diff --git a/src/WorkOS/Sync/UserSync.php b/src/WorkOS/Sync/UserSync.php
index 343d0ee..4d7b592 100644
--- a/src/WorkOS/Sync/UserSync.php
+++ b/src/WorkOS/Sync/UserSync.php
@@ -642,24 +642,49 @@ public static function get_wp_user_id_by_workos_id( string $workos_id ): ?int {
 	/**
 	 * Generate a unique username from WorkOS user data.
 	 *
+	 * Uses the email local part as the base and resolves collisions with a
+	 * short hash of the full email instead of probing sequential suffixes —
+	 * popular shared-mailbox local parts (info, sales, …) build chains deep
+	 * enough that a sequential probe walks thousands of users and exhausts
+	 * memory. Hashing keeps the lookup count constant regardless of chain
+	 * depth, and the same email always derives the same username.
+	 *
 	 * @param array $workos_user WorkOS user data.
 	 *
 	 * @return string
 	 */
 	private static function generate_username( array $workos_user ): string {
-		$email = $workos_user['email'] ?? '';
-		$base  = sanitize_user( strtok( $email, '@' ), true );
+		$email = strtolower( trim( (string) ( $workos_user['email'] ?? '' ) ) );
+		$base  = sanitize_user( explode( '@', $email, 2 )[0], true );
 
 		if ( ! $base ) {
 			$base = 'workos_user';
 		}
 
-		$username = $base;
-		$counter  = 1;
+		// Cap so the widest suffix ('_' + 12 hex) fits user_login's varchar(60).
+		$base = substr( $base, 0, 47 );
+
+		if ( ! username_exists( $base ) ) {
+			return $base;
+		}
+
+		$hash     = hash( 'sha256', $email );
+		$username = $base . '_' . substr( $hash, 0, 5 );
+
+		if ( ! username_exists( $username ) ) {
+			return $username;
+		}
+
+		$username = $base . '_' . substr( $hash, 0, 12 );
+
+		// ~2^-48 territory. The generator is deterministic, so recovery must
+		// change the input — salt with an attempt counter, never re-roll.
+		for ( $attempt = 1; $attempt <= 3; $attempt++ ) {
+			if ( ! username_exists( $username ) ) {
+				return $username;
+			}
 
-		while ( username_exists( $username ) ) {
-			$username = $base . '_' . $counter;
-			++$counter;
+			$username = $base . '_' . substr( hash( 'sha256', $email . '|' . $attempt ), 0, 12 );
 		}
 
