diff --git a/docs/superpowers/plans/2026-04-26-wordpress-content-sync-url-transformer.md b/docs/superpowers/plans/2026-04-26-wordpress-content-sync-url-transformer.md
new file mode 100644
index 0000000..c557025
--- /dev/null
+++ b/docs/superpowers/plans/2026-04-26-wordpress-content-sync-url-transformer.md
@@ -0,0 +1,634 @@
+# WordPress Content Sync URL Transformer Implementation Plan
+
+> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.
+
+**Goal:** Build the URL transformation layer that rewrites source-domain URLs to destination-domain URLs across content strings, HTML attributes, JSON strings, and serialized metadata.
+
+**Architecture:** URL transformation is implemented as a focused service under `src/Url/` with immutable mapping objects and a recursive metadata transformer. The transformer accepts explicit source/destination mappings, normalizes trailing slashes, preserves serialized data validity, and avoids WordPress database mutation so it can be tested with PHPUnit in isolation.
+
+**Tech Stack:** PHP 7.4+, WordPress 5.6+, PHPUnit, PHPStan level 6+, PHPCS with project `phpcs.xml`.
+
+---
+
+## File Structure
+
+- Create: `src/Url/UrlMapping.php` as an immutable source/destination mapping value object.
+- Create: `src/Url/UrlMappingCollection.php` as a validated list of mappings.
+- Create: `src/Url/UrlTransformer.php` for string, HTML, escaped URL, and protocol-relative replacement.
+- Create: `src/Url/MetadataUrlTransformer.php` for recursive arrays, JSON strings, and serialized PHP data.
+- Modify: `src/Plugin.php` to register `UrlTransformer` and `MetadataUrlTransformer` services.
+- Create: `tests/Unit/Url/UrlMappingCollectionTest.php`.
+- Create: `tests/Unit/Url/UrlTransformerTest.php`.
+- Create: `tests/Unit/Url/MetadataUrlTransformerTest.php`.
+
+## Task 1: URL Mapping Value Objects
+
+**Files:**
+- Create: `src/Url/UrlMapping.php`
+- Create: `src/Url/UrlMappingCollection.php`
+- Create: `tests/Unit/Url/UrlMappingCollectionTest.php`
+
+- [ ] **Step 1: Write failing mapping tests**
+
+Create `tests/Unit/Url/UrlMappingCollectionTest.php`:
+
+```php
+<?php
+
+namespace WPContentSync\Tests\Unit\Url;
+
+use PHPUnit\Framework\TestCase;
+use WPContentSync\Url\UrlMapping;
+use WPContentSync\Url\UrlMappingCollection;
+
+class UrlMappingCollectionTest extends TestCase {
+	public function test_it_normalizes_mapping_urls_without_trailing_slashes(): void {
+		$mapping = new UrlMapping( 'https://example.test/', 'https://staging.example.test/' );
+
+		self::assertSame( 'https://example.test', $mapping->sourceUrl() );
+		self::assertSame( 'https://staging.example.test', $mapping->destinationUrl() );
+	}
+
+	public function test_it_rejects_empty_mapping_urls(): void {
+		$this->expectException( \InvalidArgumentException::class );
+		$this->expectExceptionMessage( 'Source and destination URLs are required.' );
+
+		new UrlMapping( '', 'https://staging.example.test' );
+	}
+
+	public function test_it_returns_mappings_in_order(): void {
+		$first      = new UrlMapping( 'https://example.test', 'https://staging.example.test' );
+		$second     = new UrlMapping( 'https://cdn.example.test', 'https://cdn.staging.example.test' );
+		$collection = new UrlMappingCollection( array( $first, $second ) );
+
+		self::assertSame( array( $first, $second ), $collection->all() );
+	}
+
+	public function test_it_rejects_non_mapping_items(): void {
+		$this->expectException( \InvalidArgumentException::class );
+		$this->expectExceptionMessage( 'URL mapping collections only accept UrlMapping instances.' );
+
+		new UrlMappingCollection( array( new \stdClass() ) );
+	}
+}
+```
+
+- [ ] **Step 2: Run test to verify it fails**
+
+Run: `composer test -- --filter UrlMappingCollectionTest`
+
+Expected: FAIL because `WPContentSync\Url\UrlMapping` does not exist.
+
+- [ ] **Step 3: Implement `UrlMapping`**
+
+Create `src/Url/UrlMapping.php`:
+
+```php
+<?php
+
+namespace WPContentSync\Url;
+
+final class UrlMapping {
+	private string $source_url;
+	private string $destination_url;
+
+	public function __construct( string $source_url, string $destination_url ) {
+		$source_url      = $this->normalizeUrl( $source_url );
+		$destination_url = $this->normalizeUrl( $destination_url );
+
+		if ( '' === $source_url || '' === $destination_url ) {
+			throw new \InvalidArgumentException( 'Source and destination URLs are required.' );
+		}
+
+		$this->source_url      = $source_url;
+		$this->destination_url = $destination_url;
+	}
+
+	public function sourceUrl(): string {
+		return $this->source_url;
+	}
+
+	public function destinationUrl(): string {
+		return $this->destination_url;
+	}
+
+	private function normalizeUrl( string $url ): string {
+		return rtrim( trim( $url ), '/' );
+	}
+}
+```
+
+- [ ] **Step 4: Implement `UrlMappingCollection`**
+
+Create `src/Url/UrlMappingCollection.php`:
+
+```php
+<?php
+
+namespace WPContentSync\Url;
+
+final class UrlMappingCollection {
+	/**
+	 * @var array<int, UrlMapping>
+	 */
+	private array $mappings;
+
+	/**
+	 * @param array<int, UrlMapping> $mappings URL mappings.
