diff --git a/extension.json b/extension.json
index 58416a89e..4997213fe 100644
--- a/extension.json
+++ b/extension.json
@@ -48,8 +48,7 @@
 						"discussiontools-replywidget-feedback-link",
 						"discussiontools-replywidget-feedback-link-newtopic",
 						"discussiontools-replywidget-mention-prefix",
-						"discussiontools-replywidget-mention-suffix",
-						"discussiontools-signature-prefix"
+						"discussiontools-replywidget-mention-suffix"
 					]
 				},
 				{
@@ -295,8 +294,6 @@
 			"cases/modified.json",
 			"cases/reply.json",
 			"cases/unwrap.json",
-			"cases/isWikitextSigned.json",
-			"cases/isHtmlSigned.json",
 			"cases/linearWalk.json",
 			"cases/sanitize-wikitext-linebreaks.json",
 			"cases/timestamp-regex.json",
@@ -422,6 +419,9 @@
 		"discussiontoolspageinfo": {
 			"class": "MediaWiki\\Extension\\DiscussionTools\\ApiDiscussionToolsPageInfo"
 		},
+		"discussiontoolspreview": {
+			"class": "MediaWiki\\Extension\\DiscussionTools\\ApiDiscussionToolsPreview"
+		},
 		"discussiontoolssubscribe": {
 			"class": "MediaWiki\\Extension\\DiscussionTools\\ApiDiscussionToolsSubscribe",
 			"services": [
diff --git a/i18n/api/en.json b/i18n/api/en.json
index 30a6b6b30..f86045a5d 100644
--- a/i18n/api/en.json
+++ b/i18n/api/en.json
@@ -22,6 +22,9 @@
 	"apihelp-discussiontoolsgetsubscriptions-summary": "Get the subscription statuses of given topics.",
 	"apihelp-discussiontoolspageinfo-param-oldid": "The revision number to use (defaults to latest revision).",
 	"apihelp-discussiontoolspageinfo-summary": "Returns metadata required to initialize the discussion tools.",
+	"apihelp-discussiontoolspreview-param-type": "Type of message to preview",
+	"apihelp-discussiontoolspreview-param-wikitext": "Content to preview, as wikitext.",
+	"apihelp-discussiontoolspreview-summary": "Preview a message on a discussion page.",
 	"apihelp-discussiontoolssubscribe-param-commentname": "Name of the topic to subscribe to (or unsubscribe from)",
 	"apihelp-discussiontoolssubscribe-param-page": "A page on which the topic appears",
 	"apihelp-discussiontoolssubscribe-param-subscribe": "True to subscribe, false to unsubscribe",
diff --git a/i18n/api/qqq.json b/i18n/api/qqq.json
index a1df8861c..d50b1e04c 100644
--- a/i18n/api/qqq.json
+++ b/i18n/api/qqq.json
@@ -24,6 +24,9 @@
 	"apihelp-discussiontoolsgetsubscriptions-summary": "{{doc-apihelp-summary|discussiontoolsgetsubscriptions}}",
 	"apihelp-discussiontoolspageinfo-param-oldid": "{{doc-apihelp-param|discussiontoolspageinfo|oldid}}",
 	"apihelp-discussiontoolspageinfo-summary": "{{doc-apihelp-summary|discussiontoolspageinfo}}",
+	"apihelp-discussiontoolspreview-param-type": "{{doc-apihelp-param|discussiontoolspreview|type}}",
+	"apihelp-discussiontoolspreview-param-wikitext": "{{doc-apihelp-param|discussiontoolspreview|wikitext}}",
+	"apihelp-discussiontoolspreview-summary": "{{doc-apihelp-summary|discussiontoolspreview}}",
 	"apihelp-discussiontoolssubscribe-param-commentname": "{{doc-apihelp-param|discussiontoolssubscribe|commentname}}",
 	"apihelp-discussiontoolssubscribe-param-page": "{{doc-apihelp-param|discussiontoolssubscribe|page}}",
