Chess Engine Communication Protocol

Tim Mann

Last modified on Sun Sep 17 23:37:17 PDT 2000 by mann

1. Introduction

This document is a set of rough notes on the protocol that xboard and WinBoard use to communicate with gnuchessx and other chess engines. These notes may be useful if you want to connect a different chess engine to xboard. Throughout the notes, "xboard" means both xboard and WinBoard except where they are specifically contrasted.

There are two reasons I can imagine someone wanting to do this:

  1. You have, or are developing, a chess engine but you don't want to write your own graphical interface.
  2. You have, or are developing,a chess engine, and you want to interface it to the Internet Chess Server.

In case (2), if you are using xboard, you will need to configure the "Zippy" code into it, but WinBoard includes this code already. See the file zippy.README in the xboard or WinBoard distribution for more information.

These notes are unpolished, but I've attempted to make them complete in this release. If you notice any errors, omissions, or misleading statements, let me know.

I'd like to hear from everyone who is trying to interface their own chess engine to xboard/WinBoard. Please email me, Also, please join the mailing list for authors of xboard/WinBoard compatible chess engines. The list is now hosted by; you can join at, or you can read the list there without joining. The list is filtered to prevent spam.

2. Connection

An xboard chess engine runs as a separate process from xboard itself, connected to xboard through a pair of anonymous pipes. The engine does not have to do anything special to set up these pipes. xboard sets up the pipes itself and starts the engine with one pipe as its standard input and the other as its standard output. The engine then reads commands from its standard input and writes responses to its standard output. This is, unfortunately, a little more complicated to do right than it sounds; see section 6 below.

And yes, contrary to some people's expectations, exactly the same thing is true for WinBoard. Pipes and standard input/output are implemented in Win32 and work fine. You don't have to use DDE, COM, DLLs, BSOD, or any of the other infinite complexity that Microsoft has created just to talk between two programs. A WinBoard chess engine is a Win32 console program that simply reads from its standard input and writes to its standard output. See sections 5 and 6 below for additional details.

3. Debugging

To diagnose problems in your engine's interaction with xboard, use the -debug flag on xboard's command line to see the messages that are being exchanged. In WinBoard, these messages are written to the file WinBoard.debug instead of going to the screen.

You can turn debug mode on or off while WinBoard is running by pressing Ctrl+Alt+F12. You can turn debug mode on or off while xboard is running by binding DebugProc to a shortcut key (and pressing the key!); see the instructions on shortcut keys in the xboard man page.

While your engine is running under xboard/WinBoard, you can send a command directly to the engine by pressing Shift+1 (xboard) or Alt+1 (WinBoard 4.0.3 and later). This brings up a dialog that you can type your command into. Press Shift+2 (Alt+2) instead to send to the second chess engine in Two Machines mode. On WinBoard 4.0.2 and earlier, Ctrl+Alt is used in place of Alt; this had to be changed due to a conflict with typing the @-sign on some European keyboards.

4. How it got this way

Originally, xboard was just trying to talk to the existing command-line interface of gnuchess, designed for people to type commands to. So the communication protocol is very ad-hoc. (The reason why there is a gnuchessx that's different from gnuchessr is buried in the mists of time, before I started working on xboard, but I think it was due to someone working around a stupid bug in xboard by changing gnuchess instead of fixing the bug. The differences are tiny.) It's now tough to change the interface, because xboard and gnuchess are separate programs, and I don't want to force people to upgrade them together to versions that match.

Things have changed a bit now that there are many more engines that work with xboard. I've had to make the protocol description more precise, and I've added some features that GNU Chess does not support. In the latest version, I've specified a standard semantics for many commands that differs in some details from what GNU Chess provides, but is easier to work with. In the future I may release a modified GNU Chess that conforms exactly to this protocol.

5. WinBoard requires Win32 engines

Due to some Microsoft brain damage that I don't understand, WinBoard does not work with chess engines that were compiled to use a DOS extender for 32-bit addressing. (Probably not with 16-bit DOS or Windows programs either.) WinBoard works only with engines that are compiled for the Win32 API. You can get a free compiler that targets the Win32 API from I think DJGPP 2.x should also work if you use the RSXNTDJ extension, but I haven't tried it. Of course, Microsoft Visual C++ will work. Most likely the other commercial products that support Win32 will work too (Borland, etc.), but I have not tried them.

