diff --git a/src/Integrations/Divi/Design/FontNameRules.php b/src/Integrations/Divi/Design/FontNameRules.php
new file mode 100644
index 0000000..869fb4b
--- /dev/null
+++ b/src/Integrations/Divi/Design/FontNameRules.php
@@ -0,0 +1,74 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Integrations\Divi\Design;
+
+defined('ABSPATH') || exit;
+
+/**
+ * Name rules of an uploaded font (ruling W9), stricter than FontsUtility::font_add(): Divi silently strips every
+ * character but A–Z, a–z, 0–9, whitespace, _ and -; the plugin refuses them and names what Divi would store.
+ * Divi compares names with Google's and uploaded fonts case-sensitively and skips the web-safe ones; the plugin
+ * refuses any of them whatever the case, so a font never shadows another one in Divi's font lists. Pure PHP.
+ */
+final class FontNameRules
+{
+    /** = FontsUtility::FONT_NAME_MAX_LENGTH in Divi 5.15.1 (FontInstaller::nameMaxLength() reads Divi's). */
+    public const MAX_LENGTH = 100;
+
+    private const ALLOWED = '/^[A-Za-z0-9 _-]+$/D';
+
+    /** What FontsUtility::font_add() removes from a name. */
+    private const DIVI_STRIPS = '/[^A-Za-z0-9\s\_-]/';
+
+    /**
+     * @param list<string> $reserved Names of Google, web-safe and uploaded fonts (FontCatalog::reserved()).
+     * @return string|null null = valid, else the problem (German).
+     */
+    public static function check(string $name, array $reserved, int $maxLength = self::MAX_LENGTH): ?string
+    {
+        if (trim($name) === '') {
+            return sprintf('Name fehlt (1–%d Zeichen)', $maxLength);
+        }
+
+        if (preg_match(self::ALLOWED, $name) !== 1) {
+            $stored = trim((string) preg_replace(self::DIVI_STRIPS, '', $name));
+
+            return 'nur Buchstaben A–Z/a–z, Ziffern, Leerzeichen, _ und - erlaubt ('
+                . ($stored === '' ? 'Divi ließe nichts übrig)' : sprintf('Divi würde „%s“ speichern)', $stored));
+        }
+
+        if ($name !== trim($name)) {
+            return 'ohne Leerzeichen am Anfang oder Ende';
+        }
+
+        if (strlen($name) > $maxLength) {
+            return sprintf('höchstens %d Zeichen (%d angegeben)', $maxLength, strlen($name));
+        }
+
+        $taken = self::taken($name, $reserved);
+
+        return $taken === null ? null : sprintf(
+            '„%s“ gibt es schon als Google-, Web-Safe- oder hochgeladene Schrift (ohne Beachtung der '
+                . 'Groß-/Kleinschreibung)',
+            $taken
+        );
+    }
+
+    /**
+     * @param list<string> $reserved
+     * @return string|null The existing name $name collides with (case-insensitive), null when it is free.
+     */
+    public static function taken(string $name, array $reserved): ?string
+    {
+        $lower = strtolower($name);
+        foreach ($reserved as $existing) {
+            if (strtolower($existing) === $lower) {
+                return $existing;
+            }
+        }
+
+        return null;
+    }
+}
diff --git a/src/Integrations/Divi/Design/FontSignature.php b/src/Integrations/Divi/Design/FontSignature.php
new file mode 100644
index 0000000..f67e3a8
--- /dev/null
+++ b/src/Integrations/Divi/Design/FontSignature.php
@@ -0,0 +1,32 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Integrations\Divi\Design;
+
+defined('ABSPATH') || exit;
+
+/**
+ * The font format of a file by its magic bytes (ruling W9): wOF2 = woff2, wOFF = woff, 00 01 00 00 or "true" =
+ * ttf, OTTO = otf. Divi's own check (finfo) may be skipped for users with unfiltered_upload, so the font upload
+ * checks the content itself. Font collections (ttcf) and eot are no font here. Pure PHP.
