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Documentation Index

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The Source Engine AI system provides the base infrastructure that TF2 bots and any other NPCs build on. The core lives in game/server/ and centers on CAI_BaseNPC — a large class that implements scheduling, sensing, navigation, and squad logic. TF2’s bot system (CTFBot) uses the newer NextBot framework (game/server/NextBot/) layered on top of the standard player class rather than CAI_BaseNPC.

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CAI_BaseNPC

CAI_BaseNPC (game/server/ai_basenpc.h) is the foundation for all Source Engine NPCs. It provides:
  • Schedule/task system — discrete AI behaviors broken into named sequences of tasks
  • Senses — CAI_Senses manages sight and hearing, populating a memory of enemies
  • Navigator — CAI_Navigator handles pathfinding through the nav mesh to a goal
  • Motor — CAI_Motor translates path following into movement commands
  • Squad — CAI_Squad groups NPCs for coordinated behavior
// game/server/ai_basenpc.h (simplified)
class CAI_BaseNPC : public CBaseCombatCharacter, public CAI_DefMovementSink
{
public:
    virtual int  SelectSchedule();
    virtual int  TranslateSchedule( int scheduleType );
    virtual void BuildScheduleTestBits();
    virtual bool IsJumpLegal( const Vector &startPos, const Vector &apex,
                              const Vector &endPos ) const;

    CAI_Navigator   *GetNavigator()  { return m_pNavigator; }
    CAI_Motor       *GetMotor()      { return m_pMotor; }
    CAI_Senses      *GetSenses()     { return m_pSenses; }
    CAI_Squad       *GetSquad()      { return m_pSquad; }
};

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Schedule and task system

Schedules are named sequences of tasks defined with the BEGIN_SCHEDULE / DEFINE_TASK macros and stored in ai_schedule.cpp. Each task has a start and run phase; when a task completes, the NPC advances to the next one. 
// Example schedule definition pattern
BEGIN_SCHEDULE( SCHED_CHASE_ENEMY )
    DEFINE_TASK( TASK_SET_FAIL_SCHEDULE, (float)SCHED_CHASE_ENEMY_FAILED )
    DEFINE_TASK( TASK_GET_PATH_TO_ENEMY, 0 )
    DEFINE_TASK( TASK_RUN_PATH, 0 )
    DEFINE_TASK( TASK_WAIT_FOR_MOVEMENT, 0 )
END_SCHEDULE()

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Senses and memory

CAI_Senses (game/server/ai_senses.h) manages enemy detection. It runs a periodic scan, checking line-of-sight and hearing radius, then stores detected entities in CAI_Memory. Memory entries decay over time, allowing NPCs to “forget” enemies they’ve lost sight of.

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The NextBot framework

TF2 bots use the NextBot framework (game/server/NextBot/) rather than CAI_BaseNPC. NextBot provides a component-based architecture:

INextBotComponent

Base interface for all NextBot components (locomotion, body, vision, intention)

ILocomotion

Handles movement — walking, jumping, climbing ladders, falling

IVision

Manages threat detection, field-of-view filtering, and threat prioritization

IIntention / ActionFactory

Runs a behavior tree of Action<> objects that push/pop based on priority
The NextBot framework was introduced with Left 4 Dead and retrofitted into TF2 for MvM bot support. It is strictly server-side; client-side rendering of bot players uses the normal C_TFPlayer path.

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