Chess engine with API I am looking for a chess engine which provides a normal API which I can use with some high level programming language like C# or python. I went through the list of leaders (Houdini, Komodo, Stockfish and Rybka) and was not able to find any signs that these engines provides API (which surprised me and suggested that I am missing something).

To be less vague, here is what I need from this API:


be able to load specific game into the engine
be able to evaluate position of the game
be able to make a move


So, does anyone knows an engine which provides a clear API to it's functions?
As others have said, UCI is the API you want.  The full specifications of the protocol is here (the zip file extracts to a text file): http://download.shredderchess.com/div/uci.zip

It's actually very straight forward and simple, a UCI engine must respond to and reply in plain text through stdin, stdout and stderr.  In fact, you should be able to launch the executable binary of any UCI engine on your platform and issue the UCI commands manually.

UCI facilitates instructions from a GUI or other engine to tell the specific engine to do whatever it is that you want it to do, including analyse a specific PGN file (the loading of it is through the GUI or similar front-end, the raw moves are then fed to the engine in extended algebraic form, such as e4 becoming e2e4).  The second and third points of your requirements are standard with UCI, but it is up to each engine developer to support them (given how lean those requirements are, that should be most of them).

Here's an example of UCI in action on the command line:

bash-3.2$ ./stockfish-5-64 
Stockfish 5 64 by Tord Romstad, Marco Costalba and Joona Kiiski
uci
id name Stockfish 5 64
id author Tord Romstad, Marco Costalba and Joona Kiiski

option name Write Debug Log type check default false
option name Write Search Log type check default false
option name Search Log Filename type string default SearchLog.txt
option name Book File type string default book.bin
option name Best Book Move type check default false
option name Contempt Factor type spin default 0 min -50 max 50
option name Mobility (Midgame) type spin default 100 min 0 max 200
option name Mobility (Endgame) type spin default 100 min 0 max 200
option name Pawn Structure (Midgame) type spin default 100 min 0 max 200
option name Pawn Structure (Endgame) type spin default 100 min 0 max 200
option name Passed Pawns (Midgame) type spin default 100 min 0 max 200
option name Passed Pawns (Endgame) type spin default 100 min 0 max 200
option name Space type spin default 100 min 0 max 200
option name Aggressiveness type spin default 100 min 0 max 200
option name Cowardice type spin default 100 min 0 max 200
option name Min Split Depth type spin default 0 min 0 max 12
option name Threads type spin default 1 min 1 max 128
option name Hash type spin default 32 min 1 max 16384
option name Clear Hash type button
option name Ponder type check default true
option name OwnBook type check default false
option name MultiPV type spin default 1 min 1 max 500
option name Skill Level type spin default 20 min 0 max 20
option name Emergency Move Horizon type spin default 40 min 0 max 50
option name Emergency Base Time type spin default 60 min 0 max 30000
option name Emergency Move Time type spin default 30 min 0 max 5000
option name Minimum Thinking Time type spin default 20 min 0 max 5000
option name Slow Mover type spin default 80 min 10 max 1000
option name UCI_Chess960 type check default false
uciok
isready
readyok
quit
bash-3.2$ 


The commands I issued were uci, isready and quit.

To utilise the UCI commands you essentially have two main choices; you can use an existing interface (e.g. ChessX, Arena and a host of others) or you can write your own code to issue instructions to an engine or group of engines.  I believe there may already be Python wrappers for accessing UCI engines, but I'd need to double-check.  There's definitely python-chess (import chess) and pychess (import pychess), I just can't recall the extent of their functions.  I can't recall where pychess is lurking (probably github or google code), but python-chess can be installed with pip.