+	 */
+	public function __construct( array $mappings ) {
+		foreach ( $mappings as $mapping ) {
+			if ( ! $mapping instanceof UrlMapping ) {
+				throw new \InvalidArgumentException( 'URL mapping collections only accept UrlMapping instances.' );
+			}
+		}
+
+		$this->mappings = array_values( $mappings );
+	}
+
+	/**
+	 * @return array<int, UrlMapping>
+	 */
+	public function all(): array {
+		return $this->mappings;
+	}
+}
+```
+
+- [ ] **Step 5: Run test to verify it passes**
+
+Run: `composer test -- --filter UrlMappingCollectionTest`
+
+Expected: PASS with 4 tests.
+
+- [ ] **Step 6: Commit**
+
+```bash
+git add src/Url/UrlMapping.php src/Url/UrlMappingCollection.php tests/Unit/Url/UrlMappingCollectionTest.php
+git commit -m "feat: add url mapping value objects"
+```
+
+## Task 2: Plain and HTML URL Transformation
+
+**Files:**
+- Create: `src/Url/UrlTransformer.php`
+- Create: `tests/Unit/Url/UrlTransformerTest.php`
+
+- [ ] **Step 1: Write failing transformer tests**
+
+Create `tests/Unit/Url/UrlTransformerTest.php`:
+
+```php
+<?php
+
+namespace WPContentSync\Tests\Unit\Url;
+
+use PHPUnit\Framework\TestCase;
+use WPContentSync\Url\UrlMapping;
+use WPContentSync\Url\UrlMappingCollection;
+use WPContentSync\Url\UrlTransformer;
+
+class UrlTransformerTest extends TestCase {
+	public function test_it_rewrites_plain_urls(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test', 'https://staging.example.test' ),
+			)
+		);
+
+		self::assertSame(
+			'Visit https://staging.example.test/about for details.',
+			$transformer->transformString( 'Visit https://example.test/about for details.', $mappings )
+		);
+	}
+
+	public function test_it_rewrites_html_attribute_urls(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test', 'https://staging.example.test' ),
+			)
+		);
+
+		self::assertSame(
+			'<a href="https://staging.example.test/about"><img src="https://staging.example.test/image.jpg"></a>',
+			$transformer->transformString(
+				'<a href="https://example.test/about"><img src="https://example.test/image.jpg"></a>',
+				$mappings
+			)
+		);
+	}
+
+	public function test_it_rewrites_escaped_urls(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test/path?a=1&b=2', 'https://staging.example.test/path?a=1&b=2' ),
+			)
+		);
+
+		self::assertSame(
+			'https://staging.example.test/path?a=1&amp;b=2',
+			$transformer->transformString( 'https://example.test/path?a=1&amp;b=2', $mappings )
+		);
+	}
+
+	public function test_it_rewrites_protocol_relative_urls(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test', 'https://staging.example.test' ),
+			)
+		);
+
+		self::assertSame(
+			'//staging.example.test/uploads/file.pdf',
+			$transformer->transformString( '//example.test/uploads/file.pdf', $mappings )
+		);
+	}
+
+	public function test_it_supports_multiple_mappings(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test', 'https://staging.example.test' ),
+				new UrlMapping( 'https://cdn.example.test', 'https://cdn.staging.example.test' ),
+			)
+		);
+
+		self::assertSame(
+			'https://staging.example.test https://cdn.staging.example.test/image.jpg',
+			$transformer->transformString( 'https://example.test https://cdn.example.test/image.jpg', $mappings )
+		);
+	}
+}
+```
+
+- [ ] **Step 2: Run test to verify it fails**
+
+Run: `composer test -- --filter UrlTransformerTest`
+
+Expected: FAIL because `WPContentSync\Url\UrlTransformer` does not exist.
+
+- [ ] **Step 3: Implement `UrlTransformer`**
+
+Create `src/Url/UrlTransformer.php`:
+
+```php
+<?php
+
+namespace WPContentSync\Url;
+
+final class UrlTransformer {
+	public function transformString( string $value, UrlMappingCollection $mappings ): string {
+		$transformed = $value;
+
+		foreach ( $mappings->all() as $mapping ) {
+			$transformed = $this->replaceMapping( $transformed, $mapping );
+		}
+
+		return $transformed;
+	}
+
+	private function replaceMapping( string $value, UrlMapping $mapping ): string {
+		$source      = $mapping->sourceUrl();
+		$destination = $mapping->destinationUrl();
+
+		$replacements = array(
+			$source                                 => $destination,
+			str_replace( '&', '&amp;', $source )    => str_replace( '&', '&amp;', $destination ),
+			$this->toProtocolRelative( $source )   => $this->toProtocolRelative( $destination ),
+		);
+
+		return strtr( $value, $replacements );
+	}
+
+	private function toProtocolRelative( string $url ): string {
+		return preg_replace( '#^https?:#', '', $url ) ?? $url;
+	}
+}
+```
+
+- [ ] **Step 4: Run test to verify it passes**
+
+Run: `composer test -- --filter UrlTransformerTest`
+
+Expected: PASS with 5 tests.
