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.

Before Writing Network Code

Four architectural decisions that must be made before choosing sync modes or writing any synced variable. Changing them mid-implementation typically requires a full rewrite:

• Who owns this state? One owner writes; all others read. If two players can both write (e.g., a shared toggle), you need an ownership transfer protocol — writes without ownership are silently discarded.
• When does ownership transfer? On grab? Interact? Game event? OnPlayerLeft? Networking.SetOwner is locally immediate on the calling client — Networking.IsOwner(gameObject) is true synchronously after the call, and writing [UdonSynced] fields plus RequestSerialization() immediately afterwards is safe under an IsOwner guard. Concurrent SetOwner calls from multiple clients are resolved by network arrival order — there is no client-side arbitration, so accept that the loser's write is overwritten.
• What do late joiners see? State set only by one-time events (SendCustomNetworkEvent) is invisible to late joiners. Persistent state requires [UdonSynced] variables; the owner calls RequestSerialization() on join events to push current state to newcomers.
• What if the owner leaves mid-session? Without explicit OnPlayerLeft handling, the object's synced state can never change again. Decide upfront: auto-transfer to master, reset to a known default, or the next interacting player claims ownership.

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().

Common Mistakes (NEVER List)

These constraints cause either compile-time failures or silent runtime failures. Check this list before writing any UdonSharp code.

#	NEVER do this	Why it fails silently	Use instead
1	Use List<T>, Dictionary<T,K>, or any generic collection	Compile error — blocked by Udon compiler	T[] arrays, DataList, DataDictionary
2	Use async/await, System.Threading, or coroutines	Udon is single-threaded; these features do not exist	SendCustomEventDelayedSeconds()
3	Modify [UdonSynced] fields without owning the object	Change appears local but is silently reverted on next deserialization	Networking.SetOwner() before modify, then RequestSerialization()
4	Forget RequestSerialization() after modifying synced fields (Manual sync)	State changes never leave the local client — no error, no warning	Always call RequestSerialization() after modifying [UdonSynced] fields
5	Use try/catch/finally/throw	Compile error — exception handling is blocked	Defensive null checks + early return
6	Access Networking.LocalPlayer in field initializers	Field initializers run at compile time — LocalPlayer is null	Initialize in Start() or use lazy-init guard
7	Use static fields for per-instance state	Static fields are shared across all instances on the same client and are not synced	Instance fields with [UdonSynced] if sync is needed
8	Call RequestSerialization() every frame in Manual sync	Floods the ~11 KB/s network budget, causing congestion for the entire world	Throttle to 1-10 Hz with change detection; check Networking.IsClogged
9	Use LINQ (.Where, .Select, etc.) or lambda expressions	Compile error — not supported by Udon compiler	Manual for loops with named methods
10	Use Button.onClick.AddListener()	Not available in Udon — no runtime delegate support	Configure SendCustomEvent via Inspector OnClick
11	Mix Continuous and Manual sync concerns on one behaviour	Wastes bandwidth (discrete values in Continuous) or loses control (redundant RequestSerialization in Continuous)	Separate behaviours: Continuous for position/rotation, Manual for discrete state
12	Write to [UdonSynced] fields without an IsOwner guard	Non-owner writes are purely local and silently reverted on the next deserialization from the actual owner	Networking.SetOwner first if needed (locally immediate), then write under IsOwner and call RequestSerialization()
13	Use [NetworkCallable] on SDK < 3.8.1	Compiles but silently ignored at runtime — the attribute has no effect and methods never receive network calls	Verify SDK >= 3.8.1; on older SDKs use synced variables + SendCustomNetworkEvent
14	Use PhysBones/Contacts API (OnPhysBoneGrab, OnContactEnter, etc.) on SDK < 3.10.0	Compiles but silently ignored at runtime — world-side Dynamics did not exist pre-3.10.0, so callbacks never fire	Verify SDK >= 3.10.0; Dynamics for Worlds was added in 3.10.0
15	Use PlayerData persistence API on SDK < 3.7.4	Compile error — missing symbol; PlayerData, PlayerObject, and OnPlayerRestored were added in 3.7.4 and are not in the Udon whitelist before then	Verify SDK >= 3.7.4; persistence was added in 3.7.4
16	Create a .cs script without a corresponding .asset file	Script is not recognized as UdonBehaviour — "The associated script cannot be loaded", no Udon compilation	Every time a .cs is created: verify Assets/Editor/UdonSharpProgramAssetAutoGenerator.cs exists, install from references/editor-scripting.md if missing, notify the user (see Rule 8 in rules/udonsharp-constraints.md)
17	Call Debug.Log() inside Update(), PostLateUpdate(), or any per-frame event	VRChat's client-side log rate limiter silently drops excess entries; the implicit string allocation every frame causes sustained GC pressure that tanks framerate. ClientSim and Unity Editor hide both symptoms	Guard with if (debugMode && Time.frameCount % 60 == 0), or move all logging to event-driven callbacks
18	Use [UdonSynced] on a GameObject, Transform, UdonBehaviour, or any component reference	Only primitives, value types (Vector3, Quaternion, Color, etc.), string, VRCUrl, and their simple arrays are syncable. Component references either fail at compile time or are silently never serialized depending on SDK version	Sync a player ID (int) or scene object index (int) and resolve the actual reference locally on each client

