contextcost 0.1.0

Measure what a repository costs an AI coding agent to read, find what is wasting that budget, and prove the saving by re-measuring.

contextcost

What does this repository cost an AI coding agent to read, and what is wasting that budget?

Point it at a repository. It measures what reading that repository costs in tokens, works out which files are spending that budget without earning it, and then — the part nobody else does — applies its own proposal and measures the result again, so the saving it reports is a difference between two measurements rather than a sum of its own opinions.

$ contextcost .

contextcost  ~/work/example

  74,318 tokens to read this repository   ±12% estimated, no tokenizer
  12 text files · 1 binary not counted · 3 paths ignored

WHERE IT GOES
  (root)                    41,882  ████████████████████████████  56%
  vendor                    12,704  ████████·····················  17%
  src                        9,331  ██████·······················  13%

WHAT IS NOT WORTH READING
  certain  lockfile         38,905  1 file
             38,905  package-lock.json
                      package-lock.json is written by a package manager
  likely   vendored         12,704  2 files
             7,110  vendor/legacy/helpers.js
                      inside a directory named vendor/

SAVING
  74,318 → 15,033 tokens   80% saved
  Measured by walking the repository again with the proposal applied,
  not by subtracting what was dropped.

  Add to .gitignore (or run with --write-gitignore):
    /package-lock.json
    /vendor/

Install

pip install contextcost

No dependencies. Python 3.9+.

Why this exists

Every coding agent — Claude Code, Cursor, Codex, Copilot Workspace, an in-house one — spends part of its context window just working out what is in your repository. That budget is finite and it is charged per token, and most repositories quietly spend a large fraction of it on files no human and no agent will ever read: lockfiles, minified bundles, vendored dependencies, snapshot fixtures, generated clients.

The first repository this was ever pointed at had 55% of its entire context cost in a single generated CSV.

There are good tools for packing a repository into a prompt — repomix, gitingest, code2prompt, files-to-prompt. This is not one of them. Packing is a solved and crowded problem. Auditing what the packing will cost you, and reducing it with evidence, was not.

What it will not do

Stated up front, because a tool that measures something is only useful if you know where its numbers stop.

It does not use a real tokenizer. An exact count needs tiktoken — a compiled dependency with a wheel per platform. A tool whose pitch is "find out what your repo costs in ten seconds" cannot open with a build toolchain. So it approximates by character class and prints its error bound next to every total.

That bound is measured, not asserted. docs/calibrate.py encodes a corpus with cl100k_base and compares:

	error vs. a real tokenizer
median file	2.6%
95th percentile	10.0%
whole corpus (what a repository total looks like)	7.6%

The bound the tool actually prints is ±12%: the measured 95th percentile plus 20% headroom. The corpus is this repository's own files, so every commit changes it slightly, and a bound sitting exactly on the measurement would turn ordinary editing into a red build — where the tempting fix is to widen the bound, which is how a number stops meaning anything.

Two caveats that belong here rather than in a footnote. It is one tokenizer — Anthropic and most others do not publish theirs, so this is a proxy, and "byte-pair encoders land close to each other" is doing real work in that sentence. And the corpus is this repository's own files plus synthetic dense and CJK samples; it is real code and real prose, but it is not yours.

CJK is counted per script, not as one thing

Charging Chinese at the Latin rate under-counts it roughly threefold, so CJK has always been counted separately. What was wrong until recently is that it was counted as one category, and the scripts are not close to each other:

script	tokens per character
Japanese kana	0.85
Korean hangul	1.10
Chinese, simplified	1.08
Chinese, traditional	1.55

Traditional Chinese costs 44% more per character than simplified for the same sentence, because the tokenizer has far fewer merges for it. A single constant under-counted it by 30% — and traditional is what this project's author writes documentation in, so the first real user would have been the one mis-billed.

Simplified and traditional share a Unicode block, so they are told apart by looking for characters that exist only in the traditional set. Measured on prose: 27% of traditional Han characters trip that detector, and 0% of simplified ones.

