# Agent-assisted Godot development

Research notes and the resulting setup for this repo. The short version: Godot
is an editor-centric engine and a coding agent is a text-centric tool, so the
whole workflow is built around making the game fully runnable, inspectable and
verifiable from the command line — with the editor as an optional viewer rather
than a required step.

## The core problem

An agent editing a Godot project cannot see the editor. It cannot click play,
cannot look at the scene dock, and cannot tell whether a `.tscn` it wrote is
valid. Left alone it will produce code that looks right and does not run.

Everything below exists to close that gap. Three properties matter:

1. **A fast, honest failure signal.** Parse errors must surface in seconds, from
   the CLI, with the engine's own words.
2. **A way to actually play the game without a human.** Not "the tests pass" —
   the real binary, the real socket, the real loop.
3. **Source of truth in text.** Binary scenes and inspector-tuned resources are
   invisible to review and produce meaningless diffs.

## Godot's CLI, as an agent uses it

| Command | Use |
| --- | --- |
| `godot --headless --path . --import` | Rebuild the import + **global class cache**. Required after adding any `class_name`. ~2s. |
| `godot --headless --path . res://scene.tscn` | Run a scene headless with autoloads alive. |
| `godot --headless --path . --script tools/x.gd` | Run a `SceneTree` script. **No autoloads.** |
| `godot --headless --check-only -s file.gd` | Parse one file. |
| `godot --headless --path . -s addons/gut/gut_cmdln.gd -gdir=res://tests -gexit` | Run the test suite. |
| `godot --headless --path . --export-release <preset> <out>` | Build. |

Four things cost real time to discover, so they are worth stating plainly:

- **The global class cache is not built by running the game.** It lives in
  `.godot/global_script_class_cache.cfg` and is written by the editor or by
  `--import`. Add a `class_name`, run your code, and every reference to it fails
  with `Identifier "Foo" not declared in the current scope` — a message that
  looks exactly like a typo and is not one. `tools/check.sh` runs `--import`
  first for this reason.
- **Autoload singletons do not exist under `--script`.** `--script` replaces the
  main loop, so autoload names are unresolved identifiers at compile time. Any
  tool that needs an autoload has to run as a scene instead. The practical
  consequence for design: keep autoloads to the minimum that genuinely needs to
  be a node. This repo has exactly one (`Net`, because RPC routing is by node
  path); logging and CLI options are static classes, which makes them usable
  from tests and tools alike.
- **`ResourceLoader.load()` on a script with a parse error can return a non-null
  object.** Detecting failures by null-checking the return value silently passes.
  Read the engine's stderr instead — that is what `tools/check.sh` does.
- **Never call `Script.reload()` on the script currently executing.** It hangs
  the engine with no output.

## What this repo does about it

### `tools/check.sh` — the inner loop

Refreshes the class cache, loads every `.gd` in the project, greps the engine's
stderr for `Parse Error` / `SCRIPT ERROR` / `Failed to load script`, and exits
non-zero on any hit. About five seconds. This is the command to run after every
edit; it catches the entire class of "looks right, does not compile" mistakes
that an agent produces most often.

### `tools/test.sh` — GUT, headless

[GUT 9.7.1](https://github.com/bitwes/Gut) is vendored into `addons/gut/`.
Chosen over [gdUnit4](https://github.com/godot-gdunit-labs/gdUnit4) because this
project is GDScript-only, where GUT is the lighter and simpler of the two;
gdUnit4 is the better pick when C# is in play or when you want its scene-runner
utilities. Either runs headless in CI.

The suite runs in about two seconds because of an architectural choice, not a
testing trick: **the entire simulation is plain `RefCounted` objects** — no
nodes, no physics server, no rendering. A test constructs a `SimWorld`, drives
a thousand ticks, and asserts. Nothing to await, nothing to instantiate, no
frame timing. If the simulation had been built out of `CharacterBody2D` and
`Area2D`, every one of these tests would need a live SceneTree and would be
slower and flakier by an order of magnitude.

That is the single highest-leverage decision in this repo for agent-assisted
work, and it is worth stating as a general rule: **keep game logic out of nodes.**
Nodes are for presentation and input. Logic in plain objects is testable,
diffable, reviewable, and runs on a headless server for free.

