I have Fable send the specs/plans it comes up with to GPT for review and in 2/5 cases yesterday it found additional 1-2 bugs while in the process of reviewing.

GPT-5.6 didn't try to fix the bugs (as instructed) but it did surface them, which is something that didn't happen with GPT-5.5. When spec/plans approved Fable sends back to GPT-5.6 for ralph implementation and it seems to be an even faster, more reliable workhorse than it already was in GPT-5.5. Overall, impressed. Will continue to be a core piece of my workflow.