 	"apihelp-discussiontoolssubscribe-param-subscribe": "{{doc-apihelp-param|discussiontoolssubscribe|subscribe}}",
diff --git a/includes/ApiDiscussionToolsPreview.php b/includes/ApiDiscussionToolsPreview.php
new file mode 100644
index 000000000..3a8f49f26
--- /dev/null
+++ b/includes/ApiDiscussionToolsPreview.php
@@ -0,0 +1,94 @@
+<?php
+
+namespace MediaWiki\Extension\DiscussionTools;
+
+use ApiBase;
+use ApiMain;
+use ApiParsoidTrait;
+use Title;
+use Wikimedia\ParamValidator\ParamValidator;
+
+class ApiDiscussionToolsPreview extends ApiBase {
+
+	use ApiDiscussionToolsTrait;
+	use ApiParsoidTrait;
+
+	/**
+	 * @inheritDoc
+	 */
+	public function __construct( ApiMain $main, string $name ) {
+		parent::__construct( $main, $name );
+	}
+
+	/**
+	 * @inheritDoc
+	 */
+	public function execute() {
+		$params = $this->extractRequestParams();
+		$title = Title::newFromText( $params['page'] );
+
+		if ( !$title ) {
+			$this->dieWithError( [ 'apierror-invalidtitle', wfEscapeWikiText( $params['page'] ) ] );
+		}
+		if ( $params['type'] === 'topic' ) {
+			$this->requireAtLeastOneParameter( $params, 'sectiontitle' );
+		}
+
+		// Add signature if missing
+		$signature = null;
+		if ( !CommentModifier::isWikitextSigned( $params['wikitext'] ) ) {
+			$signature = $this->msg( 'discussiontools-signature-prefix' )->inContentLanguage()->text() . '~~~~';
+			// Drop opacity of signature in preview to make message body preview clearer.
+			// Extract any leading spaces outside the <span> markup to ensure accurate previews.
+			$signature = preg_replace_callback( '/^( *)(.+)$/', static function ( $matches ) {
+				list( , $leadingSpaces, $sig ) = $matches;
+				return $leadingSpaces . '<span style="opacity: 0.6;">' . $sig . '</span>';
+			}, $signature );
+		}
+
+		$result = $this->previewMessage( [
+			'type' => $params['type'],
+			'title' => $title,
+			'wikitext' => $params['wikitext'],
+			'sectiontitle' => $params['sectiontitle'],
+			'signature' => $signature,
+		] );
+
+		$this->getResult()->addValue( null, $this->getModuleName(), $result->serializeForApiResult() );
+	}
+
+	/**
+	 * @inheritDoc
+	 */
+	public function getAllowedParams() {
+		return [
+			'type' => [
+				ParamValidator::PARAM_REQUIRED => true,
+				ParamValidator::PARAM_TYPE => [
+					'reply',
+					'topic',
+				],
+				ApiBase::PARAM_HELP_MSG_PER_VALUE => [],
+			],
+			'page' => [
+				ParamValidator::PARAM_REQUIRED => true,
+				ApiBase::PARAM_HELP_MSG => 'apihelp-visualeditoredit-param-page',
+			],
+			'wikitext' => [
+				ParamValidator::PARAM_REQUIRED => true,
+				ParamValidator::PARAM_TYPE => 'text',
+			],
+			'sectiontitle' => [
+				ParamValidator::PARAM_TYPE => 'string',
+				ApiBase::PARAM_HELP_MSG => 'apihelp-edit-param-sectiontitle',
+			],
+		];
+	}
+
+	/**
+	 * @inheritDoc
+	 */
+	public function isInternal() {
+		return true;
+	}
+}
diff --git a/includes/ApiDiscussionToolsTrait.php b/includes/ApiDiscussionToolsTrait.php
index f7ae636d5..359d16a67 100644
--- a/includes/ApiDiscussionToolsTrait.php
+++ b/includes/ApiDiscussionToolsTrait.php
@@ -2,12 +2,20 @@
 