### `tools/smoke.sh` — the thing tests cannot do

Boots the real dedicated server, connects two scripted bot clients over a real
ENet socket, and asserts on the server and client logs that the whole loop
happened: handshake, lobby, portal into a dungeon, emergency escape back out.

Unit tests structurally cannot cover RPC wiring, codec round-trips over the
wire, instance transfers or client reconciliation. This does, in ~35 seconds,
with no display. The bot input lives in `ClientRuntime._bot_input()` behind
`--bot`, so the "player" driving it is the same code path a human uses.

This is the pattern to reach for whenever an agent needs to verify something
interactive: give the program a scripted-input mode and a `--autoquit`, run it
headless, and assert on structured log lines. Log markers like `SERVER_READY`
exist specifically to be grepped.

### `tools/bench.gd` — performance as a number

Runs the simulation with no transport and no rendering and prints milliseconds
per tick. Current numbers, 4 players, 60s of play:

```
boss fight     0.26 ms/tick   peak ~300 bullets   headroom x64
trash wave     0.16 ms/tick   peak  ~82 bullets   headroom x103
```

Note what this does *not* cover: it measures the simulation only, with no
transport. Per-peer snapshot encoding was measured separately (95.6us for four
filtered snapshots against 24.9us for one shared, or 0.032 ms/tick amortised).
Quoting a bench number for something the bench does not exercise is its own
version of the trap below.

A 60 Hz tick has a 16.6 ms budget, so one instance uses ~1.4% of one core. That
is the measurement that says a single server process can host dozens of
concurrent dungeons, and it took ten seconds to get because the simulation has
no engine dependencies.

## Content as code, not as `.tres`

Enemies and bosses are built by GDScript functions in `src/content/content.gd`,
not authored as `.tres` files. For agent-assisted work this is the right default:

- A boss is a readable diff. A `.tres` full of `SubResource` ids is not.
- No resource UID churn in version control.
- A test can build content inline without touching the filesystem.
- The agent can write a boss without an editor.

`tools/export_content.gd` writes `.tres` copies into `resources/` for anyone who
wants to tune numbers in the inspector, with the direction of truth documented:
code wins, port inspector changes back.

The same reasoning applies to scenes. This project has three `.tscn` files, each
a handful of nodes. Anything dynamic — the HUD, the menu — is built in code,
because a scene file for it would only be one more thing to keep in sync.

## Godot MCP servers

An MCP server lets an agent talk to a live Godot editor: inspect the scene tree,
read node properties, create nodes, run the project and read its output. The
most established option is
[Coding-Solo/godot-mcp](https://github.com/Coding-Solo/godot-mcp) (~5.5k stars,
Node ≥18):

```bash
claude mcp add godot -- npx @coding-solo/godot-mcp
```

It exposes `launch_editor`, `run_project`, `stop_project`, `get_debug_output`,
`get_godot_version`, `get_project_info`, `create_scene`, `add_node`,
`load_sprite`, `save_scene`, `export_mesh_library`, and UID helpers for 4.4+.
Set `GODOT_PATH` if `godot` is not on `PATH`.

**Where it earns its place:** projects with large hand-authored scene trees,
where the agent otherwise has to guess node names and paths. Being able to read
the real tree instead of inferring it is the biggest single quality jump in that
situation.

**Where it does not:** this repo. `run_project` and `get_debug_output` are what
`tools/server.sh` and `tools/smoke.sh` already do, more precisely and with
assertions attached; and the scene-authoring tools matter little when there are
three small scenes. It also adds a live-editor dependency, and editor
auto-reload racing against agent file writes is a known source of stale state.

The honest recommendation: **start with the headless CLI loop, add the MCP when
scene inspection becomes the bottleneck.** It is one command to add later.

## Verification traps, learned the hard way

Every entry below cost at least one round trip of reporting something as fixed
when it was not. They are specific to this setup, and they generalise.

