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chess/doc/stockfish-extended.md

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Stockfish UCI Protocol Reference

UCI Protocol Overview

Universal Chess Interface (UCI) is a text-based protocol for communication between chess engines and GUIs. Commands are line-based with ASCII encoding.

Initialization Sequence

→ uci
← id name Stockfish 16
← id author Stockfish developers
← option name Debug Log File type string default
← option name Threads type spin default 1 min 1 max 1024
← option name Hash type spin default 16 min 1 max 33554432
← [... more options ...]
← uciok

→ isready
← readyok

Core UCI Commands

Engine Identification

  • uci - Initialize UCI mode, engine responds with options and uciok
  • quit - Terminate engine process

Synchronization

  • isready - Synchronization command, engine responds readyok when ready
  • ucinewgame - Clear hash tables and reset for new game

Position Setup

position [fen <fenstring> | startpos] [moves <move1> ... <moveN>]
  • startpos - Standard starting position
  • fen <fenstring> - Custom position in FEN notation
  • moves - Apply moves from position in UCI format (e.g., e2e4, e7e8q)

Search Commands

Basic Search

go [searchmoves <move1> ... <moveN>] [ponder] [wtime <ms>] [btime <ms>]
   [winc <ms>] [binc <ms>] [movestogo <n>] [depth <n>] [nodes <n>]
   [mate <n>] [movetime <ms>] [infinite]

Parameters:

  • searchmoves - Restrict search to specific moves
  • ponder - Start pondering mode (thinking on opponent's time)
  • wtime/btime - White/black time remaining (ms)
  • winc/binc - White/black increment per move (ms)
  • movestogo - Moves until next time control
  • depth - Search to fixed depth
  • nodes - Search fixed number of positions
  • mate - Search for mate in N moves
  • movetime - Search for fixed time (ms)
  • infinite - Search until stop command

Search Control

  • stop - Stop calculating and return best move
  • ponderhit - Opponent played expected ponder move

Engine Options

setoption name <option_name> [value <value>]

Stockfish-Specific Options

Search Parameters

  • MultiPV (1-500): Number of principal variations to calculate
  • Skill Level (0-20): Playing strength limitation
  • Contempt (-100 to 100): Draw avoidance tendency
  • Analysis Contempt (Off/White/Black/Both): Contempt perspective
  • Move Overhead (0-5000ms): Time buffer for network/GUI delay
  • Slow Mover (10-1000): Time management aggressiveness
  • UCI_AnalyseMode (true/false): Optimization for analysis
  • UCI_Chess960 (true/false): Fischer Random Chess support
  • UCI_ShowWDL (true/false): Show win/draw/loss probabilities
  • UCI_LimitStrength (true/false): Enable ELO limitation
  • UCI_Elo (1320-3190): Target ELO when strength limited

Hash Tables

  • Hash (1-33554432 MB): Transposition table size
  • Clear Hash: Clear transposition table
  • Ponder (true/false): Think during opponent's turn

Hardware Configuration

  • Threads (1-1024): Search threads (typically CPU cores)
  • Use NNUE (true/false): Neural network evaluation
  • EvalFile (path): Custom NNUE evaluation file

Syzygy Tablebases

  • SyzygyPath (path): Directory containing tablebase files
  • SyzygyProbeDepth (1-100): Minimum depth for tablebase probing
  • Syzygy50MoveRule (true/false): Consider 50-move rule
  • SyzygyProbeLimit (0-7): Maximum pieces for probing

Debug Commands

Board Display

→ d
← 
 +---+---+---+---+---+---+---+---+
 | r | n | b | q | k | b | n | r | 8
 +---+---+---+---+---+---+---+---+
 | p | p | p | p | p | p | p | p | 7
 +---+---+---+---+---+---+---+---+
 |   |   |   |   |   |   |   |   | 6
 [...]
 +---+---+---+---+---+---+---+---+
   a   b   c   d   e   f   g   h

Fen: rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
Key: 8F8F01D4562F59FB
Checkers:

Evaluation

→ eval
← Total evaluation: +0.25 (white side)
← [detailed NNUE evaluation breakdown]

Performance Testing

→ bench [depth] [threads] [hash] [fenfile] [limittype] [evaltype]

