diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index f438263..90b0682 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -48,8 +48,8 @@ jobs:
         with:
           php-version: '8.4'
           extensions: intl, sodium, gd, mysqli, curl, fileinfo, json, dom, simplexml, mbstring, zip, redis, memcached, apcu
-          coverage: pcov
-          ini-values: apc.enable_cli=1, mysqli.allow_local_infile=1
+          coverage: xdebug
+          ini-values: apc.enable_cli=1, mysqli.allow_local_infile=1, xdebug.mode="develop,coverage"
       - name: Validate composer.json
         run: composer validate --no-check-publish
       - name: Install dependencies
diff --git a/tests/RequestTest.php b/tests/RequestTest.php
index 0a62d9d..5612e72 100644
--- a/tests/RequestTest.php
+++ b/tests/RequestTest.php
@@ -420,7 +420,10 @@ public function testToStringMultipart() : void
         $body .= \implode("\r\n", $files[3]) . "\r\n";
         $body .= $boundary . "--\r\n";
         $contentLength = \strlen($body);
-        self::assertSame(953, $contentLength);
+        // The fixture is embedded twice, so derive the expected length from it
+        // rather than hard-coding a size that rots when the file changes.
+        $fileSize = \strlen((string) \file_get_contents($filepath));
+        self::assertSame(931 + 2 * $fileSize, $contentLength);
         $message = $startLine . "\r\n"
             . \implode("\r\n", $headerLines) . "\r\n"
             . "\r\n"
diff --git a/tests/ResponseTest.php b/tests/ResponseTest.php
index 5b9b6ed..4f96f20 100644
--- a/tests/ResponseTest.php
+++ b/tests/ResponseTest.php
@@ -603,16 +603,20 @@ public function testToStringWithDownloadMultipart() : void
         $this->response = new Response(new RequestMock());
         $filename = __DIR__ . '/files/file.txt';
         $boundary = \md5($filename);
+        // Derived from the fixture, like testToStringWithDownload above, so the
+        // expectation cannot drift when tests/files/file.txt changes size.
+        $contents = (string) \file_get_contents($filename);
+        $fileSize = \strlen($contents);
         $body = "\r\n--{$boundary}--\r\n"
             . "Content-Type: application/octet-stream\r\n"
-            . "Content-Range: bytes 0-1/11\r\n"
+            . "Content-Range: bytes 0-1/{$fileSize}\r\n"
             . "\r\n"
-            . 'Hi'
+            . \substr($contents, 0, 2)
             . "\r\n--{$boundary}--\r\n"
             . "Content-Type: application/octet-stream\r\n"
-            . "Content-Range: bytes 4-10/11\r\n"
+            . 'Content-Range: bytes 4-' . ($fileSize - 1) . "/{$fileSize}\r\n"
             . "\r\n"
-            . "Webisters!\n"
+            . \substr($contents, 4)
             . "\r\n--{$boundary}--\r\n";
         $length = \strlen($body);
         $startLine = 'HTTP/1.1 206 Partial Content';