### Verify the behaviour, not the artefact you produced

Bullet sprites were reported fixed three times before they were. Each time the
*atlas* was checked and found correct. The actual fault was one layer down: a
`MultiMeshInstance2D` cannot display a sub-region of a texture, so the renderer
was structurally incapable of showing one cell no matter what the atlas held.
Checking the input to a broken stage will confirm the input forever.

Ask what the *player* would see, and find a way to check that.

### An ASCII dump only proves what you dumped

The same sprites were "verified" by rendering their alpha channel. The pack
animates as a colour shimmer over a fixed silhouette, so identical-looking
frames proved nothing at all. The check was real; it was measuring the wrong
channel.

### Measure the configuration that has the bug

The client-prediction gap was measured at 0.1px on a listen server and reported
fixed. A listen server runs both halves in one process on one physics tick, so
the quantity that had drifted — two independent clocks — was constant *by
construction*. The bug needed two processes to exist at all.

If a bug is described as appearing "after a while", ask what accumulates, and
make sure your harness lets it accumulate.

### Have the test tell you the setup was valid

`test_firing_is_reported_even_when_the_bullet_dies_instantly` asserts that no
`BULLET_SPAWN` event occurred, purely to prove the interesting condition was
actually reached. The first version placed a turret as the victim — and turrets
shoot, so the assertion passed on the turret's own bullets while testing
nothing. A setup check inside the test caught it.

### `git checkout` to clean up a probe reverts real work too

A one-file revert to remove a temporary debug hook also discarded a fix made to
the same file earlier in the session, and it had already been verified and
reported. Only `git status` showing the file *missing* from the staged set
caught it. Prefer editing the probe back out, or stash.

### A pattern-based edit can match twice

A `replace()` intended for the snapshot decoder also matched inside the
character decoder, which then read a field its encoder never wrote. Both are
codecs and both had the same trailing lines. Check the match count when patching
by pattern, not by line.

## CI

`.github/workflows/ci.yml` runs check → test → smoke on a container image with
Godot preinstalled. All three are headless, so nothing extra is needed.

## Sources

- [Coding-Solo/godot-mcp](https://github.com/Coding-Solo/godot-mcp)
- [GUT](https://github.com/bitwes/Gut) · [gdUnit4](https://github.com/godot-gdunit-labs/gdUnit4)
- [Godot: high-level multiplayer](https://docs.godotengine.org/en/stable/tutorials/networking/high_level_multiplayer.html)
- [Godot: command line tutorial](https://docs.godotengine.org/en/stable/tutorials/editor/command_line_tutorial.html)

### A green suite is not a quiet one

`tools/test.sh` and `tools/smoke.sh` both passed while a client logged twelve
engine errors at every startup. `ConfigFile.get_value(section, key, null)` does
not mean "no default" — it means the key is absent *and* no default was given,
so the engine logs an error for each one. Nothing caught it because the smoke
test's refutations matched `SCRIPT ERROR|Parse Error|USER ERROR`, and a plain
`ERROR:` is none of those.

It surfaced from actually running the client (`--listen --autoquit`) and reading
the output, which is worth doing after any change to startup.

`smoke.sh` now asserts no plain engine errors either, excluding by name the one
line Godot prints on every clean exit — naming it means anything else that turns
up is a real finding rather than noise to be squinted past.

### Nothing automated can see the screen

Every gate passed — 458 tests, a clean smoke run, no engine errors — while the
entire interface rendered in Godot's default style and none of the UI pack was
visible. A Control inherits its theme from Control *ancestors only*, and the
chain breaks at the first plain `Node` or `CanvasLayer`; every screen in this
game hangs off one. `get_window().theme = ...` set the property, changed
nothing, and looked correct in the code.

It took `tools/screenshot.tscn` — boot the client, save a PNG, open it — to
find. The same run then showed **Fire bound to the right mouse button**, which
was the test suite writing the player's real `user://settings.cfg`, because
rebinding calls `save()` and nothing had redirected the path.

Two lessons, and the second is the general one:

- After touching `src/ui/`, look at it. There is no substitute.
- A test that exercises a code path which writes to user state **will write to
  user state**. Redirect the path in the fixture, and assert the default is
  still the real one so the redirect cannot escape.