Default: bench 13 1 16 default depth mixed

Search Output Format

Standard Info String

info depth <d> seldepth <sd> multipv <n> score <score> nodes <n> 
     nps <n> hashfull <n> tbhits <n> time <ms> pv <move1> ... <moveN>

Fields:

  • depth - Current search depth
  • seldepth - Selective search depth
  • multipv - PV number (when MultiPV > 1)
  • score cp <n> - Evaluation in centipawns
  • score mate <n> - Mate in N moves (negative if being mated)
  • score lowerbound/upperbound - Bound type in fail-high/low
  • nodes - Nodes searched
  • nps - Nodes per second
  • hashfull - Hash table saturation (per mill)
  • tbhits - Tablebase positions found
  • time - Search time (ms)
  • pv - Principal variation (best line)
  • currmove - Currently searching move
  • currmovenumber - Move number in root move list
  • string - Free-form engine output

Win/Draw/Loss Output (UCI_ShowWDL=true)

info depth 20 score cp 15 wdl 395 604 1

WDL values in per mill: win/draw/loss from current side's perspective.

Multi-PV Example

setoption name MultiPV value 3
go depth 15

info multipv 1 depth 15 score cp 31 pv e2e4 e7e5 g1f3
info multipv 2 depth 15 score cp 20 pv d2d4 d7d5 g1f3
info multipv 3 depth 15 score cp 15 pv g1f3 g8f6 d2d4

Best Move Output

bestmove <move> [ponder <move>]
  • bestmove - Best move in UCI notation
  • ponder - Expected opponent response for pondering

Special cases:

  • bestmove (none) - No legal moves (checkmate/stalemate)
  • bestmove 0000 - Null move (analysis mode only)

Advanced Analysis Techniques

Infinite Analysis

position fen <position>
setoption name UCI_AnalyseMode value true
go infinite
[... engine thinks until stop ...]
stop

Multi-PV Analysis

setoption name MultiPV value 5
position startpos moves e2e4 e7e5
go depth 20

Mate Search

go mate 7  # Find mate in 7 moves or less

Fixed Node Search

go nodes 1000000  # Analyze exactly 1M positions

Search Move Restriction

position startpos
go searchmoves e2e4 d2d4 g1f3  # Only consider these moves

Time Management

Tournament Time Control

position startpos moves e2e4 e7e5
go wtime 300000 btime 300000 winc 2000 binc 2000 movestogo 40

5 minutes + 2 second increment, 40 moves to time control.

Sudden Death

go wtime 60000 btime 60000  # 1 minute each, no increment

Fixed Time Per Move

go movetime 5000  # Think for exactly 5 seconds

Performance Tuning

Analysis Optimization

setoption name Threads value 8
setoption name Hash value 4096
setoption name UCI_AnalyseMode value true
setoption name MultiPV value 1

Rapid/Blitz Optimization

setoption name Move Overhead value 100
setoption name Slow Mover value 50
setoption name Threads value 4

Endgame Optimization

setoption name SyzygyPath value /path/to/tablebases
setoption name SyzygyProbeDepth value 1

Error Handling

Common error responses:

  • Unknown command: <cmd> - Invalid UCI command
  • Illegal move: <move> - Move not legal in current position
  • Invalid position - FEN parsing failed
  • No such option: <name> - Unknown engine option

Protocol Extensions

Chess960 (Fischer Random)

setoption name UCI_Chess960 value true
position fen <chess960_fen> moves <move1> ...

Debug Logging

setoption name Debug Log File value debug.txt
setoption name Use Debug Log value true

NNUE Evaluation

setoption name Use NNUE value true
setoption name EvalFile value nn-[hash].nnue

Typical Usage Patterns

Game Analysis

  1. Set analysis mode and resources
  2. Load position with game moves
  3. Run infinite analysis
  4. Stop and retrieve evaluation

Opening Preparation

  1. Set MultiPV to compare variations
  2. Load opening position
  3. Search to fixed depth
  4. Compare evaluations of candidate moves

Endgame Study

  1. Configure tablebase paths
  2. Load endgame position
  3. Search for mate or optimal play
  4. Verify with tablebase hits

Engine Match

  1. Reset with ucinewgame
  2. Set time controls
  3. Apply moves incrementally
  4. Use ponder for thinking on opponent time