+
+- [ ] **Step 5: Commit**
+
+```bash
+git add src/Url/UrlTransformer.php tests/Unit/Url/UrlTransformerTest.php
+git commit -m "feat: add url string transformer"
+```
+
+## Task 3: Metadata URL Transformation
+
+**Files:**
+- Create: `src/Url/MetadataUrlTransformer.php`
+- Create: `tests/Unit/Url/MetadataUrlTransformerTest.php`
+
+- [ ] **Step 1: Write failing metadata tests**
+
+Create `tests/Unit/Url/MetadataUrlTransformerTest.php`:
+
+```php
+<?php
+
+namespace WPContentSync\Tests\Unit\Url;
+
+use PHPUnit\Framework\TestCase;
+use WPContentSync\Url\MetadataUrlTransformer;
+use WPContentSync\Url\UrlMapping;
+use WPContentSync\Url\UrlMappingCollection;
+use WPContentSync\Url\UrlTransformer;
+
+class MetadataUrlTransformerTest extends TestCase {
+	private function mappings(): UrlMappingCollection {
+		return new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test', 'https://staging.example.test' ),
+			)
+		);
+	}
+
+	public function test_it_recursively_transforms_array_metadata(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+
+		self::assertSame(
+			array(
+				'hero' => array(
+					'url' => 'https://staging.example.test/uploads/hero.jpg',
+				),
+			),
+			$transformer->transformValue(
+				array(
+					'hero' => array(
+						'url' => 'https://example.test/uploads/hero.jpg',
+					),
+				),
+				$this->mappings()
+			)
+		);
+	}
+
+	public function test_it_transforms_json_strings(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+		$result      = $transformer->transformValue(
+			'{"url":"https:\/\/example.test\/uploads\/hero.jpg"}',
+			$this->mappings()
+		);
+
+		self::assertSame( '{"url":"https:\/\/staging.example.test\/uploads\/hero.jpg"}', $result );
+	}
+
+	public function test_it_preserves_serialized_data_validity(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+		$serialized  = serialize(
+			array(
+				'url' => 'https://example.test/uploads/hero.jpg',
+			)
+		);
+
+		$result = $transformer->transformValue( $serialized, $this->mappings() );
+
+		self::assertSame(
+			array(
+				'url' => 'https://staging.example.test/uploads/hero.jpg',
+			),
+			unserialize( $result )
+		);
+	}
+
+	public function test_it_transforms_plain_string_metadata(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+
+		self::assertSame(
+			'https://staging.example.test/contact',
+			$transformer->transformValue( 'https://example.test/contact', $this->mappings() )
+		);
+	}
+}
+```
+
+- [ ] **Step 2: Run test to verify it fails**
+
+Run: `composer test -- --filter MetadataUrlTransformerTest`
+
+Expected: FAIL because `WPContentSync\Url\MetadataUrlTransformer` does not exist.
+
+- [ ] **Step 3: Implement `MetadataUrlTransformer`**
+
+Create `src/Url/MetadataUrlTransformer.php`:
+
+```php
+<?php
+
+namespace WPContentSync\Url;
+
+final class MetadataUrlTransformer {
+	private UrlTransformer $url_transformer;
+
+	public function __construct( UrlTransformer $url_transformer ) {
+		$this->url_transformer = $url_transformer;
+	}
+
+	/**
+	 * @param mixed $value Metadata value.
+	 * @return mixed
+	 */
+	public function transformValue( $value, UrlMappingCollection $mappings ) {
+		if ( is_array( $value ) ) {
+			return $this->transformArray( $value, $mappings );
+		}
+
+		if ( ! is_string( $value ) ) {
+			return $value;
+		}
+
+		if ( $this->isSerialized( $value ) ) {
+			$unserialized = unserialize( $value );
+			return serialize( $this->transformValue( $unserialized, $mappings ) );
+		}
+
+		if ( $this->isJsonObjectOrArray( $value ) ) {
+			$decoded = json_decode( $value, true );
+			if ( is_array( $decoded ) ) {
+				return wp_json_encode( $this->transformArray( $decoded, $mappings ) );
+			}
+		}
+
+		return $this->url_transformer->transformString( $value, $mappings );
+	}
+
+	/**
+	 * @param array<mixed> $value Metadata array.
+	 * @return array<mixed>
+	 */
+	private function transformArray( array $value, UrlMappingCollection $mappings ): array {
+		$transformed = array();
+
+		foreach ( $value as $key => $item ) {
+			$transformed[ $key ] = $this->transformValue( $item, $mappings );
+		}
+
+		return $transformed;
+	}
+
+	private function isSerialized( string $value ): bool {
+		return false !== @unserialize( $value );
+	}
+
+	private function isJsonObjectOrArray( string $value ): bool {
+		$trimmed = trim( $value );
+
+		return '' !== $trimmed && in_array( $trimmed[0], array( '{', '[' ), true );
+	}
+}
+```
+
+- [ ] **Step 4: Add `wp_json_encode` test stub**
+
+Modify `tests/bootstrap.php` by appending:
+
+```php
+if ( ! function_exists( 'wp_json_encode' ) ) {
+	function wp_json_encode( $value ) {
+		return json_encode( $value );
+	}
+}
+```
+
+- [ ] **Step 5: Run test to verify it passes**
+
+Run: `composer test -- --filter MetadataUrlTransformerTest`
+
+Expected: PASS with 4 tests.
+
+- [ ] **Step 6: Commit**
+
+```bash
+git add src/Url/MetadataUrlTransformer.php tests/Unit/Url/MetadataUrlTransformerTest.php tests/bootstrap.php
+git commit -m "feat: add metadata url transformer"
+```
+
+## Task 4: Service Wiring
+
+**Files:**
+- Modify: `src/Plugin.php`
+
+- [ ] **Step 1: Wire URL services into the plugin container**
+
+Modify `src/Plugin.php` imports:
+
+```php
+use WPContentSync\Url\MetadataUrlTransformer;
+use WPContentSync\Url\UrlTransformer;
+```
+
+Inside `Plugin::create()`, before the `AdminPage::class` factory, add:
+
+```php
+$container->factory(
+	UrlTransformer::class,
+	static function (): UrlTransformer {
+		return new UrlTransformer();
+	}
+);
+
+$container->factory(
+	MetadataUrlTransformer::class,
+	static function () use ( $container ): MetadataUrlTransformer {
+		return new MetadataUrlTransformer(
+			$container->get( UrlTransformer::class )
+		);
+	}
+);
+```
+
+- [ ] **Step 2: Run static analysis**
+
+Run: `composer stan`
+
+Expected: PASS with no PHPStan errors.
+
+- [ ] **Step 3: Run tests**
+
+Run: `composer test`
+
+Expected: PASS with all tests.
