Coding agents' success falls from 62.7% to 28.5% as features evolve in stages
FeatureArc tests coding agents on features that grow in stages, each building on the agent's own earlier code. Across 10 agents and 73 tasks of 2 to 10 stages, success fell from 62.7% at the first stage to 28.5% when the whole feature was tested on the final code. Inherited defects appeared in 72.9% of failed later stages, and agents left the earlier error unnoticed about 95% of the time.