running-headless-godot

Rules for making headless Godot (4.2+) development reproducible, avoiding drift and environment-specific failures.

Scope:

Godot 4.2+ (Godot 3.x is out of scope)

Required rules (highest priority):

Always use --headless --path <PROJECT_DIR> (removes cwd dependency)
Always capture logs under the project. Use 2>&1 | tee <PROJECT_DIR>/logs/<name>.log for commands that terminate themselves, and capture Godot's status from ${PIPESTATUS[0]}. When timeout is the only termination mechanism, redirect directly with > <PROJECT_DIR>/logs/<name>.log 2>&1 and capture timeout's status explicitly; do not pipe that run through tee.
Always set project-local XDG_DATA_HOME, XDG_CONFIG_HOME, and XDG_CACHE_HOME for any scripted Godot run. Set all three together — Godot writes to all three categories, and partial redirection still hits the unwritable default for the unset ones. Required to avoid CI/sandbox write failures (Can't open file for writing: ~/.config/godot/...); harmless on a developer machine because the only effect is that per-project metadata/cache live under <PROJECT_DIR>/.godot-xdg/ instead of the user's globals. Add .godot-xdg/ to .gitignore.
- These exports affect every process started from the same shell, not just godot. If the same shell session also runs non-Godot tooling that honors XDG vars (e.g. a Node/Playwright browser-automation step for web-export testing), that tooling's own cache/data lookup silently moves under <PROJECT_DIR>/.godot-xdg/ too and can fail to find things it installed elsewhere (observed: Playwright reporting a missing browser executable). Export these three vars only in the specific command/subshell that invokes godot, or explicitly unset/restore them before running unrelated tools in the same shell.
Never edit .tscn as raw text (edits must go through --headless --script)
For repeated validation commands, standardize them as tools/*.sh; after manual tweaks, re-run via the same scripts
If res://tools/godot_apply_patch.gd is missing, copy tools/godot_apply_patch.gd, resolved relative to this loaded SKILL.md, to <PROJECT_DIR>/tools/godot_apply_patch.gd before running patch commands
Keep startup smoke and logic tests separate: smoke starts run/main_scene; run_tests.gd holds project-specific logic checks

Minimum smoke wrapper template

For repeated startup verification, standardize as <PROJECT_DIR>/tools/smoke.sh. Other repeated invocations (run_tests.sh, export_web.sh, ...) follow the same shape — only the godot arguments and the log filename change.

#!/usr/bin/env bash
set -euo pipefail
PROJECT_DIR="$(cd "$(dirname "$0")/.." && pwd)"

# Project-local XDG (always; see Required rules above).
export XDG_DATA_HOME="$PROJECT_DIR/.godot-xdg/data"
export XDG_CONFIG_HOME="$PROJECT_DIR/.godot-xdg/config"
export XDG_CACHE_HOME="$PROJECT_DIR/.godot-xdg/cache"
mkdir -p "$XDG_DATA_HOME" "$XDG_CONFIG_HOME" "$XDG_CACHE_HOME" "$PROJECT_DIR/logs"

# Smoke = launch run/main_scene headlessly, quit after a few frames.
# --quit-after <N> bounds the run by frame count.
godot --headless --path "$PROJECT_DIR" --quit-after 60 \
  2>&1 | tee "$PROJECT_DIR/logs/smoke.log"
echo "godot_exit=${PIPESTATUS[0]}"

Pass criteria for smoke:

Captured exit code (${PIPESTATUS[0]}) is 0.
logs/smoke.log contains no ERROR, SCRIPT ERROR, Failed to load, or Parse Error line outside the project's known-warning whitelist (see references/headless_cli.md).
A startup-visible marker is present (e.g. a print() from the main scene's _ready, or absence of "Main scene can't be loaded").

If you get stuck, provide:

Full command lines and logs/*.log
godot --version output
Whether export_presets.cfg exists (when exporting)

Out of scope:

GUI/OS-specific setup beyond the project-local XDG wrapper
Level design, render validation, performance tuning
Game-specific scoring, win/loss rules, controls, and simulation policies

When you need details, read these files (relative to this skill's base directory):

references/headless_cli.md — CLI conventions, XDG setup, known warnings
references/export_and_import.md — export/import rules
references/scene_editing_via_godot.md — safe .tscn editing, patch JSON schema and operations
references/testing_headless.md — headless testing strategy

If unsure about Godot CLI or features, consult:

About this skill

Minimum smoke wrapper template