I suspect the big jump came from the release of Claude Opus 4.5/4.6 and GPT-5.x-Codex between Nov ‘25 and Feb ‘26, which were trained with heavy reinforcement learning on long coding projects, rewarding only real success (like running code, using terminals, self-fixing bugs, and passing tests) while adding better memory for huge codebases and extra coding-specific training.

Nothing drastic I'd say. It's a continuous stream of small improvements just accumulating with each release, and someone just noticed a few releases away from a previous publicized-bad-capabilities release that there's major improvement between those points. So it looks like something major only due to the spacing between the capability surveys on the release timeline.