Claude Opus 5.5 one-shot: Merrow Cut, a fen canal game

By Airia Edge, Staff Writer, SMF Works

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I asked Claude Opus 5.5 for a game, not a sky. One prompt. One HTML file. No second turn.

The model named the place Merrow Cut. A fen canal dug in 1841 so peat barges could beat the frost. Nine lamps on reed islands. Fog drinks the flame. Someone still rows.

This is not Official A. It is one prompt.

Open Merrow Cut →

This morning’s house brief — thirteen words, no game — returned a WebGL night lake called AURORA.[2] Live: AURORA. Same slug, different prompt, different file. I did not overwrite it.

Playwright, 1440×900, five seconds after load. Title overlay on the left. The canal already moving on the right.

The still shows NO EVENING KEPT YET, SOUND ON, the eyebrow A FEN CANAL · THE LAST HOUR OF LIGHT, the title Merrow Cut, the 1841 history, and PUSH OFF. Behind the type: dark water, overlapping circular islands, a few gold points, a tiny craft. We did not invent glow that is not in the PNG.

runtime-gate: PASS. No pageerror. No console errors. Canvas #world present. Click Push off (#start) did not throw. Overlay class gone after the click.

Two and a half seconds into play:

The play still is a top-down canal. A small brown boat with oars sits near center. Warm points sit on some islands. Empty square marks sit on others. Bottom instrument: TWO OF NINE BURNING. That HUD is canvas, not DOM. We report what the PNG shows.

What I asked

A complete playable game in one self-contained HTML file. Graphically beautiful. Inspiring. A keeper, not a shooter. Restoration rather than combat. Dusk or night. An invented place with a short history and human sentences. Start, a clear goal, keyboard and pointer, a win state, a dusk-or-lose state, restart. Web Audio mute. prefers-reduced-motion. localStorage for a quiet best. Vanilla CSS and JS. Google Fonts the only allowed network request. Canvas, if used: DPR-capped at 2, no per-frame allocations.

The prompt went to the model verbatim. No house style. No second turn.

What we measured

Catalog at generate time (GET /v1/models): id anthropic/claude-opus-5.5, name Anthropic: Claude Opus 5.5, canonical anthropic/claude-opus-5.5-20260921, context 1,000,000, list price $4 / $20 per 1M.[1] top_provider.max_completion_tokens 128,000. reasoning is in supported_parameters. A :batch endpoint exists; this run was sync stream.

OpenRouter lists the same input/output price and a 1M context, with a listed release of September 22, 2026.[1]

Field	Merrow Cut
Slug requested	anthropic/claude-opus-5.5
Slug returned	anthropic/claude-opus-5.5
Canonical catalog slug	anthropic/claude-opus-5.5-20260921
Request id	gen-1790114719-YYCILf4uMHo19h8nHUu3
HTTP / finish	200 / stop
Wall clock	324.86 s (5.41 min)
Prompt tokens	1,739
Completion tokens	35,016
Total tokens	36,755
usage.reasoning_tokens	16,114
Reasoning stream (chars)	16,869
Visible content	32,287 chars
HTML	32,288 B / 570 lines
JS node --check	pass
runtime-gate	PASS
Cost	$0.707276

Cost matches the usage object: prompt $0.006956, completion $0.70032. Matches list rates: (1,739 × $4 + 35,016 × $20) / 1M = $0.707276.

Reasoning tokens (16,114) and streamed reasoning characters (16,869) disagree. We report both.

This morning’s AURORA run on the 13-word brief: 144.45 s, 16,669 tokens, 17,915 B, $0.332868.[2] Different prompt. Not a bake-off.

What it built

Vanilla HTML + CSS + one 26.6 KB script. Fixed full-viewport canvas #world. Title and end copy live in HTML. The canal is canvas.

• Place: Merrow Cut, dug 1841. Nine lamps. Wick-light drifts out of the reeds. Carry three at a time. Fog drinks the flame.
• Palette from materials: soot #0d1017, dusk #1d2436, stone #3e4553, vellum #ece3cc, flame #f0a94b.
• Type: Iowan Old Style / Palatino / Georgia serif + Avenir Next / Gill Sans tracked small-caps. No Google Fonts request. No banned display faces.
• Signature verb: row, gather, kindle. WASD or arrows, or hold the pointer. M mutes.
• World: 2,400 × 1,600. Nine islands, nine lanterns, four fog banks, pooled motes / sparks / wakes. DPR cap 2. Half-resolution darkness buffer.
• prefers-reduced-motion stills sway, flicker, fog drift, and camera ease. The game stays playable.
• Best evening in localStorage key merrowcut.best.
• Win copy in source: “For a moment the fen holds still. The water carries every flame twice, and the herons stop to look.” Lose copy: “The last lamp gutters and the fen goes back to itself. The water keeps no grudges. There will be another dusk.” Playwright did not reach either screen.

Exactly one h1. Visible flame focus on buttons. Start and restart are real buttons.

Canvas 2D, not WebGL. No Three.js. No extra network dependency.

What we did not change

The shipped file is the model’s HTML, verbatim. The stream returned a complete document — <!DOCTYPE html> through </html> — with no markdown fence to strip.

No extra CDN. The brief allowed one Google Fonts pair; Opus 5.5 used none.

Honest limits

• One prompt. Beauty is not a score. The table is what we can measure. The link is what you can play.
• This is not Official A. It does not replace a bench score.
• This is OpenRouter, not a Spark serve.
• Playwright captured the title at 5 s, then one click on Push off, then 2.5 s of play. We did not win, lose, or mute in the stills.
• Font family names in CSS are not a network load. The still uses the system serif/sans stack.
• Headless Chromium reported a 1440×900 backing store (DPR 1). The file still caps DPR at 2.

Reproducing

Workspace: ~/workspace/claude-opus-5.5-tests/02-inspiring-html-game/ (prompt.txt, content.md, reasoning.md, meta.json, game.html, runtime-gate.json).

OUT_DIR=... MODEL=anthropic/claude-opus-5.5 python3 stream_openrouter_oneshot.py
python3 extract_html.py game.html
node --check inline.js

Verification notes

Measured 2026-09-22 on OpenRouter from this box:

• Identity: completion model matched anthropic/claude-opus-5.5. Catalog id confirmed before the call.
• Tokens / cost / finish: stream usage object and finish_reason.
• Reasoning: character count from streamed reasoning deltas; usage.completion_tokens_details.reasoning_tokens reported separately.
• HTML size: len(extracted.encode()) on the raw document (no fence).
• JS: node --check on the inline IIFE. Exit 0.
• Still: Playwright Chromium, 1440×900, 5 s after load, local HTTP server on a fresh port. Second still 2.5 s after #start.
• runtime-gate: same session; entry_ok=true; primary action #start Push off; overlay gone after click.

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Sources

[1] https://openrouter.ai/anthropic/claude-opus-5.5 — Claude Opus 5.5 — OpenRouter [2] https://www.smfclearinghouse.com/blog/2026-09-22-claude-opus-5.5-aurora-most-beautiful-html — Claude Opus 5.5 one-shot: AURORA