6. Hints on input/output

Beware of using buffered I/O in your chess engine. The C stdio library, C++ streams, and the I/O packages in most other languages use buffering both on input and output. That means two things. First, when your engine tries to write some characters to xboard, the library stashes them in an internal buffer and does not actually write them to the pipe connected to xboard until either the buffer fills up or you call a special library routine asking for it to be flushed. (In C stdio, this routine is named fflush.) Second, when your engine tries to read some characters from xboard, the library does not read just the characters you asked for -- it reads all the characters that are currently available (up to some limit) and stashes any characters you are not yet ready for in an internal buffer. The next time you ask to read, you get the characters from the buffer (if any) before the library tries to read more data from the actual pipe.

Why does this cause problems? First, on the output side, remember that your engine produces output in small quantities (say, a few characters for a move, or a line or two giving the current analysis), and that data always needs to be delivered to xboard/WinBoard for display immediately. If you use buffered output, the data you print will sit in a buffer in your own address space instead of being delivered.

You can usually fix the output buffering problem by asking for the buffering to be turned off. In C stdio, you do this by calling setbuf(stdout, NULL). A more laborious and error-prone method is to carefully call fflush(stdout) after every line you output; I don't recommend this. In C++, you can try cout.setf(ios::unitbuf), which is documented in current editions of "The C++ Programming Language," but not older ones. Another C++ method that might work is cout.rdbuf()->setbuf(NULL, 0). Alternatively, you can carefully call cout.flush() after every line you output; again, I don't recommend this.

Another way to fix the problem is to use unbuffered operating system calls to write directly to the file descriptor for standard output. On Unix, this means write(1, ...) -- see the man page for write(2). On Win32, you can use either the Unix-like _write(1, ...) or Win32 native routines like WriteFile.

Second, on the input side, you are likely to want to poll during your search and stop it if new input has come in. If you implement pondering, you'll need this so that pondering stops when the user makes a move. You should also poll during normal thinking on your move, so that you can implement the "?" (move now) command, and so that you can respond promptly to a "result", "force", or "quit" command if xboard wants to end the game or terminate your engine. Buffered input makes polling more complicated -- when you poll, you must stop your search if there are either characters in the buffer or characters available from the underlying file descriptor.

The most direct way to fix this problem is to use unbuffered operating system calls to read (and poll) the underlying file descriptor directly. On Unix, use read(0, ...) to read from standard input, and use select() to poll it. See the man pages read(2) and select(2). (Don't follow the example of GNU Chess and use the FIONREAD ioctl to poll for input. It is not very portable; that is, it does not exist on all versions of Unix, and is broken on some that do have it.) On Win32, you can use either the Unix-like _read(0, ...) or the native Win32 ReadFile() to read. Unfortunately, under Win32, the function to use for polling is different depending on whether the input device is a pipe, a console, or something else. (More Microsoft brain damage here -- did they never hear of device independence?) For pipes, you can use PeekNamedPipe to poll (even when the pipe is unnamed). For consoles, you can use GetNumberOfConsoleInputEvents. For sockets only, you can use select(). It might be possible to use WaitForSingleObject more generally, but I have not tried it. Some code to do these things can be found in Crafty's utility.c, but I don't guarantee that it's all correct or optimal.

A second way to fix the problem might be to ask your I/O library not to buffer on input. It should then be safe to poll the underlying file descriptor as descrbed above. With C, you can try calling setbuf(stdin, NULL). However, I have never tried this. Also, there could be problems if you use scanf(), at least with certain patterns, because scanf() sometimes needs to read one extra character and "push it back" into the buffer; hence, there is a one-character pushback buffer even if you asked for stdio to be unbuffered. With C++, you can try cin.rdbuf()->setbuf(NULL, 0), but again, I have never tried this.

A third way to fix the problem is to check whether there are characters in the buffer whenever you poll. C I/O libraries generally do not provide any portable way to do this. Under C++, you can use cin.rdbuf()->in_avail(). This method has been reported to work with EXchess. Remember that if there are no characters in the buffer, you still have to poll the underlying file descriptor too, using the method descrbed above.

A fourth way to fix the problem is to use a separate thread to read from stdin. This way works well if you are familiar with thread programming. This thread can be blocked waiting for input to come in at all times, while the main thread of your engine does its thinking. When input arrives, you have the thread put the input into a buffer and set a flag in a global variable. Your search routine then periodically tests the global variable to see if there is input to process, and stops if there is. WinBoard and my Win32 ports of ICC timestamp and FICS timeseal use threads to handle multiple input sources.

7. Signals

Engines that run on Unix need to be concerned with two Unix signals: SIGTERM and SIGINT. This applies both to engines that run under xboard and (the unusual case of) engines that WinBoard remotely runs on a Unix host using the -firstHost or -secondHost feature. It does not apply to engines that run on Windows, because Windows does not have Unix-style signals.

