explainx / blog
Developer am.will used Claude Opus 5 to build a playable Three.js Rocket League clone at ~27% of a 5x Max sub. Opus then drove the car to verify mechanics. Open source + Vercel demo.
Jul 25, 2026
Claude Code’s Thariq explains why Claude 5 models need less prompting theatre: delete conflicting rules, design tool interfaces, progressive disclosure, and run /doctor to rightsize CLAUDE.md and skills.
Jul 25, 2026
Official ClaudeDevs thread decoded: upgrade to claude-opus-5, run migrate + the claude-api skill, dial effort, enable Fast mode, and use new Platform tool-cache + fallback routing without invalidating prompt cache.
Jul 25, 2026
Claude Opus 5 is live — default on Max, strongest on Pro, same price as Opus 4.8. explainx.ai unpacks Anthropic’s benches, effort/cost charts, alignment story, and when to pick Opus 5 vs Fable 5.
Hours after Claude Opus 5 launched, developer am.will (@LLMJunky) dropped a five-minute clip that X immediately filed under “psychosis”: Opus 5 had built the best Rocket League–style clone he’d seen — car reflections, arena detail, playable bots — while burning only ~27% of a Claude Max 5x sub.
Then the follow-up: Opus didn’t just ship code. It played the game to verify mechanics.
This explainx.ai post is the builder decode — stack, links, what “Opus controlling the game” actually means, and how it fits Anthropic’s day-one coding pitch.
Demo pulled from am.will’s announcement thread — sound on if you want the full effect.
| Question | Answer |
|---|---|
| What shipped? | Browser Rocket Arena — RL-style car soccer |
| Who? | am.will / @LLMJunky |
| Model? | Claude Opus 5 on Max 5x |
| Credit burn? | ~27% of the 5x sub (per author) |
| Play? | rl-opus5.vercel.app |
| Code? | github.com/am-will/rl-opus |
| Stack? | Three.js + Rapier + Vite + TS |
| Twist? | Opus drove the car to verify mechanics |
| vs Fable/Sol? | Same author: Opus wins detail; Fable/Sol prior demos exist |
Anthropic’s launch sold Opus 5 as near-Fable intelligence at half the price with Frontier-Bench SOTA. am.will’s thread is the complementary vibe: I expected a cheaper Fable and got a model that owns games and 3D polish.
Concrete claims from the thread:
am-will/rl-opusThat last point matters for builders: this is the kind of long-horizon UI/game work ClaudeDevs said Opus 5 was built for (developer migrate guide).
From the follow-up: “This is Opus controlling the game entirely.”
Read this carefully:
That distinction is the interesting part for agent harnesses. Verification used to mean unit tests and screenshots. Here verification means closing the gameplay loop — the same direction as Anthropic’s early-access quotes about models opening pages at mobile widths and fixing fold bugs before handoff.
If you’re building product agents, the pattern is: ship artifact → instrument play/use → feed failures back. Games make that visceral; CRUD apps can do the same with scripted browsers.
From the public Rocket Arena README:
npm install
npm run dev
| Layer | Choice |
|---|---|
| Render | Three.js |
| Physics | Rapier |
| Tooling | Vite + TypeScript |
| Local play | Game is the first screen (no marketing landing) |
| Modes | 1v1 bot · 2v2 bots · practice · infinite boost |
| Input | Remappable keyboard + standard gamepads (RL layout) |
| Online | Esc → Online; host browser + Cloudflare Worker rooms |
| Physics constants | Community RL reverse-engineering (1 uu ≈ 1 cm) |
Notable gameplay fidelity called out in-repo: Rocket League kickoff spawns, demolitions only when supersonic, boost pads left alone at 100%, goal pyro + slow-mo, wheel ember trails for supersonic, bot roles (closest ball vs support).
Online note: Vercel serves static/game; a Cloudflare Worker holds WebSockets (server/ in the repo) — already deployed on the free tier for ~130 matches/day per the README.
am.will linked an earlier seed prompt along the lines of: Build a polished browser-based 3D Rocket League–style game using Three.js… first screen is the actual game… use real physics… Then iterated: practice mode, infinite boost, steering tweaks, audio, orb boost pads, rounded goals/ceilings.
That matches Anthropic’s Claude 5 context engineering advice more than prompt theatre: clear product intent + iterative refinement + rich running artifact, not a 4,000-word system prompt.
Also pairs with thin prompts / thick artifacts — the game is the thick artifact.
| Model | Author’s read (thread) |
|---|---|
| Opus 5 | Best clone seen; car/reflections/detail; ~27% of Max 5x |
| Fable 5 | Very good prior one-shot; weaker small details vs Opus |
| GPT-5.6 Sol (high) | Solid earlier demo; Fable “mogged” that run |
This is one developer’s longitudinal taste test, not a blind arena. Still useful: same person, same genre, multiple frontier models — Opus 5 wins on polish for this task class.
claude-opus-5 (migrate guide)CLAUDE.md — gotchas only; don’t hobble with “never use X library” bans (context engineering)Build a polished browser Rocket League–style arena using Three.js + Rapier.
First screen = the game. Keyboard + gamepad. 1v1 bot. Real boost/jump/flip feel.
After each feature, open the app and play 20 seconds to verify steering and camera.
Later the same day, am.will posted that multiplayer was working — plug in Xbox / PS5 / Steam controllers, join with a room code. That lines up with the README’s Cloudflare Worker rooms: one browser hosts the match authority; the joiner’s car is predicted locally and corrected toward the host.
For agentic coding, that is a second verification surface: not only “does 1v1 against a bot feel right?” but “does networked input still feel fair?” Opus 5’s long-horizon story is exactly the kind of stack that can keep those threads alive without you re-pasting the architecture every turn.
ls and npm test.If your team evaluates models on “write a React form,” you will miss this delta. Add one visual + interactive task to your private harness: Three.js toy, canvas tool, or small game loop with a play-script.
Using Three.js + Rapier in a Vite app, build a minimal playable car-arena:
drive, jump, boost, one ball, one goal. First screen is the game.
After implementing, open the app and play for 30 seconds.
Fix the first three feel bugs you notice (camera, steering, collision).
Commit after each fix with a one-line note of what felt wrong.
Run that on Opus 5, Fable, and Sol. Score on playability, not LOC.
Sources: X — Opus 5 Rocket Arena announcement · X — Opus playing · Play · GitHub am-will/rl-opus · Anthropic Opus 5
Demo claims and credit usage as reported by the developer on July 24–25, 2026. Re-verify the live Vercel URL, repo, and your own Max limits before treating this as a reproducible recipe.