Replay solved incidents. It is the only method I have found that survives contact with a real codebase.
Pick three bugs you personally fixed and know the root cause of. Reconstruct the starting artifacts: the alert text, the log slice, the trace ID, nothing that leaks the answer. Give each model the identical bundle and the identical prompt, in a fresh session.
Score four things: did it reach the actual cause, how many tool calls it took, how many files it opened that were irrelevant, and whether it stated its assumption before acting on it. Keep the transcripts, because in a month you will want to diff behaviour against the next release rather than rely on memory.
Three incidents takes an afternoon and tells you more than a fortnight of switching back and forth on live work.