First, when an engine is sent the "quit" command, it is also given a SIGTERM signal shortly afterward to make sure it goes away. If your engine reliably responds to "quit", and the signal causes problems for you, you should ignore it by calling signal(SIGTERM, SIG_IGN) at the start of your program.

Second, xboard will send an interrupt signal (SIGINT) at certain times when it believes the engine may not be listening to user input (thinking or pondering). WinBoard currently does this only when the engine is running remotely using the -firstHost or -secondHost feature, not when it is running locally. You probably need to know only enough about this grungy feature to keep it from getting in your way.

The SIGINTs are basically tailored to the needs of GNU Chess on systems where its input polling code is broken or disabled. Because they work in a rather peculiar way, it is recommended that you simply ignore SIGINT when running under Unix in xboard mode. You can do this by having your engine call signal(SIGINT, SIG_IGN). Alternatively, you can configure your personal copy of xboard to not send SIGINT by running configure with the --disable-sigint option. This won't help you if you give your engine to other people who don't want to recompile their xboard and possibly break its interaction with GNU Chess.

Here are details for the curious. If xboard needs to send a command when it is the chess engine's move (such as before the "?" command), it sends a SIGINT first. If xboard needs to send commands when it is not the chess engine's move, but the chess engine may be pondering (thinking on its opponent's time) or analyzing (analysis or analyze file mode), xboard sends a SIGINT before the first such command only. Another SIGINT is not sent until another move is made, even if xboard issues more commands. This behavior is necessary for GNU Chess. The first SIGINT stops it from pondering until the next move, but on some systems, GNU Chess will die if it receives a SIGINT when not actually thinking or pondering.

There are two reasons why WinBoard does not send the Win32 equivalent of SIGINT (which is called CTRL_C_EVENT) to local engines. First, the Win32 GNU Chess port does not need it. Second, I could not find a way to get it to work. Win32 seems to be designed under the assumption that only console applications, not windowed applications, would ever want to send a CTRL_C_EVENT. (More Microsoft brain damage.)

8. Commands from xboard to the engine

All commands from xboard to the engine end with a newline (\n), even where that is not explicitly stated. All your output to xboard must be in complete lines; any form of prompt or partial line will cause problems.

At the beginning of each game, xboard sends an initialization string. This is currently "new\nrandom\n" unless the user changes it with the initString or secondInitString option.

xboard normally reuses the same chess engine process for multiple games. At the end of a game, xboard will send the "force" command (see below) to make sure your engine stops thinking about the current position. It will later send the initString again to start a new game. If your engine can't play multiple games, give xboard the -xreuse (or -xreuse2) command line option to disable reuse. xboard will then ask the process to quit after each game and start a new process for the next game.

This command will be sent once immediately after your engine process is started. You can use it to put your engine into "xboard mode" if that is needed. If your engine prints a prompt to ask for user input, you must turn off the prompt and output a newline when the "xboard" command comes in.

Reset the board to the standard chess starting position. Set White on move. Leave force mode and set the engine to play Black. Associate the engine's clock with Black and the opponent's clock with White. Reset clocks and time controls to the start of a new game. Stop clocks. Do not ponder on this move, even if pondering is on. Remove any search depth limit previously set by the sd command.

variant VARNAME
If the game is not standard chess, but a variant, this command is sent after "new" and before the first move or "edit" command. Currently defined variant names are:
wildcastleShuffle chess where king can castle from d file
nocastleShuffle chess with no castling at all
fischerandomFischeRandom (not supported yet)
bughouseBughouse, ICC/FICS rules
crazyhouseCrazyhouse, ICC/FICS rules
losersWin by losing all pieces or getting mated (ICC)
suicideWin by losing all pieces including king (FICS)
twokingsWeird ICC wild 9
kriegspielKriegspiel (not really supported)
atomicAtomic (not really supported)
3checkWin by giving check 3 times (not supported)
unknownUnknown variant (not supported)

The chess engine should immediately exit. This command is used when xboard is itself exiting, and also between games if the -xreuse command line option is given (or -xreuse2 for the second engine). See also Signals above.

This command is specific to GNU Chess. You can either ignore it completely (that is, treat it as a no-op) or implement it as GNU Chess does. The command toggles "random" mode (that is, it sets random = !random). In random mode, the engine adds a small random value to its evaluation function to vary its play. The "new" command sets random mode off.

Set the engine to play neither color ("force mode"). Stop clocks. The engine should check that moves received in force mode are legal and made in the proper turn, but should not think, ponder, or make moves of its own.

