What they did
Bailo runs C64AIToolChain, a workspace that gives coding agents a cross-compiler, a VICE emulator they can drive over a socket, filming tools and a set of project rules. The day after Fable 5.2 came out, he gave it a one-line brief in Claude Code: study the project, then make the first level of a high-end C64 run-and-gun game in the style of the late-80s classics, under a new name. The agent wrote a new engine in 6502 assembly with C for startup and the HUD, wrote Python scripts that generate the rock textures, sprites and level layout, profiled each frame by raster line, and built a checker that pauses the emulator and compares the screen to the map. A vision model was also asked to look at frames for defects. Bailo watched the game and stepped in twice.
How it went
About two and a half hours from first message to final build, 37 minutes of it a pause. The result is a full playable level: a 4,096 by 1,280 pixel world scrolling in eight directions, three enemy types, weapon power-ups, checkpoints, lava and a title screen, in under 47 KB. Profiling cut the frame cost from line 253 to about 213 to 233 of 312, with no late frames. Bailo spotted a flicker on the right edge that every automated check missed; the agent then moved its checker, found 14 bad samples in 60 and fixed it with triple buffering. There is no boss or ending yet.
Worth knowing
The agent did not start from zero: the toolchain already held an earlier scroller, its diary and the rules. The write-up was drafted by Claude Opus 5.5 from the session diary for the author's review. A second report of stutter turned out to be the agent's own checker left running with a breakpoint, so run measurements on a separate emulator instance.