A context window is the maximum amount of text, measured in tokens, that a large language model can process in a single request. This includes both the input (the prompt, code, documents provided) and the output (the model’s response).
Context window size directly impacts what a coding agent can do. A small context window means the agent can only see a few files at a time. A large context window (100K+ tokens) allows the agent to ingest entire codebases, understand cross-file dependencies, and make coordinated changes across dozens of files in a single pass.
As of 2026, context windows range from 8K tokens (older models) to 1M+ tokens (Claude, Gemini). The practical utility depends not just on size but on how well the model retrieves and reasons over information within that window.