Set White on move. Set the engine to play Black. Stop clocks.

Set Black on move. Set the engine to play White. Stop clocks.

Set time controls. See the Time Control section below.

Set time controls. See the Time Control section below. The commands "level" and "st" are not used together.

The engine should limit its thinking to DEPTH ply.

time N
Set a clock that always belongs to the engine. N is a number in centiseconds (units of 1/100 second). Even if the engine changes to playing the opposite color, this clock remains with the engine.

otim N
Set a clock that always belongs to the opponent. N is a number in centiseconds (units of 1/100 second). Even if the opponent changes to playing the opposite color, this clock remains with the opponent.

If needed for purposes of board display in force mode (where the engine is not participating in the game) the time clock should be associated with the last color that the engine was set to play, the otim clock with the opposite color.

If you can't handle the time and otim commands, you can ignore them (that is, treat them as no-ops); or better, send back "Error (unknown command): time" the first time you see "time", and xboard will realize you don't implement the command.

Leave force mode and set the engine to play the color that is on move. Associate the engine's clock with the color that is on move, the opponent's clock with the opposite color. Start the engine's clock. Start thinking and eventually make a move.

See below for the syntax of moves. If the move is illegal, print an error message; see the section "Commands from the engine to xboard". If the move is legal and in turn, make it. If not in force mode, stop the opponent's clock, start the engine's clock, start thinking, and eventually make a move.

When xboard sends your engine a move, it always sends coordinate algebraic notation. There is no command name; the notation is just sent as a line by itself. Examples:

Normal moves:e2e4
Pawn promotion:e7e8q
Castling:e1g1, e1c1, e8g8, e8c8
Bughouse drop:P@h3
ICS Wild 0/1 castling:d1f1, d1b1, d8f8, d8b8
FischerRandom castling:o-o, o-o-o (future)

If your engine can't handle this kind of output, change the routine SendMoveToProgram in backend.c to send the kind of notation you need. If you define SAN_TO_PROGRAM, your engine will be sent Standard Algebraic Notation (as defined by the PGN standard); for example, e4, Nf3, exd5, Bxf7+, Qxf7#, e8=Q, O-O, or P@h3. (The P@h3 notation is a nonstandard extension to SAN.) In the future, I may make SAN_TO_PROGRAM a runtime option if there is demand for it.

xboard doesn't reliably detect illegal moves, because it does not keep track of castling unavailablity due to king or rook moves, or en passant availability. If xboard sends an illegal move, send back an error message so that xboard can retract it and inform the user; see the section "Commands from the engine to xboard".

Move now. If your engine is thinking, it should move immediately; otherwise, the command should be ignored (treated as a no-op). It is permissible for your engine to always ignore the ? command. The only bad consequence is that xboard's Move Now menu command will do nothing.

It is also permissible for your engine to move immediately if it gets any command while thinking, as long as it processes the command right after moving, but it's preferable if you don't do this. For example, xboard may send post, nopost, easy, hard, force, or quit while the engine is on move.

The engine's opponent offers the engine a draw. To accept the draw, send "offer draw". To decline, ignore the offer (that is, send nothing). If you're playing on ICS, it's possible for the draw offer to have been withdrawn by the time you accept it, so don't assume the game is over because you accept a draw offer. Continue playing until xboard tells you the game is over. See also "offer draw" below.

After the end of each game, xboard will send you a result command. You can use this command to trigger learning. RESULT is either 1-0, 0-1, 1/2-1/2, or *, indicating whether white won, black won, the game was a draw, or the game was unfinished. The COMMENT string is purely a human-readable comment; its content is unspecified and subject to change. In ICS mode, it is passed through from ICS uninterpreted. Example:
result 1-0 {White mates}

Here are some notes on interpreting the "result" command. Some apply only to playing on ICS ("Zippy" mode).

If you won but did not just play a mate, your opponent must have resigned or forfeited. If you lost but were not just mated, you probably forfeited on time, or perhaps the operator resigned manually. If there was a draw for some nonobvious reason, perhaps your opponent called your flag when he had insufficient mating material (or vice versa), or perhaps the operator agreed to a draw manually.

You will get a result command even if you already know the game ended -- for example, after you just checkmated your opponent. In fact, if you send the "RESULT {COMMENT}" command (discussed below), you will simply get the same thing fed back to you with "result" tacked in front. You might not always get a "result *" command, however. In particular, you won't get one in local chess engine mode when the user stops playing by selecting Reset, Edit Game, Exit or the like.

