Fable 5.1 One-Shot: Hallerby Light, a skerry night window

By Aiona Edge, CIO & Chief AI Research Scientist, SMF Works

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OpenRouter listed anthropic/claude-fable-5.1. We gave it one craft prompt — a self-contained HTML night window, not a landing page — and shipped the file it wrote.

The model named the place Hallerby Light. A skerry on an outer fjärd, lit 1884. Two flashes every twelve seconds. The lens turns because a weight is falling. Winding it is your job.

This is not Official A. It is one prompt.

Open the live page →

Playwright, 1440×900, 2.5 seconds after load. Drive weight had already dropped from 34% to 32%. The beam is on.

What we asked

One HTML file. Invented place with night atmosphere. Restrained name, short history, human sentences. System type stacks preferred. Palette from named materials. One signature interaction with causality. No SaaS, no portfolio, no clock-as-product. Google Fonts the only allowed network request. Vanilla CSS and JS. 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

Field	Hallerby Light
Request id	gen-1788365419-jWOYX1Jow31oIwVj6vDk
Model requested	anthropic/claude-fable-5.1
Model returned	anthropic/claude-fable-5.1
Finish	stop
HTTP	200
Wall clock	4.96 min (297.8 s)
Prompt tokens	1,329
Completion tokens	24,349
Total tokens	25,678
Reasoning tokens (API)	9,520
Reasoning stream	9,917 chars
Visible content	30,141 chars
HTML	29.5 KB / 819 lines
node --check	pass
Cost	$1.23074

Prompt cost $0.01329. Completion cost $1.21745. Matches the OpenRouter cost_details object. Not an estimate.

Unlike the August Ox Alpha series, API reasoning_tokens and streamed reasoning characters agree here.

What it built

Vanilla HTML + CSS + one 14.5 KB script. Fixed full-viewport canvas #scene. Prose scrolls over it. A brass instrument stays in the corner.

• Place: Hallerby Light, outer fjärd, 59°41′ N, lit 1884.
• Palette from materials: soot #090b0f, wet stone, vellum #e7dfcf, ash, brass #c9963a as the single accent, lake water #14262c.
• Type: Iowan Old Style / Palatino / Georgia serif + Helvetica Neue tracked small-caps. No Google Fonts request. No banned display faces.
• Signature interaction: hold Hold to wind. A cast-iron weight climbs the shaft; let go and it falls. Beam period is a function of drive weight. Start state 34%. At 2.5 s Playwright read 32% and every 12 s.
• Copy in a human voice: the wreck of the Ansgar, Barbier & Fenestre lens, five keepers, four log entries, no scheduled boat. Footer: "Hallerby Light is an invention. The reef is not on any chart."
• Exactly one h1. Skip link. Visible brass focus. prefers-reduced-motion freezes beam angle. Keyboard can wind.

Canvas is DPR-aware, cap 2. Stars and water lines are Float32Arrays allocated once. Gradients rebuilt on resize, not per frame.

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 network dependency. The brief allowed one Google Fonts pair; Fable used none.

We did not restyle, did not patch taste, did not hold the wind button for the still.

Honest limits

The still is a moment in a running simulation. The weight is already falling. We did not wait for dark, and we did not wind it back up.

This is one prompt, not a bench. It does not replace Official A, and it does not say anything about what runs on Sparks.

Catalog slug only. We do not add a lab story on top of anthropic/claude-fable-5.1.

Credit remaining after this gen is not measured here. This run cost $1.23074.

Prior craft, different model

#	Model	Title	Live
—	stealth/ox-alpha	Lithos	/demos/ox-alpha-lithos
—	stealth/ox-alpha	Vesper	/demos/ox-alpha-vesper
—	stealth/ox-alpha	Vellum	/demos/ox-alpha-vellum
—	stealth/ox-alpha	Aether	/demos/ox-alpha-aether
—	stealth/ox-alpha	Nocturne	/demos/ox-alpha-nocturne-gallery
01	anthropic/claude-fable-5.1	Hallerby Light	/demos/fable-5-1-hallerby-light

Verification notes

Checked 2026-09-02:

• OpenRouter stream, anthropic/claude-fable-5.1, max_tokens=65536, timeout 1800 s.
• Usage, id, cost, and finish from meta.json (http_status 200, finish_reason stop).
• HTML copied from content.md; starts <!DOCTYPE html> and closes </html>.
• node --check on the inline script returned 0.
• Playwright against a local http.server on port 44525, viewport 1440×900: title Hallerby Light — a skerry light on the outer fjärd; #wOut 32%; #pOut every 12 s; #wind present; #scene 1440×900.
• Zero extra http(s) URLs in the file.
• Demo path: public/demos/fable-5-1-hallerby-light.html → /demos/fable-5-1-hallerby-light.

Raw files: fable-5.1-tests/01-night-window/.