Sync Mode Quick Decision

Changing every frame (position, rotation)?    -> Continuous sync
Changing on user action (toggle, score)?      -> Manual sync + RequestSerialization()
No sync needed (local UI, effects)?           -> NoVariableSync
Need reliable one-shot calls with params?     -> [NetworkCallable] (SDK 3.8.1+)
Temporary effect for all players, no state?   -> SendCustomNetworkEvent (no synced vars)

For detailed decision trees, data budget, and minimization principles, see rules/udonsharp-sync-selection.md.

Sync Debugging Quick Decision

When sync "looks correct locally but doesn't work for others":

Remote players don't see my state change?
  ├── Did I call RequestSerialization() after writing? (Manual sync) → Add it
  ├── Does the local player own the object?                          → Networking.SetOwner() first
  └── Using Continuous sync for button/toggle state?                → Switch to Manual + RequestSerialization()

RequestSerialization() called but still not syncing?
  ├── Is Networking.IsClogged == true?           → Throttle; retry after delay
  └── Writing in OnPreSerialization scope?       → Move write before OnPreSerialization fires

Late joiners don't see current state?
  ├── State set only on event (e.g., player trigger)?  → Also set in Start() + RequestSerialization() on owner
  └── Using SendCustomNetworkEvent for persistent state? → Use [UdonSynced] variables instead

OnOwnershipTransferred not firing on a remote client?
  └── On the caller, the callback fires synchronously inside SetOwner — confirm the calling client called Networking.SetOwner(LocalPlayer, gameObject), and that remote clients resolve the same gameObject reference (scene path or prefab GUID)

Reference Loading Guide

Load only what you need. Over-loading wastes tokens; under-loading causes critical mistakes.

Task	MANDATORY READ	Optional	Do NOT Load
Writing networking/sync code	networking.md, networking-antipatterns.md	networking-bandwidth.md, sync-examples.md	dynamics.md, web-loading.md, image-loading-vram.md
Building UI/menus	patterns-ui.md, events.md	patterns-core.md, api.md	networking-bandwidth.md, dynamics.md, web-loading.md
Implementing persistence (save/load)	persistence.md	patterns-networking.md, events.md	dynamics.md, web-loading.md, image-loading-vram.md
Downloading strings/images from web	web-loading.md	web-loading-advanced.md, image-loading-vram.md	dynamics.md, persistence.md, networking-bandwidth.md
Using PhysBones/Contacts/Constraints	dynamics.md, events.md	patterns-networking.md, api.md	web-loading.md, image-loading-vram.md, persistence.md
Optimizing performance (Update loops)	patterns-performance.md	patterns-utilities.md, api.md	dynamics.md, web-loading.md, persistence.md
Building a video player	patterns-video.md	events.md, web-loading.md	dynamics.md, persistence.md, image-loading-vram.md
Debugging/troubleshooting	troubleshooting.md	constraints.md, networking.md, testing.md	patterns-*.md, dynamics.md, web-loading.md
Debugging ownership / sync conflicts	networking.md, troubleshooting.md	networking-antipatterns.md	dynamics.md, web-loading.md
Writing new UdonSharp scripts (not sure if sync needed)	constraints.md	networking.md	dynamics.md, web-loading.md, image-loading-vram.md
Creating new UdonSharp scripts	editor-scripting.md	troubleshooting.md	networking.md, dynamics.md

