FOS Sync Doctor
Version 1.0.0 · requires FOS Essentials Core 1.1.0 or later
Audits the synchronised behaviours of your world for the mistakes that never raise an error — the ones you only discover when a player joins your instance and sees a different world from everyone else.
Free. Requires FOS Essentials Core 1.1.0.
Menu: FOS Essentials > FOS Sync Doctor > Audit.
Nothing is added to your world
Section titled “Nothing is added to your world”This pack contains editor code only. No component, no prefab, no behaviour. It reads your scripts and reports; it never runs in game and costs nothing at runtime.
That is also why it ships without a Runtime assembly or a Prefabs folder, unlike every
other FOS pack: there would be nothing to put in them.
What it checks
Section titled “What it checks”Synchronised variables with no OnDeserialization — error
Section titled “Synchronised variables with no OnDeserialization — error”The single most common late-joiner bug. The values arrive, and nothing applies them. For the player who joined, the world stays in its initial state — permanently.
A field carrying [FieldChangeCallback] is not reported: the callback fires on
deserialisation, so that pattern is already correct.
RequestSerialization() inside Start() — warning
Section titled “RequestSerialization() inside Start() — warning”VRChat documents that a serialisation requested from Start() does not reach players who
join later. Send it from an event, or from OnPlayerJoined.
Manual sync that never serialises — warning
Section titled “Manual sync that never serialises — warning”Manual sync sends nothing until you ask. Without a single RequestSerialization() the
variables are, in practice, local.
Serialising with no ownership check — warning
Section titled “Serialising with no ownership check — warning”Only the owner can serialise. Called by anyone else, RequestSerialization() silently
does nothing — the most confusing failure in Udon, because everything looks fine locally.
Continuous sync carrying an array or a string — warning
Section titled “Continuous sync carrying an array or a string — warning”Continuous sync is capped at roughly 200 bytes per behaviour. A variable-length field goes over it without warning. Manual sync carries far more.
No sync mode declared — note
Section titled “No sync mode declared — note”Without [UdonBehaviourSyncMode] the mode defaults to Any and stays changeable in the
inspector, where it can be altered by accident.
A broadcast event acting on the local player — warning
Section titled “A broadcast event acting on the local player — warning”The one that costs the most time to understand. A network event sent to every client runs
on every client, and Networking.LocalPlayer is a different player on each of them. A
teleport meant for one person moves the whole instance.
Player events used without isLocal — warning
Section titled “Player events used without isLocal — warning”OnPlayerTriggerEnter and its siblings fire on every client, for every player
entering. Without the check, a stranger walking through your volume triggers the effect on
your machine.
A network event whose name starts with _ — warning
Section titled “A network event whose name starts with _ — warning”VRChat refuses to run such an event remotely. The call leaves, nothing happens, and nothing is logged.
Certain, and probable
Section titled “Certain, and probable”Findings come in two kinds, and the window never mixes them up.
Certain findings are established by inspecting the compiled types: which fields carry
[UdonSynced], which sync mode is declared, whether OnDeserialization is overridden.
These are facts.
Probable findings are deduced from the text of your scripts, because the body of a
method cannot be inspected any other way. “This script never calls
RequestSerialization” is wrong if another script calls it for you. Those findings carry
a probable tag and are worth a look rather than a blind fix.
Comments and string literals are stripped before anything is searched, so a script explaining “no ownership check here” in a comment is not mistaken for one that performs the check.
An audit tool that cannot tell you how much to trust it is not worth much.
What it does not do
Section titled “What it does not do”- It does not read your scene, only your scripts. A behaviour that is perfectly written but wired to the wrong object is not something this tool can see.
- It does not check performance, lighting or Quest budgets. VRWorldToolkit does that well, and for free.
- It does not touch anything. Every finding is a report; the fixes are yours to make.
By default the FOS packs themselves are skipped — you are auditing your world, not this line. The toggle exists if you want to look.