Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add oborchers/fractional-cto --skill using-python-package-principlesgit clone --depth 1 https://github.com/oborchers/fractional-ctoWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/oborchers/fractional-cto/using-python-package-principles)<a href="https://agentmods.dev/skills/oborchers/fractional-cto/using-python-package-principles"><img src="https://agentmods.dev/badge/skills/oborchers/fractional-cto/using-python-package-principles/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/oborchers/fractional-cto/using-python-package-principles"><img src="https://agentmods.dev/badge/skills/oborchers/fractional-cto/using-python-package-principles.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00068 | $0.01071 |
| Opus 5.5 | $0.00027 | $0.00428 |
| Sonnet 5.5 | $0.00014 | $0.00214 |
| Haiku 4.5 | $0.00007 | $0.00107 |
Grade A, and why
using-python-package-principles scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 3d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 55 lines — stays where its author put it; the contents beside it link to each section on GitHub.
These are not suggestions. They are research-backed standards drawn from FastAPI, Pydantic, httpx, Ruff, uv, Polars, Rich, pytest, Hatch, Flask, attrs, Django, and 30+ PEPs.
How to Access Skills
Use the Skill tool to invoke any skill by name. When invoked, follow the skill's guidance directly.
Python Package Principles
| Skill | Triggers On |
|---|---|
python-package:project-structure |
src/ vs flat layout, directory organization, __init__.py design, _internal/ convention, py.typed marker, monorepo vs single-package |
python-package:pyproject-toml |
pyproject.toml configuration, PEP 621 metadata, build backends (hatchling/setuptools/flit/maturin), PEP 735 dependency groups, PEP 639 SPDX licenses, dynamic versioning, entry points |
python-package:code-quality |
Ruff configuration, mypy strict mode, modern type hints (PEP 695/649), str | None vs Optional, pre-commit hooks, formatting rules |
python-package:testing-strategy |
pytest configuration, test organization, fixtures, parametrize, coverage (80-90%), async testing, Hypothesis, nox/tox, snapshot testing |
python-package:ci-cd |
GitHub Actions workflows, test matrix, trusted publishing (OIDC), release automation, caching, Dependabot/Renovate, SLSA/Sigstore, reusable workflows |
python-package:documentation |
MkDocs Material vs Sphinx, mkdocstrings/Griffe, Diataxis framework, Google-style docstrings, versioned docs, code examples in docs |
python-package:versioning-releases |
SemVer vs CalVer, PEP 440, Keep a Changelog, Towncrier, Conventional Commits, release process, deprecation strategy |
python-package:cli-architecture |
CLI framework selection (Click/Typer/argparse), cli.py vs cli/ directory, __main__.py delegation, exit codes, [project.scripts] entry points, subcommand organization |
python-package:api-design |
Public API surface (__all__), progressive disclosure, exception hierarchy, async/sync dual API, plugin architecture (pluggy/entry points/protocols), dependency injection |
python-package:packaging-distribution |
Pure Python vs compiled extensions, wheel format, platform tags, maturin/scikit-build-core, cibuildwheel, package size, namespace packages |
python-package:security-supply-chain |
Trusted publishing (OIDC), Sigstore/PEP 740, SLSA framework, pip-audit, SECURITY.md, dependency scanning, OpenSSF Scorecard |
python-package:developer-experience |
One-command dev setup, CONTRIBUTING.md, Makefile/justfile, issue/PR templates, code of conduct, governance models, README best practices |
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 3d ago First seen · 55 lines · 68 tokens per session scan A 2d9a709481c7
using-python-package-principles is a skill published in the GitHub repository oborchers/fractional-cto (32 stars, last pushed 3d ago), licensed MIT. It adds 68 tokens to every session and 1,071 once invoked, about $0.0003 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-10-13.
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