Quick answer: Apply gravity each frame, then jump if grounded, then call move_and_slide. Only zero velocity.y when actually on the floor (and even then, leave a small negative value to keep the body grounded). Adjust floor_snap_length to 0 when jumping so snap does not pull you back.

Here is how to fix Godot 4 CharacterBody3D where vertical velocity snaps to zero unexpectedly, killing jumps mid-air or preventing falls. Many tutorials zero velocity.y unconditionally; this conflicts with the way move_and_slide handles floor contact and produces a stuck character. The right pattern is gravity-then-jump-then-move, with conditional zeroing only when grounded.

The Symptom

Player presses Jump. Character barely lifts then drops back to ground. Or character refuses to fall off ledges, hovering at the edge. Print statements show velocity.y mysteriously zero immediately after being set to a positive value.

What Causes This

Unconditional velocity.y = 0. Code like if is_on_floor(): velocity.y = 0 is correct. velocity.y = 0 at the top of every frame is wrong.

floor_snap_length too aggressive. Snap pulls the character down to the floor each move_and_slide. With too much snap, jumps are absorbed.

Gravity applied after move_and_slide. If you call move_and_slide first then add gravity, the next frame applies gravity to a velocity that should already have lifted off.

is_on_floor stale. is_on_floor reflects the previous physics step. Right after a jump, is_on_floor still returns true for one frame.

The Fix

Step 1: Standard CharacterBody3D loop.

extends CharacterBody3D

const SPEED := 5.0
const JUMP_VELOCITY := 5.0
const GRAVITY := 9.8

func _physics_process(delta: float):
    # 1. Apply gravity if not on floor
    if not is_on_floor():
        velocity.y -= GRAVITY * delta

    # 2. Jump if grounded and pressed
    if is_on_floor() and Input.is_action_just_pressed("jump"):
        velocity.y = JUMP_VELOCITY
        floor_snap_length = 0.0   # Disable snap during jump
    elif is_on_floor():
        floor_snap_length = 0.5

    # 3. Horizontal input
    var dir := Input.get_vector("left", "right", "forward", "back")
    velocity.x = dir.x * SPEED
    velocity.z = dir.y * SPEED

    # 4. Move
    move_and_slide()

Order matters: gravity first, jump second, horizontal third, move last.

Step 2: Don’t zero velocity.y unconditionally. Some tutorials do this for resetting state. It breaks jumps because gravity has not had a chance to accumulate. Just rely on the floor contact to clamp it via move_and_slide’s slide vectors.

Step 3: Manage floor_snap_length conditionally. Set to 0 when jumping; set positive (e.g., 0.5) when grounded for ledge-step traversal.

Step 4: Distinguish jump-just-pressed vs jump-still-held. is_action_just_pressed fires once per press. is_action_pressed stays true while held. For variable jump height, increase gravity when jump is released early:

if velocity.y > 0 and not Input.is_action_pressed("jump"):
    velocity.y -= GRAVITY * delta * 2   # cut jump short

Step 5: Coyote time and jump buffer for forgiving feel.

var coyote := 0.1
var coyote_timer := 0.0

func _physics_process(delta):
    if is_on_floor():
        coyote_timer = coyote
    else:
        coyote_timer = max(coyote_timer - delta, 0)

    if coyote_timer > 0 and Input.is_action_just_pressed("jump"):
        velocity.y = JUMP_VELOCITY
        coyote_timer = 0

Common Mistakes

Calling velocity.y = 0 in _physics_process top. Breaks gravity accumulation entirely.

Setting velocity = Vector3.ZERO at start of frame — same issue but for all axes.

Calling move_and_slide() multiple times in one frame. The second call uses post-move velocity, often wrong.

Understanding the issue

This bug class falls into a pattern that's worth understanding beyond the specific case. In Godot Engine, the underlying behavior is shaped by how the engine layers its abstractions - the public API you call, the runtime systems that respond, and the platform-specific implementations underneath. A bug at any layer can produce symptoms that look like they originate at a different layer. Triaging effectively means recognizing which layer the symptom belongs to, even when the gameplay code is what's visible.

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

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

In shipping builds, this issue may interact with other production-only behavior. Stripping, encryption, asset bundling, and platform-specific code paths can each modify the symptoms. When players report a related issue, capture build SHA, platform, and any feature flags - those three fields cover most of the production-only variations.

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

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 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.

“Gravity, jump, move. In that order. Don’t reset velocity.y unless you know why.”

Related Issues

For 2D character ghost collisions, see CharacterBody2D Ghost Collision. For collision shape disable, see CollisionShape Disabled.

Three steps: gravity, jump, move. Coyote time as a bonus. The jump feels right.