 namespace MediaWiki\Extension\DiscussionTools;
 
+use ApiMain;
+use ApiResult;
+use DerivativeContext;
+use DerivativeRequest;
+use IContextSource;
 use MediaWiki\MediaWikiServices;
 use MediaWiki\Revision\RevisionRecord;
 use Title;
 use Wikimedia\Parsoid\Utils\DOMCompat;
 use Wikimedia\Parsoid\Utils\DOMUtils;
 
+/**
+ * Random methods we want to share between API modules.
+ */
 trait ApiDiscussionToolsTrait {
 	/**
 	 * @param RevisionRecord $revision
@@ -29,9 +37,99 @@ trait ApiDiscussionToolsTrait {
 		return $parser->parse( $container, $title );
 	}
 
+	/**
+	 * Given parameters describing a reply or new topic, transform them into wikitext using Parsoid,
+	 * then preview the wikitext using the legacy parser.
+	 *
+	 * @param array $params Associative array with the following keys:
+	 *  - `type` (string) 'topic' or 'reply'
+	 *  - `title` (Title) Context title for wikitext transformations
+	 *  - `wikitext` (string) Content of the message
+	 *  - `sectiontitle` (string) Content of the title, when `type` is 'topic'
+	 *  - `signature` (string|null) Wikitext signature to add to the message
+	 * @return ApiResult action=parse API result
+	 */
+	protected function previewMessage( array $params ): ApiResult {
+		$wikitext = $params['wikitext'];
+		$title = $params['title'];
+		$signature = $params['signature'] ?? null;
+
+		switch ( $params['type'] ) {
+			case 'topic':
+				$wikitext = CommentUtils::htmlTrim( $wikitext );
+				if ( $signature !== null ) {
+					$wikitext .= $signature;
+				}
+
+				if ( $params['sectiontitle'] ) {
+					$wikitext = "== " . $params['sectiontitle'] . " ==\n" . $wikitext;
+				}
+
+				break;
+
+			case 'reply':
+				$doc = DOMUtils::parseHTML( '' );
+
+				$container = CommentModifier::prepareWikitextReply( $doc, $wikitext );
+
+				if ( $signature !== null ) {
+					CommentModifier::appendSignature( $container, $signature );
+				}
+				$list = CommentModifier::transferReply( $container );
+				$html = DOMCompat::getOuterHTML( $list );
+
+				$wikitext = $this->transformHTML( $title, $html )[ 'body' ];
+
+				break;
+		}
+
+		$apiParams = [
+			'action' => 'parse',
+			'title' => $title->getPrefixedText(),
+			'text' => $wikitext,
+			'pst' => '1',
+			'preview' => '1',
+			'disableeditsection' => '1',
+			'prop' => 'text|modules|jsconfigvars',
+		];
+
+		$context = new DerivativeContext( $this->getContext() );
+		$context->setRequest(
+			new DerivativeRequest(
+				$context->getRequest(),
+				$apiParams,
+				/* was posted? */ true
+			)
+		);
+		$api = new ApiMain(
+			$context,
+			/* enable write? */ false
+		);
+
+		$api->execute();
+		return $api->getResult();
+	}
+
 	/**
 	 * @param RevisionRecord $revision
 	 * @return array
 	 */
 	abstract protected function requestRestbasePageHtml( RevisionRecord $revision ): array;
+
+	/**
+	 * @param Title $title
+	 * @param string $html
+	 * @param int|null $oldid
+	 * @param string|null $etag
+	 * @return array
+	 */
+	abstract protected function transformHTML(
+		Title $title, string $html, int $oldid = null, string $etag = null
+	): array;
+
+	/**
+	 * @return IContextSource
+	 */
+	abstract public function getContext();
+
 }
diff --git a/includes/CommentModifier.php b/includes/CommentModifier.php
index eab1ce196..cba357ce1 100644
--- a/includes/CommentModifier.php
+++ b/includes/CommentModifier.php
@@ -541,6 +541,31 @@ class CommentModifier {
 		}
 	}
 
