diff --git a/apps/files_external/lib/Lib/Storage/SFTPReadStream.php b/apps/files_external/lib/Lib/Storage/SFTPReadStream.php
index 7aaeac0040aa7..42dde1d778dc4 100644
--- a/apps/files_external/lib/Lib/Storage/SFTPReadStream.php
+++ b/apps/files_external/lib/Lib/Storage/SFTPReadStream.php
@@ -125,6 +125,8 @@ private function seekTo(int $offset): void {
 		$this->internalPosition = $offset;
 		$this->readPosition = $offset;
 		$this->buffer = '';
+		// a previous read may have reached the end of the file, reading from the new position has to work again
+		$this->eof = false;
 		$this->request_chunk(256 * 1024);
 	}
 
diff --git a/apps/files_external/tests/Storage/SFTPReadStreamTest.php b/apps/files_external/tests/Storage/SFTPReadStreamTest.php
new file mode 100644
index 0000000000000..52c3fc2dbaff6
--- /dev/null
+++ b/apps/files_external/tests/Storage/SFTPReadStreamTest.php
@@ -0,0 +1,118 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Files_External\Tests\Storage;
+
+use OCA\Files_External\Lib\Storage\SFTPReadStream;
+use phpseclib3\Net\SFTP;
+use phpseclib3\Net\SSH2;
+use PHPUnit\Framework\MockObject\MockObject;
+
+/**
+ * Unit tests for the SFTP read stream wrapper.
+ *
+ * The SFTPReflection seam (invokeSftp / getSftpProperty) is stubbed by a fake
+ * server that answers READ requests from an in-memory file, so seeking and
+ * end-of-file handling can be exercised without a live SFTP server.
+ */
+class SFTPReadStreamTest extends \Test\TestCase {
+	protected function setUp(): void {
+		parent::setUp();
+		// The stream uses the NET_SFTP_* global constants; phpseclib defines them
+		// lazily the first time an SFTP object is constructed (no connection made).
+		if (!defined('NET_SFTP_HANDLE')) {
+			new SFTP('localhost');
+		}
+	}
+
+	private function openStream(string $content): SFTPReadStream&MockObject {
+		$state = new \stdClass();
+		$state->packetType = null;
+		$state->requests = [];
+
+		$sftp = $this->createMock(SFTP::class);
+		$sftp->method('realpath')->willReturnCallback(fn ($p) => $p);
+
+		$stream = $this->getMockBuilder(SFTPReadStream::class)
+			->onlyMethods(['invokeSftp', 'getSftpProperty'])
+			->getMock();
+
+		$stream->method('getSftpProperty')->willReturnCallback(fn ($s, $prop) => match ($prop) {
+			'bitmap' => SSH2::MASK_LOGIN,
+			'packet_type' => $state->packetType,
+			default => null,
+		});
+
+		$stream->method('invokeSftp')->willReturnCallback(function ($s, $method, $args = []) use ($state, $content) {
+			switch ($method) {
+				case 'send_sftp_packet':
+					if ($args[0] === NET_SFTP_OPEN) {
+						$state->requests[] = ['type' => NET_SFTP_HANDLE, 'data' => pack('N', 6) . 'handle'];
+					} elseif ($args[0] === NET_SFTP_READ) {
+						$packet = $args[1];
+						$handleLength = unpack('N', substr($packet, 0, 4))[1];
+						[, $high, $low, $length] = unpack('N3', substr($packet, 4 + $handleLength));
+						$offset = $high * (2 ** 32) + $low;
+						if ($offset >= strlen($content)) {
+							$state->requests[] = ['type' => NET_SFTP_STATUS, 'data' => pack('N', NET_SFTP_STATUS_EOF)];
+						} else {
+							$state->requests[] = ['type' => NET_SFTP_DATA, 'data' => pack('N', 0) . substr($content, $offset, $length)];
+						}
+					}
+					return null;
+				case 'get_sftp_packet':
+					$response = array_shift($state->requests);
+					$state->packetType = $response['type'];
+					return $response['data'];
+				case 'close_handle':
+					return true;
+			}
+			return null;
+		});
+
+		$stream->context = stream_context_create(['sftp' => ['session' => $sftp, 'size' => strlen($content)]]);
+		$opened = '';
+		$this->assertTrue($stream->stream_open('sftpread://file.mp4', 'r', 0, $opened));
+		return $stream;
+	}
+
+	private function readAll(SFTPReadStream $stream): string {
+		$data = '';
+		while (!$stream->stream_eof()) {
+			$data .= $stream->stream_read(8192);
+		}
+		return $data;
+	}
+
+	public function testReadUntilEof(): void {
+		$stream = $this->openStream('0123456789');
+
+		$this->assertSame('0123456789', $this->readAll($stream));
+		$this->assertTrue($stream->stream_eof());
+		$this->assertSame(10, $stream->stream_tell());
+	}
+
+	public function testSeekReadsFromTheNewPosition(): void {
+		$stream = $this->openStream('0123456789');
+
+		$this->assertTrue($stream->stream_seek(4));
+		$this->assertSame(4, $stream->stream_tell());
+		$this->assertSame('456789', $this->readAll($stream));
+	}
+
+	public function testSeekBackAfterEof(): void {
+		$stream = $this->openStream('0123456789');
+		$this->readAll($stream);
+
+		// e.g. the movie preview reads the 'moov' atom at the end of a video and then copies the file from the start
+		$this->assertTrue($stream->stream_seek(0));
+		$this->assertFalse($stream->stream_eof());
+		$this->assertSame('0123456789', $this->readAll($stream));
+	}
+}