UpperSupervisor/horizon-modpack-compatibility

What it does

Hopper is a desktop helper for players of Horizon, a Minecraft: Java Edition 26.3 rewrite in Rust. Java mod jars do not run on Horizon, so players cannot tell which mods they can keep. Hopper answers that per mod with one of four verdicts:

• Built in: Horizon already provides the feature, mapped in rules/builtin.toml, so no jar is needed.
• Ported: a Rust version exists in Horizon's mod registry, the index.json built by horizon-rust/horizon-mods; the mapping back to the Java mod lives in rules/ports.toml.
• Works: Horizon's own horizon-launcher mods check <slug> --json accepts the mod and carries over its blocks, items, recipes and worldgen.
• Not yet: anything else, which can be voted for on a most-wanted board whose counts are approximate by design.

It also reads whole packs — a mods folder, a Prism or MultiMC instance, a .mrpack or a CurseForge zip — and splits them into mods that come along, mods that are no longer needed and mods that are missing. It can install the first group into Horizon in one click, and it can watch individual mods and report when a Horizon version appears. The main audience is Horizon players migrating an existing pack; porters can use the wanted board to decide what to port next.

Getting started

You need Windows 10 or 11 and an install of Horizon 26.3 on the same PC; Hopper locates it by itself. Download Hopper-1.3.5-portable-x64.exe — a single file with no installer and no admin rights. The build is not code-signed yet, so SmartScreen may warn on first launch; choose More info, then Run anyway.

1. Launch Hopper next to your Horizon installation.
2. Type a mod name, or drag a jar, your mods folder or a modpack onto the window. Supported pack inputs are a mods folder, a Prism or MultiMC instance, a .mrpack, or a CurseForge zip.
3. Read the verdict — Built in, Ported, Works or Not yet — and press the button beneath it: Install, Add to Horizon, Want it too, or How to turn it on for built-in features.
4. For a pack, use Add the 30 to Horizon-style button to install all compatible mods at once; per-mod results are behind Show details.

Mods that cannot be identified (typically CurseForge-only mods) show Not sure yet and are counted as not yet in pack totals. There is no account and no payment.

Constraints and data handling

• Platform: the packaged release is Windows 10/11 x64 only.
• Dependency: mod installation requires Horizon 26.3 on the same machine; the inspection of jars can be done without it, but installing cannot.
• Identification data: mod hashes and names are sent to Modrinth's POST /v2/version_files to identify jars; a dropped jar is read from fabric.mod.json, META-INF/neoforge.mods.toml or META-INF/mods.toml first. Icons come from Modrinth's CDN and are cached locally. Horizon's registry index is downloaded to check ports. Votes go to the vote board together with a random per-install ID that is generated on first start; there is no telemetry and packs/watchlists stay local.
• CurseForge matching: CurseForge's API requires a key, so CurseForge-only mods are matched by name and may be uncertain.
• Install integrity: files are installed only if listed in Horizon's registry, checked against their SHA-256 and, when the project has a key, a minisign signature. Revoked files never install and existing copies are disabled. Content mods (.hmod) go to horizon-mods/ and native mods (.hnmod) to horizon-native-mods/; native mods run unsandboxed and Horizon asks for trust on first use.
• Votes: one vote per mod per install ID, rate-limited per address; counts are indicative, not exact.
• Build from source: Rust 1.85+ plus the Tauri prerequisites; the repo layout puts jar reading and verdicts in crates/hopper-core, the vote board in server/, and the desktop UI in app/. License is MIT.

HotRepo's take

Hopper entered HotRepo's new-project list for the Rust language on 2026-11-01, with 107 stars gained in the roughly 11.5-hour window from 2026-10-31 23:20 UTC to 2026-11-01 10:45 UTC, and 4 forks. That is the entire recorded history here — one snapshot and one star sample — so it establishes early pickup rather than a trend. What stands out technically is that Hopper defers verdicts to Horizon's own launcher check rather than reimplementing compatibility, and keeps the Java-to-Rust mappings in reviewable TOML files, which makes the answer set auditable and community-extensible.