diff --git a/tests/src/Protocol/ActivityPub/ProcessorMock.php b/tests/src/Protocol/ActivityPub/ProcessorMock.php
new file mode 100644
index 0000000000..8d509bf9de
--- /dev/null
+++ b/tests/src/Protocol/ActivityPub/ProcessorMock.php
@@ -0,0 +1,40 @@
+<?php
+/**
+ * @copyright Copyright (C) 2010-2021, the Friendica project
+ *
+ * @license GNU AGPL version 3 or any later version
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ *
+ */
+
+namespace Friendica\Test\src\Protocol\ActivityPub;
+
+use Friendica\Protocol\ActivityPub\Processor;
+
+/**
+ * Class ProcessorMock
+ *
+ * Exposes protected methods for test in the inherited class
+ *
+ * @method static string addMentionLinks(string $body, array $tags)
+ * @method static string normalizeMentionLinks(string $body)
+ */
+class ProcessorMock extends Processor
+{
+	public static function __callStatic($name, $arguments)
+	{
+		return self::$name(...$arguments);
+	}
+}
diff --git a/tests/src/Protocol/ActivityPub/ProcessorTest.php b/tests/src/Protocol/ActivityPub/ProcessorTest.php
new file mode 100644
index 0000000000..894e90af28
--- /dev/null
+++ b/tests/src/Protocol/ActivityPub/ProcessorTest.php
@@ -0,0 +1,87 @@
+<?php
+
+namespace Friendica\Test\src\Protocol\ActivityPub;
+
+use PHPUnit\Framework\TestCase;
+
+class ProcessorTest extends TestCase
+{
+	public function dataNormalizeMentionLinks(): array
+	{
+		return [
+			'one-link-@' => [
+				'expected' => '@[url=https://example.com]Example[/url]',
+				'body'     => '[url=https://example.com]@Example[/url]',
+			],
+			'one-link-#' => [
+				'expected' => '#[url=https://example.com]Example[/url]',
+				'body'     => '[url=https://example.com]#Example[/url]',
+			],
+			'one-link-!' => [
+				'expected' => '![url=https://example.com]Example[/url]',
+				'body'     => '[url=https://example.com]!Example[/url]',
+			],
+			'wrong-hash-char' => [
+				'expected' => '[url=https://example.com]%Example[/url]',
+				'body'     => '[url=https://example.com]%Example[/url]',
+			],
+			'multiple-links' => [
+				'expected' => '@[url=https://example.com]Example[/url] #[url=https://example.com]Example[/url] ![url=https://example.com]Example[/url]',
+				'body'     => '[url=https://example.com]@Example[/url] [url=https://example.com]#Example[/url] [url=https://example.com]!Example[/url]',
+			],
+			'already-correct-format' => [
+				'expected' => '@[url=https://example.com]Example[/url] #[url=https://example.com]Example[/url] ![url=https://example.com]Example[/url]',
+				'body'     => '@[url=https://example.com]Example[/url] #[url=https://example.com]Example[/url] ![url=https://example.com]Example[/url]',
+			],
+			'mixed-format' => [
+				'expected' => '@[url=https://example.com]Example[/url] #[url=https://example.com]Example[/url] ![url=https://example.com]Example[/url] @[url=https://example.com]Example[/url] #[url=https://example.com]Example[/url] ![url=https://example.com]Example[/url]',
+				'body'     => '[url=https://example.com]@Example[/url] [url=https://example.com]#Example[/url] [url=https://example.com]!Example[/url] @[url=https://example.com]Example[/url] #[url=https://example.com]Example[/url] ![url=https://example.com]Example[/url]',
+			],
+		];
+	}
+
+	/**
+	 * @dataProvider dataNormalizeMentionLinks
+	 *
+	 * @param string $expected
+	 * @param string $body
+	 */
+	public function testNormalizeMentionLinks(string $expected, string $body)
+	{
+		$this->assertEquals($expected, ProcessorMock::normalizeMentionLinks($body));
+	}
+
+	public function dataAddMentionLinks(): array
+	{
+		return [
+			'issue-10603' => [
+				'expected' => '@[url=https://social.wake.st/users/liaizon]liaizon@social.wake.st[/url] @[url=https://friendica.mrpetovan.com/profile/hypolite]hypolite@friendica.mrpetovan.com[/url] yes<br /><br />',
+				'body'     => '@liaizon@social.wake.st @hypolite@friendica.mrpetovan.com yes<br /><br />',
+				'tags'     => [
+					[
+						'type' => 'Mention',
+						'href' => 'https://social.wake.st/users/liaizon',
+						'name' => '@liaizon@social.wake.st'
+					],
+					[
+						'type' => 'Mention',
+						'href' => 'https://friendica.mrpetovan.com/profile/hypolite',
+						'name' => '@hypolite@friendica.mrpetovan.com'
+					]
+				],
+			],
+		];
+	}
+
+	/**
+	 * @dataProvider dataAddMentionLinks
+	 *
+	 * @param string $expected
+	 * @param string $body
+	 * @param array $tags
+	 */
+	public function testAddMentionLinks(string $expected, string $body, array $tags)
+	{
+		$this->assertEquals($expected, ProcessorMock::addMentionLinks($body, $tags));
+	}
+}