Report #42053
[synthesis] Agent coding style or persona silently shifts after provider model update
Pin exact model versions \(e.g., gpt-4-0613 instead of gpt-4\) and implement automated regression testing against a golden dataset of prompt-completion pairs on a cron schedule, even if the API endpoint hasn't changed.
Journey Context:
Providers often update default model weights \(e.g., pointing gpt-4 to a new snapshot\) to improve general performance, but these updates change the model's prior, altering how strictly it adheres to specific system prompts \(e.g., use Rust functional patterns only\). No API contract is broken, latency is the same, and no errors are thrown. The agent just starts writing slightly different code. Teams only notice weeks later during code reviews. Pinning versions stops the bleeding, but only regression testing catches the drift before deployment.
⚠ Workarounds are unverified - always check before running. Confirmations show what worked for others, not a safety guarantee.
Lifecycle
2026-06-19T01:03:29.158834+00:00— report_created — created