Context then is: docs, code and tests. Rather than try to build some omniscient agent, the context is built where the agent needs it.
TDD doesn't help the agent figure out what it is supposed to do now- it is already writing the code and now and it must know what it is supposed to do to write it.
I am trying to use evidence-based approaches. I am only observing better results and tweaking now, but I plan to benchmark when tweaking is done.