CosmicClash/docs/TECH_STACK.md

# Tech stack

What this project is built with, and why each piece was chosen, sourced from
the project's own docs and code comments. Where the reasoning for a choice
isn't recorded anywhere, this doc says so rather than guessing.

## Engine: Godot 4.7

The whole game — client and dedicated server alike — is one Godot 4.7
project, GDScript only. There is no C#, ONNX, or .NET code involved at
runtime anywhere in the shipped product.

**Why Godot, specifically:** legal, not technical. Per `README.md`'s
"Legality" section, the concept of "vehicle soccer" cannot be copyrighted,
but Rocket League's specific expression of it can be. Building on Unreal or
Unity — the engines Psyonix and most Rocket-League-likes use — would invite
comparison to that specific expression. Using a different engine (Godot) and
different vehicles (space ships instead of cars) is a deliberate part of
keeping the project's own expression original and legally distinct.

## Physics: Jolt Physics

Set via `Game/project.godot`'s `3d/physics_engine="Jolt Physics"` — Godot
4's alternative physics backend, not the engine's own default
(`GodotPhysics3D`). All ship and ball movement is force/torque-based
(`_integrate_forces`), never kinematic.

**Why Jolt over Godot's default physics:** not written down anywhere in the
project's own docs, but per the project owner, the goal was a physics engine
whose behavior isn't tied to Godot's own release cycle — so upgrading to a
future major Godot version doesn't silently change how the game feels, the
way an engine-version upgrade has repeatedly worried Rocket League's own
playerbase (players have specifically flagged that Unreal Engine
upgrades risk changing timestep/continuous-collision behavior enough to
break muscle memory built over thousands of hours).

This holds up under scrutiny. `GodotPhysics3D` (Godot's built-in default) is
an internal engine subsystem, versioned and rewritten alongside Godot itself,
and has a real history of behavior changing across Godot releases — for
example a kinematic-body regression introduced between 4.3-dev4 and
4.3-dev5, and collision-detection differences reported across the 4.0 line.
Jolt, by contrast, is developed as an independent upstream library
([jrouwe/JoltPhysics](https://github.com/jrouwe/JoltPhysics)) with its own
semantic versioning and a user base beyond just Godot, so its collision
behavior changes on its own release cadence rather than as a side effect of
unrelated Godot core work. It's a real reduction in coupling, not a
complete guarantee: Godot still pins (and can bump) a specific Jolt version
per release, and Godot's own Jolt *integration layer* can itself introduce
differences — e.g. a sleeping `RigidBody3D` wakes differently under Jolt
than under `GodotPhysics3D` when another body approaches it.

Separately documented, and a real consequence either physics backend would
share: **Jolt is not bit-deterministic** across platforms or even across
differing contact orderings on the same platform, and Godot exposes no world
snapshot/restore API. That fact is why the multiplayer architecture is
server-authoritative with client-side prediction of only the local ship,
rather than rollback/resimulation netcode — rollback would require
deterministic replay, which no physics engine choice here provides
(`multiplayer-next.md` §1, decision 1).

## Multiplayer transport: Godot's built-in `MultiplayerAPI` over ENet

The default and fully-supported transport is `ENetMultiplayerPeer` —
Godot's built-in high-level multiplayer networking, direct-IP over UDP,
port 7777 by default. A thin `NetTransport` abstraction
(`Game/scripts/net_transport.gd`) exists specifically so a second transport
(Steam) can be swapped in without touching the rest of the networking code.

Design choices layered on top of the built-in peer, and why:

- **`ENetMultiplayerPeer.server_relay` is forced to `false`.** It defaults
  to `true`, which lets any client `rpc()` any other client *through the
  server* — incompatible with a server-authoritative model. Called out in
  `multiplayer-next.md` §2.1 as "the single highest-value one-line security
  change in the document."
- **Manual multiplayer polling**, not Godot's automatic idle-frame poll.
  `NetworkManager` calls `set_multiplayer_poll_enabled(false)` because the
  automatic poll runs on the idle frame, which would tax every RPC issued
  from `_physics_process` up to a full frame in each direction — unacceptable
  for a physics-tick-rate game.
- **Server-authoritative simulation with client-side prediction of the
  local ship and ball only; no world rollback.** Direct consequence of
  Jolt's non-determinism (see above).
- **A custom binary wire format** (`net_codec.gd`) rather than raw RPC
  argument marshalling, for compact, quantised input/snapshot packets sent
  at high frequency — no stated alternative was considered in the docs, but
  the packet-size/channel-intent design in `multiplayer-next.md` §2 is
  extensive and deliberate.

## Optional multiplayer transport: Steam (GodotSteam)

`Game/scripts/steam_transport.gd` implements the same `NetTransport`
interface using `SteamMultiplayerPeer` over Steam's SDR (Steam Datagram
Relay), from a custom GodotSteam-patched Godot build (not stock Godot —
`STEAM.md`). It is entirely optional: the default build and every CI check
use ENet only, and a build without the `steam` feature is fully functional
without it.

**Why it's optional and why raw ENet remains primary:** `multiplayer-next.md`
states plainly that "Docker/VPS is the primary v1 deployment path. Raw ENet
self-hosting needs port forwarding, and SDR is Phase 7 — so [the ENet
phases] ship something that works on LAN or a VPS and nowhere else." Steam/SDR
is being added later specifically to remove the port-forwarding requirement
and to supply verified player identity — direct-IP ENet's slot-reclaim logic
is keyed by display name today, which is insecure against a public server
(see `multiplayer-next.md`).

