The biggest problem with Gemini is that its performance degrades the longer you use it for coding. Is it just me?

Filling the large context does that yes.

But a good agents.md, starting from a clean slate, and specifying which key files to look into and follow the standards allows me to build gigantic projects even I struggle to keep in my head structurally.

thats not just the context growing issue, hallucinations is a thing

Seems maybe you’re keeping a forever-session and multiple independent tasks end up overstaying in context?

I would say either start new sessions for new tasks or limit the context to something smaller than 1M.

I usually start with research/planning session, this goes into a detailed implementation plan and then a new session for the actual implementation.

If it's complex problem maybe a review/adversarial step between plan and implementation.

Also with forever-session any time you take a longer break (depends on model and provider as to how long) you will push an entire big context again without caching even if you don't need it. With 1M context this gets expensive.