Pattern Selection Guide

Six pattern files cover different domains. Use this quick routing to pick the right one:

Building a UI, menu, or HUD?           -> patterns-ui.md
VR finger/touch interaction on Canvas? -> patterns-ui.md
Modular app with multiple screens?     -> patterns-ui.md
Syncing state across players?           -> patterns-networking.md
Optimizing Update() or heavy loops?     -> patterns-performance.md
Heavy rebuild, replay, or reset/cancel?  -> patterns-performance.md
Playing or streaming video?             -> patterns-video.md
Need array helpers, event bus, or       -> patterns-utilities.md
  pseudo-delegates?
Basic interactions, timers, audio,      -> patterns-core.md
  pickups, or teleportation?
Station + trigger zone detection?       -> troubleshooting.md

Multiple concerns? Load the primary pattern file plus its dependencies. For example, a synced video player needs both patterns-video.md and patterns-networking.md.

Template Selection Guide

17 templates cover common starting points. Pick the closest match and adapt:

Starting Point	Template	Key Feature
Interaction & Objects
Interactive object (click/use)	BasicInteraction.cs	Cooldown, toggle, audio feedback
Synced toggle / shared object	SyncedObject.cs	Ownership guard, FieldChangeCallback, late-joiner init
Per-player movement settings	PlayerSettings.cs	Walk/run/jump speed via trigger zone
Contact-based collision detection	ContactReceiver.cs	OnContactEnter/Exit, avatar vs world, debounce (SDK 3.10.0+)
State & Game Logic
State machine / game flow	StateMachine.cs	Timed transitions, synced state, late-joiner safety
Game with undo/history	UndoableGameManager.cs	byte[] history, NetworkCallable OwnerProcessMove/Undo/Reset
Object pool (player slots)	MasterManagedPlayerPool.cs	FIFO ring buffer, master-managed, OnPlayerJoined/Left
Persistence & Data
Save/load player data	DataPersistence.cs	PlayerData API, OnPlayerRestored, auto-save (SDK 3.7.4+)
Networking Patterns
Rate-limited sync (slider drag)	RateLimitedSync.cs	0.15s cooldown, last-write-wins
Batched sync (rapid events)	BatchedSync.cs	Idempotent schedule, 0.2s delay, single packet
Congestion-aware retry	CloggedRetrySync.cs	IsClogged check, linear back-off, MaxRetries
Dual local+synced copy	DualCopySync.cs	Local working copy + synced transport, dirty flag
Pack multiple values into one field	PackedStateSync.cs	3 ints in one Vector3, reduced sync overhead
Utilities
Array helpers (List<T> alternative)	ArrayUtils.cs	Add, Remove, Contains, FindIndex, Shuffle for arrays
Event bus (pub/sub)	EventBus.cs	Subscriber list (max 32), RegisterListener/RaiseEvent
Custom editor inspector	CustomInspector.cs	UdonSharpGUI, Undo, proxy sync
Auto-generate .asset for new scripts	UdonSharpProgramAssetAutoGenerator.cs	AssetPostprocessor, domain-reload-only, auto-compile

Multiple needs? Start with the template closest to your primary concern, then pull patterns from others. For example, a synced game with undo needs UndoableGameManager.cs as the base plus patterns from RateLimitedSync.cs for throttling.

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
3.10.3	VRCPlayerApi.isVRCPlus, VRCRaycast (avatar), Mirror render-order fix

Note: SDK versions below 3.9.0 are deprecated as of December 2, 2025. New world uploads are no longer possible.

