For AI agents: llms.txt is a short guide that sends you to the rules your problem needs, and rules.json holds every rule of this playbook as JSON.

Cost

Open source. MIT licence, and the Python Package Index is free to use.

How to do it

pip passes all five tests. Its first release was in October 2008, and 26.2.2 shipped on 20 October 2026. It had 105 million downloads in the week to 20 October 2026. In the Stack Overflow Developer Survey 2026, 42.1% of developers used it. It is MIT licensed. The venv module ships with Python, so nothing else needs installing. Choose something else when installs are slow or you want a lock file built in, where uv is fast and popular. uv fails the five-year test, since its first release was in February 2024 and it is still version 0.13. Record that choice in a decision record. Pin every version, as Make builds repeatable says.

Examples

Using these examples: they show one way to follow this rule. Keep the behaviour and the check, not the syntax. Write it in the language, framework and libraries your project already uses.

Shell example

python3.14 -m venv .venv
.venv/bin/pip install django
.venv/bin/pip freeze > requirements.txt
.venv/bin/pip install -r requirements.txt

Python example

import re
from pathlib import Path

PINNED = re.compile(r"^[A-Za-z0-9_.\-\[\]]+==\S+$")

def unpinned(requirements: Path) -> list[str]:
    lines = [line.strip() for line in requirements.read_text().splitlines()]
    return [line for line in lines if line and not line.startswith("#") and not PINNED.match(line)]

How to check

The project has a .venv folder that is not committed. requirements.txt pins every package with ==. Every build runs pip install -r requirements.txt inside the virtual environment.

Sources

• pip documentation
• venv

Related rules

• Make builds repeatable in Engineering