+
+- [ ] **Step 4: Commit**
+
+```bash
+git add src/Plugin.php
+git commit -m "feat: register url transformation services"
+```
+
+## Task 5: Full URL Transformer Verification
+
+**Files:**
+- Verify all files created or modified in Tasks 1-4.
+
+- [ ] **Step 1: Run Composer validation**
+
+Run: `composer validate --strict`
+
+Expected: PASS.
+
+- [ ] **Step 2: Run PHPCS**
+
+Run: `composer lint`
+
+Expected: PASS.
+
+- [ ] **Step 3: Run PHPStan**
+
+Run: `composer stan`
+
+Expected: PASS.
+
+- [ ] **Step 4: Run PHPUnit**
+
+Run: `composer test`
+
+Expected: PASS with the existing foundation tests plus URL transformer tests.
+
+- [ ] **Step 5: Manual smoke check in a PHP shell**
+
+Run:
+
+```bash
+php -r "require 'vendor/autoload.php'; $m=new WPContentSync\Url\UrlMappingCollection([new WPContentSync\Url\UrlMapping('https://example.test','https://staging.example.test')]); $t=new WPContentSync\Url\UrlTransformer(); echo $t->transformString('https://example.test/about',$m), PHP_EOL;"
+```
+
+Expected output:
+
+```text
+https://staging.example.test/about
+```
+
+- [ ] **Step 6: Commit any verification fixes**
+
+```bash
+git status --short
+git add src tests
+git commit -m "chore: verify url transformer"
+```
+
+## Self-Review
+
+**Spec coverage in this URL transformer plan:**
+- Plain text URL replacement is covered in Task 2.
+- HTML attribute URL replacement is covered in Task 2 because the transformer rewrites URLs inside HTML strings without parsing/mutating markup structure.
+- Escaped URLs are covered in Task 2 through `&amp;` replacement.
+- Protocol-relative URLs are covered in Task 2.
+- Multiple source/destination mappings are covered in Tasks 1 and 2.
+- JSON strings, serialized PHP arrays, and nested metadata arrays are covered in Task 3.
+- GUID and permalink transformation rules are represented by the same string transformer used for any URL-bearing field; later content-handler phases will decide which post fields should be passed through it.
+
+**Deferred by design:**
+- Admin UI for editing URL mappings remains in the later admin workflow phase.
+- Database mutation and post/meta import logic remain in content-handler phases.
+- Large-scale search/replace progress reporting remains in sync engine phases.
diff --git a/src/Plugin.php b/src/Plugin.php
index 36c9e4e..4b1838a 100644
--- a/src/Plugin.php
+++ b/src/Plugin.php
@@ -11,6 +11,8 @@ use WPContentSync\Admin\AdminPage;
 use WPContentSync\Logging\LoggerInterface;
 use WPContentSync\Logging\OptionLogger;
 use WPContentSync\Settings\SettingsRepository;
+use WPContentSync\Url\MetadataUrlTransformer;
+use WPContentSync\Url\UrlTransformer;
 
 final class Plugin {
 	private Container $container;
@@ -36,6 +38,22 @@ final class Plugin {
 			}
 		);
 
+		$container->factory(
+			UrlTransformer::class,
+			static function (): UrlTransformer {
+				return new UrlTransformer();
+			}
+		);
+
+		$container->factory(
+			MetadataUrlTransformer::class,
+			static function () use ( $container ): MetadataUrlTransformer {
+				return new MetadataUrlTransformer(
+					$container->get( UrlTransformer::class )
+				);
+			}
+		);
+
 		$container->factory(
 			AdminPage::class,
 			static function () use ( $container ): AdminPage {
diff --git a/src/Url/MetadataUrlTransformer.php b/src/Url/MetadataUrlTransformer.php
new file mode 100644
index 0000000..9142c14
--- /dev/null
+++ b/src/Url/MetadataUrlTransformer.php
@@ -0,0 +1,142 @@
+<?php
+
+namespace WPContentSync\Url;
+
+final class MetadataUrlTransformer {
+	private UrlTransformer $url_transformer;
+
+	public function __construct( UrlTransformer $url_transformer ) {
+		$this->url_transformer = $url_transformer;
+	}
+
+	/**
+	 * Transform URLs within a metadata value.
+	 *
+	 * @param mixed                $value    Metadata value.
+	 * @param UrlMappingCollection $mappings URL mappings.
+	 * @return mixed
+	 */
+	public function transformValue( $value, UrlMappingCollection $mappings ) {
+		if ( is_array( $value ) ) {
+			return $this->transformArray( $value, $mappings );
+		}
+
+		if ( ! is_string( $value ) ) {
+			return $value;
+		}
+
+		if ( $this->looksSerialized( trim( $value ) ) && trim( $value ) !== $value ) {
+			return $value;
+		}
+
+		if ( $this->looksSerialized( $value ) ) {
+			$unserialized = $this->unserializeValue( $value );
+
+			if ( ! $unserialized['valid'] || $this->containsObject( $unserialized['value'] ) ) {
+				return $value;
+			}
+
+			// phpcs:ignore -- Required to reserialize metadata with updated string lengths.
+			return serialize( $this->transformValue( $unserialized['value'], $mappings ) );
+		}
+
+		if ( $this->isJsonObjectOrArray( $value ) ) {
+			$decoded = json_decode( $value, true );
+
+			if ( is_array( $decoded ) && JSON_ERROR_NONE === json_last_error() ) {
+				$encoded = wp_json_encode( $this->transformArray( $decoded, $mappings ) );
+
+				if ( is_string( $encoded ) ) {
+					return $encoded;
+				}
+			}
+
+			return $value;
+		}
+
+		return $this->url_transformer->transformString( $value, $mappings );
+	}
+
+	/**
+	 * Transform URLs within a metadata array.
+	 *
+	 * @param array<mixed>         $value    Metadata array.
+	 * @param UrlMappingCollection $mappings URL mappings.
