BITT

A small BitTorrent client for macOS, for people who already have a tracker site.

BITT lives in the menu bar. The app is native SwiftUI and the BitTorrent engine is plain Swift compiled into the same binary — one process, about 3.5 MB, no runtime to install and no third-party dependencies.

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Scope — this is deliberately small

BITT is built for one situation: you get .torrent files and magnet links from a site you already use, and you want them downloaded without fuss.

It works with any torrent or magnet that carries trackers, which is effectively everything a site hands you. Private trackers always attach their announce URL with your passkey; public sites attach a long list.

It does not do DHT (BEP 5) or PEX (BEP 11), and that is a decision rather than a gap:

1. Private trackers forbid both. Those torrents set the private flag, and announcing them to the DHT is a good way to lose your account. For the intended user it is not merely unnecessary, it is harmful.
2. Site magnets already carry trackers, so the DHT would add nothing in practice.
3. A DHT node is never idle. It answers UDP from the whole world whether or not you are downloading, which works against the point of a small client — and it is another ~1,000 lines to carry, roughly a quarter of the engine again.

The consequence: a bare magnet with only xt=urn:btih:… and no &tr= will not work. BITT says so in the add dialog instead of leaving it stuck at 0%.

If that trade ever needs revisiting the order would be: enforce the private flag properly, then PEX, then DHT.

What it does

• .torrent files, magnet links, and http(s) URLs that point at a .torrent
• Fetches a magnet's file list from peers (BEP 9 / BEP 10 ut_metadata)
• HTTP and UDP trackers (BEP 15), compact peer lists including IPv6 (BEP 23)
• Rarest-first piece selection with an endgame, tit-for-tat choking with an optimistic unchoke
• Verifies SHA-1 on every piece before it reaches the disk; bad data is refetched
• Resumes by hash-checking what is already on disk — no separate state file to corrupt
• Upload and download speed limits shared across torrents
• Ratio and cumulative shared bytes, counted across restarts
• Asks the router to forward the listening port (NAT-PMP, then UPnP IGD) and tells you plainly when it could not
• Seeds in the background with no window open
• Liquid Glass on floating surfaces on macOS 26+, older materials below that

Install

Download the latest .dmg from Releases, open it, and drag BITT to Applications. It is signed with a Developer ID and notarised by Apple, so it opens without a security warning.

Or build it yourself:

git clone https://github.com/ninjait07/bitt.git
cd bitt
bash macapp/build.sh --install

Needs Xcode Command Line Tools. The build compiles the app, draws the icon, writes the bundle and copies it to /Applications. --dmg writes a disk image to dist/, and --out <dir> puts it somewhere else. ./release.sh <version> does the signed, notarised, stapled build that goes out as a release.

Layout

The engine exists twice, on purpose.

macapp/Sources/Engine/   the Swift engine — this is what the app runs
macapp/Sources/          the SwiftUI app: menu bar, window, settings
macapp/EngineTests/      125 tests for the Swift engine
pytorrent/               the same protocol in Python — reference and CLI
torrentdl                command line client built on the Python engine
tests/                   34 tests for the Python engine

The Python engine came first and was validated against real swarms. The Swift port is checked against it on every test run, which is what makes the port trustworthy.

Testing

bash macapp/run-tests.sh    # 125 tests, Swift engine
python3 tests/test_units.py # 28 tests, Python engine
python3 tests/test_swarm.py #  6 tests, real sockets on loopback

run-tests.sh builds its fixtures with the Python engine and then holds the Swift engine against them:

• Every wire frame is compared byte for byte — handshake, request, piece, bitfield, ut_metadata, the extended handshake (which depends on bencode key order), and the percent-encoding of a tracker announce
• Every value read from a real .torrent matches, including all piece hashes
• Real transfers over real sockets: Swift to Swift through a magnet link, and a Swift leecher downloading from the Python seeder

Both engines have downloaded the Debian 13.7 netinst ISO (756 MiB) to completion from the public swarm, against qBittorrent, Transmission, Deluge and rqbit peers at 9–11 MiB/s, with the resulting SHA-256 matching Debian's published checksum and zero failed pieces.

Known limits

• No DHT or PEX — see Scope above
• No connection encryption (MSE/PE), so some ISPs may throttle
• No BitTorrent v2 (btmh: magnets)
• Releases are signed and notarised; a local build.sh build is ad-hoc signed and will trip Gatekeeper if copied to another machine

Licence

GPL-3.0. See LICENSE.

BitTorrent is a trademark of Rainberry, Inc. BITT is not affiliated with or endorsed by them. What you choose to download with it is your own responsibility.