CS2 air acceleration: sv_airaccelerate 12, capped by a 30 u/s wish speed

How it was measured

Air acceleration is one of the few CS2 mechanics with no demo oracle worth using: a demo records where a player went, not the branch the server took, and any curve fitted to airborne positions will reproduce the trajectory without recovering the rule. So this one was taken from the binary, and the constants confirmed from the running game's own convar list.

The function is unchanged from classic Source. Compute the desired speed, clamp it to sv_air_max_wishspeed, subtract the component of current velocity already along the wish direction, and if anything is left, add acceleration along that direction — but limited by the smaller of the remaining difference and sv_airaccelerate × the FULL wish speed × surface friction × frame time.

The exploit is in that last sentence. addSpeed uses the capped wish speed, accelSpeed uses the uncapped one. Point the wish direction almost perpendicular to your velocity and the projection stays small, so addSpeed stays positive even at high speed, while the acceleration term remains large. Air-strafing is not a bug in the physics; it is the direct consequence of two different wish speeds appearing in two adjacent lines.

One structural detail matters for anyone porting this: CS2 applies acceleration per user command in its subtick pipeline, not once per tick. The air function reads its frame time from the subtick interval and maintains per-subtick base-velocity accumulators, and one-step replay errors on the ground quantize at exactly accelerate × maxspeed × dt = 21.48 u/s, which is the signature of the same structure.

AirAccelerate, transcribed from the current CS2 server binary

wishSpd  = min(wishSpeed, sv_air_max_wishspeed)   // 30
addSpeed = wishSpd - dot(velocity, wishdir)
if addSpeed > 0:
    accelSpeed = min(sv_airaccelerate * wishSpeed * surfaceFriction * frametime,
                     addSpeed)                     // note: FULL wishSpeed here
    velocity += accelSpeed * wishdir