diff --git a/tests/Unit/ClassNameSyntaxTest.php b/tests/Unit/ClassNameSyntaxTest.php
new file mode 100644
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--- /dev/null
+++ b/tests/Unit/ClassNameSyntaxTest.php
@@ -0,0 +1,247 @@
+<?php
+
+declare(strict_types=1);
+
+namespace SMF\Tests\Unit;
+
+use FilesystemIterator;
+use PHPUnit\Framework\Attributes\CoversNothing;
+use PHPUnit\Framework\TestCase;
+use RecursiveDirectoryIterator;
+use RecursiveIteratorIterator;
+
+/**
+ * Checks that SMF names its own classes with ::class rather than in a string.
+ *
+ * A name in a string is not a name PHP ever checks. phplint sees a valid string,
+ * php-cs-fixer does not read inside strings, and nothing resolves the name until
+ * the moment it is used, which for a background task is a cron run on somebody
+ * else's forum. Written as a resolver, the compiler resolves the name against
+ * the file's imports, so a misspelling is a name that is simply not there.
+ *
+ * This only sees a name spelled out in one literal. A name assembled at run time,
+ * as the menus do with __NAMESPACE__ . '\\Home::call', is invisible here; the
+ * class half of it is not written down for anything to check either.
+ */
+#[CoversNothing]
+class ClassNameSyntaxTest extends TestCase
+{
+	/*****************
+	 * Class constants
+	 *****************/
+
+	/**
+	 * The directories that hold the classes SMF ships.
+	 */
+	private const ROOTS = [
+		'Sources',
+		'Themes',
+	];
+
+	/**
+	 * The files that ship outside those directories.
+	 */
+	private const ENTRY_POINTS = [
+		'index.php',
+		'cron.php',
+		'SSI.php',
+		'proxy.php',
+		'subscriptions.php',
+	];
+
+	/**
+	 * The names that have to stay in a string, and why.
+	 *
+	 * Keyed by the file, with '/' as the separator whatever the platform uses,
+	 * so that moving the code around a file does not invalidate the reason.
+	 *
+	 * Anything added here needs a comment saying why a resolver cannot express
+	 * it. "The class does not exist yet" is not one of them: ::class resolves at
+	 * compile time and never asks the autoloader, so it is happy to name a class
+	 * that has not been loaded, or one that is never loaded at all.
+	 */
+	private const ALLOWED = [
+		// A namespace prefix, which is never a class.
+		'Sources/ActionRouter.php' => [
+			'SMF\\Actions',
+		],
+		// The alias class_alias() is about to create. Writing the name it is
+		// creating as a resolver would claim the alias already exists.
+		'Sources/Actions/Admin/PackageManager.php' => [
+			'SMF\\Actions\\Admin\\PackageManager',
+		],
+		// Two Unicode helper functions, which only some data files define.
+		// Functions have no resolver.
+		'Sources/Punycode.php' => [
+			'SMF\\Unicode\\idna_maps_deviation',
+			'SMF\\Unicode\\idna_maps_not_std3',
+		],
+	];
+
+	/****************
+	 * Public methods
+	 ****************/
+
+	public function testEveryClassNameIsWrittenAsAResolverRatherThanAString(): void
+	{
+		$found = [];
+
+		foreach ($this->shippedFiles() as $file) {
+			$relative = $this->relative($file);
+
+			foreach ($this->classNameLiterals($file) as [$line, $literal]) {
+				if (\in_array($literal, self::ALLOWED[$relative] ?? [], true)) {
+					continue;
+				}
+
+				$found[] = $relative . ':' . $line . ' names ' . $literal;
+			}
+		}
+
+		$this->assertSame([], $found, 'A class name is written as a string. Write it as a ::class resolver, or add it to ' . self::class . '::ALLOWED with the reason it cannot be one.');
+	}
+
+	/**
+	 * An exception whose reason has gone stale is an exception nobody notices,
+	 * so the list is only allowed to name things that are really still there.
