diff --git a/apps/cli/lib/native-php/site-setup.ts b/apps/cli/lib/native-php/site-setup.ts
index 138ccdc604..87232cadd2 100644
--- a/apps/cli/lib/native-php/site-setup.ts
+++ b/apps/cli/lib/native-php/site-setup.ts
@@ -23,6 +23,11 @@ const WP_CONFIG_TRANSFORMER_PATH = path.resolve(
 
 const DEFAULT_WP_CONFIG_CONSTANTS = { DB_NAME: 'wordpress' } as const;
 
+// Defined only when the site has not set them itself. Theme development mode
+// makes new theme pattern files and theme.json edits apply on the next load
+// instead of waiting out core's caches.
+const SITE_DEFAULT_WP_CONFIG_CONSTANTS = { WP_DEVELOPMENT_MODE: 'theme' } as const;
+
 type Logger = ( ...args: Parameters< typeof console.log > ) => void;
 
 export async function ensureWpConfig(
@@ -45,6 +50,11 @@ require_once $transformer_path;
 
 $transformer = WP_Config_Transformer::from_file( $wp_config_path );
 $transformer->define_constants( $constants );
+foreach ( json_decode( $argv[4] ?? '', true ) ?: array() as $name => $value ) {
+	if ( ! $transformer->constant_exists( $name ) ) {
+		$transformer->define_constant( $name, $value );
+	}
+}
 $transformer->to_file( $wp_config_path );
 `;
 
@@ -74,6 +84,7 @@ $transformer->to_file( $wp_config_path );
 				WP_CONFIG_TRANSFORMER_PATH,
 				wpConfigPath,
 				JSON.stringify( constants ),
+				JSON.stringify( SITE_DEFAULT_WP_CONFIG_CONSTANTS ),
 			],
 			{ phpVersion, signal }
 		);
diff --git a/apps/cli/lib/native-php/tests/site-setup.test.ts b/apps/cli/lib/native-php/tests/site-setup.test.ts
index 1d202a9c8c..ccad856fc7 100644
--- a/apps/cli/lib/native-php/tests/site-setup.test.ts
+++ b/apps/cli/lib/native-php/tests/site-setup.test.ts
@@ -18,7 +18,7 @@ async function getWrittenConstants(
 	runPhpCommand.mockClear();
 	await ensureWpConfig( '/nonexistent-site', '8.4', new AbortController().signal, config );
 	const args = runPhpCommand.mock.calls[ 0 ][ 0 ] as string[];
-	return JSON.parse( args[ args.length - 1 ] );
+	return JSON.parse( args[ args.length - 2 ] );
 }
 
 describe( 'ensureWpConfig', () => {
@@ -33,6 +33,14 @@ describe( 'ensureWpConfig', () => {
 		expect( constants.WP_ENVIRONMENT_TYPE ).toBe( 'local' );
 	} );
 
+	it( 'defaults theme development mode without replacing a site’s own', async () => {
+		runPhpCommand.mockClear();
+		await ensureWpConfig( '/nonexistent-site', '8.4', new AbortController().signal );
+		const args = runPhpCommand.mock.calls[ 0 ][ 0 ] as string[];
+		expect( JSON.parse( args[ args.length - 1 ] ) ).toEqual( { WP_DEVELOPMENT_MODE: 'theme' } );
+		expect( args[ 1 ] ).toContain( 'constant_exists( $name )' );
+	} );
+
 	it( 'writes SCRIPT_DEBUG when script debug is enabled', async () => {
 		const constants = await getWrittenConstants( { enableScriptDebug: true } );