> For the complete documentation index, see [llms.txt](https://docs.allout.game/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.allout.game/scripting/networking-fundamentals.md).

# Networking Fundamentals

All Out is multiplayer-first, but CSL is designed so you can write gameplay almost like it’s singleplayer.

The key idea is:

{% hint style="info" %}
All gameplay state is automatically synced from the server to clients. You do not need to write RPCs or “network spawn” things.
{% endhint %}

## Authority (who decides what?)

As a rule of thumb:

* **Server**: decides gameplay state (damage, rewards, spawns, win/loss, collisions-by-query, etc)
* **Client**: renders and plays cosmetics (UI, particles, screen shake, sound)

In practice, most of your code will run on both server and clients — you just guard *what work* runs where.

## Local vs server checks

Two checks you’ll use constantly:

* `is_local_or_server()`: for gameplay UI and input handling (runs on server + local client)
* `is_local()`: for purely cosmetic effects (runs only on the local client)

Example:

```go
Player :: class : Player_Base {
    ao_late_update :: method(dt: float) {
        // Use for gameplay UI and inputs (runs on server + local client)
        if is_local_or_server() {
            // draw_ability_button(this, Shoot_Ability, 0);
            // Handle inputs that affect game state
            // Movement is handled automatically
        }

        // Use for purely cosmetic effects (runs only on local client)
        if is_local() {
            // UI.text(..., "Waiting for host to start the game...");
            // Particle effects, cosmetic UI, etc.
        }
    }
}
```

{% hint style="warning" %}
If you run cosmetic-only work on the server, you’re wasting CPU (and potentially doing it multiple times). Keep cosmetics local.
{% endhint %}

## Client-specific state

Sometimes you need *local* state that should not affect gameplay and doesn’t need replication:

* UI open/close toggles
* Camera shake timers
* “last hovered item” in an inventory UI

Keep these as fields, but only read/write them behind `is_local()` checks.

```go
Player :: class : Player_Base {
    // Local-only UI state
    inventory_open: bool;

    ao_late_update :: method(dt: float) {
        if !is_local() return;

        // Toggle UI (example)
        // if UI.button(...) inventory_open = !inventory_open;
    }
}
```

## Multiplayer safety rules

* **Avoid global variables for player state.** If each player needs their own value, store it on `Player`.
* **Be careful with “singletons”.** If you truly need a global manager, make sure it doesn’t accidentally store per-player data.
* **Prefer deterministic server state.** The client should react to the server’s state, not invent it.

## Debugging networking issues

* Add `log_info(...)` on the server paths (not just local) so you can see authoritative decisions.
* If something “works locally” but not for other players, it’s usually:
  * code guarded by `is_local()` that should be server-authoritative, or
  * a global variable being overwritten by multiple players.

## Related docs

* [Adding Player Logic](/scripting/player-model.md)
* [Game/Frame Lifecycle](/scripting/game-frame-lifecycle.md)


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