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	Search Hints
constraints.md	C# feature availability in UdonSharp; blocked features; syncable types; attributes; DataList vs array decision guidance; advanced workarounds (object array pseudo-struct, VRCUrl array sync)	List, async, try/catch, LINQ, generics, DataList, DataDictionary, DataList vs array, when to use DataList, VRCUrl array, VRCUrl sync, pseudo-struct, object array cast, multi-field state container
networking.md	Ownership model, sync modes, RequestSerialization, NetworkCallable, network events, data limits	UdonSynced, SetOwner, BehaviourSyncMode, FieldChangeCallback, OnDeserialization, master leave, ownership cascade
networking-bandwidth.md	Bandwidth throttling, bit packing, synced data size examples, debugging, owner-centric architecture	IsClogged, bandwidth, throttle, bit packing, data budget, IsMaster
networking-antipatterns.md	6 anti-patterns to avoid; 5 advanced sync patterns with template links	anti-pattern, race condition, ownership fight, late-joiner, PackedStateSync, BatchedSync
persistence.md	Storage layer decision tree (local/synced/PlayerData/PlayerObject); PlayerData/PlayerObject API (SDK 3.7.4+); per-player save data; storage usage query API (SDK 3.10.0+)	storage layer, decision tree, local variable, PlayerData, PlayerObject, OnPlayerRestored, SetInt, TryGetInt, GetPlayerDataStorageUsage, GetPlayerDataStorageLimit, RequestStorageUsageUpdate, OnPersistenceUsageUpdated, storage quota, storage usage, which storage, when to use PlayerData
dynamics.md	PhysBones, Contacts, VRC Constraints (SDK 3.10.0+)	PhysBone, ContactReceiver, ContactSender, VRCConstraint, OnContactEnter
patterns-core.md	Initialization, interaction, player detection, timer, audio, pickup, animation, UI, teleportation, lazy init guard, remote players	Interact, OnEnable, Initialize, AudioSource, VRCPickup, Animator, UI, TeleportTo, remote players, GetRemotePlayers, exclude local player, FindAll alternative
patterns-networking.md	Object pooling, NetworkCallable patterns, persistence integration, dynamics integration, synced game state, delayed event debounce, string join for array sync	pool, MasterManagedPlayerPool, NetworkCallable, DamageReceiver, game state, debounce, state machine, string join, array sync, paragraph separator, U+2029
patterns-performance.md	Partial class pattern, update handler, PostLateUpdate, spatial query, platform optimization, frame budget Stopwatch, heavy processing architecture (rebuild, replay, reset/cancel), rate limit resolver, GameObject lookup cost tiers	Update, PostLateUpdate, Bounds, AnimatorHash, performance, mobile, PC, Stopwatch, frame budget, SendCustomEventDelayedFrames, heavy processing, rebuild, replay, reset, cancel, operation log, authoritative data, derived state, cursor rebuild, rate limit, URL scheduler, video load queue, GameObject.Find, Find cost, lookup cost tier, SerializeField vs Find, silent failure on rename
patterns-utilities.md	Array helpers (List alternatives), event bus, GameObject relay communication, pseudo-struct double-cast, abstract class callback, cancellable delayed event, re-entrance guard, UdonEvent pseudo-delegate	ArrayUtils, EventBus, relay, subscriber, FindIndex, ShuffleArray, object array, pseudo struct, double cast, abstract class, callback, interface alternative, cancellable timer, re-entrance, emitting guard, UdonEvent, pseudo delegate
patterns-ui.md	UI/Canvas patterns: immobilize guard, avatar-scale-aware UI, FOV-responsive positioning, platform-adaptive layout, dynamic player list, scroll input abstraction, lookup-table localization, toggle-animator bridge, settings persistence via PlayerObject, listener-based menu events, finger touch interaction, modular app architecture	Canvas, UI, menu, Immobilize, avatar scale, FOV, platform, Quest, VR, desktop, player list, scroll, localization, language, Toggle, Animator, PlayerObject, settings, persistence, listener, broadcast, finger touch, fingertip, haptic, FingerPointer, FingerTouchCanvas, touch canvas, app architecture, AppModule, AppManager, plugin lifecycle, CanvasGroup transition
patterns-video.md	Video player state machine, server-time playback sync, late joiner sync, AVPro Blit buffering, error retry with fallback, synced playlist/queue, platform URL selection	video player, AVPro, VRCUnityVideoPlayer, BaseVRCVideoPlayer, playback sync, server time, GetServerTimeInMilliseconds, late joiner, VRCGraphics.Blit, OnVideoReady, OnVideoError, retry, fallback, playlist, queue, shuffle, repeat, Quest URL
web-loading.md	String/Image downloading, VRCJson, Trusted URLs	VRCStringDownloader, VRCImageDownloader, VRCJson, DataDictionary, VRCUrl
image-loading-vram.md	Advanced VRAM management for image loading: Destroy vs Dispose, double-buffer fade, stock mode, mipmap bias	VRAM, texture memory, memory leak, Destroy, Dispose, double buffer, fade, mipmap, TextureInfo
web-loading-advanced.md	Advanced data loading: Base64 texture embedding via StringDownloader, cross-platform compression, URL double-key indexing, LRU decode cache	Base64, LoadRawTextureData, StringDownloader texture, DXT1, ETC_RGB4, UNITY_ANDROID, LRU cache, packed resources, binary format
api.md	VRCPlayerApi, Networking, enums reference	GetPlayers, playerId, isMaster, isLocal, GetPosition, SetVelocity, Drone, VRCDroneApi
events.md	All Udon events (including OnPlayerRestored, OnContactEnter)	OnPlayerJoined, OnPlayerLeft, OnPlayerTriggerEnter, OnOwnershipTransferred
editor-scripting.md	Editor scripting, proxy system, and UdonSharpProgramAsset auto-generation	UdonSharpEditor, UdonSharpBehaviourProxy, SerializedObject, UdonSharpProgramAsset, auto-generate, AssetPostprocessor, .asset missing
sync-examples.md	Sync pattern examples (Local/Events/SyncedVars)	Continuous, Manual, NoVariableSync, sync example
troubleshooting.md	Common errors and solutions	NullReference, compile error, sync not working, FieldChangeCallback, VRCStation, seated player, trigger zone, OnPlayerTriggerEnter not firing, station collider, position polling, OnStationEntered
sdk-migration.md	SDK migration guide (3.7 to 3.10), version-by-version changes and checklists	migration, deprecated, upgrade, 3.7, 3.8, 3.9, 3.10
testing.md	Testing and debugging guide: ClientSim editor testing, Build and Test (single and multi-client), Debug.Log patterns, pre-release cleanup, testing checklist	ClientSim, Build and Test, multi-client, late joiner test, debug, Debug.Log, ownership test, sync test, testing checklist

