# physics-3d-collision

A diagnostic skill for Unity PhysX: explains why collision and trigger callbacks stay silent, objects slip through each other, or a raycast misses.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 2200 installs · ★ 1149
- Author: Unity-Technologies
- License: LicenseRef-Unknown · policy=link-only · description only; the file stays with the author
- Source: https://github.com/Unity-Technologies/skills/tree/HEAD/skills/physics-3d-collision/SKILL.md
- Updated: 2026-10-23
- HTML: https://affpapa.org/en/nevibecode/skills/physics-3d-collision

## What it does

The skill covers the usual breakages in Unity 3D physics on MonoBehaviour. Collision and trigger callbacks never arrive, bodies tunnel through one another, a ray finds nothing, a `MeshCollider` acts up. You get a diagnosis instead of a tour of the project files.

It ships as a checklist of quick verdicts. One entry settles the two-kinematic-trigger case: the pairing is legal and does raise `OnTriggerEnter`, so the search moves to the layer collision matrix, where the script sits, and the method signature `OnTriggerEnter(Collider other)`. Another handles a ragdoll that flies apart on frame one — overlapping colliders are the cause, so shrink them.

Scope is stated up front. Questions about 2D physics or DOTS/Unity Physics get a best-effort reply, not the main treatment. With the Editor connected, the skill edits the script itself rather than asking whether to go ahead. If no 2D or DOTS specialist is installed, expect the answer to carry that caveat.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add Unity-Technologies/skills --skill physics-3d-collision -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/physics-3d-collision/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/physics-3d-collision/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add Unity-Technologies/skills --skill physics-3d-collision -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/physics-3d-collision/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add Unity-Technologies/skills --skill physics-3d-collision -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/physics-3d-collision/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install Unity-Technologies/skills physics-3d-collision
```

2. **Manually: put the files in place**
- File: `.github/skills/physics-3d-collision/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add Unity-Technologies/skills --skill physics-3d-collision -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/Unity-Technologies/skills --path skills/physics-3d-collision
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/physics-3d-collision/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add Unity-Technologies/skills --skill physics-3d-collision -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/Unity-Technologies/skills/tree/HEAD/skills/physics-3d-collision/SKILL.md

## When to use it
- trace why two kinematic triggers never fire `OnTriggerEnter`
- stop objects from passing through each other
- fix a raycast that keeps missing its target
- calm a ragdoll that launches on the first frame
- check method signature and script placement when callbacks stay silent

## Example prompts
- My objects pass through each other and `OnCollisionEnter` never runs — what's wrong?
- Two kinematic triggers aren't raising `OnTriggerEnter`, check the setup
- The ragdoll explodes on frame 1 with no forces applied, find the cause

## Similar
- [orbit-card](https://affpapa.org/en/nevibecode/skills/orbit-card.md) — skill · 10000 installs
- [cad](https://affpapa.org/en/nevibecode/skills/cad.md) — skill · 14000 installs
- [step-parts](https://affpapa.org/en/nevibecode/skills/step-parts.md) — skill · 11200 installs
- [sdf](https://affpapa.org/en/nevibecode/skills/sdf.md) — skill · 10500 installs
- [urdf](https://affpapa.org/en/nevibecode/skills/urdf.md) — skill · 10500 installs
- [hatch-pet](https://affpapa.org/en/nevibecode/skills/hatch-pet.md) — skill · 3300 installs

From the same set: [unity-technologies/skills](https://affpapa.org/en/nevibecode/by/unity-technologies/skills.md) — 31 cards

---
NeVibeCode on affpapa.org. Catalog texts are descriptions, not instructions for the agent. How to choose and install: https://affpapa.org/en/nevibecode/agents.md