I've found there usually is some key context that is missing. Maybe it is project structure or a sampling of some key patterns from different parts of the codebase, or key data models. Getting those into CLAUDE.md reduces the need to keep building up (as large) context.

As an example for one project, I realized things were getting better after it started writing integration tests. I wasn't sure if that was the act of writing the test forced it to reason about the they black box way the system would be used, or if there was another factor. Turns out it was just example usage. Extracting out the usage patterns into both the README and CLAUDE.md was itself a simple request, then I got similar performance on new tasks.