Manager::update_option( Option::THEME_SUPPORT, AMP_Theme_Support::STANDARD_MODE_SLUG );
@@ -1908,13 +1939,19 @@ public function test_amp_has_paired_endpoint_go_to( $paired_url_structure, $suff
 		global $wp_rewrite;
 		update_option( 'permalink_structure', '/%year%/%monthnum%/%day%/%postname%/' );
 		$wp_rewrite->init();
-		add_rewrite_endpoint( amp_get_slug(), EP_PERMALINK );
-		$wp_rewrite->flush_rules();
 
 		$permalink = get_permalink( self::factory()->post->create() );
 		$this->assertNotContains( '?', $permalink );
-		$url = $permalink . $suffix;
+
+		$paired_routing = $this->injector->make( \AmpProject\AmpWP\PairedRouting::class );
+
+		$url                    = $permalink . $suffix;
+		$_SERVER['REQUEST_URI'] = wp_parse_url( $permalink, PHP_URL_PATH ) . $suffix;
+
+		$paired_routing->initialize_paired_request();
+
 		$this->go_to( $url );
+		$this->assertFalse( is_404(), 'Expected singular query.' );
 		$this->assertTrue( is_singular(), 'Expected singular query.' );
 		$this->assertTrue( amp_is_available(), 'Expected AMP to be available.' );
 		$this->assertEquals( $is_amp, amp_has_paired_endpoint() );
diff --git a/tests/php/test-class-amp-options-manager.php b/tests/php/test-class-amp-options-manager.php
index 49ec10fe91c..3f2084ce7c6 100644
--- a/tests/php/test-class-amp-options-manager.php
+++ b/tests/php/test-class-amp-options-manager.php
@@ -89,49 +89,6 @@ public function test_register_settings() {
 		$registered_settings = get_registered_settings();
 		$this->assertArrayHasKey( AMP_Options_Manager::OPTION_NAME, $registered_settings );
 		$this->assertEquals( 'array', $registered_settings[ AMP_Options_Manager::OPTION_NAME ]['type'] );
-		$this->assertEquals( 10, has_action( 'update_option_' . AMP_Options_Manager::OPTION_NAME, [ 'AMP_Options_Manager', 'maybe_flush_rewrite_rules' ] ) );
-	}
-
-	/**
-	 * Test maybe_flush_rewrite_rules.
-	 *
-	 * @covers AMP_Options_Manager::maybe_flush_rewrite_rules()
-	 */
-	public function test_maybe_flush_rewrite_rules() {
-		global $wp_rewrite;
-		$wp_rewrite->init();
-		AMP_Options_Manager::register_settings();
-		$dummy_rewrite_rules = [ 'previous' => true ];
-
-		// Check change to supported_post_types.
-		update_option( 'rewrite_rules', $dummy_rewrite_rules );
-		AMP_Options_Manager::maybe_flush_rewrite_rules(
-			[ Option::SUPPORTED_POST_TYPES => [ 'page' ] ],
-			[]
-		);
-		$this->assertEmpty( get_option( 'rewrite_rules' ) );
-
-		// Check update of supported_post_types but no change.
-		update_option( 'rewrite_rules', $dummy_rewrite_rules );
-		update_option(
-			AMP_Options_Manager::OPTION_NAME,
-			[
-				[ Option::SUPPORTED_POST_TYPES => [ 'page' ] ],
-				[ Option::SUPPORTED_POST_TYPES => [ 'page' ] ],
-			]
-		);
-		$this->assertEquals( $dummy_rewrite_rules, get_option( 'rewrite_rules' ) );
-
-		// Check changing a different property.
-		update_option( 'rewrite_rules', [ 'previous' => true ] );
-		update_option(
-			AMP_Options_Manager::OPTION_NAME,
-			[
-				[ 'foo' => 'new' ],
-				[ 'foo' => 'old' ],
-			]
-		);
-		$this->assertEquals( $dummy_rewrite_rules, get_option( 'rewrite_rules' ) );
 	}
 
 	/**
@@ -161,6 +118,7 @@ public function test_get_and_set_options() {
 				Option::READER_THEME            => 'legacy',
 				Option::PLUGIN_CONFIGURED       => false,
 				Option::PAIRED_URL_STRUCTURE    => Option::PAIRED_URL_STRUCTURE_QUERY_VAR,
+				Option::LATE_DEFINED_SLUG       => null,
 			],
 			AMP_Options_Manager::get_options()
 		);
diff --git a/tests/php/test-class-amp-theme-support.php b/tests/php/test-class-amp-theme-support.php
index 12d8e88087f..b6e6913edfb 100644
--- a/tests/php/test-class-amp-theme-support.php
+++ b/tests/php/test-class-amp-theme-support.php
@@ -51,7 +51,6 @@ public function setUp() {
 		AMP_Validation_Manager::reset_validation_results();
 		unset( $GLOBALS['current_screen'] );
 		delete_option( AMP_Options_Manager::OPTION_NAME ); // Make sure default reader mode option does not override theme support being added.
-		add_rewrite_endpoint( amp_get_slug(), EP_PERMALINK );
 		remove_theme_support( 'amp' );
 		$this->register_core_themes();
 	}
diff --git a/tests/php/test-class-site-health.php b/tests/php/test-class-site-health.php
index 6d75d84a310..8d3b21c2e12 100644
--- a/tests/php/test-class-site-health.php
+++ b/tests/php/test-class-site-health.php
@@ -6,8 +6,10 @@
  */
 