+	 * @return array<mixed>
+	 */
+	private function transformArray( array $value, UrlMappingCollection $mappings ): array {
+		$transformed = array();
+
+		foreach ( $value as $key => $item ) {
+			$transformed[ $key ] = $this->transformValue( $item, $mappings );
+		}
+
+		return $transformed;
+	}
+
+	private function looksSerialized( string $value ): bool {
+		return 'N;' === $value || 'b:0;' === $value || 1 === preg_match( '/^(?:a|O|s|i|d|b):/', $value );
+	}
+
+	/**
+	 * @return array{valid: bool, value: mixed}
+	 */
+	private function unserializeValue( string $value ): array {
+		if ( 'N;' === $value ) {
+			return array(
+				'valid' => true,
+				'value' => null,
+			);
+		}
+
+		// phpcs:ignore WordPress.PHP.NoSilencedErrors.Discouraged, WordPress.PHP.DiscouragedPHPFunctions.serialize_unserialize -- Metadata may be stored in PHP serialized format.
+		$unserialized = @unserialize( $value, array( 'allowed_classes' => false ) );
+
+		if ( false === $unserialized && 'b:0;' !== $value ) {
+			return array(
+				'valid' => false,
+				'value' => null,
+			);
+		}
+
+		// phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.serialize_serialize -- Strict validation avoids transforming malformed serialized-looking strings.
+		if ( serialize( $unserialized ) !== $value ) {
+			return array(
+				'valid' => false,
+				'value' => null,
+			);
+		}
+
+		return array(
+			'valid' => true,
+			'value' => $unserialized,
+		);
+	}
+
+	private function isJsonObjectOrArray( string $value ): bool {
+		$trimmed = trim( $value );
+
+		return '' !== $trimmed && in_array( $trimmed[0], array( '{', '[' ), true );
+	}
+
+	/**
+	 * @param mixed $value Value to inspect.
+	 */
+	private function containsObject( $value ): bool {
+		if ( is_object( $value ) ) {
+			return true;
+		}
+
+		if ( ! is_array( $value ) ) {
+			return false;
+		}
+
+		foreach ( $value as $item ) {
+			if ( $this->containsObject( $item ) ) {
+				return true;
+			}
+		}
+
+		return false;
+	}
+}
diff --git a/src/Url/UrlMapping.php b/src/Url/UrlMapping.php
new file mode 100644
index 0000000..0bd7e38
--- /dev/null
+++ b/src/Url/UrlMapping.php
@@ -0,0 +1,84 @@
+<?php
+
+namespace WPContentSync\Url;
+
+final class UrlMapping {
+	private string $source_url;
+	private string $destination_url;
+
+	public function __construct( string $source_url, string $destination_url ) {
+		$source_url      = $this->normalizeUrl( $source_url );
+		$destination_url = $this->normalizeUrl( $destination_url );
+
+		if ( '' === $source_url || '' === $destination_url ) {
+			throw new \InvalidArgumentException( 'Source and destination URLs are required.' );
+		}
+
+		if ( ! $this->isAbsoluteUrl( $source_url ) || ! $this->isAbsoluteUrl( $destination_url ) ) {
+			throw new \InvalidArgumentException( 'Source and destination URLs must include a scheme and host.' );
+		}
+
+		$this->source_url      = $source_url;
+		$this->destination_url = $destination_url;
+	}
+
+	public function sourceUrl(): string {
+		return $this->source_url;
+	}
+
+	public function destinationUrl(): string {
+		return $this->destination_url;
+	}
+
+	private function normalizeUrl( string $url ): string {
+		$url   = trim( $url );
+		$parts = wp_parse_url( $url );
+
+		if ( ! is_array( $parts ) || ! isset( $parts['path'] ) || '/' !== $parts['path'] ) {
+			return $url;
+		}
+
+		unset( $parts['path'] );
+
+		return $this->buildUrl( $parts );
+	}
+
+	private function isAbsoluteUrl( string $url ): bool {
+		$parts = wp_parse_url( $url );
+
+		return is_array( $parts ) && ! empty( $parts['scheme'] ) && ! empty( $parts['host'] );
+	}
+
+	/**
+	 * @param array<string, mixed> $parts URL parts.
+	 */
+	private function buildUrl( array $parts ): string {
+		$url = (string) $parts['scheme'] . '://';
+
+		if ( isset( $parts['user'] ) ) {
+			$url .= (string) $parts['user'];
+
+			if ( isset( $parts['pass'] ) ) {
+				$url .= ':' . (string) $parts['pass'];
+			}
+
+			$url .= '@';
+		}
+
+		$url .= (string) $parts['host'];
+
+		if ( isset( $parts['port'] ) ) {
+			$url .= ':' . (string) $parts['port'];
+		}
+
+		if ( isset( $parts['query'] ) ) {
+			$url .= '?' . (string) $parts['query'];
+		}
+
+		if ( isset( $parts['fragment'] ) ) {
+			$url .= '#' . (string) $parts['fragment'];
+		}
+
+		return $url;
+	}
+}
diff --git a/src/Url/UrlMappingCollection.php b/src/Url/UrlMappingCollection.php
new file mode 100644
index 0000000..744a480
--- /dev/null
+++ b/src/Url/UrlMappingCollection.php
@@ -0,0 +1,37 @@
+<?php
+
+namespace WPContentSync\Url;
+
+final class UrlMappingCollection {
+	/**
+	 * @var array<int, UrlMapping>
+	 */
+	private array $mappings;
+
+	/**
+	 * @param array<int, UrlMapping> $mappings URL mappings.