Templates (assets/templates/)

Template	Purpose
BasicInteraction.cs	Interactive object with Interact() handler
SyncedObject.cs	Network-synced object (Manual sync, ownership guard, late-joiner init flag)
PlayerSettings.cs	Per-player movement settings (walk/run/jump speed)
StateMachine.cs	State machine with synced state and transitions
DataPersistence.cs	PlayerData save/load with OnPlayerRestored (SDK 3.7.4+)
ContactReceiver.cs	Contact receiver for world-side collision detection (SDK 3.10.0+)
CustomInspector.cs	Custom editor inspector with UdonSharpEditor
MasterManagedPlayerPool.cs	Master-managed player object pool; FIFO ring buffer; OnPlayerJoined/Left; VerifyAssignments after master handoff
EventBus.cs	Subscriber list event bus (max 32 listeners); RegisterListener/UnregisterListener/RaiseEvent; in-place compaction
ArrayUtils.cs	List<T> alternatives: Add, Contains, AddUnique, Remove, RemoveAt, Insert for GameObject[]; FindIndex/ShuffleArray for int[]
UndoableGameManager.cs	History/undo sync with byte[] state history; NetworkCallable OwnerProcessMove/OwnerUndo/OwnerReset
PackedStateSync.cs	Pack 3 ints into one Vector3 UdonSynced field; OnPreSerialization/OnDeserialization
RateLimitedSync.cs	0.15s sync cooldown with _syncLocked/_changeCounter; _OnSyncUnlock callback
DualCopySync.cs	Local + synced copy with _dirty flag; strict OnPreSerialization/OnDeserialization separation
BatchedSync.cs	Idempotent ScheduleBatchedSync with 0.2s BatchDelay; _FlushBatch delayed callback
CloggedRetrySync.cs	Networking.IsClogged check; linear back-off (RetryDelay * retryCount); MaxRetries=5
UdonSharpProgramAssetAutoGenerator.cs	AssetPostprocessor that auto-creates UdonSharpProgramAsset for new scripts

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