+	 */
+	public function testNothingInTheListOfExceptionsHasGoneStale(): void
+	{
+		$stale = [];
+
+		foreach (self::ALLOWED as $relative => $literals) {
+			$file = $this->boardDir() . DIRECTORY_SEPARATOR . strtr($relative, ['/' => DIRECTORY_SEPARATOR]);
+
+			$present = [];
+
+			if (is_file($file)) {
+				foreach ($this->classNameLiterals($file) as [, $literal]) {
+					$present[] = $literal;
+				}
+			}
+
+			foreach ($literals as $literal) {
+				if (!\in_array($literal, $present, true)) {
+					$stale[] = $relative . ' no longer names ' . $literal;
+				}
+			}
+		}
+
+		$this->assertSame([], $stale, 'The list of names allowed to stay in a string mentions something that is not there any more. Remove the entry.');
+	}
+
+	/******************
+	 * Internal methods
+	 ******************/
+
+	/**
+	 * Every PHP file the forum ships, entry points included.
+	 *
+	 * @return array<int, string>
+	 */
+	private function shippedFiles(): array
+	{
+		$files = [];
+
+		foreach (self::ROOTS as $dir) {
+			$root = $this->boardDir() . DIRECTORY_SEPARATOR . $dir;
+
+			$iterator = new RecursiveIteratorIterator(
+				new RecursiveDirectoryIterator($root, FilesystemIterator::SKIP_DOTS),
+			);
+
+			foreach ($iterator as $file) {
+				if ($file->isFile() && $file->getExtension() === 'php') {
+					$files[] = $file->getPathname();
+				}
+			}
+		}
+
+		foreach (self::ENTRY_POINTS as $name) {
+			$files[] = $this->boardDir() . DIRECTORY_SEPARATOR . $name;
+		}
+
+		// So that a failure reads the same way wherever it happens.
+		sort($files);
+
+		return $files;
+	}
+
+	/**
+	 * The class names one file writes down as strings, as [line, name] pairs.
+	 *
+	 * @param string $file The file to read.
+	 * @return array<int, array{int, string}>
+	 */
+	private function classNameLiterals(string $file): array
+	{
+		$code = (string) file_get_contents($file);
+
+		// Tokenising sixteen megabytes of source to find a handful of strings is
+		// worth avoiding, and the Unicode data files are most of that weight.
+		// Every literal we want opens with a quote and reaches a namespace
+		// separator within a couple of characters.
+		if (preg_match('/[\'"]\\\\{0,2}SMF\\\\/', $code) !== 1) {
+			return [];
+		}
+
+		$found = [];
+
+		// A docblock saying @var SMF\Utils is not a name the code uses, and
+		// there are thousands of those, which is why this reads tokens and not
+		// lines.
+		foreach (token_get_all($code) as $token) {
+			if (!\is_array($token) || $token[0] !== T_CONSTANT_ENCAPSED_STRING) {
+				continue;
+			}
+
+			$literal = $this->unquote($token[1]);
+
+			// Every segment has to be non-empty, which leaves out the bare
+			// 'SMF\Tasks\' that TaskRunner and ReportedContent build a name
+			// from at run time.
+			if (preg_match('/^\\\\?SMF(\\\\[A-Za-z0-9_]+)+(::[A-Za-z0-9_]+)?$/', $literal) !== 1) {
+				continue;
+			}
+
+			$found[] = [$token[2], ltrim($literal, '\\')];
+		}
+
+		return $found;
+	}
+
+	/**
+	 * The text of a quoted string, with its escapes resolved.
+	 *
+	 * @param string $token The token, quotes and all.
+	 */
+	private function unquote(string $token): string
+	{
+		$body = substr($token, 1, -1);
+
+		// A class name in a double-quoted string is an interpolation waiting to
+		// happen, so the codebase writes them single-quoted, where the only two
+		// escapes are the quote and the backslash itself.
+		return $token[0] === "'"
+			? strtr($body, ['\\\\' => '\\', '\\\'' => '\''])
+			: stripcslashes($body);
+	}
+
+	/**
+	 * A path as it would be written in a bug report.
+	 *
+	 * @param string $file An absolute path inside the checkout.
+	 */
+	private function relative(string $file): string
+	{
+		return strtr(substr($file, \strlen($this->boardDir()) + 1), [DIRECTORY_SEPARATOR => '/']);
+	}
+
+	/**
+	 * The root of the checkout, resolved so the walked paths start with it.
+	 */
+	private function boardDir(): string
+	{
+		return (string) realpath(TESTS_BOARDDIR);
+	}
+}