A Four-Model Build-Off: Carbon Finished Last in Two of Three Tests — at 1/30th of Fable's Bill

AI for Mortals (Pat Simmons) put Gemini 4 Carbon up against Claude Fable 5.2, Claude Opus 5.5 and GPT-6 Astra on the same three briefs: a pixel-perfect website clone, a product launch film, and a 1940s dogfight flight simulator. Every model got the exact same prompt, ran in its own lab's tool on high effort, and each build was run once; Opus and Astra ran on November 6, Fable and Carbon on November 10–11. The author's caveat up front: these are big builds, not a proxy for everyday work.

Test 1: a pixel-perfect clone of the Agrumea site

Rebuild agrumeafarm.it pixel for pixel — physics loader, 3D jar carousel and page transitions, every asset made from scratch, run through a published clone methodology with a style-assertion gate. The ranking: Fable 5.2 first ($594.80, 533 minutes), Opus 5.5 second ($60.14, 141 minutes), GPT-6 Astra third ($90.63, 68 minutes), Carbon fourth ($10.86, 99 minutes).

Test 2: a launch film for Motion

A 30–45 second product launch film at 1920×1080, 30 fps, that has to be clearly better than Motion's own launch film — interface, pacing and story all improved. The ranking: Opus 5.5 first ($14.23, 48 minutes), Fable 5.2 second ($97.62, 248 minutes), GPT-6 Astra third ($14.55, 34 minutes), Carbon fourth ($6.82, 68 minutes).

Test 3: a 1940s dogfight flight simulator

A browser-playable air-combat simulator: a realistic 20×20 km coastline built in Blender, two historically accurate 1940s propeller fighters, a real flight model (stalls, energy management, G-force blackout), ballistics, per-part damage and enemy AI that actually dogfights. The ranking: Opus 5.5 first ($106.92, 376 minutes), Fable 5.2 second ($190.88, 243 minutes), Carbon third ($10.00, 102 minutes), GPT-6 Astra fourth ($21.25, 79 minutes). The only test where Carbon did not finish last.

The bill: all three builds, Carbon at 1/30th of Fable

Total cost across the three builds, priced at each model's published API rates: Carbon $27.68 (269 minutes), Astra $126.43 (181 minutes), Opus 5.5 $181.29 (564 minutes), Fable 5.2 $883.30 (1,024 minutes). Per test, Carbon ran 10–50 times cheaper than Fable. The author's reservations: part of Fable's flight-sim log was lost when a disk filled up, so its tokens and cost may be undercounted; and once Carbon's introductory price ends on December 1, it will cost the same per token as Opus 5.5, so much of the savings will shrink.