 		return $username;
diff --git a/tests/wpunit/UserSyncFindOrCreateTest.php b/tests/wpunit/UserSyncFindOrCreateTest.php
index 02015cc..047fa3c 100644
--- a/tests/wpunit/UserSyncFindOrCreateTest.php
+++ b/tests/wpunit/UserSyncFindOrCreateTest.php
@@ -242,4 +242,21 @@ public function test_generates_unique_username_on_conflict(): void {
 		$this->assertNotSame( 'conflicting', $result->user_login );
 		$this->assertStringStartsWith( 'conflicting', $result->user_login );
 	}
+
+	/**
+	 * Test the created username carries the deterministic email-hash suffix on conflict.
+	 */
+	public function test_creates_user_with_hash_suffix_when_local_part_taken(): void {
+		self::factory()->user->create( [ 'user_login' => 'info', 'user_email' => 'other-info@example.com' ] );
+
+		$result = UserSync::find_or_create_wp_user(
+			[
+				'id'    => 'user_info_hash_test',
+				'email' => 'info@acme-widgets.com',
+			]
+		);
+
+		$this->assertInstanceOf( \WP_User::class, $result );
+		$this->assertSame( 'info_48f25', $result->user_login );
+	}
 }
diff --git a/tests/wpunit/UserSyncGenerateUsernameTest.php b/tests/wpunit/UserSyncGenerateUsernameTest.php
new file mode 100644
index 0000000..ee79e46
--- /dev/null
+++ b/tests/wpunit/UserSyncGenerateUsernameTest.php
@@ -0,0 +1,203 @@
+<?php
+/**
+ * Tests for UserSync::generate_username().
+ *
+ * @package WorkOS\Tests\Wpunit
+ */
+
+namespace WorkOS\Tests\Wpunit;
+
+use Closure;
+use lucatume\WPBrowser\TestCase\WPTestCase;
+use ReflectionMethod;
+use WorkOS\Sync\UserSync;
+
+/**
+ * Username generation tests.
+ *
+ * The generator must derive usernames in O(1) lookups regardless of how many
+ * users share the same email local part — the previous sequential probe
+ * (info, info_1, info_2, …) walked the whole chain and exhausted memory on
+ * deep chains (CONS-513). Expected hash values are precomputed sha256 hex
+ * digests of the lowercased email (or email + '|' + attempt for salted
+ * retries), independent of the implementation.
+ */
+class UserSyncGenerateUsernameTest extends WPTestCase {
+
+	/**
+	 * Set up each test.
+	 */
+	public function setUp(): void {
+		parent::setUp();
+
+		remove_all_actions( 'user_register' );
+	}
+
+	/**
+	 * Invoke the private username generator via reflection.
+	 *
+	 * @param string $email Email to derive a username from.
+	 *
+	 * @return string Generated username.
+	 */
+	private function generate( string $email ): string {
+		$method = new ReflectionMethod( UserSync::class, 'generate_username' );
+		$method->setAccessible( true );
+
+		return $method->invoke( null, [ 'email' => $email ] );
+	}
+
+	/**
+	 * Create a user occupying the given login.
+	 *
+	 * @param string $login Login to occupy.
+	 */
+	private function seed_login( string $login ): void {
+		self::factory()->user->create(
+			[
+				'user_login' => $login,
+				'user_email' => md5( $login ) . '@seed.test',
+			]
+		);
+	}
+
+	/**
+	 * Collision-ladder scenarios.
+	 *
+	 * Each case seeds the usernames the email's derivation collides with,
+	 * then states the expected outcome — reading top to bottom walks the
+	 * full ladder: bare base → 5-hex suffix → 12-hex widening → salted
+	 * retries.
+	 *
+	 * @return array<string, array{0: Closure, 1: string, 2: string}>
+	 */
+	public function username_scenario_provider(): array {
+		$seed = static function ( string ...$logins ): Closure {
+			return static function ( self $test ) use ( $logins ): void {
+				foreach ( $logins as $login ) {
+					$test->seed_login( $login );
+				}
+			};
+		};
+
+		return [
+			'bare local part when base is free'            => [
+				$seed(),
+				'info@acme-widgets.com',
+				'info',
+			],
+			'5-hex email hash suffix when base is taken'   => [
+				$seed( 'info' ),
+				'info@acme-widgets.com',
+				'info_48f25',
+			],
+			'widened 12-hex suffix when 5-hex name taken'  => [
+				$seed( 'info', 'info_48f25' ),
+				'info@acme-widgets.com',
+				'info_48f257791ee0',
+			],
+			'first salted retry when widened name taken'   => [
+				$seed( 'info', 'info_48f25', 'info_48f257791ee0' ),
+				'info@acme-widgets.com',
+				'info_b1dc4def5ca9',
+			],
+			'second salted retry when first salt taken'    => [
+				$seed( 'info', 'info_48f25', 'info_48f257791ee0', 'info_b1dc4def5ca9' ),
+				'info@acme-widgets.com',
+				'info_023b84c3d9b3',
+			],
+			'long local part capped to 47 chars'           => [
+				$seed(),
+				'international-wholesale-distribution-and-logistics-coordination@globex.test',
+				'international-wholesale-distribution-and-logist',
+			],
+			'widened suffix on capped base lands on 60'    => [
+				$seed(
+					'international-wholesale-distribution-and-logist',
+					'international-wholesale-distribution-and-logist_6a3ae'
+				),
+				'international-wholesale-distribution-and-logistics-coordination@globex.test',
+				'international-wholesale-distribution-and-logist_6a3aeb9adeab',
+			],
+			'workos_user fallback when local part empty'   => [
+				$seed(),
+				'@example.com',
+				'workos_user',
+			],
+		];
+	}
+
+	/**
+	 * Test the generator walks the collision ladder deterministically.
+	 *
+	 * @dataProvider username_scenario_provider
+	 *
+	 * @param Closure $arrange  Seeds the usernames the scenario collides with.
+	 * @param string  $email    Email to derive a username from.
+	 * @param string  $expected Expected username.
+	 */
+	public function test_derives_expected_username( Closure $arrange, string $email, string $expected ): void {
+		$arrange( $this );
+
+		$username = $this->generate( $email );
+
+		$this->assertSame( $expected, $username );
+		$this->assertLessThanOrEqual( 60, strlen( $username ) );
+	}
+
+	/**
+	 * Test the same email derives the identical username across runs.
+	 */
+	public function test_same_email_derives_identical_username_across_runs(): void {
+		$this->seed_login( 'info' );
+
+		$first  = $this->generate( 'info@acme-widgets.com' );
+		$second = $this->generate( 'info@acme-widgets.com' );
+
+		$this->assertSame( $first, $second );
+	}
+
+	/**
+	 * Test lookup count stays constant no matter how deep the existing chain is.
+	 *
+	 * This is the regression test for the production OOM: the old sequential
+	 * probe performed one lookup per existing info_* user. With 150 seeded
+	 * users it would need 152 lookups; the generator must need exactly 2.
+	 */
+	public function test_lookup_count_is_constant_for_deep_username_chains(): void {
+		$this->seed_login( 'info' );
+		for ( $i = 1; $i <= 150; $i++ ) {
+			$this->seed_login( 'info_' . $i );
+		}
+
+		$lookups = 0;
+		$counter = static function ( $user_id ) use ( &$lookups ) {
+			++$lookups;
+
+			return $user_id;
+		};
+
+		add_filter( 'username_exists', $counter );
+		$username = $this->generate( 'info@acme-widgets.com' );
+		remove_filter( 'username_exists', $counter );
+
+		$this->assertSame( 'info_48f25', $username );
+		$this->assertSame( 2, $lookups );
+	}
+
+	/**
+	 * Test emails sharing a truncated base still derive distinct usernames.
+	 *
+	 * The hash is computed from the full email, never the truncated base, so
+	 * truncation must not create collisions.
+	 */
+	public function test_truncated_bases_still_get_distinct_usernames(): void {
+		$this->seed_login( 'international-wholesale-distribution-and-logist' );
+
+		$first  = $this->generate( 'international-wholesale-distribution-and-logistics-coordination@globex.test' );
+		$second = $this->generate( 'international-wholesale-distribution-and-logistics-department@globex.test' );
+
+		$this->assertSame( 'international-wholesale-distribution-and-logist_6a3ae', $first );
+		$this->assertSame( 'international-wholesale-distribution-and-logist_1c20f', $second );
+	}
+}