Here is the uncomfortable thing I have landed on after a year of watching coding agents succeed and fail on real work: the model is almost never the bottleneck. Claude Code and Codex are both more than capable of the feature you are asking for. What breaks the run is that the agent cannot see the truth it is supposed to build against. The story. The acceptance criteria. The architecture decision it is meant to respect. The test that already exists for the thing it is about to rewrite.
So it guesses. The guess is locally reasonable and globally wrong, and you spend the afternoon unwinding it. The instinct is to reach for a smarter model. The fix is to give the model your backlog.
Why pasting context stops working
Most of us feed an agent context by pasting it. You paste the ticket, a few file paths, maybe a paragraph of background, and you let it run. This works for a self-contained task and falls apart the moment the work touches the rest of the system.
The reason is simple. Pasted context is a snapshot, and snapshots go stale inside the same session. The agent makes a change on step three that invalidates the assumption you pasted on step one, but the pasted text does not update, so by step seven it is reasoning about a version of the project that no longer exists. You are not giving it context. You are giving it a photograph of context and asking it to navigate a moving room.