+	 */
+	public function __construct( array $mappings ) {
+		foreach ( $mappings as $mapping ) {
+			if ( ! $mapping instanceof UrlMapping ) {
+				throw new \InvalidArgumentException( 'URL mapping collections only accept UrlMapping instances.' );
+			}
+		}
+
+		$this->mappings = array_values( $mappings );
+
+		usort(
+			$this->mappings,
+			static function ( UrlMapping $left, UrlMapping $right ): int {
+				return strlen( $right->sourceUrl() ) <=> strlen( $left->sourceUrl() );
+			}
+		);
+	}
+
+	/**
+	 * @return array<int, UrlMapping>
+	 */
+	public function all(): array {
+		return $this->mappings;
+	}
+}
diff --git a/src/Url/UrlTransformer.php b/src/Url/UrlTransformer.php
new file mode 100644
index 0000000..c64cdce
--- /dev/null
+++ b/src/Url/UrlTransformer.php
@@ -0,0 +1,65 @@
+<?php
+
+namespace WPContentSync\Url;
+
+final class UrlTransformer {
+	public function transformString( string $value, UrlMappingCollection $mappings ): string {
+		$replacements = array();
+
+		foreach ( $mappings->all() as $mapping ) {
+			$replacements = array_merge( $replacements, $this->replacementsForMapping( $mapping ) );
+		}
+
+		if ( array() === $replacements ) {
+			return $value;
+		}
+
+		return preg_replace_callback(
+			$this->replacementPattern( array_keys( $replacements ) ),
+			static function ( array $matches ) use ( $replacements ): string {
+				return $replacements[ $matches[0] ];
+			},
+			$value
+		) ?? $value;
+	}
+
+	/**
+	 * @return array<string, string>
+	 */
+	private function replacementsForMapping( UrlMapping $mapping ): array {
+		$source      = $mapping->sourceUrl();
+		$destination = $mapping->destinationUrl();
+
+		return array(
+			$source                              => $destination,
+			str_replace( '&', '&amp;', $source ) => str_replace( '&', '&amp;', $destination ),
+			$this->toProtocolRelative( $source ) => $this->toProtocolRelative( $destination ),
+			str_replace( '&', '&amp;', $this->toProtocolRelative( $source ) ) => str_replace( '&', '&amp;', $this->toProtocolRelative( $destination ) ),
+		);
+	}
+
+	/**
+	 * @param array<int, string> $sources Replacement sources.
+	 */
+	private function replacementPattern( array $sources ): string {
+		usort(
+			$sources,
+			static function ( string $left, string $right ): int {
+				return strlen( $right ) <=> strlen( $left );
+			}
+		);
+
+		$quoted = array_map(
+			static function ( string $source ): string {
+				return preg_quote( $source, '~' );
+			},
+			$sources
+		);
+
+		return '~(?<![A-Za-z0-9.-])(?:' . implode( '|', $quoted ) . ')(?![A-Za-z0-9.@-])~';
+	}
+
+	private function toProtocolRelative( string $url ): string {
+		return preg_replace( '#^https?:#', '', $url ) ?? $url;
+	}
+}
diff --git a/tests/Unit/PluginTest.php b/tests/Unit/PluginTest.php
new file mode 100644
index 0000000..9c5a1e5
--- /dev/null
+++ b/tests/Unit/PluginTest.php
@@ -0,0 +1,34 @@
+<?php
+
+namespace WPContentSync\Tests\Unit;
+
+use PHPUnit\Framework\TestCase;
+use WPContentSync\Container;
+use WPContentSync\Plugin;
+use WPContentSync\Url\MetadataUrlTransformer;
+use WPContentSync\Url\UrlTransformer;
+
+class PluginTest extends TestCase {
+	public function test_it_registers_url_transformation_services(): void {
+		$container = $this->getPluginContainer( Plugin::create() );
+
+		$url_transformer      = $container->get( UrlTransformer::class );
+		$metadata_transformer = $container->get( MetadataUrlTransformer::class );
+
+		self::assertInstanceOf( UrlTransformer::class, $url_transformer );
+		self::assertInstanceOf( MetadataUrlTransformer::class, $metadata_transformer );
+		self::assertSame( $url_transformer, $container->get( UrlTransformer::class ) );
+		self::assertSame( $metadata_transformer, $container->get( MetadataUrlTransformer::class ) );
+	}
+
+	private function getPluginContainer( Plugin $plugin ): Container {
+		$reflection = new \ReflectionClass( $plugin );
+		$property   = $reflection->getProperty( 'container' );
+		$property->setAccessible( true );
+
+		/** @var Container $container */
+		$container = $property->getValue( $plugin );
+
+		return $container;
+	}
+}
diff --git a/tests/Unit/Url/MetadataUrlTransformerTest.php b/tests/Unit/Url/MetadataUrlTransformerTest.php
new file mode 100644
index 0000000..52c446d
--- /dev/null
+++ b/tests/Unit/Url/MetadataUrlTransformerTest.php
@@ -0,0 +1,134 @@
+<?php
+
+namespace WPContentSync\Tests\Unit\Url;
+
+use PHPUnit\Framework\TestCase;
+use WPContentSync\Url\MetadataUrlTransformer;
+use WPContentSync\Url\UrlMapping;
+use WPContentSync\Url\UrlMappingCollection;
+use WPContentSync\Url\UrlTransformer;
+
+class MetadataUrlTransformerTest extends TestCase {
+	private function mappings(): UrlMappingCollection {
+		return new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test', 'https://staging.example.test' ),
+			)
+		);
+	}
+
+	public function test_it_recursively_transforms_array_metadata(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+
+		self::assertSame(
+			array(
+				'hero'  => array(
+					'url' => 'https://staging.example.test/uploads/hero.jpg',
+				),
+				'count' => 3,
+				'flag'  => true,
+			),
+			$transformer->transformValue(
+				array(
+					'hero'  => array(
+						'url' => 'https://example.test/uploads/hero.jpg',
+					),
+					'count' => 3,
+					'flag'  => true,
+				),
+				$this->mappings()
+			)
+		);
+	}
+
+	public function test_it_transforms_json_strings(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+		$result      = $transformer->transformValue(
+			'{"url":"https:\/\/example.test\/uploads\/hero.jpg"}',
+			$this->mappings()
+		);
+
+		self::assertSame( '{"url":"https:\/\/staging.example.test\/uploads\/hero.jpg"}', $result );
+	}
+
+	public function test_it_preserves_serialized_data_validity(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+		// phpcs:ignore -- Test fixture requires native serialized metadata.