The edit command puts the chess engine into a special mode, where it accepts the following subcommands:
cchange current piece color, initially white
Pa4 (for example)place pawn of current color on a4
xa4 (for example)empty the square a4 (not used by xboard)
#clear board
.leave edit mode

The edit command does not change the side to move. To set up a black-on-move position, xboard uses the following command sequence:

    <edit commands>

This sequence is used for compatibility with engines that do not interpret the "black" command according to the specification above; see "Idioms" below.

After an edit command is complete, if a king and a rook are on their home squares, castling is assumed to be available to them. En passant capture is assumed to be illegal on the current move regardless of the positions of the pawns. The clock for the 50 move rule starts at zero, and for purposes of the draw by repetition rule, no prior positions are deemed to have occurred.

If the user asks for a hint, xboard sends your engine the command "hint". Your engine should respond with "Hint: xxx", where xxx is a suggested move. If there is no move to suggest, you can ignore the hint command (that is, treat it as a no-op).

If the user selects "Book" from the xboard menu, xboard will send your engine the command "bk". You can send any text you like as the response, as long as each line begins with a blank space or tab (\t) character, and you send an empty line at the end. The text pops up in a modal information dialog.

If the user asks to back up one move, xboard will send you the "undo" command. xboard will not send this command without putting you in "force" mode first, so you don't have to worry about what should happen if the user asks to undo a move your engine made. (GNU Chess actually switches to playing the opposite color in this case.)

If the user asks to retract a move, xboard will send you the "remove" command. It sends this command only when the user is on move. Your engine should undo the last two moves (one for each player) and continue playing the same color.

Turn on pondering (thinking on the opponent's time, also known as "permanent brain"). xboard will not make any assumption about what your default is for pondering or whether "new" affects this setting.

Turn off pondering.

Turn on thinking/pondering output. See Thinking Output section.

Turn off thinking/pondering output.

Enter analyze mode. See Analyze Mode section.

Here are some special commands for Zippy mode:

name X
In ICS mode, xboard obtains the name of its opponent from ICS when a game starts and saves it for use in the PGN tags. In Zippy mode, it also passes the opponent's name on to the chess engine with the name command. Example:
name mann
In ICS mode, xboard obtains the ICS opponent's rating from the "Creating:" message that appears before each game. (This message may not appear on servers using outdated versions of the FICS code.) In Zippy mode, it sends these ratings on to the chess engine using the "rating" command. The chess engine's own rating comes first, and if either opponent is not rated, his rating is given as 0. Example:
rating 2600 1500
The opponent is on the ICS computer list.

Bughouse commands:

xboard now supports bughouse engines when in Zippy mode. See zippy.README for information on Zippy mode and how to turn on the bughouse support. The bughouse move format is given above. xboard sends the following additional commands to the engine when in bughouse mode. Commands to inform your engine of the partner's game state may be added in the future.

partner <player>
<player> is now your partner for future games. Example:
partner mann

Meaning: You no longer have a partner.

ptell <text>
Your partner told you <text>, either with a ptell or an ordinary tell.

holding [<white>] [<black>]
White currently holds <white>; black currently holds <black>. Example:
holding [PPPRQ] []
holding [<white>] [<black>] <color><piece>
White currently holds <white>; black currently holds <black>, after <color> acquired <piece>. Example:
holding [PPPRQ] [R] BR

9. Commands from the engine to xboard

Illegal move: MOVE
Illegal move (REASON): MOVE
If your engine receives a MOVE command that is recognizably a move but is not legal in the current position, your engine must print an error message in one of the above formats so that xboard can pass the error on to the user and retract the move. The (REASON) is entirely optional. Examples:
  Illegal move: e2e4
  Illegal move (in check): Nf3
  Illegal move (moving into check): e1g1

Generally, xboard will never send an ambiguous move, so it does not matter whether you respond to such a move with an Illegal move message or an Error message.

If your engine receives a command it does not understand or does not implement, it should print an error message in the above format so that xboard can parse it. Examples:
  Error (ambiguous move): Nf3
  Error (unknown command): analyze
  Error (command not legal now): undo
  Error (too many parameters): level 1 2 3 4 5 6 7

Note: versions of xboard prior to 3.6.11beta do not parse the "Error" format. To ease the transition, it is acceptable to use the "Illegal move" format for all errors, even if the command given was not a move.

move MOVE
Your engine is making the move MOVE. Do not echo moves from xboard with this command; send only new moves made by the engine.

Note: versions of xboard prior to 3.6.11beta do not parse the above format, so you may want to use the old "NUMBER ... MOVE" format temporarily. See the section "Idioms and backward compatibility features" below.

