diff --git a/.github/changelog/content-parser-interface b/.github/changelog/content-parser-interface
new file mode 100644
index 0000000..4715d62
--- /dev/null
+++ b/.github/changelog/content-parser-interface
@@ -0,0 +1,4 @@
+Significance: minor
+Type: added
+
+Add extensible content parser support and a JSON preview endpoint for AT Protocol records.
diff --git a/includes/class-atmosphere.php b/includes/class-atmosphere.php
index 8110424..8ab16b3 100644
--- a/includes/class-atmosphere.php
+++ b/includes/class-atmosphere.php
@@ -44,6 +44,9 @@ public function init(): void {
 		// Plugin integrations.
 		Load::init();
 
+		// JSON preview for AT Protocol records.
+		\add_action( 'template_redirect', array( $this, 'preview' ) );
+
 		// Post lifecycle hooks.
 		\add_action( 'transition_post_status', array( $this, 'on_status_change' ), 10, 3 );
 
@@ -145,6 +148,43 @@ public function serve_wellknown_publication(): void {
 		exit;
 	}
 
+	/**
+	 * Serve a JSON preview of the AT Protocol record for a post.
+	 *
+	 * Append ?atproto to a singular post URL to see the document
+	 * record JSON. Requires the edit_posts capability.
+	 */
+	public function preview(): void {
+		// phpcs:ignore WordPress.Security.NonceVerification.Recommended
+		if ( ! isset( $_GET['atproto'] ) || ! \is_singular() ) {
+			return;
+		}
+
+		if ( ! \current_user_can( 'edit_posts' ) ) {
+			return;
+		}
+
+		$post = \get_queried_object();
+
+		if ( ! $post instanceof \WP_Post ) {
+			return;
+		}
+
+		if ( ! \in_array( $post->post_type, Backfill::syncable_post_types(), true ) ) {
+			\status_header( 404 );
+			exit;
+		}
+
+		$transformer = new Document( $post );
+		$record      = $transformer->transform();
+
+		\status_header( 200 );
+		\header( 'Content-Type: application/json; charset=utf-8' );
+		// phpcs:ignore WordPress.WP.AlternativeFunctions.json_encode_json_encode
+		echo \wp_json_encode( $record, JSON_PRETTY_PRINT | JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE );
+		exit;
+	}
+
 	/**
 	 * Handle post status transitions.
 	 *
diff --git a/includes/content-parser/interface-content-parser.php b/includes/content-parser/interface-content-parser.php
new file mode 100644
index 0000000..4f11827
--- /dev/null
+++ b/includes/content-parser/interface-content-parser.php
@@ -0,0 +1,42 @@
+<?php
+/**
+ * Content parser interface for AT Protocol content formats.
+ *
+ * Plugins can implement this interface to provide custom content
+ * parsers for the site.standard.document content union field.
+ *
+ * @package Atmosphere
+ */
+
+namespace Atmosphere\Content_Parser;
+
+\defined( 'ABSPATH' ) || exit;
+
+/**
+ * Content parser contract.
+ */
+interface Content_Parser {
+
+	/**
+	 * Parse WordPress post content into an AT Protocol content object.
+	 *
+	 * The returned array must include a '$type' key identifying the
+	 * lexicon type (e.g. 'at.markpub.markdown').
+	 *
+	 * Receives raw post content so parsers can choose their own
+	 * strategy: parse_blocks() for block-aware parsing, or
+	 * apply_filters( 'the_content', ... ) for rendered HTML.
+	 *
+	 * @param string   $content Raw post content (post_content).
+	 * @param \WP_Post $post    The WordPress post object.
+	 * @return array AT Protocol content object.
+	 */
+	public function parse( string $content, \WP_Post $post ): array;
+
+	/**
+	 * The lexicon NSID this parser produces.
+	 *
+	 * @return string e.g. 'at.markpub.markdown'.
+	 */
+	public function get_type(): string;
+}
diff --git a/includes/transformer/class-document.php b/includes/transformer/class-document.php
index c96788e..564280e 100644
--- a/includes/transformer/class-document.php
+++ b/includes/transformer/class-document.php
@@ -13,6 +13,7 @@
 
 \defined( 'ABSPATH' ) || exit;
 
+use Atmosphere\Content_Parser\Content_Parser;
 use function Atmosphere\build_at_uri;
 use function Atmosphere\get_did;
 use function Atmosphere\sanitize_text;
@@ -55,10 +56,13 @@ public function transform(): array {
 			'publishedAt' => $this->to_iso8601( $this->object->post_date_gmt ),
 		);
 
