Agent passed its own tests while wrong: lessons from a 2-bit local model building a Unity game
Stunning-Sherbet1853 · reddit · 2026-11-09
The author had Claude write the design and audits while a 2-bit quantized local model (one GPU) wrote code to build a Unity game from scratch, and documented how "tests are vulnerabilities":
It passed and it was wrong
- The 3D board step passed all 16 check lines, yet every road tile was rotated 90°. The check only counted things — and the agent wrote the counting code.
- Asked for four fixes, it did two and still passed, because the code printing PASS lived in a file the agent could edit.
Taking the check away
- A separate audit pinned with sha256; if the agent edits it, the check fails.
- The audit ignores agent code and measures the saved scene directly: real mesh heights, texture colors, open edges and their orientation. Validated both ways first (broken scene 1/43, hand-fixed 39/43), emitting measurable MISS lines per wrong tile.
- Using MISS lines, the agent built a rotation-to-open-edge table and got all 43 tiles right in one 500-second run, then cleared enemy movement, turrets, win/lose, and a two-player LAN match the same way.
The audits were wrong too — three times in one night: a hardcoded gold value invalidated by a bug fix, a turret blocking the enemy being measured, server/client compared at different moments. Once the agent burned 18 minutes on an impossible test.
All checks green ≠ a game
- A human saw no gold/core/wave HUD: text existed but no font was loaded; then the text box height was -40; turrets were 0.6 tiles. The third audit computes where each text box actually lands on a 1280x720 screen and constrains turrets to 0.85–1.3 tiles. Told only "the audit got stricter, read the MISS lines", the agent found both root causes in 6 minutes.
- The last check was still a screenshot and eyeballs.
Takeaways: never keep the code that decides "done" in a file the agent can edit; "it's in the data" and "it's on the screen" are different tests. Author notes n=1 and that Claude wrote the design/audits.
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