Will your codebase fit in the context window? How to measure it (and trim to fit)

"Just paste the repo into the model" runs into a hard wall: the context window. Paste too much and you get a truncation error, or — worse — the model silently drops the earliest files and answers from a partial picture. The fix is to treat "will it fit?" as a number you compute before you paste.

Step 1: estimate tokens without calling an API

You don't need a network round-trip to get a usable estimate. For source code, a blend of two signals is within ~5–10% of real BPE tokenizers:

• Characters ÷ ~3.6 — code tokenizes denser than prose (more punctuation and identifiers).
• Count of word/symbol runs × ~1.15 — a second signal that corrects the char estimate on symbol-heavy files.

Average the two and you have a fast, offline token estimate. Good enough to answer "does it fit?"

Step 2: check it against the model you're targeting

Context windows vary a lot, so budget against the specific model:

Report the bundle as a percentage of the target window — "48K tokens = 24% of 200K" tells you at a glance whether you have room left for the actual conversation.

Step 3: if it's over budget, trim by importance — not at random

When a repo is too big, the naive move (truncate the end) throws away whoever files happen to be last. Better: omit the largest file bodies first, but keep every file listed. The model still sees the full project map (so it knows payments/refund.ts exists) even if that file's body didn't make the cut.

With ctxpack this is one flag:

npx github:trongtruong110-ux/ctxpack . --fit 60000 -o context.md

ctxpack: 220 files packed
  tokens: ~59,400
  trimmed: 34 file(s) omitted to fit 60,000 tokens

Every file is still named in the index; only the biggest bodies are dropped to hit the budget.

The habit

1. Estimate before you paste — "does it fit?" is answerable up front.
2. Budget per model — a bundle that fits Gemini may blow Claude's window.
3. Trim by size, keep the map — a partial bundle that still lists every file beats a truncated one that hides what's missing.

ctxpack is MIT-licensed and free: https://github.com/trongtruong110-ux/ctxpack. How do you currently decide what to include when a repo is too big for one prompt?