Latitude Compat Backport Playbook

Latitude — Compat Backport Playbook (Authoritative)

This skill is used whenever porting fixes between Minecraft versions (1.21.x), especially:

• main target: 1.21.11 (main)
• compat target: 1.21.1 (compat/1.21.1)

Goal: make backports boring and safe.

Default mode: STRICT — behavior must match across targets.

• Allowed exceptions only when vanilla signatures/behavior truly diverge between MC versions.
• Any exception must be documented in the commit/PR as: Pragmatic exception: <why> including the exact method signature difference.

Golden rules

1. One variable at a time: one fix, one commit, one test.
2. Prefer cherry-pick of known-good commits over manual reimplementation.
3. Do not mix backport fixes with unrelated refactors.
4. Always confirm:
   • branch
   • mod_version
   • jar name
   • tags
5. If a change touches worldgen, require at least one runtime proof.

Prep checklist (must pass)

On source branch (e.g., main):

• fix is committed and tested
• commit(s) are small and focused
• optional debug is gated and OFF by default

On target branch (e.g., compat/1.21.1):

• baseline builds cleanly before backport
• working tree is clean (or stash)

Backport procedure (mandatory order)

Step 1 — Baseline build on target branch

git checkout compat/1.21.1
git pull
$env:GRADLE_USER_HOME = "$PWD\.gradle-user-home"
.\gradlew clean build --no-daemon

If baseline fails, STOP and fix baseline first.

Step 2 — Identify minimal commits to port

Preferred: port only commits that touch:

• the specific mixin/feature/fix
• required config entries (mixins.json)
• required debug gating

Avoid porting:

• docs
• extracted sources
• tooling folders
• large unrelated refactors

Gather commit list using:

• git log --oneline <sourceBranch> -- <filePath>
• git show <hash> --name-only

Step 3 — Cherry-pick onto target

From target branch:

git cherry-pick <hash1> <hash2> ...

If conflicts:

• resolve by matching target MC signatures (method params/owners)
• do not change fix behavior while resolving compile errors
• keep edits minimal and localized

Step 4 — Build on target

$env:GRADLE_USER_HOME = "$PWD\.gradle-user-home"
.\gradlew clean build --no-daemon

If build fails:

• paste first ~60 lines of error
• adjust only signatures/descriptors until it compiles

Step 5 — Minimal runtime proof (no “new world fatigue”)

Require exactly one proof run:

• start client
• generate a small area near a known test coordinate (or equator zone)
• confirm the fix triggers (via visual result or gated logs/counters)

If old chunks contain the old bug:

• delete only the relevant region file (r.<x>.<z>.mca) to regen

Step 6 — Version + tag correctness (do NOT skip)

On the target branch, ensure gradle.properties:

• mod_version=<release>+<mcVersion>

Then:

• build jars again
• confirm jar names match mod_version

Tag must point to the commit that produces that jar:

git tag -f v<mod_version>
git push origin -f v<mod_version>

Common failure patterns (and required fixes)

A) “I can’t checkout branch: gradle.properties would be overwritten”

Fix:

• git stash push -m "temp: gradle.properties bump"
• checkout
• build
• later git stash pop on the original branch

B) “I built but jar name is wrong”

Cause:

• target branch still has old mod_version Fix:
• update gradle.properties on THAT branch
• commit
• rebuild

C) “Incompatible mods found” during dev run

Cause:

• leftover jars in run/mods from a different MC target Fix:
• clear run/mods/* and run/.fabric/processedMods/*

D) Mixins apply failures on older target

Fix:

• remove non-essential mixin (especially precipitation hooks) if it fails apply
• keep the core guard/fix mixins
• re-run, then optionally reintroduce later

Required assistant output format

When asked to backport:

1. Source branch + target branch
2. Commit list to cherry-pick (with reasons)
3. Exact commands (PowerShell)
4. Expected success outputs
5. Minimal runtime proof steps
6. Version/tag actions
7. Rollback strategy

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