+	/**
+	 * Transfer comment DOM nodes into a list node, as if adding a reply, but without requiring a
+	 * ThreadItem.
+	 *
+	 * @param DocumentFragment $container Container of comment DOM nodes
+	 * @return Element $node List node
+	 */
+	public static function transferReply( DocumentFragment $container ): Element {
+		$services = MediaWikiServices::getInstance();
+		$dtConfig = $services->getConfigFactory()->makeConfig( 'discussiontools' );
+		$replyIndentation = $dtConfig->get( 'DiscussionToolsReplyIndentation' );
+
+		$doc = $container->ownerDocument;
+
+		// Like addReply(), but we make our own list
+		$list = $doc->createElement( $replyIndentation === 'invisible' ? 'dl' : 'ul' );
+		while ( $container->childNodes->length ) {
+			$item = $doc->createElement( $replyIndentation === 'invisible' ? 'dd' : 'li' );
+			self::whitespaceParsoidHack( $item );
+			$item->appendChild( $container->firstChild );
+			$list->appendChild( $item );
+		}
+		return $list;
+	}
+
 	/**
 	 * Create a container of comment DOM nodes from wikitext
 	 *
diff --git a/modules/dt.init.js b/modules/dt.init.js
index 80df07b59..6b7a3310c 100644
--- a/modules/dt.init.js
+++ b/modules/dt.init.js
@@ -1,5 +1,4 @@
 var controller = require( './controller.js' ),
-	config = require( './config.json' ),
 	uri = new mw.Uri();
 
 /**
@@ -74,6 +73,5 @@ module.exports = {
 	HeadingItem: require( './HeadingItem.js' ),
 	CommentItem: require( './CommentItem.js' ),
 	utils: require( './utils.js' ),
-	logger: require( './logger.js' ),
-	config: config
+	logger: require( './logger.js' )
 };
diff --git a/modules/dt.ui.ReplyWidget.js b/modules/dt.ui.ReplyWidget.js
index 2bb7f348a..f754dd8da 100644
--- a/modules/dt.ui.ReplyWidget.js
+++ b/modules/dt.ui.ReplyWidget.js
@@ -1,8 +1,6 @@
 var controller = require( 'ext.discussionTools.init' ).controller,
-	modifier = require( 'ext.discussionTools.init' ).modifier,
 	utils = require( 'ext.discussionTools.init' ).utils,
 	logger = require( 'ext.discussionTools.init' ).logger,
-	dtConf = require( 'ext.discussionTools.init' ).config,
 	ModeTabSelectWidget = require( './ModeTabSelectWidget.js' ),
 	ModeTabOptionWidget = require( './ModeTabOptionWidget.js' ),
 	licenseMessages = require( './licenseMessages.json' ),
@@ -708,7 +706,6 @@ ReplyWidget.prototype.preparePreview = function ( wikitext ) {
 		return $.Deferred().resolve().promise();
 	}
 
-	var indent = dtConf.replyIndentation === 'invisible' ? ':' : '*';
 	wikitext = wikitext !== undefined ? wikitext : this.getValue();
 	wikitext = utils.htmlTrim( wikitext );
 	var title = this.isNewTopic && this.commentController.sectionTitle.getValue();
@@ -728,40 +725,22 @@ ReplyWidget.prototype.preparePreview = function ( wikitext ) {
 	if ( !wikitext ) {
 		parsePromise = $.Deferred().resolve( null ).promise();
 	} else {
-		wikitext = this.commentController.doIndentReplacements( wikitext, indent );
-
-		if ( !modifier.isWikitextSigned( wikitext ) ) {
-			// Add signature.
-			var signature = mw.msg( 'discussiontools-signature-prefix' ) + '~~~~';
-			// Drop opacity of signature in preview to make message body preview clearer.
-			// Extract any leading spaces outside the <span> markup to ensure accurate previews.
-			signature = signature.replace( /^( *)(.+)$/, function ( _, leadingSpaces, sig ) {
-				return leadingSpaces + '<span style="opacity: 0.6;">' + sig + '</span>';
-			} );
-			wikitext += signature;
-		}
-		if ( title ) {
-			wikitext = '== ' + title + ' ==\n' + wikitext;
-		}
 		this.previewRequest = parsePromise = controller.getApi().post( {
-			action: 'parse',
-			text: wikitext,
-			pst: true,
-			preview: true,
-			disableeditsection: true,
-			prop: [ 'text', 'modules', 'jsconfigvars' ],
-			title: this.pageName
+			action: 'discussiontoolspreview',
+			type: this.isNewTopic ? 'topic' : 'reply',
+			page: this.pageName,
+			wikitext: wikitext,
+			sectiontitle: title
 		} );
 	}
-	// TODO: Add list context
 
