Fix: Godot 4 CharacterBody3D Floor Snap on Stairs

Player runs down stairs. Each step launches them into the air for a frame. Sound effect for landing fires repeatedly. CharacterBody3D’s default snap distance doesn’t reach the next step.

The Symptom

Going down stairs: bouncing or stuttering. Going up: bumping into the riser, then either step-up or stop. Audio events tied to grounded state fire repeatedly.

What Causes This

CharacterBody3D’s move_and_slide checks for a floor below within floor_snap_length each tick. When the ground drops away by more than the snap distance (the next step in a staircase), the body becomes airborne, gravity applies, then re-touches the next step the frame after.

The Fix

Step 1: Set floor_snap_length.

extends CharacterBody3D

@export var speed := 5.0
@export var jump_velocity := 5.0

func _ready() -> void:
    floor_snap_length = 0.5            # up to 0.5m drop holds the body to ground
    floor_max_angle = deg_to_rad(45)   # anything steeper is a wall
    floor_constant_speed = true       # constant horizontal on slopes
    floor_stop_on_slope = false        # keep momentum on hills

Step 2: Build stairs as a slope. Easiest. Replace step geometry with a single ramp mesh (or a Box collider rotated to ~30°). The character slides up/down without per-step snapping.

Step 3: Or, write step detection. When a horizontal collision is below step_height, raycast for a higher floor and snap.

func _physics_process(delta: float) -> void:
    velocity.y -= GRAVITY * delta
    velocity.x = input_dir.x * speed
    velocity.z = input_dir.z * speed
    move_and_slide()

    # Step-up after a wall hit
    if get_slide_collision_count() > 0:
        var col = get_slide_collision(0)
        if col.get_normal().y < 0.1:   # a wall
            var step = try_step_up(col.get_position())
            if step:
                global_position.y += STEP_HEIGHT

Verifying

Walk up and down a staircase. Position.y should change smoothly. is_on_floor() should stay true. Tracked landing/grounding signals fire only when actually leaving the floor (jump, fall).

Understanding the issue

AI bugs are emergent. The code is correct in isolation; the behavior emerges from interaction with other systems. Reproducing means controlling the interaction; fixing means deciding which interaction was wrong.

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

This bug class disproportionately affects late-stage development. The work to surface it is interactive testing in realistic conditions, which only really happens after the gameplay is in place and assets are populated. Catching it early requires deliberate testing of conditions that look unimportant.

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

After applying the fix, the verification step has three parts: confirm the original repro is resolved, confirm no obvious regressions in adjacent functionality, and (for shipping titles) deploy to a small player cohort first and watch the crash and report rates. Each step catches something the others miss.

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

There's almost always a less obvious case where the same problem applies. The reported case is the one a player hit; the related cases hide because they're rarer or affect fewer players. After fixing the reported case, search the codebase for the pattern - one fix often unlocks several.

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

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

Before applying any fix, gather enough context to be confident you're addressing the actual cause and not a similar-looking symptom. The cheapest diagnostic step is reproducing the bug deterministically - if you can't get the same failure twice in a row, your fix attempts will be hard to evaluate. Lock down the reproduction first.

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

Modern engine versions ship better tooling for this kind of issue than older versions. If you're on an older release, the diagnostic step may take significantly longer because the tools you'd want don't exist yet. Sometimes the right answer is upgrading rather than fighting through limited tooling.

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

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

Document the fix and its rationale in the commit message or attached engineering doc. Future engineers will encounter related issues; the rationale tells them whether your fix is reusable or specific to the case at hand. Without rationale, the fix gets reverted or copied incorrectly.

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.

“Snap length covers the step. Slope collider over per-step. Constant speed on slopes. Stairs feel solid.”

Related Issues

For physics interpolation jitter, see interpolation jitter. For input deadzone, see deadzone.

Snap length. Slope. Smooth.