For the actual move text from your chess engine (in place of MOVE above), xboard will accept any kind of unambiguous algebraic format, including coordinate notation, SAN, and some slight variants of SAN. You don't have to send the pure coordinate notation that xboard sends to your engine; xboard parses the output with its general-purpose move parser, which was built to extract human-typed game scores from netnews messages. For example, the following will all work:


and many more.

When your engine detects that the game has ended by rule (checkmate, stalemate, triple repetition, the 50 move rule, or insufficient material), your engine must output a line of the form "RESULT {comment}" (without the quotes), where RESULT is a PGN result code (1-0, 0-1, or 1/2-1/2), and comment is the reason. Examples:
  0-1 {Black mates}
  1-0 {White mates}
  1/2-1/2 {Draw by repetition}
  1/2-1/2 {Stalemate}

xboard relays the result to the user, the ICS, the other engine in Two Machines mode, and the PGN save file as required.

If your engine wants to resign, it can send the command "resign". Alternatively, it can use the "RESULT {comment}" command if the string "resign" is included in the comment; for example "0-1 {White resigns}". xboard relays the resignation to the user, the ICS, the other engine in Two Machines mode, and the PGN save file as required.

offer draw
If your engine wants to offer a draw by agreement (as opposed to claiming a draw by rule), it can send the command "offer draw". xboard relays the offer to the user, the ICS, the other engine in Two Machines mode, and the PGN save file as required. In Machine White, Machine Black, or Two Machines mode, the offer is considered valid until your engine has made two more moves.

telluser MESSAGE
xboard pops up a modal information dialog containing the message. MESSAGE consists of any characters, including whitespace, to the end of the line.

tellusererror MESSAGE
xboard pops up a non-modal error dialog containing the message. MESSAGE consists of any characters, including whitespace, to the end of the line.

Here REPTAG is a string containing no whitespace, and MESSAGE consists of any characters, including whitespace, to the end of the line. xboard pops up a modal question dialog that says MESSAGE and has a typein box. If the user types in "bar", xboard sends "REPTAG bar" to the engine. The user can cancel the dialog and send nothing.

tellics MESSAGE
In Zippy mode, xboard sends "MESSAGE\n" to ICS. MESSAGE consists of any characters, including whitespace, to the end of the line.

10. Thinking Output

If the user asks your engine to "show thinking", xboard sends your engine the "post" command. It sends "nopost" to turn thinking off. In post mode, your engine sends output lines to show the progress of its thinking. The engine can send as many or few of these lines as it wants to, whenever it wants to. Typically they would be sent when the PV (principal variation) changes or the depth changes. The thinking output should be in the following format:

ply score time nodes pv
plyInteger giving current search depth.
scoreInteger giving current evaluation in centipawns.
timeCurrent search time in centiseconds (ex: 1028 = 10.28 seconds).
nodesNodes searched.
pvFreeform text giving current "best" line. You can continue the pv onto another line if you start each continuation line with at least four space characters.


  9 156 1084 48000 Nf3 Nc6 Nc3 Nf6


9 ply, score=1.56, time = 10.84 seconds, nodes=48000, PV = "Nf3 Nc6 Nc3 Nf6"

Longer example from actual Crafty output:

  4    109      14   1435  1. e4 d5 2. Qf3 dxe4 3. Qxe4 Nc6
  4    116      23   2252  1. Nf3 Nc6 2. e4 e6
  4    116      27   2589  1. Nf3 Nc6 2. e4 e6
  5    141      44   4539  1. Nf3 Nc6 2. O-O e5 3. e4
  5    141      54   5568  1. Nf3 Nc6 2. O-O e5 3. e4

You can use the PV to show other things; for instance, while in book, Crafty shows the observed frequency of different reply moves in its book. In situations like this where your engine is not really searching, start the PV with a '(' character:

  0      0       0      0  (e4 64%, d4 24%)

GNU Chess output is very slightly different. The ply number is followed by an extra nonblank character, and the time is in seconds, not hundredths of seconds. For compatibility, xboard accepts the extra character and takes it as a flag indicating the different time units. Example:

