Claude Code ships with a 1M token context window for Opus. You can turn it off:
CLAUDE_CODE_DISABLE_1M_CONTEXT=1I think you should. We don't need 1M tokens.
The smart zone barely grew when the window went from 200k to 1M. Still around 100k. Past that, things get lost in the middle. Chroma confirmed it still holds across Claude 4, GPT-4.1, and Gemini 2.5. A bigger window isn't a smarter model. You just get more room to bury the useful bit.
A million tokens every turn is also a great way to burn through your usage limit fast.
Even if Anthropic fixes both tomorrow, I'd still disable it.
This can be difficult to understand at first, because it feels like you're having a conversation with the model. But you're not. It receives the full history every turn and predicts the next token. Every tool call, every stack trace, every dead end you debugged. All of it, every turn. Not just your new message.
A smaller ceiling can act as a signal. When you hit it, you know your context has grown too big. Compact. Develop the discipline to split up your work and be more deliberate about what's in the window.
Constraint can be wonderful things.