godot-gdscript-headless-testing

Godot GDScript headless testing (4.x)

Run GDScript tests from the command line, without the editor GUI, and get a real process exit code CI can act on. Targets Godot 4.7 headless CLI.

When to use

• Use when a Godot project has no testing addon installed and needs a fast way to verify GDScript logic (pure functions, resource loading, autoload state) from a terminal or CI pipeline.
• Use when wiring a CI job that must fail the build when a .gd test fails.
• Use when debugging why a godot --headless invocation hangs, opens a window, or exits 0 despite failing assertions.

When not to use: GDScript syntax or language features themselves → godot-gdscript; export/build pipeline and platform templates → godot-export (its own --headless use case, producing a binary, not running tests).

Workflow

1. Confirm the binary resolves headless. Godot 4.x ships --headless built in (no export template needed); run godot --headless --version and confirm it prints a version string, not a GUI window.
2. On a fresh checkout, import before running tests. .godot/ is normally not committed, so a clean checkout has no import cache: class_name types fail to resolve (Identifier "X" not declared in the current scope) and imported assets fail to load (No loader found for resource: res://...). Run godot --headless --path <project_dir> --import once first, in CI and locally.
3. Write the runner as a SceneTree script, not a Node scene. A SceneTree script's _initialize() runs once before any frame — enough for pure-logic tests and no .tscn required to launch.
4. Track pass/fail counts yourself and call quit(<code>) explicitly. Do not use bare assert() to fail a test. Godot does not turn the process exit code non-zero on push_error() by itself — the runner must count failures and call quit(1). Worse, a failed assert() inside _initialize() (official/debug build) prints SCRIPT ERROR: Assertion failed and stops execution before quit() runs, so the process never exits and CI hangs until its own timeout. Use an assert_eq()-style helper that records the failure and keeps going.
5. Invoke with godot --headless --path <project_dir> --script res://<runner>.gd and read the process exit code, not just stdout, from the shell or CI step. --script accepts both a res://-relative path and an absolute filesystem path (e.g. a runner outside the project folder); either works.
6. Redirect stdout and stderr to files when scripting the invocation from a wrapper shell (PowerShell, some CI runners). push_error() output goes to stderr and can be dropped or reordered when only stdout is captured live.
7. Add a step timeout in CI. Even with the assert() pitfall avoided, an await that never resolves (Pattern #2) hangs the runner forever; a timeout-minutes on the CI step is a backstop CI-side, not a substitute for backing every await with a timeout node.

Patterns

1. Minimal SceneTree test runner with a real exit code

# res://test_runner.gd — run with:
#   godot --headless --path . --script res://test_runner.gd
extends SceneTree

var passed := 0
var failed := 0

func _initialize() -> void:
    test_add()
    print("Results: %d passed, %d failed" % [passed, failed])
    quit(1 if failed > 0 else 0)   # non-zero exit fails the CI step

func assert_eq(actual, expected, label: String) -> void:
    if actual == expected:
        passed += 1
    else:
        failed += 1
        push_error("FAIL %s: expected %s, got %s" % [label, expected, actual])

func test_add() -> void:
    assert_eq(2 + 2, 4, "test_add")

Verified against Godot 4.7.2: godot --headless --path . --script res://test_runner.gd prints Results: N passed, M failed to stdout, routes push_error lines to stderr, and returns process exit code 0 when failed == 0, 1 otherwise.

2. Testing something that needs a frame, a timer, or a signal

extends SceneTree

var passed := 0
var failed := 0

func _initialize() -> void:
    await run_tests()
    print("Results: %d passed, %d failed" % [passed, failed])
    quit(1 if failed > 0 else 0)   # track and report failures here too

func assert_eq(actual, expected, label: String) -> void:
    if actual == expected:
        passed += 1
    else:
        failed += 1
        push_error("FAIL %s: expected %s, got %s" % [label, expected, actual])

func run_tests() -> void:
    # `root` is not inside the tree yet during _initialize(): a Timer started now
    # errors ("not inside the tree") and its `timeout` never fires. Wait one frame.
    await process_frame
    var timer_node := Timer.new()
    timer_node.one_shot = true   # default Timer restarts after timeout
    root.add_child(timer_node)
    timer_node.start(0.1)
    await timer_node.timeout
    # assertions here can rely on the node having been in the tree for a frame
    assert_eq(timer_node.is_stopped(), true, "timer_fires_once")
    timer_node.queue_free()

_initialize() may await, which is what makes this pattern work for anything that needs a node to actually enter the tree, a timer to fire, or a signal to emit — none of which happen before the engine has processed at least one frame. Use the same passed/failed counter and assert_eq() helper as Pattern #1; a version of this pattern that always calls quit(0) can never fail a build.

3. CI step (GitHub Actions) that gates on the exit code

- name: Import project (populates .godot/ on a fresh checkout)
  run: godot --headless --path . --import
- name: Run GDScript tests
  timeout-minutes: 5
  run: godot --headless --path . --script res://test_runner.gd

The import step is required on a clean checkout — without it, class_name types and imported resources fail to resolve. No extra flag is needed for the test step itself: the runner already fails the job on a non-zero exit code from run:; the discipline lives in the runner script's quit() call, not in the CI configuration. timeout-minutes is a backstop against a hung await (see Pitfalls), not a substitute for backing every await with a timeout node.

Pitfalls

• Script "does nothing" or opens the editor window → missing --headless, or the script path is wrong. --script accepts a res://-relative path resolved against --path <project_dir>, and also an absolute filesystem path — both work.
• Identifier "X" not declared in the current scope, or a resource fails to load, only on a fresh checkout → .godot/ (the import cache) is normally not committed, so class_name types and imported assets aren't resolved yet. Run godot --headless --path <project_dir> --import once before the test step.
• A failed assert() hangs instead of failing the test → in an official/debug build, a failed assert() inside _initialize() prints SCRIPT ERROR: Assertion failed and stops that function before it reaches quit() — the process never exits and CI waits until its own timeout. Use an assert_eq() counter (Pattern #1) instead of bare assert() in test runners.
• Exit code stays 0 despite failed assertions → the runner never called quit(1), or (Pattern #2) it always calls quit(0) regardless of failures. Track failures yourself and call quit() explicitly with a code that reflects them; do not rely on assert() or push_error() alone to change the exit code.
• _initialize() runs before nodes, timers, or signals exist → logic that needs a frame to have processed must await a signal or a timer before asserting; see Pattern #2. root itself is not inside the tree yet, so a Timer added and started there errors and its timeout never fires (the runner hangs) — await process_frame first.
• Output looks empty or out of order from a wrapper shell → some shells (PowerShell in particular) can reorder or drop a native process's live stdout/stderr. Redirect both streams to files and read the files after the process exits, instead of trusting the live console.
• Runner never terminates → a SceneTree script keeps running until something calls quit(). A test that awaits a signal that never fires hangs the job forever — always back an await with a timeout node as a fallback, and set timeout-minutes on the CI step as a backstop.

Related skills

• godot-gdscript — the language syntax and node lifecycle this pattern's runner script itself uses.
• godot-export — headless CLI export/build, a different --headless use case.