How to estimate if your codebase fits an AI model's context window before pasting
Developers feeding entire codebases into AI models often hit context window limits, causing truncation errors or silent data loss where the model answers from incomplete information. A practical workaround involves estimating token count offline using a formula that blends character count and word/symbol runs, achieving roughly 5–10% accuracy compared to real tokenizers. Since context windows differ significantly across models — 200K for Claude, 400K for GPT-5, and 1M for GPT-4.1 and Gemini 2.5 Pro — developers should budget their code bundle against the specific model being used. When a repository is too large, the recommended approach is to omit the largest file bodies first while keeping all filenames listed, so the model retains a full project map. An open-source CLI tool called ctxpack automates this trimming process and is available free under the MIT license on GitHub.
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