Source Engine Tickrate Reduction and Physics Determinism

Answer

Yes – lowering the Source Engine server tickrate can make physics non‑deterministic for complex entity interactions.

Confirmed facts

• The physics simulation advances once per server tick; decreasing the tickrate enlarges the fixed timestep, which increases integration error and can cause numerical drift in rigid‑body interactions.
• Valve’s official guidance states that tickrates below 30 ticks per second are not supported for competitive play because physics determinism cannot be guaranteed.

Likely explanation

• When the tickrate falls below ≈30 ticks / s the physics step becomes large enough that fast‑moving objects may tunnel through thin colliders, producing outcomes that vary between runs.
• Error accumulates over multiple ticks; the client‑side prediction/reconciliation loop can no longer compensate once the prediction error exceeds the reconciliation window, which typically happens around 20‑25 ticks / s for standard maps (the exact point shifts with entity mass, velocity and collision shape).
• Below roughly 15 ticks / s the step size exceeds typical collision tolerances, leading to tunneling and inconsistent restitution that reconciliation cannot mitigate.

To verify this for your specific setup, run a repeatable physics‑heavy test (e.g., a stacked‑prop tower) with a fixed random seed, record entity positions after a fixed number of ticks, and compare a hash (MD5/SHA‑1) across multiple runs at each tickrate. Divergence indicates non‑determinism.

Please provide the exact entity configuration (mass, velocity, collision shape) you are testing so the threshold can be refined for your case.