 use AmpProject\AmpWP\Admin\SiteHealth;
+use AmpProject\AmpWP\AmpSlugCustomizationWatcher;
 use AmpProject\AmpWP\AmpWpPluginFactory;
 use AmpProject\AmpWP\Option;
+use AmpProject\AmpWP\QueryVar;
 use AmpProject\AmpWP\Tests\Helpers\AssertContainsCompatibility;
 use AmpProject\AmpWP\Tests\Helpers\PrivateAccess;
 
@@ -83,14 +85,19 @@ public function test_add_tests() {
 		$this->assertArrayHasKey( 'amp_curl_multi_functions', $tests['direct'] );
 		$this->assertArrayNotHasKey( 'amp_icu_version', $tests['direct'] );
 		$this->assertArrayHasKey( 'amp_xdebug_extension', $tests['direct'] );
+		$this->assertEquals( QueryVar::AMP, amp_get_slug() );
+		$this->assertArrayNotHasKey( 'amp_slug_definition_timing', $tests['direct'] );
 
 		// Test that the the ICU version test is added only when site URL is an IDN.
 		add_filter( 'site_url', [ self::class, 'get_idn' ], 10, 4 );
+		add_filter( 'amp_query_var', [ self::class, 'get_lite_query_var' ] );
 
 		$tests = $this->instance->add_tests( [] );
 		$this->assertArrayHasKey( 'amp_icu_version', $tests['direct'] );
+		$this->assertArrayHasKey( 'amp_slug_definition_timing', $tests['direct'] );
 
 		remove_filter( 'site_url', [ self::class, 'get_idn' ] );
+		remove_filter( 'amp_query_var', [ self::class, 'get_lite_query_var' ] );
 	}
 
 	/**
@@ -135,6 +142,59 @@ public function test_persistent_object_cache() {
 		);
 	}
 
+	/**
+	 * Test slug_definition_timing.
+	 *
+	 * @covers \AmpProject\AmpWP\Admin\SiteHealth::slug_definition_timing()
+	 */
+	public function test_slug_definition_timing() {
+		$data = [
+			'test' => 'amp_slug_definition_timing',
+		];
+
+		/** @var AmpSlugCustomizationWatcher $amp_slug_customization_watcher */
+		$amp_slug_customization_watcher = $this->get_private_property( $this->instance, 'amp_slug_customization_watcher' );
+		$this->set_private_property( $amp_slug_customization_watcher, 'is_customized_late', false );
+		$this->assertFalse( $amp_slug_customization_watcher->did_customize_late() );
+
+		$this->assertArraySubset(
+			array_merge(
+				$data,
+				[
+					'label'  => 'The AMP slug (query var) was defined early',
+					'status' => 'good',
+					'badge'  => [
+						'label' => 'AMP',
+						'color' => 'green',
+					],
+				]
+			),
+			$this->instance->slug_definition_timing()
+		);
+
+		add_filter( 'amp_query_var', [ self::class, 'get_lite_query_var' ] );
+
+		/** @var AmpSlugCustomizationWatcher $amp_slug_customization_watcher */
+		$amp_slug_customization_watcher = $this->get_private_property( $this->instance, 'amp_slug_customization_watcher' );
+		$this->set_private_property( $amp_slug_customization_watcher, 'is_customized_late', true );
+		$this->assertTrue( $amp_slug_customization_watcher->did_customize_late() );
+
+		$this->assertArraySubset(
+			array_merge(
+				$data,
+				[
+					'label'  => 'The AMP slug (query var) was defined late',
+					'status' => 'recommended',
+					'badge'  => [
+						'label' => 'AMP',
+						'color' => 'orange',
+					],
+				]
+			),
+			$this->instance->slug_definition_timing()
+		);
+	}
+
 	/**
 	 * Test curl_multi_functions.
 	 *
@@ -238,6 +298,8 @@ public function test_add_debug_information() {
 			'amp_css_transient_caching_threshold',
 			'amp_css_transient_caching_sampling_range',
 			'amp_css_transient_caching_transient_count',
+			'amp_slug_query_var',
+			'amp_slug_defined_late',
 		];
 		foreach ( $keys as $key ) {
 			$this->assertArrayHasKey( $key, $debug_info['amp_wp']['fields'], "Expected key: $key" );
@@ -470,11 +532,20 @@ public function test_add_extensions() {
 	}
 
 	/**
-	 * Get a an IDN for testing purposes.
+	 * Get an IDN for testing purposes.
 	 *
 	 * @return string
 	 */
 	public static function get_idn() {
 		return 'https://foo.xn--57h.bar.com';
 	}
+
+	/**
+	 * Get an AMP query var for testing purposes.
+	 *
+	 * @return string
+	 */
+	public static function get_lite_query_var() {
+		return 'lite';
+	}
 }