Claude built a browser football game, then self-played thousands of games to tune it

MDawg74 · reddit · 2026-07-21

A developer says Claude built a browser football game and then tuned it by playing thousands of self-play matches against NFL baselines.

The game, CHALK, is a free browser football game where the user acts as the coordinator and draws routes while the AI runs all 22 players.
Claude wrote the entire game in one HTML file, without an engine or framework.
The interesting part is the testing loop: Claude wrote a Playwright harness, ran thousands of headless plays, compared the stats to NFL averages, and traced bad numbers back to specific bugs.
After fixing issues such as an offensive line blocking bug, the metrics moved close to NFL ranges, including sack rate, interception rate, and fumbles per drive.
The author also used the AI-vs-AI demo itself as a regression test to catch freeze bugs.

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