+		$serialized  = serialize(
+			array(
+				'url' => 'https://example.test/uploads/hero.jpg',
+			)
+		);
+
+		$result = $transformer->transformValue( $serialized, $this->mappings() );
+
+		self::assertSame(
+			array(
+				'url' => 'https://staging.example.test/uploads/hero.jpg',
+			),
+			// phpcs:ignore -- Assertion verifies the transformed serialized metadata remains valid.
+			unserialize( $result )
+		);
+	}
+
+	public function test_it_leaves_invalid_serialized_strings_unchanged(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+		$value       = 'a:1:{s:3:"url";s:37:"https://example.test/uploads/hero.jpg";} trailing';
+
+		self::assertSame( $value, $transformer->transformValue( $value, $this->mappings() ) );
+	}
+
+	public function test_it_leaves_whitespace_wrapped_serialized_strings_unchanged(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+		// phpcs:ignore -- Test fixture requires native serialized metadata.
+		$value       = ' ' . serialize( array( 'url' => 'https://example.test/uploads/hero.jpg' ) ) . ' ';
+
+		self::assertSame( $value, $transformer->transformValue( $value, $this->mappings() ) );
+	}
+
+	public function test_it_preserves_serialized_false_and_null_values(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+
+		self::assertSame( 'b:0;', $transformer->transformValue( 'b:0;', $this->mappings() ) );
+		self::assertSame( 'N;', $transformer->transformValue( 'N;', $this->mappings() ) );
+	}
+
+	public function test_it_leaves_serialized_objects_unchanged(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+		$value       = 'O:8:"stdClass":1:{s:3:"url";s:27:"https://example.test/inside";}';
+
+		self::assertSame( $value, $transformer->transformValue( $value, $this->mappings() ) );
+	}
+
+	public function test_it_leaves_serialized_payloads_with_nested_objects_unchanged(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+		$value       = 'a:2:{s:3:"url";s:27:"https://example.test/inside";s:6:"object";O:8:"stdClass":0:{}}';
+
+		self::assertSame( $value, $transformer->transformValue( $value, $this->mappings() ) );
+	}
+
+	public function test_it_leaves_invalid_json_strings_unchanged(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+		$value       = '{"url":"https://example.test/missing"';
+
+		self::assertSame( $value, $transformer->transformValue( $value, $this->mappings() ) );
+	}
+
+	public function test_it_transforms_nested_json_arrays(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+
+		self::assertSame(
+			'[{"url":"https:\/\/staging.example.test\/one"},{"count":2}]',
+			$transformer->transformValue( '[{"url":"https:\/\/example.test\/one"},{"count":2}]', $this->mappings() )
+		);
+	}
+
+	public function test_it_transforms_plain_string_metadata(): void {
+		$transformer = new MetadataUrlTransformer( new UrlTransformer() );
+
+		self::assertSame(
+			'https://staging.example.test/contact',
+			$transformer->transformValue( 'https://example.test/contact', $this->mappings() )
+		);
+	}
+}
diff --git a/tests/Unit/Url/UrlMappingCollectionTest.php b/tests/Unit/Url/UrlMappingCollectionTest.php
new file mode 100644
index 0000000..af5a471
--- /dev/null
+++ b/tests/Unit/Url/UrlMappingCollectionTest.php
@@ -0,0 +1,55 @@
+<?php
+
+namespace WPContentSync\Tests\Unit\Url;
+
+use PHPUnit\Framework\TestCase;
+use WPContentSync\Url\UrlMapping;
+use WPContentSync\Url\UrlMappingCollection;
+
+class UrlMappingCollectionTest extends TestCase {
+	public function test_it_normalizes_mapping_urls_without_trailing_slashes(): void {
+		$mapping = new UrlMapping( 'https://example.test/', 'https://staging.example.test/' );
+
+		self::assertSame( 'https://example.test', $mapping->sourceUrl() );
+		self::assertSame( 'https://staging.example.test', $mapping->destinationUrl() );
+	}
+
+	public function test_it_rejects_empty_mapping_urls(): void {
+		$this->expectException( \InvalidArgumentException::class );
+		$this->expectExceptionMessage( 'Source and destination URLs are required.' );
+
+		new UrlMapping( '', 'https://staging.example.test' );
+	}
+
+	public function test_it_rejects_mapping_urls_without_scheme_and_host(): void {
+		$this->expectException( \InvalidArgumentException::class );
+		$this->expectExceptionMessage( 'Source and destination URLs must include a scheme and host.' );
+
+		new UrlMapping( 'https://', 'https://staging.example.test' );
+	}
+
+	public function test_it_preserves_query_and_fragment_trailing_slashes(): void {
+		$mapping = new UrlMapping(
+			'https://example.test/?redirect=/',
+			'https://staging.example.test/#/'
+		);
+
+		self::assertSame( 'https://example.test?redirect=/', $mapping->sourceUrl() );
+		self::assertSame( 'https://staging.example.test#/', $mapping->destinationUrl() );
+	}
+
+	public function test_it_returns_mappings_by_longest_source_url_first(): void {
+		$short      = new UrlMapping( 'https://example.test', 'https://staging.example.test' );
+		$long       = new UrlMapping( 'https://example.test/uploads', 'https://staging.example.test/uploads' );
+		$collection = new UrlMappingCollection( array( $short, $long ) );
+
+		self::assertSame( array( $long, $short ), $collection->all() );
+	}
+
+	public function test_it_rejects_non_mapping_items(): void {
+		$this->expectException( \InvalidArgumentException::class );
+		$this->expectExceptionMessage( 'URL mapping collections only accept UrlMapping instances.' );
+
+		new UrlMappingCollection( array( new \stdClass() ) );
+	}
+}
