Why do bullets pass through walls in Godot?

Discrete physics integrates position once per tick. A fast body may move past a thin wall between steps. Set continuous_cd = true on the RigidBody to enable continuous collision detection (sweep test).

Should I just make walls thicker?

Pragmatic and effective. Walls thicker than (max body speed * physics tick) won't be tunneled. For 60Hz physics and 50 m/s bullets, walls need to be at least 0.85m thick to be safe.

CharacterBody3D has safe_margin: a small offset where collisions are pre-resolved. Larger margin = less tunneling at the cost of slight detachment from surfaces. Default 0.001 is fine for normal speeds.

Fix: Godot 4 Thin Collider Tunneled by Fast Body

Quick answer: Set continuous_cd = true on the fast RigidBody. Or thicken the thin wall to be wider than (max_speed * physics_tick). For CharacterBody3D, increase safe_margin.

Bullets pass through walls. The wall is 0.1m thick; the bullet moves 1m per physics tick. Bullet starts on one side of the wall and ends on the other; collision missed.

The Symptom

Fast objects pass through thin colliders. Slow speeds collide normally. Reproduces consistently above some speed threshold.

The Fix

Continuous CD on the bullet.

extends RigidBody3D

func _ready() -> void:
    continuous_cd = true   # sweep test against thin objects

Continuous CD performs a sweep test from previous to current position. Catches the wall regardless of thickness.

Or thicken walls. Cheap. Reliable. Limit max body speed to (wall_thickness / tick_seconds) and you don’t need continuous CD.

Or raycast bullet logic. For very-fast projectiles (laser, hitscan), don’t simulate physics — raycast each tick from old to new position and apply damage / spawn impact at the hit.

CharacterBody3D safe_margin

extends CharacterBody3D

func _ready() -> void:
    safe_margin = 0.05   # cm-scale margin

Resolves penetration before move_and_slide. Reduces tunneling without continuous CD.

Verifying

Test with a fast projectile and thin wall. Without fix: passes. With continuous_cd or thick wall: collides reliably.

Understanding the issue

The challenge with physics-related bugs is reproducibility. A symptom you see in a 30 fps build may vanish at 60 fps because the integrator's step size changed. Reproducing reliably means controlling both your inputs and the engine's tick rate.

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 Godot. 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

Verifying this fix in isolation is straightforward: reproduce the bug, apply the change, confirm the bug no longer reproduces. The harder verification is regression - did this fix introduce a new bug elsewhere? Run your standard regression suite, plus any tests that exercise the same code path with different inputs.

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

For shipping titles with a long support window, watch for this issue resurfacing after dependency updates. Engine upgrades, driver updates, OS releases - each one can resurface a bug class you thought you'd fixed because the underlying behavior changed slightly. Regression tests catch the obvious ones; player reports catch the rest.

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

Performance implications matter when this bug class scales with player count or asset count. A bug that fires once per session is annoying; a bug that fires once per frame compounds. After fixing, profile the affected code path under realistic load. The fix that's correct for one entity may be too slow for ten thousand.

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 Godot-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

The tooling around this bug class matters as much as the fix itself. Good logging, accessible profilers, and clear error messages turn 30-minute investigations into 5-minute ones. If your project doesn't have visibility into this code path, the first fix should add the visibility - the second fix uses it.

Within Godot, 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

Edge cases for this class of issue often involve specific timing: the first frame after a state change, the last frame before a transition, frames where multiple subsystems update simultaneously. Reproducing these reliably is part of what makes the bug class hard to test.

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.

“Continuous CD on fast bodies. Or raycast hitscan. Or thicker walls.”

Related Issues

For physics interpolation jitter, see interpolation. For PhysicsServer3D direct state, see direct state.

Sweep or thicken. Tunneling stops.