UdonSharp Skill
Why This Skill Matters
UdonSharp looks like regular Unity C# scripting — until you hit its hidden walls. Many standard C# features (List<T>, async/await, try/catch, LINQ, generics) silently fail or refuse to compile in Udon. Networking is even more treacherous: modifying a synced variable without ownership produces no error — it just does nothing. Forgetting RequestSerialization means your state changes never leave your machine. Standard single-player local testing gives zero signal about these networking bugs because there is only one player.
Every rule in this skill exists because UdonSharp's default behavior is to fail silently. Read the Rules before generating any code.
Core Principles
- Constraints First — Assume standard C# features are blocked until verified. Check
udonsharp-constraints.md before using any API.
- Ownership Before Mutation — Only the owner of an object can modify its synced variables. Always
SetOwner → modify → RequestSerialization.
- Late Joiner Correctness — State must be correct for players who join after events have occurred. Design for re-serialization, not just live updates.
- Sync Minimization — Every synced variable costs bandwidth (see data budget in
udonsharp-sync-selection.md). Derive what you can locally; sync only the source of truth.
- Event-Driven, Not Polling — Use
OnDeserialization, [FieldChangeCallback], and SendCustomEvent instead of checking state in Update().
Overview
SDK Coverage: 3.7.1 - 3.10.2 (as of March 2026)
Rules (Constraints & Networking)
Compile constraints and networking rules are defined in always-loaded Rules:
| Rule File | Contents |
|---|
rules/udonsharp-constraints.md | Blocked features, code generation rules, attributes, syncable types |
rules/udonsharp-networking.md | Ownership, sync modes, RequestSerialization, NetworkCallable |
rules/udonsharp-sync-selection.md | Sync pattern selection, data budget, minimization principles |
After installation, place these in the agent's rules directory for automatic loading.
SDK Versions
| SDK Version | Key Features |
|---|
| 3.7.1 | Added StringBuilder, RegularExpressions, System.Random |
| 3.7.4 | Added Persistence API (PlayerData/PlayerObject) |
| 3.7.6 | Multi-platform Build & Publish (PC + Android simultaneously) |
| 3.8.0 | PhysBone dependency sorting, Drone API (VRCDroneInteractable) |
| 3.8.1 | [NetworkCallable] attribute, parameterized network events, NetworkEventTarget.Others/.Self |
| 3.9.0 | Camera Dolly API, Auto Hold pickup simplification |
| 3.10.0 | VRChat Dynamics for Worlds (PhysBones, Contacts, VRC Constraints) |
| 3.10.1 | Bug fixes and stability improvements |
| 3.10.2 | EventTiming extensions, PhysBones fixes, shader time globals |
Note: SDK versions below 3.9.0 are deprecated as of December 2, 2025. New world uploads are no longer possible.
Web Search
When to Search
| Scenario | Action |
|---|
| New SDK version support | Check official docs for latest API |
| "Is this possible?" questions | Verify feasibility in official docs |
| Unknown errors | Refer to official troubleshooting |
| New feature usage | Retrieve latest code examples |
Search Strategy
# Official documentation search
WebSearch: "feature or API name site:creators.vrchat.com"
# UdonSharp API reference
WebSearch: "API name site:udonsharp.docs.vrchat.com"
# Error investigation: VRChat official forums
WebSearch: "error message site:ask.vrchat.com"
# Error investigation: Canny (bug reports / known issues)
WebSearch: "error message site:feedback.vrchat.com"
# Error investigation: GitHub Issues
WebSearch: "error message UdonSharp site:github.com"
Official Resources
| Resource | URL | Contents |
|---|
| VRChat Creators | creators.vrchat.com/worlds/udon/ | Official Udon / SDK documentation |
| UdonSharp Docs | udonsharp.docs.vrchat.com | UdonSharp API reference |
| VRChat Forums | ask.vrchat.com | Q&A, solutions |
| VRChat Canny | feedback.vrchat.com | Bug reports, known issues |
| GitHub | github.com/vrchat-community | Samples and libraries |
References
| File | Contents |
|---|
constraints.md | C# feature availability in UdonSharp |
networking.md | Networking pattern details (including NetworkCallable) |
persistence.md | PlayerData/PlayerObject (SDK 3.7.4+) |
dynamics.md | PhysBones, Contacts, VRC Constraints (SDK 3.10.0+) |
patterns.md | Buttons, pickups, player detection, object pools, etc. |
web-loading.md | String/Image downloading, VRCJson, Trusted URLs |
api.md | VRCPlayerApi, Networking, enums reference |
events.md | All Udon events (including OnPlayerRestored, OnContactEnter) |
editor-scripting.md | Editor scripting and proxy system |
sync-examples.md | Sync pattern examples (Local/Events/SyncedVars) |
troubleshooting.md | Common errors and solutions |
Templates (assets/templates/)
| Template | Purpose | Path |
|---|
BasicInteraction.cs | Interactive object | assets/templates/BasicInteraction.cs |
SyncedObject.cs | Network-synced object | assets/templates/SyncedObject.cs |
PlayerSettings.cs | Player movement settings | assets/templates/PlayerSettings.cs |
CustomInspector.cs | Custom editor inspector | assets/templates/CustomInspector.cs |
Hooks
| Hook | Platform | Purpose |
|---|
validate-udonsharp.ps1 | Windows (PowerShell) | PostToolUse constraint validation |
validate-udonsharp.sh | Linux/macOS (Bash) | PostToolUse constraint validation |
Quick Reference
CHEATSHEET.md - One-page quick reference