+ */
+final class FontSignature
+{
+    /** Bytes detect() needs from the start of the file. */
+    public const HEADER_BYTES = 4;
+
+    private const MAGIC = [
+        'wOF2' => 'woff2',
+        'wOFF' => 'woff',
+        "\x00\x01\x00\x00" => 'ttf',
+        'true' => 'ttf',
+        'OTTO' => 'otf',
+    ];
+
+    /** @return 'woff2'|'woff'|'ttf'|'otf'|null */
+    public static function detect(string $header): ?string
+    {
+        return self::MAGIC[substr($header, 0, self::HEADER_BYTES)] ?? null;
+    }
+}
diff --git a/tests/Unit/Integrations/Divi/Design/FontNameRulesTest.php b/tests/Unit/Integrations/Divi/Design/FontNameRulesTest.php
new file mode 100644
index 0000000..dda2197
--- /dev/null
+++ b/tests/Unit/Integrations/Divi/Design/FontNameRulesTest.php
@@ -0,0 +1,64 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Unit\Integrations\Divi\Design;
+
+use LIT\MCP\Integrations\Divi\Design\FontNameRules;
+use PHPUnit\Framework\TestCase;
+
+final class FontNameRulesTest extends TestCase
+{
+    private const RESERVED = ['Open Sans', 'Roboto', 'Arial', 'Times New Roman', 'Hausschrift'];
+
+    public function testLettersDigitsSpacesUnderscoresAndHyphensAreAllowed(): void
+    {
+        self::assertNull(FontNameRules::check('Inter Display_2-Bold', self::RESERVED));
+        self::assertNull(FontNameRules::check('X', self::RESERVED));
+    }
+
+    public function testOtherCharactersAreRefusedWithTheNameDiviWouldStore(): void
+    {
+        $problem = (string) FontNameRules::check('Grotesk (Pro)!', self::RESERVED);
+        self::assertStringContainsString('nur Buchstaben', $problem);
+        self::assertStringContainsString('„Grotesk Pro“', $problem);
+
+        self::assertStringContainsString('„Mller“', (string) FontNameRules::check('Müller', self::RESERVED));
+        self::assertStringContainsString('nur Buchstaben', (string) FontNameRules::check('a.b', self::RESERVED));
+        self::assertStringContainsString('nur Buchstaben', (string) FontNameRules::check("Tab\tName", self::RESERVED));
+        self::assertStringContainsString('nichts übrig', (string) FontNameRules::check('«»', self::RESERVED));
+    }
+
+    public function testEmptyAndPaddedNamesAreRefused(): void
+    {
+        self::assertStringContainsString('1–100 Zeichen', (string) FontNameRules::check('', self::RESERVED));
+        self::assertStringContainsString('1–100 Zeichen', (string) FontNameRules::check('   ', self::RESERVED));
+        self::assertStringContainsString('Anfang oder Ende', (string) FontNameRules::check(' Inter', self::RESERVED));
+        self::assertStringContainsString('Anfang oder Ende', (string) FontNameRules::check('Inter ', self::RESERVED));
+    }
+
+    public function testTheLengthLimitIsOneHundredCharactersOrDivisBound(): void
+    {
+        self::assertNull(FontNameRules::check(str_repeat('a', 100), self::RESERVED));
+        self::assertStringContainsString(
+            'höchstens 100 Zeichen (101 angegeben)',
+            (string) FontNameRules::check(str_repeat('a', 101), self::RESERVED)
+        );
+        self::assertStringContainsString(
+            'höchstens 50 Zeichen',
+            (string) FontNameRules::check(str_repeat('a', 51), self::RESERVED, 50)
+        );
+    }
+
+    public function testNamesOfGoogleWebSafeAndUploadedFontsAreTakenWhateverTheCase(): void
+    {
+        self::assertSame('Open Sans', FontNameRules::taken('open sans', self::RESERVED));
+        self::assertSame('Arial', FontNameRules::taken('ARIAL', self::RESERVED), 'web-safe fonts too (Divi skips them)');
+        self::assertSame('Hausschrift', FontNameRules::taken('Hausschrift', self::RESERVED));
+        self::assertNull(FontNameRules::taken('Open Sans Pro', self::RESERVED));
+        self::assertNull(FontNameRules::taken('Roboto', []));
+
+        $problem = (string) FontNameRules::check('times new roman', self::RESERVED);
+        self::assertStringContainsString('„Times New Roman“ gibt es schon', $problem);
+    }
+}
diff --git a/tests/Unit/Integrations/Divi/Design/FontSignatureTest.php b/tests/Unit/Integrations/Divi/Design/FontSignatureTest.php
new file mode 100644
index 0000000..28bd5c5
--- /dev/null
+++ b/tests/Unit/Integrations/Divi/Design/FontSignatureTest.php
@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Unit\Integrations\Divi\Design;
+
+use LIT\MCP\Integrations\Divi\Design\FontSignature;
+use PHPUnit\Framework\TestCase;
+
+final class FontSignatureTest extends TestCase
+{
+    public function testTheFourFormatsAreRecognizedByTheirFirstFourBytes(): void
+    {
+        self::assertSame('woff2', FontSignature::detect("wOF2\x00\x01\x00\x00\x00\x00\x12\x34"));
+        self::assertSame('woff', FontSignature::detect("wOFF\x00\x01\x00\x00"));
+        self::assertSame('ttf', FontSignature::detect("\x00\x01\x00\x00\x00\x0c\x00\x80"));
+        self::assertSame('ttf', FontSignature::detect('true' . "\x00\x0c"), 'Apple TrueType');
+        self::assertSame('otf', FontSignature::detect("OTTO\x00\x0a\x00\x80"));
+    }
+
+    public function testEverythingElseIsNoFont(): void
+    {
+        self::assertNull(FontSignature::detect(''));
+        self::assertNull(FontSignature::detect('wOF'), 'too short');
+        self::assertNull(FontSignature::detect('wof2'), 'case matters');
+        self::assertNull(FontSignature::detect('ttcf'), 'font collections are not accepted');
+        self::assertNull(FontSignature::detect("\x89PNG\r\n\x1a\n"));
+        self::assertNull(FontSignature::detect('<?php evil();'));
+        self::assertNull(FontSignature::detect("\x00wOF2"), 'the magic must start at byte 0');
+    }
+}