Pack Doctor

Understand what your Fabric modpack actually loads. Pack Doctor is a read-only diagnostic tool for modpack authors and players who want clear, evidence-based reports about material tags and recipe compatibility.

Get started

1. Install Minecraft 1.21.1, Java 21, Fabric Loader 0.20.1 or newer, and Fabric API 0.116.17+1.21.1.
2. Put the Pack Doctor JAR in your client profile's mods folder.
3. Open a world or join a server, press P, and select Run scan. The key can be reassigned in Controls.
4. Select a finding to inspect identifiers, evidence, confidence and recommendations. Export a UTF-8 report to pack-doctor-reports inside your game directory.

What it checks

• Installed content: local mod IDs, versions, declared dependencies, and item/block namespace counts.
• Material tags: narrow conventional tags such as c:ingots/tin and c:ingots/copper. Similar names alone are never treated as proof of a conflict.
• Recipe acceptance: recipes that accept only part of a material tag are flagged for review, since progression restrictions can be intentional.
• Empty ingredients: supported recipes with a required ingredient that has no matching items are reported as confirmed snapshot errors.
• Crafting dependencies: informational notices identify outputs with no entry path within the supported crafting graph. These notices do not claim that an item is unobtainable.

Clear results, conservative conclusions

Findings are separated into confirmed errors, likely incompatibilities, and informational notices. Each includes a stable code, technical identifiers, supporting evidence, confidence and a recommendation. Filter by severity and category, browse the mod inventory, or export the complete report.

Supported recipes and limitations

Version 0.1.0 supports the exact vanilla shaped/shapeless crafting, smelting, blasting, smoking, campfire cooking and stonecutting recipe classes with vanilla ingredients. Modded JSON recipes using those standard classes can be inspected.

Custom recipe classes, custom ingredients, special recipes and smithing are explicitly skipped. Loot, mining, trading, world generation, fluids, energy, machines, quests and progression systems are outside MVP coverage. A shared material tag is evidence of a relationship, not proof that every recipe should allow substitution.

In singleplayer, diagnostics capture the integrated server's loaded recipe data on its owning thread. On remote servers, only client-synchronized data is visible; hidden server data is neither requested nor bypassed. The mod inventory describes the client installation, not the server's installed mods.

Read-only by design

Pack Doctor does not modify recipes, resources, configs, mods or saved worlds. It sends no custom network packets. Registry capture is batched on the game thread; analysis and export run in a background worker. Changing worlds or reloading data invalidates the report.

Exports contain game/mod versions, diagnostic codes, identifiers, evidence and coverage limits. They do not read configurations, passwords, tokens, player names or server addresses.

Initial alpha: validation and release status

24 automated tests passed, including actual Minecraft/Fabric API tests. A real Loom development client was used to check the P key, scan, filters, details, mod inventory, report export and invalidation after /reload.

The test installation contained 1,333 items and 1,290 recipes, with 1,249 supported recipes. These figures describe the test installation only; your pack's results will differ.

This is an alpha release. The final JAR's metadata and mixin refmap have been verified, but a separate ordinary-launcher installation, remote-server session and large third-party modpacks have not yet been manually validated. Please include the finding code and relevant mod versions when reporting a problem.

Client-side Fabric mod. Minecraft 1.21.1 only. MIT license.