One-shot AI port of a 39,000-line C game breaks down

A builder handed three coding agents the same job: take a 2.1MB single-file C shooter, about 600,000 tokens of source, and rewrite it as one HTML file running three.js. No follow-up prompts, no back-and-forth. Just one shot, on a job that blows past the context window of every model involved.

Claude Code on Opus 5 finished fastest, in 21 minutes, and produced 1,759 lines. Qwen3 8x27B, run locally in FP8 on vLLM with the full 262,144-token context on an RTX 6000 Pro, took far longer and produced far less: 949 lines in 4 hours 18 minutes under one harness, 1,056 lines in 1 hour 40 minutes under another. Both Qwen runs got rated bad. Opus did better but still only scored okay, per the full writeup and code on r/LocalLLaMA.

The output sizes tell the real story. A 39,000-line C source file becoming 949 to 1,759 lines of HTML isn't compression, it's the model giving up early or skipping most of the game's logic. None of the three runs produced something close to a full port.

The useful part is what stayed constant across the failures. Two different harnesses, running on completely different infrastructure, hit the same wall. That rules out a harness bug or a prompting quirk specific to one tool. The task itself, a single unbroken instruction to port 600k tokens of legacy C in one pass, is what's too big to hand a model whole.

For anyone doing large migrations with agents, that's the actionable read: this size of job needs to be broken into pieces small enough to verify, not thrown at a model as one instruction and trusted to come back complete.