diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index 6d3c2bc..05de7a8 100644
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -25,4 +25,26 @@
 	// OCP/Guzzle dev dependencies are enough for the pure unit tests.
 	require_once __DIR__ . '/../vendor/autoload.php';
 	require_once __DIR__ . '/../vendor-bin/nextcloud-ocp/vendor/autoload.php';
+
+	// The nextcloud/ocp package is a stub collection without an autoload
+	// section (psalm scans it as extra files), so OCP\… must be mapped by
+	// hand for tests that mock OCP interfaces.
+	spl_autoload_register(function (string $class): void {
+		if (str_starts_with($class, 'OCP\\')) {
+			$file = __DIR__ . '/../vendor-bin/nextcloud-ocp/vendor/nextcloud/ocp/'
+				. str_replace('\\', '/', $class) . '.php';
+			if (file_exists($file)) {
+				require_once $file;
+			}
+		}
+	});
+
+	// \OC::$SERVERROOT appears as a default argument in resolveLogDir(); PHP
+	// evaluates default arguments on every call, so the class must exist even
+	// when the mocked config never lets that default win.
+	if (!class_exists('OC')) {
+		class OC {
+			public static string $SERVERROOT = '/srv/nextcloud';
+		}
+	}
 }
diff --git a/tests/unit/Http/Client/LoggingClientServiceTest.php b/tests/unit/Http/Client/LoggingClientServiceTest.php
new file mode 100644
index 0000000..697ffd2
--- /dev/null
+++ b/tests/unit/Http/Client/LoggingClientServiceTest.php
@@ -0,0 +1,92 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * SPDX-FileCopyrightText: 2026 [ernolf] Raphael Gradenwitz <raphael.gradenwitz@googlemail.com>
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\AdminAuditHttpClient\Tests\Unit\Http\Client;
+
+use OCA\AdminAuditHttpClient\Http\Client\LoggingClientService;
+use OCP\Http\Client\IClientService;
+use OCP\IConfig;
+use OCP\IRequest;
+use PHPUnit\Framework\TestCase;
+use Psr\Log\NullLogger;
+
+class LoggingClientServiceTest extends TestCase {
+	private function service(array $stringValues, array $values = []): LoggingClientService {
+		$config = $this->createMock(IConfig::class);
+		$config->method('getSystemValueString')->willReturnCallback(
+			fn (string $key, string $default = ''): string => $stringValues[$key] ?? $default,
+		);
+		$config->method('getSystemValue')->willReturnCallback(
+			fn (string $key, $default = '') => $values[$key] ?? $default,
+		);
+
+		return new LoggingClientService(
+			$this->createMock(IClientService::class),
+			new NullLogger(),
+			$config,
+			$this->createMock(IRequest::class),
+		);
+	}
+
+	private function resolveLogDir(LoggingClientService $service): string {
+		$ref = new \ReflectionMethod($service, 'resolveLogDir');
+		return $ref->invoke($service);
+	}
+
+	public function testExplicitLogdirWinsAndTrailingSlashIsTrimmed(): void {
+		$service = $this->service(
+			['audit_http_client_logdir' => '/var/log/nextcloud/http_client/'],
+			['datadirectory' => '/srv/nc/data'],
+		);
+		$this->assertSame('/var/log/nextcloud/http_client', $this->resolveLogDir($service));
+	}
+
+	public function testAuditLogSiblingOutsideTheDataDirectory(): void {
+		$service = $this->service(
+			['logfile_audit' => '/var/log/nextcloud/audit.log'],
+			['datadirectory' => '/srv/nc/data'],
+		);
+		$this->assertSame('/var/log/nextcloud/client', $this->resolveLogDir($service));
+	}
+
+	public function testAuditLogInsideTheDataDirectoryUsesTheDataDirSubfolder(): void {
+		$service = $this->service(
+			['logfile_audit' => '/srv/nc/data/audit.log'],
+			['datadirectory' => '/srv/nc/data'],
+		);
+		$this->assertSame('/srv/nc/data/admin_audit_http_client_logs', $this->resolveLogDir($service));
+	}
+
+	public function testLogfileSiblingWhenNoAuditLogIsConfigured(): void {
+		$service = $this->service(
+			['logfile' => '/var/log/nextcloud/nextcloud.log'],
+			['datadirectory' => '/srv/nc/data'],
+		);
+		$this->assertSame('/var/log/nextcloud/client', $this->resolveLogDir($service));
+	}
+
+	public function testDefaultsToTheDataDirSubfolder(): void {
+		$service = $this->service(
+			[],
+			['datadirectory' => '/srv/nc/data'],
+		);
+		$this->assertSame('/srv/nc/data/admin_audit_http_client_logs', $this->resolveLogDir($service));
+	}
+
+	public function testAuditLogWinsOverLogfile(): void {
+		$service = $this->service(
+			[
+				'logfile_audit' => '/var/log/nextcloud-audit/audit.log',
+				'logfile' => '/var/log/nextcloud/nextcloud.log',
+			],
+			['datadirectory' => '/srv/nc/data'],
+		);
+		$this->assertSame('/var/log/nextcloud-audit/client', $this->resolveLogDir($service));
+	}
+}