 	return parsePromise.then( function ( response ) {
-		widget.$preview.html( response ? response.parse.text : '' );
+		widget.$preview.html( response ? response.discussiontoolspreview.parse.text : '' );
 
 		if ( response ) {
-			mw.config.set( response.parse.jsconfigvars );
-			mw.loader.load( response.parse.modulestyles );
-			mw.loader.load( response.parse.modules );
+			mw.config.set( response.discussiontoolspreview.parse.jsconfigvars );
+			mw.loader.load( response.discussiontoolspreview.parse.modulestyles );
+			mw.loader.load( response.discussiontoolspreview.parse.modules );
 		}
 
 		mw.hook( 'wikipage.content' ).fire( widget.$preview );
diff --git a/modules/modifier.js b/modules/modifier.js
index 0a8af29c7..11ee7de97 100644
--- a/modules/modifier.js
+++ b/modules/modifier.js
@@ -363,57 +363,11 @@ function addSiblingListItem( previousItem ) {
 	return listItem;
 }
 
-/**
- * Check whether wikitext contains a user signature.
- *
- * @param {string} wikitext
- * @return {boolean}
- */
-function isWikitextSigned( wikitext ) {
-	wikitext = utils.htmlTrim( wikitext );
-	// Contains ~~~~ (four tildes), but not ~~~~~ (five tildes), at the end.
-	return /([^~]|^)~~~~$/.test( wikitext );
-}
-
-/**
- * Check whether HTML node contains a user signature.
- *
- * @param {HTMLElement} container
- * @return {boolean}
- */
-function isHtmlSigned( container ) {
-	// Good enough?…
-	var matches = container.querySelectorAll( 'span[typeof="mw:Transclusion"][data-mw*="~~~~"]' );
-	if ( matches.length === 0 ) {
-		return false;
-	}
-	var lastSig = matches[ matches.length - 1 ];
-	// Signature must be at the end of the comment - there must be no sibling following this node, or its parents
-	var node = lastSig;
-	while ( node ) {
-		// Skip over whitespace nodes
-		while (
-			node.nextSibling &&
-			node.nextSibling.nodeType === Node.TEXT_NODE &&
-			utils.htmlTrim( node.nextSibling.textContent ) === ''
-		) {
-			node = node.nextSibling;
-		}
-		if ( node.nextSibling ) {
-			return false;
-		}
-		node = node.parentNode;
-	}
-	return true;
-}
-
 module.exports = {
 	addReplyLink: addReplyLink,
 	addListItem: addListItem,
 	removeAddedListItem: removeAddedListItem,
 	addSiblingListItem: addSiblingListItem,
 	unwrapList: unwrapList,
-	isWikitextSigned: isWikitextSigned,
-	isHtmlSigned: isHtmlSigned,
 	sanitizeWikitextLinebreaks: sanitizeWikitextLinebreaks
 };
diff --git a/tests/qunit/modifier.test.js b/tests/qunit/modifier.test.js
index 7f44739c0..6bb421d83 100644
--- a/tests/qunit/modifier.test.js
+++ b/tests/qunit/modifier.test.js
@@ -122,33 +122,6 @@ QUnit.test( '#unwrapList', function ( assert ) {
 	} );
 } );
 
-QUnit.test( 'isWikitextSigned', function ( assert ) {
-	var cases = require( '../cases/isWikitextSigned.json' );
-
-	cases.forEach( function ( caseItem ) {
-		assert.strictEqual(
-			modifier.isWikitextSigned( caseItem.wikitext ),
-			caseItem.expected,
-			caseItem.msg
-		);
-	} );
-} );
-
-QUnit.test( 'isHtmlSigned', function ( assert ) {
-	var cases = require( '../cases/isHtmlSigned.json' );
-
-	cases.forEach( function ( caseItem ) {
-		var container = document.createElement( 'div' );
-		container.innerHTML = caseItem.html;
-
-		assert.strictEqual(
-			modifier.isHtmlSigned( container ),
-			caseItem.expected,
-			caseItem.msg
-		);
-	} );
-} );
-
 QUnit.test( 'sanitizeWikitextLinebreaks', function ( assert ) {
 	var cases = require( '../cases/sanitize-wikitext-linebreaks.json' );