It definitely isn't. LLMs often end up stuck in weird corners they just don't get and need someone familiar with the theory of what they're working on to unstick them. If the agent is the same model as the code generator it won't be able to on its own.

I was getting to stuck state with Gemini and to lesser extent with Sonnet 4, but my cases were resolved by Opus. I think it is mostly due to size of the task and if you split it in advance to smaller chunks, all these models has much higher probability to resolve.

Skill issue