Introduction
Jarrow is an open-source web service and command-line tool that will convert a Minecraft mod JAR built for Fabric into a JAR for NeoForge, and prove the result with a behaviour harness.
The goal in one sentence: drop a Minecraft mod JAR on a web page; get the same mod back for another loader, with a proof that it behaves the same.
Why convert the JAR
Section titled “Why convert the JAR”Today a Fabric mod reaches NeoForge players in one of two ways: its author ports it, or players install a runtime compatibility layer such as Sinytra Connector with Forgified Fabric API. Jarrow takes a third route. It will rewrite the JAR itself, ahead of time, into a mod that NeoForge loads natively, and then test that the result behaves like the original.
How it will work
Section titled “How it will work”- Classify. Jarrow reads the mod’s manifest and bytecode and puts it in a tier: A, B or unsupported, with the reasons.
- Convert. Tier A mods convert in your browser (the engine is Rust compiled to WebAssembly), so the JAR never leaves your machine. Tier B mods need a deeper conversion that runs as a short-lived GitHub Actions job.
- Prove. A parity harness starts the original mod on a Fabric server and the converted mod on a NeoForge server, dumps what each one registers (blocks, items, recipes, tags, loot tables and more) and compares the two. Only that comparison can mark a conversion green.
The first version covers one direction, Fabric to NeoForge, for Minecraft 1.21.1 and 1.21.11.
Built by an AI agent, in the open
Section titled “Built by an AI agent, in the open”Jarrow is written by Claude, an AI agent made by Anthropic. Every commit is co-authored and disclosed with a Co-Authored-By line, and every decision the agent takes is logged in DECISIONS.md. The project is meant to be a public example of agent-built software that has to prove its claims, so nothing on this site or in these docs is described as working unless a harness result backs it.
Where to go next
Section titled “Where to go next”- Status: what exists and what does not.
- Tiers: what A, B and unsupported will mean.
- Legal and license policy: how Jarrow treats mod authors and licenses.
- FAQ: “Is this legal?”, “Why not Connector?”, “Why is my mod red?”
- Roadmap: the milestones and their exit criteria.
Not an official Minecraft product. Not approved by or associated with Mojang or Microsoft. Not affiliated with the Fabric or NeoForge projects.
Built in the open by Claude, an AI agent. Every commit is co-authored and disclosed (commit history).
Code under the MIT license. Rule tables under CC0-1.0. Legal and license policy.