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How to use the Port a Mod Faithfully agent skill

Updated · written with AI from the project's own documentation and checked against our catalogue data

Port a Mod Faithfully is an agent skill that helps an AI coding agent move an existing mod to a newer game version or a different mod loader while keeping its values, behaviour and quirks. It is aimed at modders reviving old Minecraft Forge mods, developers porting Skyrim LE plugins to SE/AE, and maintainers continuing a long port another agent started. After this guide you will know what the skill changes about how the agent works, how to install it, how to trigger it, and what it needs and touches on your machine.

Port a Mod Faithfullyrehan-remade/universal-modder · MIT

Quick start

  1. 1Download and unzip the skill. Download the ZIP from this page and unzip it into ~/.claude/skills/ or a project's .claude/skills/ folder.
  2. 2Or upload it in the Claude apps. In the Claude apps, go to Settings → Capabilities → Skills and upload the ZIP.
  3. 3Pin the original release. Give the agent the exact jar or source, its hash, where it came from, and its changelog so the reference is fixed.
  4. 4Check the license. Have the agent find the license and record it with a link in the status document, then decide whether publishing is allowed.
  5. 5Build a private reference. Decompile the jar with Vineflower, fall back to CFR where needed, and remap names to readable ones.
  6. 6Port unit by unit. Work through the subsystems in dependency order, with tests that catch drift and a status entry after each unit.
  7. 7Audit against the original. Compare the port with the original, including real-client checks, before calling a unit done.

What the skill makes the agent do

An agent without this skill tends to treat an old mod as a loose idea to rewrite. With the skill, the original release is the exact specification. The agent pins one source release, builds a private reference from it, maps what the original contains, extracts data with scripts rather than retyping it, ports unit by unit with tests, and checks the result against the original.

The skill names the main failure as drift: values recalled from memory, mechanics borrowed from a later rewrite, quiet improvements, and renderers that look roughly right. It also asks the agent to check the original's license before anything is published.

  • Pin one original release and treat it as the only gameplay source.
  • Build a private decompiled reference and keep it out of public repositories.
  • Map every subsystem into keep, adapt, replace, remove or missing.
  • Extract registration data with scripts and re-run them instead of hand-editing.
  • Port in dependency order, with tests and real-client checks.
  • Keep a status document so a later session or agent can resume the work.

Install the port-a-mod skill

You can install the skill in one of two ways. For Claude Code, download the ZIP from this page and unzip it into ~/.claude/skills/ to make it available everywhere, or into a project's .claude/skills/ folder to limit it to that project. In the Claude apps, you can instead upload the ZIP under Settings → Capabilities → Skills.

After installing, the folder should contain SKILL.md and the references/ folder, which holds references/minecraft-1-7-10-forge-to-1-20-fabric.md.

  • Claude Code, all projects: unzip into ~/.claude/skills/.
  • Claude Code, one project: unzip into the project's .claude/skills/.
  • Claude apps: upload the ZIP under Settings → Capabilities → Skills.
~/.claude/skills/

How to trigger the skill with natural requests

You do not need a special command. The skill is meant to be used when you ask to update, port, revive or re-implement a mod that already exists, or to continue such a port. Name the mod, the release you have, and the target, so the agent can pin the right source.

Good requests state the source version and the destination. A request to continue a port should say where the existing work lives so the agent reads the status document first.

  • "Port my old Forge 1.7.10 mod to Fabric 1.20.1. The jar is in ./original and it's MIT licensed."
  • "Revive this tModLoader 1.3 mod for 1.4 and keep all the balance values."
  • "Continue the Skyrim LE to SE port of this plugin; another agent started it last week."

Examples of a port the skill handles

The skill's own worked example is a Thaumcraft 4.2.3.5 port from Minecraft 1.7.10 Forge to 1.20.1 Fabric. It describes 859 original classes becoming about 77,000 lines of port code, covered by 323 GameTests and scripted real-client scenarios, across many sessions and two agent accounts. Your project will be smaller or larger, but the same steps apply.