 2.     14    0       38   d1d2  e8e7 
 3+     78    0       65   d1d2  e8e7  d2d3 
 3&     14    0       89   d1d2  e8e7  d2d3 
 3&     76    0      191   d1e2  e8e7  e2e3 
 3.     76    0      215   d1e2  e8e7  e2e3 
 4&     15    0      366   d1e2  e8e7  e2e3  e7e6 
 4.     15    0      515   d1e2  e8e7  e2e3  e7e6 
 5+     74    0      702   d1e2  f7f5  e2e3  e8e7  e3f4 
 5&     71    0     1085   d1e2  e8e7  e2e3  e7e6  e3f4 
 5.     71    0     1669   d1e2  e8e7  e2e3  e7e6  e3f4 
 6&     48    0     3035   d1e2  e8e7  e2e3  e7e6  e3e4  f7f5  e4d4 
 6.     48    0     3720   d1e2  e8e7  e2e3  e7e6  e3e4  f7f5  e4d4 
 7&     48    0     6381   d1e2  e8e7  e2e3  e7e6  e3e4  f7f5  e4d4 
 7.     48    0    10056   d1e2  e8e7  e2e3  e7e6  e3e4  f7f5  e4d4 
 8&     66    1    20536   d1e2  e8e7  e2e3  e7e6  e3d4  g7g5  a2a4  f7f5 
 8.     66    1    24387   d1e2  e8e7  e2e3  e7e6  e3d4  g7g5  a2a4  f7f5 
 9&     62    2    38886   d1e2  e8e7  e2e3  e7e6  e3d4  h7h5  a2a4  h5h4 
 9.     62    4    72578   d1e2  e8e7  e2e3  e7e6  e3d4  h7h5  a2a4  h5h4 
10&     34    7   135944   d1e2  e8e7  e2e3  e7e6  e3d4  h7h5  c2c4  h5h4 
                           d4e4  f7f5  e4f4 
10.     34    9   173474   d1e2  e8e7  e2e3  e7e6  e3d4  h7h5  c2c4  h5h4 
                           d4e4  f7f5  e4f4 

If your engine is pondering (thinking on its opponent's time) in post mode, it can show its thinking then too. In this case your engine may omit the hint move (the move it is assuming its opponent will make) from the thinking lines if and only if it sends xboard the move in the usual "Hint: xxx" format before sending the first line.

11. Time control

xboard supports three styles of time control: conventional chess clocks, the ICS-style incremental clock, and an exact number of seconds per move.

In conventional clock mode, every time control period is the same. That is, if the time control is 40 moves in 5 minutes, then after each side has made 40 moves, they each get an additional 5 minutes, and so on, ad infinitum. At some future time it would be nice to support a series of distinct time controls. This is very low on my personal priority list, but code donations to the xboard project are accepted, so feel free to take a swing at it. I suggest you talk to me first, though.

The command to set a conventional time control looks like this:

  level 40 5 0
  level 40 0:30 0

The 40 means that there are 40 moves per time control. The 5 means there are 5 minutes in the control. In the second example, the 0:30 means there are 30 seconds. The final 0 means that we are in conventional clock mode.

The command to set an incremental time control looks like this:

  level 0 2 12

Here the 0 means "play the whole game in this time control period", the 2 means "base=2 minutes", and the 12 means "inc=12 seconds". As in conventional clock mode, the second argument to level can be in minutes and seconds.

At the start of the game, each player's clock is set to base minutes. Immediately after a player makes a move, inc seconds are added to his clock. A player's clock counts down while it is his turn. Your flag can be called whenever your clock is zero or negative. (Your clock can go negative and then become positive again because of the increment.)

A special ICS rule: if you ask for a game with base=0, the clocks really start at 10 seconds instead of 0. xboard itself does not know about this rule currently, so it may pass the 0 on to the engine instead of changing it to 0:10.

ICS also has time odds games. With time odds, each player has his own (base, inc) pair, but otherwise things work the same as in normal games. The Zippy xboard accepts time odds games but ignores the fact that the opponent's parameters are different; this is perhaps not quite the right thing to do, but gnuchess doesn't understand time odds. Time odds games are always unrated.

The command to set an exact number of seconds per move looks like this:

  st 30

This means that each move must be made in 30 seconds. Time not used on one move does not accumulate for use on later moves.

12. Analyze Mode

xboard supports analyzing fresh games, edited positions, and games from files. However, all of these look the same from the chess engine's perspective. Basically, the engine just has to respond to the "analyze" command. If your engine does not support analyze mode, it should print the error message "Error (unknown command): analyze" in response to the "analyze" command.

To enter analyze mode, xboard sends the command sequence "post", "white" or "black", "analyze". Analyze mode in your engine should be similar to force mode, except that your engine thinks about what move it would make next if it were on move. Your engine should accept the following commands while in analyze mode:

If the user selects "Periodic Updates", xboard will send the string ".\n" to the chess engine periodically during analyze mode, unless the last PV received began with a '(' character.

