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An AIN file is a nodegraph for Source engine games. It stores information relating to info_nodes and their proximity and visibility for use by the AI for movement around a map. AIN files are automatically generated by the game. If there is no existing AIN file specific to that map, or if it was generated for an older version of that map, the AIN file ( <MAP_NAME>.ain ) is generated by the game. The message "Node graph out of date. Rebuilding..." is printed to the screen when a build or rebuild is occurring.
The graph files are stored in the ..\<game>\maps\graphs directory.
You can force the game to recreate a nav mesh by typing nav_generate in the console.