diff --git a/tests/Unit/Url/UrlTransformerTest.php b/tests/Unit/Url/UrlTransformerTest.php
new file mode 100644
index 0000000..9a7cd1a
--- /dev/null
+++ b/tests/Unit/Url/UrlTransformerTest.php
@@ -0,0 +1,162 @@
+<?php
+
+namespace WPContentSync\Tests\Unit\Url;
+
+use PHPUnit\Framework\TestCase;
+use WPContentSync\Url\UrlMapping;
+use WPContentSync\Url\UrlMappingCollection;
+use WPContentSync\Url\UrlTransformer;
+
+class UrlTransformerTest extends TestCase {
+	public function test_it_rewrites_plain_urls(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test', 'https://staging.example.test' ),
+			)
+		);
+
+		self::assertSame(
+			'Visit https://staging.example.test/about for details.',
+			$transformer->transformString( 'Visit https://example.test/about for details.', $mappings )
+		);
+	}
+
+	public function test_it_rewrites_html_attribute_urls(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test', 'https://staging.example.test' ),
+			)
+		);
+
+		self::assertSame(
+			'<a href="https://staging.example.test/about"><img src="https://staging.example.test/image.jpg"></a>',
+			$transformer->transformString(
+				'<a href="https://example.test/about"><img src="https://example.test/image.jpg"></a>',
+				$mappings
+			)
+		);
+	}
+
+	public function test_it_rewrites_escaped_urls(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test/path?a=1&b=2', 'https://staging.example.test/path?a=1&b=2' ),
+			)
+		);
+
+		self::assertSame(
+			'https://staging.example.test/path?a=1&amp;b=2',
+			$transformer->transformString( 'https://example.test/path?a=1&amp;b=2', $mappings )
+		);
+	}
+
+	public function test_it_rewrites_protocol_relative_urls(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test', 'https://staging.example.test' ),
+			)
+		);
+
+		self::assertSame(
+			'//staging.example.test/uploads/file.pdf',
+			$transformer->transformString( '//example.test/uploads/file.pdf', $mappings )
+		);
+	}
+
+	public function test_it_supports_multiple_mappings(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test', 'https://staging.example.test' ),
+				new UrlMapping( 'https://cdn.example.test', 'https://cdn.staging.example.test' ),
+			)
+		);
+
+		self::assertSame(
+			'https://staging.example.test https://cdn.staging.example.test/image.jpg',
+			$transformer->transformString( 'https://example.test https://cdn.example.test/image.jpg', $mappings )
+		);
+	}
+
+	public function test_it_does_not_cascade_mapping_destinations_into_other_sources(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://a.example.test', 'https://b.example.test' ),
+				new UrlMapping( 'https://b.example.test', 'https://c.example.test' ),
+			)
+		);
+
+		self::assertSame(
+			'https://b.example.test/page https://c.example.test/page',
+			$transformer->transformString( 'https://a.example.test/page https://b.example.test/page', $mappings )
+		);
+	}
+
+	public function test_it_does_not_rewrite_partial_host_matches(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test', 'https://staging.example.test' ),
+			)
+		);
+
+		self::assertSame(
+			'https://example.test.evil/path https://staging.example.test/path',
+			$transformer->transformString(
+				'https://example.test.evil/path https://example.test/path',
+				$mappings
+			)
+		);
+	}
+
+	public function test_it_does_not_rewrite_userinfo_host_lookalikes(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test', 'https://staging.example.test' ),
+			)
+		);
+
+		self::assertSame(
+			'https://example.test@evil.test/path https://staging.example.test/path',
+			$transformer->transformString(
+				'https://example.test@evil.test/path https://example.test/path',
+				$mappings
+			)
+		);
+	}
+
+	public function test_it_prefers_more_specific_overlapping_mappings(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test', 'https://staging.example.test' ),
+				new UrlMapping( 'https://example.test/uploads', 'https://media.staging.example.test/uploads' ),
+			)
+		);
+
+		self::assertSame(
+			'https://media.staging.example.test/uploads/image.jpg',
+			$transformer->transformString( 'https://example.test/uploads/image.jpg', $mappings )
+		);
+	}
+
+	public function test_it_rewrites_escaped_protocol_relative_urls(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test/path?a=1&b=2', 'https://staging.example.test/path?a=1&b=2' ),
+			)
+		);
+
+		self::assertSame(
+			'//staging.example.test/path?a=1&amp;b=2',
+			$transformer->transformString( '//example.test/path?a=1&amp;b=2', $mappings )
+		);
+	}
+}
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index dbea835..586b930 100644
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -125,6 +125,32 @@ if ( ! function_exists( 'esc_url_raw' ) ) {
 	}
 }
 
+if ( ! function_exists( 'wp_parse_url' ) ) {
+	/**
+	 * Minimal URL parser for unit tests.
+	 *
+	 * @param string $url URL to parse.
+	 * @return array<string, mixed>|false
+	 */
+	function wp_parse_url( $url ) {
+		// phpcs:ignore WordPress.WP.AlternativeFunctions.parse_url_parse_url -- Test stub for WordPress' wp_parse_url().
+		return parse_url( $url );
+	}
+}
+
+if ( ! function_exists( 'wp_json_encode' ) ) {
+	/**
+	 * Minimal JSON encoder for unit tests.
+	 *
+	 * @param mixed $value Value to encode.
+	 * @return string|false
+	 */
+	function wp_json_encode( $value ) {
+		// phpcs:ignore -- Test stub for WordPress' wp_json_encode().
+		return json_encode( $value );
+	}
+}
+
 if ( ! function_exists( 'get_option' ) ) {
 	/**
 	 * Minimal WordPress option reader for unit tests.