I think this is where tools like OpenSpec [1] may help. The deterioration in quality is because the context is degrading, often due to incomplete or amibiguous requests from the coder. With a more disciplined way of creating and persisting locally the specs for the work, especially if the agent got involved in creating that too, you'll have a much better chance of keeping the agent focussed and aligned.

[1] - https://openspec.dev/