Other cases named in the skill are a Skyrim LE plugin moving to SE/AE and a tModLoader 1.3 mod moving to 1.4.

  • Forge 1.7.10 mod to modern Fabric or NeoForge.
  • Skyrim LE plugin to SE or AE.
  • tModLoader 1.3 mod to 1.4.
  • Splitting a long port into units, each with its own tests and a status entry.

What the skill needs and the safety notes

You need the original mod jar or source for the release you are porting. The skill refers to decompiler tools such as Vineflower and CFR, and to MCP mapping CSVs for Minecraft Forge mods. It also mentions javap and Loom. You need a target development environment for the port.

The skill runs scripts and uses the network. Its process includes running a tokenizer script over decompiled code and re-running it, and it asks the agent to fetch and read sources and mapping files. Review what the agent runs before you let it proceed, and keep the decompile out of any public repository, as the skill requires.

  • Requires: the original jar or source, a decompiler such as Vineflower or CFR, MCP mapping CSVs for Forge mods, and a target environment.
  • Runs scripts: yes. Uses network: yes.
  • Keep the decompile private: it never ships and is never committed to a public repo.
  • Check the original's license before publishing any part of the port.

Tips for a good result

Tell the agent the license before it starts. The skill says that for closed-source or all-rights-reserved mods, a private port stays private unless the author gives permission. Ask the author and keep their answer in the status document.

Do not use this skill to add new features to a mod you already maintain, or to mix content from one game into another. The skill is built to preserve behaviour rather than improve it, and mashups belong to a different skill.

  • State the license and the pinned release in your first message.
  • Keep the status document in the project so new sessions can resume.
  • Write the project rule in AGENTS.md that the original release is the only gameplay source.
  • Ask for audits against the original, not only for passing builds.

What people use it for

  • Reviving an old Minecraft Forge 1.7.10 mod on modern Fabric or NeoForge while keeping its values.
  • Porting a Skyrim LE plugin to Skyrim SE or AE.
  • Moving a tModLoader 1.3 mod to tModLoader 1.4.
  • Continuing a multi-session port that another agent started, using the status document.
  • Keeping a private port of a closed-source mod for your own pack, after the author's answer is recorded.

Frequently asked questions

How do I install the port-a-mod agent skill?

Download the ZIP from this page and unzip it into ~/.claude/skills/ for all projects, or into a project's .claude/skills/ folder for one project. In the Claude apps, upload the ZIP under Settings → Capabilities → Skills.

What does the port-a-mod skill require?

You need the original mod jar or source for the release you are porting, decompiler tools such as Vineflower or CFR, and MCP mapping CSVs for Minecraft Forge mods. You also need a target development environment for the port.

Is the port-a-mod skill free to use?

The skill is listed under the MIT licence. The licence terms are on this page; this guide does not state any price.

Does the skill run scripts or use the network?

Yes on both counts. The skill runs scripts, such as a tokenizer that extracts registration data, and it uses the network. Review what the agent runs before you approve it.

Which mods can I port with this skill?

The skill names old Forge 1.7.10 or 1.12 Minecraft mods moving to Fabric or NeoForge, Skyrim LE plugins moving to SE/AE, and tModLoader 1.3 mods moving to 1.4. The skill also says to check the license before publishing any port.

How is this different from simply asking an agent to update my mod?

This skill makes the agent treat the original release as the exact specification, extract data with scripts, and check the port against the original with tests. It is built to preserve behaviour rather than add improvements, and it is not for mixing content from another game, which belongs to the mashup-mods skill.

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Our verdict

Use this skill if you are reviving or porting an existing mod and want the agent to keep its values and quirks rather than improve them, or if you are continuing a long port across sessions. Pick something else if you want new features added to a mod you already maintain, or if you are mixing content between games. Check the original mod's license first, because it decides whether the port can be published.

Download the skill