For most of this project's life that bound read ±12%, and it had been chosen rather than measured — the comment beside it cited a calibration script that did not exist. When the script was finally written, the true figure was more than four times worse, and fixing the ratios it exposed (source code is 4.14 characters per token, not the 3.15 that had been reasoned out; dense content is bimodal and no single ratio fits it) is what produced the table above. That is recorded in estimate.py rather than quietly corrected, because a tool that argues against unverified numbers should say when it shipped one.

It will not decide the ambiguous cases for you. Findings carry a confidence: certain (the file says what it is, or its name is reserved by the tool that wrote it), likely (a strong path convention), and possible. That last tier — mostly large data files — is never excluded automatically, because a large CSV is waste in a web app and is the entire subject in an analysis repository, and nothing visible from the file system tells those apart. Those are listed separately, with the rule's reasoning, for you to judge. --include-possible moves them in.

It never edits your repository unless you ask. The default output is a proposal. --write-gitignore appends it and tells you exactly what it wrote.

It has no users yet. This is a new tool. The estimator's error bound is measured against a reference tokenizer, and the reduction is measured rather than estimated, but neither of those is the same as having been run against a thousand repositories by people who did not write it.

How the saving is verified

This is the part worth being suspicious of in any tool that claims one, so here is the mechanism in full.

1. Walk the repository, respecting .gitignore. Attribute a cost to every file.
2. Classify what looks wasteful, with quoted evidence per file.
3. Turn the findings into ignore patterns.
4. Walk the repository again with those patterns applied.
5. Compare the files that disappeared against the files that were proposed. Those two sets must be equal. If a pattern took anything extra — a docs/ rule that also caught docs/guide/writing.md, or a PNG sitting beside a minified bundle — the patterns are narrowed to exact paths, the repository is walked a third time, and the report says the narrowing happened.

Step 5 is the one that matters. A saving computed by adding up what a tool decided to drop cannot tell the difference between a pattern that worked and a pattern that matched too much: the number goes up either way.

On an ordinary small project

The example below is generated by docs/build_docs.py: a small web project with some source, a lockfile, a bundle, a vendored widget and a snapshot file. Nobody would call it bloated.

37,603 tokens to read 16 text files (estimated, ±12% — see below for why there is no tokenizer).

file	tokens	rule	confidence
package-lock.json	6,764	lockfile	certain
dist/bundle.min.js	3,582	minified	certain
vendor/legacy/widget.js	1,043	vendored	likely
src/generated/schema.js	924	generated	certain
tests/__snapshots__/app.test.js.snap	850	snapshot	likely

Excluding those leaves 23,788 tokens — a 37% reduction, and that number is the difference between two walks of the repository, not a sum of what was dropped.

Usage

contextcost                       # measure the current directory
contextcost path/to/repo          # measure somewhere else
contextcost --json                # machine-readable, for scripts and CI
contextcost --include-possible    # also act on large data files
contextcost --write-gitignore     # append the proposal to .gitignore
contextcost --no-gitignore        # count files git would hide
contextcost --top 20              # more rows per section

The exit code is 1 when something confidently wasteful was found and 0 when it was not, so this works as a CI check:

- name: Keep the context budget honest
  run: pipx run contextcost --quiet

That exit code will bite you under set -e. "Found something" is not an error, but bash -e cannot tell the difference and will abort your script on it. This tool's own CI failed on exactly that the first time it ran. When you want the output rather than the verdict, say so:

contextcost --json > cost.json || true

As a library

from contextcost.reduce import reduce_repository

result = reduce_repository("path/to/repo")
print(result.before, "->", result.after)   # both measured
print(result.patterns)                     # what to add to .gitignore
print(result.deferred)                     # what it refused to decide

walk_repository, classify and reduce_repository are all usable separately, and every dataclass has as_dict().

Development

python -m pytest -q                      # 101 tests, no configuration needed
python -m ruff check src tests docs
python docs/build_docs.py                # regenerate the figures and README
python docs/build_docs.py --check        # CI fails if they are stale

The figures above are generated from a real run against a generated example repository, and CI fails if the README's numbers drift from what the code actually produces.

Licence

MIT.