Minecraft 1.21.1 changelog

1.2.4 — Horseshoes compatibility

• Fix the horn and Horseshoes equipment slots drawing on top of each other.
• Fix shift-click transfers for horns, horseshoes, and chested-donkey cargo.
• Fix the inventory accessor conflict with Horseshoes, including legacy-save migration.
• Run all 16 GameTests both with and without Horseshoes 1.3.3 before publishing.

1.2.3 — Backported fixes

Based on origin/1.21.1 (74277a7), in branch MrInterBugs/backport-1.21.1.

Backported fixes

• Store the horn in a dedicated one-slot inventory, separate from the saddle, armor, and chest cargo. Adding a chest no longer moves or unequips the horn.
• Synchronize horn state on inventory mutations and use the inventory as the authoritative server value. Removing a horn immediately removes its protection.
• Correct shift-click boundaries for equipment, cargo, all 36 player slots, and the horn slot.
• Save horns under UnicornHornItem for every horse type. Recover horns from the extra chest-inventory slot used by previous 1.21.1 versions, without duplicating them into cargo.
• Drop separately stored horns once on forced death. Respect bypasses_invulnerability damage, including /kill and the void.
• Filter vanilla unicorn potion and arrow variants out of both creative-tab and creative-search lists. Supply fallback translations for vanilla variants already in inventories.
• Keep potion creation and conversions exclusively in the brewing stand. Remove the obsolete potion-crafting definitions and invalid horn recipe. Migrate only the eight tipped arrows recipe and its unlock to the current resource directory/format, using a component ingredient to require the correct potion.
• Restrict unicorn bottles to the valid gunpowder/dragon-breath conversions. Reject unrelated vanilla effect ingredients and avoid turning no-op mixes of legacy vanilla unicorn bottles into fake conversion recipes.
• Let REI discover the real arrow recipe instead of adding a duplicate manual display. Keep all five valid brewing paths visible, including Awkward splash/lingering potion + horn.
• Render a tapered 3D horn on the forehead while retaining the inventory icon. On 1.21.1 this uses a custom-model-data override on a temporary render-only stack; saved/equipped stacks are unchanged.
• Position the horn slot below the other equipment and move its background to match.
• Register both environment source sets with Loom and remove the duplicate unremapped Fabric API client dependency so the development/GameTest runtime loads remapped dependencies correctly.

Version-specific behavior retained

Minecraft 1.21.1 stays on Java 21 and Yarn mappings. The existing REI and optional RyoamicLights integrations remain enabled; it does not use the 26.1.2 JEI integration or client-item JSON format. Native 1.21.1 potion items already handle drinking, bottle returns, and lingering/arrow duration scaling, so the newer consumable/duration components are not copied into this version.

The release workflow already selects Java 21 and Minecraft 1.21.1 and retains those values.

Validation

./gradlew build compiles the common/client code, packages remapped release/source jars, and runs 16 Minecraft GameTests on Java 21. Tests cover lethal damage, immediate removal, chest attachment, horse/donkey saves, old chest-slot migration, horn and ordinary shift-clicking, brewing, rejection of crafting-table potion conversions, eight-arrow crafting, drinking/bottle returns, /kill/death drops, creative-search duplicates, and item translation keys.

The report is build/reports/gametest/results.xml; logs are in build/run/gameTest/logs/. The integration suite also runs against the actual Horseshoes 1.3.3 mod; the minimal Prism 1.21.1 test instance includes both mods. The existing 26.1.2 test installation is unaffected by the backport.

Horseshoes compatibility follow-up

• Restore the horn slot to (26, 18) beside the saddle; Horseshoes keeps its (8, 54) slot. Move the client background with the horn's actual slot.
• Identify the horn and player slots by their inventory, instead of assuming the horn is last. Handle all equipment slots before cargo when shift-clicking, including Horseshoes on chested donkeys. Run this handler before Horseshoes' fixed-index quick-move handler.
• Namespace the native-horse inventory accessor to avoid colliding with Horseshoes' own getInventory() method. This also preserves legacy chest-slot migration while both mods are loaded.
• Add regression checks for non-overlapping horse/donkey slots and independent horn/horseshoe transfers without loss or duplication. CI runs with and without Horseshoes 1.3.3; release builds require both configurations to pass.

Run the integration checks with ./gradlew build runGameTest -PtestHorseshoes=true. The optional dependency is only added to the development runtime; it is not bundled or required by the mod.