Universal Chess Interface

The Universal Chess Interface (UCI) is an open communication protocol that enables chess engines to communicate with user interfaces.^[1][2]

History

In November 2000, the UCI protocol was released. Designed by Rudolf Huber and Stefan Meyer-Kahlen, the author of Shredder, UCI rivals the older "Chess Engine Communication Protocol" introduced with XBoard/WinBoard.

In 2002, Chessbase, the chess software company which markets Fritz, began to support UCI, which had previously been supported by only a few interfaces and engines.

As of 2021^[update], well over 300 engines are known to directly support UCI.

Design

By design, UCI assigns some tasks to the user interface (i.e., presentation layer) which have traditionally been handled by the engine (at the business layer) itself.^{[citation needed]}

Most notably, the opening book is usually expected to be handled by the UI, by simply selecting moves to play until it is out of book, and only then starting up the engine for calculation in the resulting position. UCI does not specify any on-disk format for the opening book. Different UIs usually have their own proprietary formats.^{[citation needed]}

While the UI can also take responsibility for handling endgame tablebases, this is arguably better handled in the engine itself, as having tablebase information can be useful for considering possible future positions.^[3]

Stefan-Meyer Kahlen's UCI protocol in Shredder uses a variation of long algebraic notation for moves. A "nullmove" from an engine to a GUI should be sent as 0000.^[4]

Examples

• e2e4
• e7e5
• e1g1 (white short castling)
• e7e8q (for promotion)

Commands

There are many commands used to communicate with an engine. ^[5]

UCI Commands List^[5]

Command	Usage	Description	Response
quit	quit	Closes the chess engine	—N/a
uci	uci	Tells the engine to switch to UCI mode	uciok
ucinewgame	ucinewgame	Used to let the engine know that the next position it receives will be from a different game.	—N/a
isready	isready	Used to synchronize the engine. Use after sending a command that might take some time.	readyok
setoption	setoption name <id> [value <x>]	Used to change the internal parameters of a chess engine.	—N/a
position	position [fen <fenstring> | startpos ] moves <move1> .... <movei>	Used to tell the engine to setup the provided position.	—N/a
go	go [infinite | depth <depth> | nodes | mate] searchmoves <move1> .... <movei> ponder wtime <x> btime <x> winc <x> binc <x> movestogo <x> nodes <x> movetime <x> perft <x>	Tells the engine to start calculation.	bestmove <move> ponder <expected>
stop	stop	Tells the engine to halt calculation as soon as possible. But after 5seconds.	—N/a
ponderhit	ponderhit	Tells the engine that the player has played the expected move.	—N/a

There are also some optional commands implemented in engines like Stockfish.^[5]

Optional UCI Commands^[5]

Command	Usage	Description	Response
bench	bench [ttSize] [threads] [limit] [fenFile] [limitType]	Runs a benchmark test on the engine.	Test data
speedtest	speedtest [threads] [hash (MiB)] [runtime (s)]	Measures the speed of the computer running the engine.	Test data
d	d	Prints an ASCII representation of the board in the current state.	ASCII Board
eval	eval	Displays the static evaluation of the board.	Scaled Evaluation
compiler	compiler	Prints information about the engine's compiler.	Compiler info
export_net	export_net [filenameBigNet] [filenameSmallNet]	Exports the currently loaded network into a file.	—N/a
flip	flip	Flips the side to move.	—N/a
help	help	In Stockfish, this command prints information about the chess engine and links to the Github.	Help text.
license	license	Same as help command.	Help text.

Features

The uci_limitstrength parameter tells engines with this feature to play at a lower level. The uci_elo parameter specifies the Elo rating at which the engine will aim to play. Engines that have implemented uci_elo include Delfi, Fritz, Hiarcs, Houdini, Junior, Rybka, Shredder, Sjeng and Stockfish.

Variants

The UCI has been modified to play some other games, and chess variants. Some of these are:^[6]

• Universal Shogi Interface (USI), a dialect for shogi;^[7]
• Universal Chinese Chess Interface (UCCI), a dialect for xiangqi.^[8]

Each of these protocols may also define variants of Portable Game Notation (PGN) and Forsyth–Edwards Notation (FEN). The XBoard CECP is said to require far fewer changes to support variants.^[6]

See also

• Chess Engine Communication Protocol (XBoard protocol)
• Shredder
• XBoard

References

1. "Chess engines". Business World Online. 2017-04-06. Archived from the original on 2021-01-28. Retrieved 2018-06-14.
2. Torres, JC (2014-06-23). "Lichess embraces blind players with new chess site features". SlashGear. Retrieved 2018-06-14.
3. Chess Life. United States Chess Federation. 2003.
4. "UCI protocol". www.wbec-ridderkerk.nl. Retrieved 2018-06-14.
5. "UCI & Commands | Stockfish Docs". Stockfish Documentation. 2025-11-23. Retrieved 2026-02-02.
6. Evert. "UCI protocol for chess variants". TalkChess.com.
7. "The Universal Shogi Interface (USI)". hgm.nubati.net.
8. "中国象棋电脑应用规范(五)：中国象棋通用引擎协议". www.xqbase.com.

External links

• List of UCI Engines (continuously updated)
• UCI technical specification download page