Fix: Unity Mesh Collider Convex Required for Rigidbody

Here is how to fix the “non-convex MeshCollider with non-kinematic Rigidbody is no longer supported” error in Unity. You imported a mesh, added a MeshCollider and a Rigidbody, and Unity throws this error. The object either falls through the floor, does not collide with anything, or the MeshCollider simply disables itself. This is a hard constraint from the PhysX engine and has no workaround — you must change your collider setup.

The Symptom

You have a GameObject with a MeshCollider and a non-kinematic Rigidbody. Unity logs the error: “Non-convex MeshCollider with non-kinematic Rigidbody is no longer supported in Unity X.X.” The collider is either ignored entirely or the Rigidbody behaves erratically. Objects clip through geometry, fall through floors, or produce no collision callbacks.

This error appears at runtime, not in the editor. In older Unity versions (pre-2019), this was silently allowed but caused unstable physics. Current versions explicitly reject the configuration.

What Causes This

PhysX requires all dynamic collision shapes to be convex. A convex shape is one where any line segment between two interior points stays entirely inside the shape — no indentations, holes, or concavities. Concave-to-concave collision detection is exponentially more expensive and numerically unstable in real-time simulation. Unity enforces this by requiring MeshColliders on dynamic Rigidbodies to have the Convex flag enabled.

The problem surfaces when you use a detailed game mesh (a spaceship, a character, a building) as a collider. These meshes are almost always concave. Enabling Convex on them produces a “shrink-wrap” approximation that fills in all the concavities, which often does not match the visual shape at all.

The Fix

Option 1: Enable Convex for simple shapes. If the mesh is roughly convex already (a barrel, a crate, a ball), just enable the Convex toggle. Unity will generate a convex hull with up to 255 triangles.

// Enable convex at runtime if needed
var meshCollider = GetComponent<MeshCollider>();
meshCollider.convex = true;

// Verify the cooking result
Debug.Log($"Convex: {meshCollider.convex}, Bounds: {meshCollider.bounds}");

Option 2: Use a compound collider. For complex shapes, approximate the mesh with child GameObjects containing primitive colliders. A spaceship might use a BoxCollider for the hull, two small BoxColliders for wings, and a CapsuleCollider for the nose. Unity treats all colliders in the Rigidbody’s hierarchy as a single compound shape:

// Structure in the hierarchy:
// Spaceship (Rigidbody, NO collider on this object)
//   Hull (BoxCollider)
//   LeftWing (BoxCollider)
//   RightWing (BoxCollider)
//   Nose (CapsuleCollider)

// The child objects should NOT have Rigidbodies.
// They inherit the parent Rigidbody automatically.
// Each child can be positioned/rotated to match the mesh shape.

Option 3: Use convex decomposition. For meshes that are too complex for manual compound colliders, use a convex decomposition tool like V-HACD. This breaks a concave mesh into multiple convex pieces automatically. Import the decomposed meshes and add a convex MeshCollider to each piece as a child of the Rigidbody:

// After V-HACD decomposition, you get multiple convex meshes
// Add each as a child with its own convex MeshCollider
foreach (Mesh piece in decomposedMeshes)
{
    var child = new GameObject("CollisionPiece");
    child.transform.SetParent(transform, false);
    var mc = child.AddComponent<MeshCollider>();
    mc.sharedMesh = piece;
    mc.convex = true;
}

Understanding the issue

UI is where most player-visible bugs live because UI is what players actually look at. A subtle data bug invisible elsewhere becomes glaring when it produces a wrong label or a stuck button.

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

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

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

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

The diagnostic tools available depend on your engine and platform. Use the engine's native profilers and debug overlays before reaching for external tools. The native tools have context that external tools lack - they know which subsystem owns the code, which assets are loaded, and what state the engine is in.

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

Boundary conditions deserve specific testing attention. What happens when the input is zero, maximum, negative, or NaN? What happens at the start of a session vs hours in? What happens at the boundary between two systems handling the same data? These are where bugs hide and where regression tests are most valuable.

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.

“The golden rule: if it moves with physics, it must be convex. If it is scenery and never moves, it can be concave. If it moves but is script-driven, make it kinematic and keep it concave.”

Why This Works

PhysX’s collision detection algorithms (GJK and EPA) only work with convex shapes for dynamic objects. When you create a compound collider, PhysX treats each child collider as a separate convex shape that moves together. The collision is mathematically correct because each individual shape is convex, even though the combined shape they approximate is concave. This is the same approach AAA games use — no shipped game uses concave dynamic colliders.

Related Issues

If your MeshCollider is convex but the collision shape does not match the visual mesh closely enough, consider increasing the mesh resolution before cooking or switching to a compound collider. For trigger-only colliders that do not need physics response, see Fix: Unity AddForce Not Working on Rigidbody for kinematic trigger setups.

If your Rigidbody falls through the floor despite having a collider, check that continuous collision detection is enabled for fast-moving objects.

Dynamic means convex. Static means anything. Kinematic means anything. No exceptions.