claude-slash-goal-example

An example of using Claude Code's /goal to build the same side-scrolling space shooter in three rendering eras — each one an independent build, each verified by a deterministic headless playtest.

The three versions

Dir	Era	Look
v1-70s-vector/	1970s vector / oscilloscope	single-colour line strokes, Asteroids-style
v2-8bit/	1980s/90s 8-bit	colourful pixel sprites, arcade palette
v3-modern/	modern	parallax, particles, glow, screen shake

Each is its own standalone game with era-appropriate gameplay (not one engine with a swappable skin), and each is verified independently.

Screenshots

v1 — 70s vector / oscilloscope

Title	Gameplay

v2 — 80s/90s 8-bit

Title	Gameplay

v3 — modern

Title	Gameplay (browser)

(v3's gameplay shot is captured in a browser, where the HUD renders correctly; the headless playtest renderer stubs text drawing — see "Running a version" below.)

Why this example exists

A first attempt at this with /goal produced a game that passed every clause of the goal and was almost unplayable to look at — a provably-correct game with no art. The lesson:

A /goal condition is a product spec, not just a verification spec. The agent optimises exactly what the condition measures. If the condition only measures "it builds and the levels are completable," that is all you get.

docs/GOAL-PROMPT.md (with docs/PRD.md as the spec) is the corrected approach: it requires the qualities that make it a real game — distinct enemies, bosses and weapons, a real difficulty curve, the era's visual identity — expressed as things a scripted-gameplay playtest can prove, not as adjectives.

Running a version

Each version builds to a single self-contained index.html. Open it in a browser to play:

• Arrows / WASD — move
• Space — fire
• P — pause, M — mute, Enter — launch

Headless, deterministic verification:

cd v1-70s-vector && node playtest.mjs   # (or v2-8bit / v3-modern)

The playtest drives a scripted bot through all five levels via real in-game controls and prints: per-level COMPLETABLE/FAILED, the invariants (no-unreachable, no-softlock, boss-difficulty-monotonic, hitreg), a challenge check (a deliberately naive bot must fail the later levels while a skilled bot clears them), determinism over three fixed-seed runs, and per-version visual assertions. Bot-driven screenshots land in each version's shots/ (v3: screenshots/) directory.

Note: the v3 headless playtest renderer stubs text drawing, so HUD text does not appear in v3's headless captures (it renders correctly in a browser — see v3-modern/screenshots/v3-browser-hud.png). The other assertions are unaffected.

How it was made

A Claude Code session was given the prompt in docs/GOAL-PROMPT.md against the spec in docs/PRD.md, and used /goal to build and verify each version. The literal per-version /goal condition strings were derived from the PRD by that session and are not reproduced here verbatim; the prompt and PRD are the reproducible artifacts.

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Built with Claude Code.