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

1. Constraints First — Assume standard C# features are blocked until verified. Check udonsharp-constraints.md before using any API.
2. Ownership Before Mutation — Only the owner of an object can modify its synced variables. Always SetOwner → modify → RequestSerialization.
3. Late Joiner Correctness — State must be correct for players who join after events have occurred. Design for re-serialization, not just live updates.
4. 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.
5. 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