diff --git a/lib/tests/filelib_test.php b/lib/tests/filelib_test.php
index 4bed342c6d6..f0414ad7473 100644
--- a/lib/tests/filelib_test.php
+++ b/lib/tests/filelib_test.php
@@ -2076,6 +2076,56 @@ EOF;
             ],
         ];
     }
+
+    /**
+     * Tests that readfile_accel() triggers the expected debugging message when a non-empty
+     * output buffer is detected, using both a file path and a stored_file input.
+     *
+     * This test runs a CLI script in a separate process to isolate buffer manipulation.
+     * This is necessary because readfile_accel() uses ob_get_clean() and ob_end_flush(),
+     * which interfere with PHPUnit's internal output buffer enforcement and cause risky
+     * test errors.
+     *
+     * The CLI script simulates a non-empty output buffer, calls the readfile_accel(), and
+     * prints any debugging output. The test then captures that output and asserts that the
+     * correct debugging message was generated.
+     *
+     * @covers ::readfile_accel
+     */
+    public function test_readfile_accel_with_path_and_stored_file(): void {
+        $this->resetAfterTest();
+
+        // Construct the command to run the CLI script with a custom constant defined.
+        $scriptpath = __DIR__ . '/fixtures/readfile_accel_debug_cli.php';
+        $cmd = 'php -r ' . escapeshellarg("define('PHPUNIT_READFILE_ACCEL_TEST', true); require '$scriptpath';");
+
+        $pipes = [];
+        $process = proc_open($cmd, [
+            1 => ['pipe', 'w'],
+            2 => ['pipe', 'w'],
+        ], $pipes);
+
+        $stdout = stream_get_contents($pipes[1]);
+        $stderr = stream_get_contents($pipes[2]);
+
+        fclose($pipes[1]);
+        fclose($pipes[2]);
+
+        $exitcode = proc_close($process);
+
+        $output = $stdout . $stderr;
+
+        // Debug just in case the subprocess fails.
+        $this->assertSame(0, $exitcode);
+
+        // Validate that both path-based and stored_file debugging messages are present.
+        $filename = "readfile_accel.txt";
+        $filepath = '/tmp/' . $filename;
+        $this->assertStringContainsString('Non-empty default output handler buffer detected while serving the file ' .
+            $filepath . '. Buffer contents (first 20 characters): test text', $output);
+        $this->assertStringContainsString('Non-empty default output handler buffer detected while serving the file ' .
+            $filename . '. Buffer contents (first 20 characters): test text', $output);
+    }
 }
 
 /**
diff --git a/lib/tests/fixtures/readfile_accel_debug_cli.php b/lib/tests/fixtures/readfile_accel_debug_cli.php
new file mode 100644
index 00000000000..4731bf3fa96
--- /dev/null
+++ b/lib/tests/fixtures/readfile_accel_debug_cli.php
@@ -0,0 +1,112 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle 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 General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Cli script for testing readfile_accel function.
+ *
+ * @package    core
+ * @subpackage fixtures
+ * @copyright  2025 Catalyst IT
+ * @author     Trisha Milan <trishamilan@catalyst-au.net>
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+define('CLI_SCRIPT', true);
+
+require_once(__DIR__ . '/../../../config.php');
+require_once($CFG->libdir . '/filelib.php');
+
+if (!defined('PHPUNIT_READFILE_ACCEL_TEST')) {
+    echo 'This script is only intended to be run via PHPUnit.';
+    exit(1);
+}
+
+$testdb = moodle_database::get_driver_instance($CFG->dbtype, $CFG->dblibrary);
+$testdb->connect($CFG->dbhost, $CFG->dbuser, $CFG->dbpass, $CFG->dbname, $CFG->phpunit_prefix);
+$DB = $testdb;
+
+set_debugging(DEBUG_DEVELOPER, true);
+$CFG->tempdir = '/tmp';
+
+/**
+ * Runs readfile_accel() with a file path or a stored_file to trigger the buffer check.
+ *
+ * @param string|stored_file $input
+ * @param string $mimetype
+ * @param bool $accelerate
+ */
+function run_readfile_accel_test(string|stored_file $input, string $mimetype, bool $accelerate): void {
+    try {
+        ob_start();
+        echo "test text";
+        $_SERVER['REQUEST_METHOD'] = 'GET';
+        readfile_accel($input, $mimetype, $accelerate);
+    } catch (Exception $e) {
+        echo $e->getMessage() . "\n";
+    }
+}
+
+try {
+    // Prepare test file.
+    $filename = "readfile_accel.txt";
+    // Generate temporary local file for testing.
+    $path = "$CFG->tempdir/$filename";
+    file_put_contents($path, "\nMoodle test data\n");
+
+    // Populate {files} table.
+    $fs = get_file_storage();
+    $filerecord = [
+        'contextid' => context_system::instance()->id,
+        'component' => 'test',
+        'filearea' => 'readfile',
+        'itemid' => 0,
+        'filepath' => '/',
+        'filename' => $filename,
+    ];
+    $storedfile = null;
+    $filerecord['filename'] = $fs->get_unused_filename(
+        $filerecord['contextid'],
+        $filerecord['component'],
+        $filerecord['filearea'],
+        $filerecord['itemid'],
+        $filerecord['filepath'],
+        $filerecord['filename']
+    );
+    $storedfile = $fs->create_file_from_pathname($filerecord, $path);
+    $mimetype = get_mimetype_for_sending($storedfile->get_filename());
+    $accelerate = true;
+
+    // Run the test with direct path.
+    run_readfile_accel_test($path, $mimetype, $accelerate);
+
+    // Run the test with direct stored_file.
+    run_readfile_accel_test($storedfile, $mimetype, $accelerate);
+} finally {
+    // Clean up {files} table.
+    if (!is_null($fs)) {
+        @$fs->delete_area_files(
+            $filerecord['contextid'],
+            $filerecord['component'],
+            $filerecord['filearea'],
+            $filerecord['itemid']
+        );
+    }
+
+    // Clean up testing file.
+    if ($path !== "") {
+        @unlink($path);
+    }
+}