Context window
Everything the model sees on each model provider request. Finite, model-specific, and the only surface through which the model perceives anything.
It’s a single sequence of tokens: the system prompt, the conversation so far, every tool result the harness has fed back in. If something is in that sequence, the model can use it; if it isn’t, the model doesn’t know it exists, not your codebase, not the file you edited yesterday, not the instruction you gave three sessions ago. Anything outside the window has to be brought in, usually via a tool call, before it can affect anything.
Finite means it fills up. Every turn appends more, your messages, the model’s responses, tool results, and a long session will eventually hit the limit, forcing compaction or clearing. It also means everything in the window competes: each token you load is one less available for the rest, and content you didn’t need still occupies the model’s attention. The practical stance is to treat the window as a budget, load what the task needs, leave the rest out.
Avoid: “memory”, the context window is working state and doesn’t persist across sessions. Memory is a separate concept layered on top.
Usage:
“Can I just paste the whole monorepo into the prompt?”
“The context window’s 200k tokens, that’s maybe a fifth of the repo. Pick the files the task touches, leave the rest behind a tool call.”
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