## Dedicated server hosting: Docker (primary) or native systemd

The dedicated server is not a separately-written service — it's the same
Godot project, exported headless (`res://scenes/server_boot.tscn`) via
Godot's own `--export-release "Linux Dedicated Server"` preset. Two
deployment paths are documented (`SERVER.md`):

- **Docker**, the primary path: a multi-stage `Dockerfile` builds the
  export inside a pinned `barichello/godot-ci:4.7.1` image and produces a
  slim `ubuntu:24.04` runtime image. `make verify-phase6` builds it, runs it,
  joins two independent client processes to it, and asserts on match/goal/
  arena-rotation behaviour — this is also the entire Phase 6 GitHub Actions
  workflow.
- **Native systemd**, for a VPS: copy the exported binary to
  `/opt/cosmic-clash`, run it as a dedicated `cosmicclash` service user via
  `deploy/cosmic-clash-server.service`.

Per the project owner, Docker was chosen as the primary path for three
reasons: it gives a pinned, reproducible build environment that behaves
identically across local development, CI, and hosted production servers
(rather than three separately-drifting setups); it's portable across
hosting providers instead of assuming a specific Linux distro/init system
the way the systemd unit does; and it's the tooling the team is already
most familiar with. That matches what's independently visible in the repo —
`SERVER.md` documents Docker as the one path CI actually exercises
(`make verify-phase6`), while the systemd unit is native-deployment
documentation only, with no automated verification of its own.

## AI opponents: reinforcement learning, trained out-of-process, run in pure GDScript

Two entirely separate pieces, deliberately joined only at a JSON file:

- **Training** (Python, not shipped): [Godot RL Agents](https://github.com/edbeeching/godot_rl_agents)
  (`godot-rl==0.8.2`, vendored bridge plugin at `Game/addons/godot_rl_agents`,
  MIT-licensed) drives self-play PPO via **Stable-Baselines3** (`==2.4.0`)
  over **PyTorch** (`==2.13.0`) and **Gymnasium** (`==1.0.0`), against a
  headless instance of the actual game (`training/train.py` launches real
  parallel `godot --headless` processes from source — the training
  environment *is* the game, not a reimplementation of its physics).
  `training/requirements.txt` pins these versions strictly, because the
  training code (`ship_action_codec.gd`, `train.py`) depends on specific
  library-internal behaviour (godot_rl's discrete-action-space branch,
  SB3's logit layout, Gymnasium's dict-key sorting) that an unpinned
  upgrade could silently change mid-curriculum.
- **In-game inference** (`Game/scripts/policy_network.gd`): the trained
  checkpoint is exported to a small JSON file
  (`training/export_policy.py`) and evaluated at runtime by a hand-written,
  dependency-free GDScript MLP. `Game/addons/godot_rl_agents/VENDORED.md`
  notes the plugin's ONNX/C# files are present upstream but unused here —
  they require the .NET Godot build, which this project does not use.

**Why this split, rather than shipping ONNX/.NET inference:** stated
directly in `TRAINING.md` — "the trained policy is exported to a small JSON
file and runs **inside the game** in pure GDScript — shipped bots need no
Python, no .NET, no network." Keeping the shipped game GDScript-only (no
.NET Godot build) is consistent with the rest of the stack.

## Tooling (not shipped with the game)

- **`mcp/godot-mcp`** (git submodule, Node/TypeScript) — drives a live
  Godot editor/runtime instance for AI-assisted development; not part of
  the game.
- **`mcp/blender-mcp`** (git submodule, Python/`uv`) — drives a live
  Blender instance for generating original 3D assets (ships, arenas), for
  the same originality reasons covered under "Why Godot" above.
- **`tools/blender/`, `tools/textures/`** — standalone Python scripts
  (Blender's embedded Python, plus texture generators) used to produce the
  project's original meshes and textures.

## What's deliberately absent

- **No C# or .NET runtime anywhere in the shipped game or server.** The
  "C# backend" in `README.md`'s early framing was never built. A backend
  service *is* now planned for matchmaking (see below), but nothing has
  chosen C# for it — that framing predates every real decision here.
- **No HTTP/WebSocket/gRPC layer for simulation traffic** — the live game uses
  ENet/Steam SDR over UDP via Godot's own `MultiplayerAPI`. The matchmaking
  control plane now has an authenticated Go REST/WebSocket boundary for queue,
  proposal, assignment and recovery traffic; simulation remains on ENet/SDR.
- **No ONNX or other ML runtime in the shipped game** — see "AI opponents"
  above.

## Planned, not yet built

- **The remaining Go matchmaking control-plane deployment** — independently
  runnable matcher, allocator and maintenance roles backed by PostgreSQL and
  Redis, deployed on provider-portable Kubernetes with Agones-managed game
  fleets. The authenticated API boundary exists; durable production wiring and
  provider deployment remain. The cloud provider remains deliberately
  replaceable; the application stack is locked.
  This is a 1.0 launch blocker and the single largest departure from "one
  Godot project, no backend". See [`MATCHMAKING.md`](MATCHMAKING.md).