The Model Was Fine. My Token Assumptions Weren't.

The model was never the problem, and that is exactly why the bug took three days to find. My ticket-classification service started returning the fallback label for long, non-English messages shortly after I moved the inference path to a cheaper endpoint, and every instinct pointed at the new model. The real culprit was a token-counting mismatch that silently truncated the prompt before the model ever saw the classification instruction. The Symptom The failure was remarkably consistent, which made it even more misleading. Messages under roughly two thousand characters classified correctly, while longer ones, especially in German and Japanese, fell through to a generic "other" bucket with a perfectly valid JSON response. The parser was not the issue, the prompt had not changed in weeks, and the retry logic never fired because the endpoint returned a normal 200 status. My first assumption was that the cheaper model was simply weaker at long-context reasoning, so I ran a controlled comparison using the same fifty tickets against the previous endpoint. The old path classified all fifty correctly, the new one failed on nineteen, and that result seemed to confirm the model-quality theory. What bothered me was the distribution: the failures clustered exactly where the input length crossed a threshold, and no ticket under that threshold ever failed. The Reproduction To isolate the variable, I needed a clean environment where I could swap endpoints without touching the production deployment, and MonkeyCode's free server option turned out to be a practical debugging tool. The project is open source, and its free model access let me replay the failing tickets without spending my own quota, so I spun up a disposable instance and pointed the same harness at the same prompt. Disclosure: This article was prepared as part of MonkeyCode's product outreach. The reproduction took about twenty minutes, and the result was identical on every retry: long inputs failed, short inputs passed.