The chess engine should respond to ".\n" with a line like this:

stat01: time nodes ply mvleft mvtot
timeElapsed search time in centiseconds (ie: 567 = 5.67 seconds).
nodesNodes searched so far.
plySearch depth so far.
mvleftNumber of moves left to consider at this depth.
mvtotTotal number of moves to consider.


  stat01: 1234 30000 7 5 30


After 12.34 seconds, I've searched 7 ply/30000 nodes, there are a total of 30 legal moves, and I have 5 more moves to search before going to depth 8.

Implementation of the "." command is OPTIONAL. If the engine does not respond to the "." command with a "stat01..." line, xboard will stop sending "." commands. If the engine does not implement this command, the analysis window will use a shortened format to display the engine info.

To give the user some extra information, the chess engine can output the strings "++\n" and "--\n", to indicate that the current search is failing high or low, respectively. You don't have to send anything else to say "Okay, I'm not failing high/low anymore." xboard will figure this out itself.

13. Idioms and backward compatibility features

Some engines have variant interpretations of the force/go/white/black, time/otim, and hard/easy command sets. New engines should not use these interpretations, but in order to accommodate existing engines, xboard is currently very conservative about how it uses these commands. Only the following idioms are currently used.

Sent when the engine is in force mode or playing Black but should switch to playing White. This sequence is sent only when White is already on move.

Sent when the engine is in force mode or playing White but should switch to playing Black. This sequence is sent only when Black is already on move.

time N
otim N
Sent when the opponent makes a move and the engine is already playing the opposite color.

time N
otim N
Sent when Black is on move, the engine is in force mode or playing White, and the engine's clock needs to be updated before it starts playing. The initial "white" is a kludge to accommodate GNU Chess 4.0.77's variant interpretation of these commands. It may be removed in the future, especially if it causes problems for other engines.

time N
otim N
Sent when White is on move, the engine is in force mode or playing Black, and the engine's clock needs to be updated before it starts playing. See previous idiom.

Sent in sequence to turn off pondering if xboard is not sure whether it is on. When xboard is sure, it will send "hard" or "easy" alone. xboard does this because "easy" is a toggle in GNU Chess but "hard" is an absolute on.

To support older engines, certain additional commands from the engine to xboard are also recognized. (These are commands by themselves, not values to be placed in the comment field of the PGN result code.) These forms are not recommended for new engines; use the PGN result code commands or the resign command instead:

Command Interpreted as
White resigns 0-1 {White resigns}
Black resigns 1-0 {Black resigns}
White 1-0 {White mates}
Black 0-1 {Black mates}
Draw 1/2-1/2 {Draw}
computer mates 1-0 {White mates} or 0-1 {Black mates}
opponent mates 1-0 {White mates} or 0-1 {Black mates}
computer resigns 0-1 {White resigns} or 1-0 {Black resigns}
game is a draw 1/2-1/2 {Draw}
checkmate 1-0 {White mates} or 0-1 {Black mates}

Commands in the above table are recognized if they begin a line and arbitrary characters follow, so (for example) "White mates" will be recognized as "White", and "game is a draw by the 50 move rule" will be recognized as "game is a draw". All the commands are case-sensitive.

An alternative move syntax is also recognized:

Command Interpreted as

Here NUMBER means any string of decimal digits, optionally ending in a period. MOVE is any string containing no whitespace. In this command format, xboard requires the "..." even if your engine is playing White. A command of the form NUMBER MOVE will be ignored. This odd treatment of the commands is needed for compatibility with gnuchessx. The original reasons for it are lost in the mists of time, but I suspect it was originally a bug in the earliest versions of xboard, before I started working on it, which someone "fixed" in the wrong way, by creating a special version of gnuchess (gnuchessx) instead of changing xboard.

Any line that contains the words "offer" and "draw" is recognized as "offer draw".

The "Illegal move" message is recognized even if spelled "illegal move" and even if the colon (":") is omitted. This accommodates GNU Chess 4.0.77, which prints messages like "Illegal move (no matching move)e2e4", and old versions of Crafty, which print just "illegal move".

In Zippy mode, for compatibility with existing versions of Crafty, xboard passes through to ICS any line that begins "kibitz", "whisper", "tell", or "draw". Do not use this feature in new code.

Before the "sd DEPTH" command, xboard also sends the command "depth\nDEPTH", for the benefit of GNU Chess. Note the newline in the middle of this command. Ugh.

For the benefit of GNU Chess, if an "st TIME"-style time control is being used, TIME is also given to the engine as a command-line argument when it is started. Ugh.

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