-		// Publication reference.
+		// Publication reference (required by spec).
 		$pub_tid = \get_option( 'atmosphere_publication_tid' );
 		if ( $pub_tid ) {
 			$record['site'] = build_at_uri( get_did(), 'site.standard.publication', $pub_tid );
+		} else {
+			// Fall back to site URL for standalone documents.
+			$record['site'] = \untrailingslashit( \get_home_url() );
 		}
 
 		// Relative path.
@@ -89,6 +93,12 @@ public function transform(): array {
 			$record['textContent'] = $text_content;
 		}
 
+		// Parsed rich content (open union).
+		$content = $this->get_content();
+		if ( ! empty( $content ) ) {
+			$record['content'] = $content;
+		}
+
 		// Tags.
 		$tags = $this->collect_tags( $this->object );
 		if ( ! empty( $tags ) ) {
@@ -140,6 +150,43 @@ public function get_rkey(): string {
 		return $rkey;
 	}
 
+	/**
+	 * Get parsed content for the document's content union field.
+	 *
+	 * @return array|null Parsed content object or null.
+	 */
+	private function get_content(): ?array {
+		if ( empty( \trim( $this->object->post_content ) ) ) {
+			return null;
+		}
+
+		/**
+		 * Filters the content parser used for site.standard.document records.
+		 *
+		 * Return a Content_Parser instance to provide a parser.
+		 * Return null to disable the content field entirely.
+		 *
+		 * @param Content_Parser|null $parser The content parser. Default: null.
+		 * @param \WP_Post            $post   The WordPress post.
+		 */
+		$parser = \apply_filters( 'atmosphere_content_parser', null, $this->object );
+
+		if ( ! $parser instanceof Content_Parser ) {
+			return null;
+		}
+
+		$content = $parser->parse( $this->object->post_content, $this->object );
+
+		/**
+		 * Filters the parsed content object before adding to the document record.
+		 *
+		 * @param array          $content The parsed content object.
+		 * @param \WP_Post       $post    The WordPress post.
+		 * @param Content_Parser $parser  The parser that produced the content.
+		 */
+		return \apply_filters( 'atmosphere_document_content', $content, $this->object, $parser );
+	}
+
 	/**
 	 * Render post content to plain text.
 	 *
diff --git a/tests/phpunit/tests/transformer/class-stub-parser.php b/tests/phpunit/tests/transformer/class-stub-parser.php
new file mode 100644
index 0000000..b869820
--- /dev/null
+++ b/tests/phpunit/tests/transformer/class-stub-parser.php
@@ -0,0 +1,36 @@
+<?php
+/**
+ * Stub content parser for testing.
+ *
+ * @package Atmosphere
+ */
+
+namespace Atmosphere\Tests\Transformer;
+
+use Atmosphere\Content_Parser\Content_Parser;
+
+/**
+ * Stub content parser that returns raw content as-is.
+ */
+class Stub_Parser implements Content_Parser {
+
+	/**
+	 * {@inheritDoc}
+	 */
+	public function get_type(): string {
+		return 'test.stub.parser';
+	}
+
+	/**
+	 * {@inheritDoc}
+	 *
+	 * @param string   $content Raw post content.
+	 * @param \WP_Post $post    The WordPress post object.
+	 */
+	public function parse( string $content, \WP_Post $post ): array { // phpcs:ignore VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable
+		return array(
+			'$type' => 'test.stub.parser',
+			'text'  => $content,
+		);
+	}
+}
diff --git a/tests/phpunit/tests/transformer/class-test-document.php b/tests/phpunit/tests/transformer/class-test-document.php
new file mode 100644
index 0000000..3915ac3
--- /dev/null
+++ b/tests/phpunit/tests/transformer/class-test-document.php
@@ -0,0 +1,195 @@
+<?php
+/**
+ * Tests for the Document transformer content parser integration.
+ *
+ * @package Atmosphere
+ * @group atmosphere
+ * @group transformer
+ */
+
+namespace Atmosphere\Tests\Transformer;
+
+require_once __DIR__ . '/class-stub-parser.php';
+
+use WP_UnitTestCase;
+use Atmosphere\Transformer\Document;
+
+/**
+ * Document transformer tests.
+ */
+class Test_Document extends WP_UnitTestCase {
+
+	/**
+	 * Test that content field is absent when no parser is registered.
+	 */
+	public function test_content_absent_without_parser() {
+		$post = self::factory()->post->create_and_get(
+			array( 'post_content' => 'Some content here.' )
+		);
+
+		$transformer = new Document( $post );
+		$record      = $transformer->transform();
+
+		$this->assertArrayNotHasKey( 'content', $record );
+	}
+
+	/**
+	 * Test that content field is present when a parser is registered via filter.
+	 */
+	public function test_content_present_with_parser_filter() {
+		\add_filter(
+			'atmosphere_content_parser',
