Yes this “premature termination”, becomes particularly evident when you switch out Opus/Sonnet with a weaker LLM, and also happens more often in Codex CLI with GPT-5.

Since one of the replies asked for an example: the agent works for a bit and just stops. We’ve all seen cases where the agent simply says “ok, let me read the blah.py to understand the context better”, and just stops. It has essentially forgotten to use a tool for its next edit or read etc.