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Official agent skill

Fastmod

by pydantic in pydantic/monty

Use fastmod to make mass code updates to avoid many repetitive changes.

OfficialMITAuto-check passedDevelopment

Install Fastmod

skills CLI
$ npx skills add pydantic/monty --skill fastmod -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI
$ gh skill install pydantic/monty fastmod --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy
$ git clone --depth 1 https://github.com/pydantic/monty.git skills-src && mkdir -p .claude/skills && cp -r skills-src/.agents/skills/fastmod .claude/skills/fastmod && rm -rf skills-src

Use ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.

Claude Code skills documentation · loads skills from .claude/skills/

Facts

Skill name
fastmod
GitHub stars
8.6k
Token cost
~176 tokens
SKILL.md length
72 words
Files
1
Skills in repo
9
Repo updated
First seen
Licence
MIT

At a glance

Use fastmod to make mass code updates to avoid many repetitive changes.

  • Development work in your project
  • SKILL.md covers Instructions and Examples
  • Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md

What it does

Fastmod is an agent skill from pydantic/monty, published by the product's own GitHub organization. Use fastmod to make mass code updates to avoid many repetitive changes.

Its SKILL.md is about 180 tokens, which your agent loads only when the skill is triggered. It is a single SKILL.md file with no bundled scripts.

It sits in Development. The repository describes itself as: A minimal, secure Python interpreter written in Rust for use by AI. The licence is MIT.

When your agent uses it

  • Development work in your project

Example prompts

  • “/fastmod”

What it can do on your machine

Read from SKILL.md and the folder at commit 5915273. It shows what the files ask for, not the result of running them.

  • Tool permissions

    Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

    From allowed-tools in the SKILL.md frontmatter.

  • Runs code

    No scripts in the folder and no shell commands in SKILL.md (its code samples are bash).

    From the folder's file list and the shell code blocks in SKILL.md.

  • Network

    No URLs in SKILL.md.

    From URLs in SKILL.md, links to its own repository left out.

  • Credentials

    Names no API keys, tokens, secrets or passwords.

    From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Fastmod loads about 176 tokens when it runs. Until then it costs about 20 tokens; SKILL.md has 72 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it
~20
When it runs · the whole SKILL.md, loaded when a task matches
~176

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Auto-check passed

The automated check found no risky patterns in SKILL.md.

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from pydantic/monty at commit 5915273, republished under its MIT licence (© pydantic). 72 words, ~176 tokens.

Download SKILL.mdSave it as .claude/skills/fastmod/SKILL.md (or your agent's skills folder).
name
fastmod
description
Use fastmod to make mass code updates to avoid many repetitive changes.

fastmod

Instructions

You can occasionally use fastmod or sed to make mass updates to the codebase and avoid wasting tokens changing each case one at a time.

Before making many repetitive changes to the codebase, consider using fastmod --accept-all.

THINK HARD about how best to use fastmod as it can dramatically improve your productivity.

Examples

Example of switching the py_type function to take heap: &Heap instead of a generic H: HeapAccess parameter:

bash
fastmod --accept-all 'fn py_type<H: HeapAccess>\((.+?), heap: &H\b' 'fn py_type($1, heap: &Heap'

© pydantic, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

Just SKILL.md in .agents/skills/fastmod of pydantic/monty.

Open the folder on GitHubat commit 5915273

Compare with similar skills

Fastmod next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.

Fastmod compared with similar skills
SkillStarsUsed inTokensAuto-checkLicenceRepo updated
Fastmod this skillpydantic/monty8.6k—~176Automated safety check: PassMIT
Vercel Composition Patternssupabase/supabase111k58 repos~726Automated safety check: PassMIT
Finishing a Development Branchobra/superpowers297k5 repos~1.9kAutomated safety check: PassMIT
Typescript Advanced Typesrolling-scopes/rsschool-app10k25 repos~4.2kAutomated safety check: PassMPL-2.0
PR Babysitteropeninterpreter/openinterpreter69k3 repos~4.2kAutomated safety check: PassApache-2.0
Code Review ChecklistshareAI-lab/learn-claude-code78k5 repos~1.1kAutomated safety check: PassMIT

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Categories

Questions about Fastmod

What does Fastmod do?

Use fastmod to make mass code updates to avoid many repetitive changes. Fastmod is an agent skill from pydantic/monty, published by the product's own GitHub organization. Use fastmod to make mass code updates to avoid many repetitive changes.

When should I use Fastmod?

Fastmod fits situations like: development work in your project.

How do I install Fastmod in Claude Code?

Run `npx skills add pydantic/monty --skill fastmod -a claude-code`. Or copy the skill folder (.agents/skills/fastmod in pydantic/monty) into .claude/skills/fastmod in your project. Claude Code loads it when a task matches its description.

How do I install Fastmod in Codex?

Run `npx skills add pydantic/monty --skill fastmod -a codex`. Or copy the skill folder (.agents/skills/fastmod in pydantic/monty) into .agents/skills/fastmod in your project. Codex loads it when a task matches its description.

Can I use Fastmod in Cursor, Gemini CLI or GitHub Copilot?

Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add pydantic/monty --skill fastmod -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/fastmod, .gemini/skills/fastmod, .github/skills/fastmod and .opencode/skills/fastmod in your project.

What does Fastmod need to run?

SKILL.md names no scripts, command-line tools or credentials: Fastmod is instructions for the agent only.

Does Fastmod access the network?

SKILL.md contains no URLs. Any network use would come from the scripts or tools the agent runs. This is read from the text; nothing was executed.

Is Fastmod safe to install?

Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.

What licence does Fastmod use?

Fastmod is published under the MIT licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does Fastmod use?

About 176 tokens (SKILL.md is roughly 704 characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full.

What are the alternatives to Fastmod?

Skills that share tags, products or a category with Fastmod: Vercel Composition Patterns (supabase/supabase, 111k stars), Finishing a Development Branch (obra/superpowers, 297k stars), Typescript Advanced Types (rolling-scopes/rsschool-app, 10k stars) and PR Babysitter (openinterpreter/openinterpreter, 69k stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains Fastmod?

pydantic (a GitHub organization, an official publisher) maintains it in pydantic/monty, which has 8,607 GitHub stars. The repository holds 9 skills in this directory. The repository was last updated on October 9, 2026.

Source: pydantic/monty on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.