Fix: Unity Physics Trigger Not Firing on Fast Rigidbody

A bullet flies through a damage trigger volume. Sometimes the damage applies, sometimes the bullet passes clean through with no OnTriggerEnter event. The bullet’s speed is well within game logic ranges, but the trigger is being tunneled at high frame deltas.

Why Discrete Misses Triggers

Unity’s default collision mode is Discrete. At each fixed-update tick, the physics engine checks if a Rigidbody’s collider overlaps any other collider. If the Rigidbody moved fast enough between ticks to be on one side of a trigger at tick N and the other side at tick N+1, no overlap is detected at either tick, so no OnTriggerEnter fires.

The fixed timestep defaults to 0.02 seconds (50 Hz). At 50 m/s, a body moves 1 m per tick. If a trigger’s thickness along the motion vector is less than 1 m, tunneling happens.

The Fix: Continuous Collision Detection

On the fast Rigidbody:

GetComponent<Rigidbody>().collisionDetectionMode = CollisionDetectionMode.Continuous;

Or set it in the Inspector under the Rigidbody component. The four modes:

• Discrete — default, cheap, miss-prone above ~1 collider thickness per tick.
• Continuous — CCD against static colliders and kinematic Rigidbodies. Use for projectiles and fast objects hitting world geometry.
• Continuous Dynamic — CCD against all other CCD-enabled bodies too. Use when two fast bodies must collide accurately with each other.
• Continuous Speculative — cheaper alternative using speculative contacts; works for both static and dynamic but less accurate.

For triggers specifically — the static trigger volume doesn’t need any setting. The flag is on the moving body.

Required Setup Quirks

CCD has caveats:

• The Rigidbody must not be Kinematic. Kinematic bodies don’t use CCD at all; they teleport.
• The moving collider should be Convex on a MeshCollider, or a primitive (Sphere/Box/Capsule). Concave MeshColliders on Rigidbodies aren’t supported.
• Continuous Dynamic only matters between two CCD-enabled Rigidbodies; one Discrete and one Continuous Dynamic still misses each other.

Alternate Fix: Replace Trigger with OverlapBox

For one-shot collision points (e.g., a projectile’s impact check), skip the trigger and use a swept query:

void FixedUpdate()
{
    Vector3 from = transform.position;
    Vector3 to = from + rigidbody.linearVelocity * Time.fixedDeltaTime;
    if (Physics.SphereCast(from, radius, rigidbody.linearVelocity.normalized,
        out RaycastHit hit, (to - from).magnitude, triggerMask, QueryTriggerInteraction.Collide))
    {
        OnHitTrigger(hit.collider);
    }
}

QueryTriggerInteraction.Collide tells SphereCast to include trigger colliders in its hits. This gives you the exact contact point and surface normal — better than OnTriggerEnter, which only tells you that a collider overlaps.

Increase Fixed Tick Rate

For project-wide improvement, lower the fixed timestep:

// Edit → Project Settings → Time
// Fixed Timestep: 0.01  (100 Hz, default 0.02)

Doubling the tick rate halves per-tick travel distance. Cost is double physics CPU; helpful if tunneling appears in many places.

Verifying

Add a counter for triggers entered. Launch the projectile through a thin trigger volume 100 times at varying speeds. Before the fix, miss rate scales with speed. After enabling Continuous, every pass registers an OnTriggerEnter.

Understanding the issue

Game physics is a contract between authoring (the body, mass, collision shapes you set) and the solver (how the engine integrates them per tick). Bugs at this boundary usually surface as 'the values look right but the behavior is wrong' - a sign that one side of the contract isn't honoring the other.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

Bugs of this class are particularly easy to ship past internal QA because they often depend on specific runtime conditions - hardware combinations, network states, or asset configurations that QA didn't reproduce. Players hit them in the wild, file reports that are hard to repro, and the bug accumulates negative reviews while engineering tries to recreate the failure mode.

At the engine level, the behavior comes from a deliberate design decision in Unity. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

For shipping games, the safest verification is a staged rollout. Apply the fix to 1% of players for 24 hours; watch the affected metric; expand if green. Skipping the staged rollout means the verification is the entire player base, which is too high a stakes for most fixes.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

Related bug classes often share the same root cause. If you find yourself fixing this issue, look for cousins: similar symptoms in adjacent systems, the same data flow but a different value, or the same fix pattern in another module. The catalog of 'we've seen this before' becomes valuable institutional knowledge.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

Live games surface this bug class at scale. What's a rare edge case in development becomes a daily occurrence once you have a few thousand concurrent players. The class isn't 'this player has a unique setup'; it's 'one in N thousand sessions will trigger this exact combination'.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

If this issue manifests under high load (many actors, many particles, many network connections), profile the post-fix code path with realistic counts. The original cost was a bug; the new cost is real work, and real work has a budget.

Diagnostic approach

Diagnosing this class of bug benefits from a structured approach: confirm the symptom, isolate the variables, hypothesize the cause, and verify the hypothesis before writing fix code. Skipping the isolation step is the most common mistake; without it, fixes often address symptoms while the underlying cause continues to produce other variations.

For Unity-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

Third-party plugins often provide better diagnostics for their own behavior than the engine does. If the affected code is in a plugin, check the plugin's documentation for debug modes, verbose logging, or inspector tools - these can save hours of investigation when they exist.

Within Unity, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Platform-specific edge cases are worth enumerating explicitly. iOS handles backgrounding differently than Android; Windows handles focus changes differently than macOS. A fix that works on the development platform may not work on every target. Test on each shipping platform deliberately.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

When this bug class affects multiple teams (often the case for cross-system issues), early communication prevents duplicate work. The team that owns the symptom may not own the cause. A 15-minute conversation at the start of triage often saves hours of independent investigation.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“Discrete + Fast + Thin = Missed Event. Pick one to change. Continuous CD is usually the cheapest fix.”

Set Continuous per-prefab on projectiles, not project-wide — CCD cost adds up across hundreds of bodies.