+			static fn() => new Stub_Parser()
+		);
+
+		$post = self::factory()->post->create_and_get(
+			array( 'post_content' => 'Hello world.' )
+		);
+
+		$transformer = new Document( $post );
+		$record      = $transformer->transform();
+
+		$this->assertArrayHasKey( 'content', $record );
+		$this->assertSame( 'test.stub.parser', $record['content']['$type'] );
+		$this->assertSame( 'Hello world.', $record['content']['text'] );
+
+		\remove_all_filters( 'atmosphere_content_parser' );
+	}
+
+	/**
+	 * Test that returning null from the parser filter disables content.
+	 */
+	public function test_content_disabled_with_null_filter() {
+		\add_filter( 'atmosphere_content_parser', '__return_null' );
+
+		$post = self::factory()->post->create_and_get(
+			array( 'post_content' => 'Some content.' )
+		);
+
+		$transformer = new Document( $post );
+		$record      = $transformer->transform();
+
+		$this->assertArrayNotHasKey( 'content', $record );
+
+		\remove_all_filters( 'atmosphere_content_parser' );
+	}
+
+	/**
+	 * Test that a non-Content_Parser return from the filter is ignored.
+	 */
+	public function test_content_ignored_with_invalid_parser() {
+		\add_filter(
+			'atmosphere_content_parser',
+			static fn() => 'not a parser'
+		);
+
+		$post = self::factory()->post->create_and_get(
+			array( 'post_content' => 'Some content.' )
+		);
+
+		$transformer = new Document( $post );
+		$record      = $transformer->transform();
+
+		$this->assertArrayNotHasKey( 'content', $record );
+
+		\remove_all_filters( 'atmosphere_content_parser' );
+	}
+
+	/**
+	 * Test that content field is absent for empty post content.
+	 */
+	public function test_content_absent_for_empty_content() {
+		\add_filter(
+			'atmosphere_content_parser',
+			static fn() => new Stub_Parser()
+		);
+
+		$post = self::factory()->post->create_and_get(
+			array( 'post_content' => '' )
+		);
+
+		$transformer = new Document( $post );
+		$record      = $transformer->transform();
+
+		$this->assertArrayNotHasKey( 'content', $record );
+
+		\remove_all_filters( 'atmosphere_content_parser' );
+	}
+
+	/**
+	 * Test the atmosphere_document_content filter can modify parsed content.
+	 */
+	public function test_document_content_filter() {
+		\add_filter(
+			'atmosphere_content_parser',
+			static fn() => new Stub_Parser()
+		);
+
+		\add_filter(
+			'atmosphere_document_content',
+			static function ( array $content ) {
+				$content['modified'] = true;
+				return $content;
+			}
+		);
+
+		$post = self::factory()->post->create_and_get(
+			array( 'post_content' => 'Hello.' )
+		);
+
+		$transformer = new Document( $post );
+		$record      = $transformer->transform();
+
+		$this->assertArrayHasKey( 'content', $record );
+		$this->assertTrue( $record['content']['modified'] );
+
+		\remove_all_filters( 'atmosphere_content_parser' );
+		\remove_all_filters( 'atmosphere_document_content' );
+	}
+
+	/**
+	 * Test that site field falls back to home URL without publication TID.
+	 */
+	public function test_site_fallback_to_home_url() {
+		\delete_option( 'atmosphere_publication_tid' );
+
+		$post = self::factory()->post->create_and_get();
+
+		$transformer = new Document( $post );
+		$record      = $transformer->transform();
+
+		$this->assertArrayHasKey( 'site', $record );
+		$this->assertSame( \untrailingslashit( \get_home_url() ), $record['site'] );
+	}
+
+	/**
+	 * Test that site field uses AT-URI when publication TID exists.
+	 */
+	public function test_site_uses_at_uri_with_publication_tid() {
+		\update_option( 'atmosphere_publication_tid', 'test-tid-123' );
+		\update_option( 'atmosphere_did', 'did:plc:test' );
+
+		$post = self::factory()->post->create_and_get();
+
+		$transformer = new Document( $post );
+		$record      = $transformer->transform();
+
+		$this->assertArrayHasKey( 'site', $record );
+		$this->assertStringStartsWith( 'at://', $record['site'] );
+		$this->assertStringContainsString( 'site.standard.publication', $record['site'] );
+		$this->assertStringContainsString( 'test-tid-123', $record['site'] );
+
+		\delete_option( 'atmosphere_publication_tid' );
+		\delete_option( 'atmosphere_did' );
+	}
+
+	/**
+	 * Test the collection NSID.
+	 */
+	public function test_collection() {
+		$post        = self::factory()->post->create_and_get();
+		$transformer = new Document( $post );
+
+		$this->assertSame( 'site.standard.document', $transformer->get_collection() );
+	}
+}