compat-check

Minecraft Mod Compatibility Check

Verify a mod has an actual build for the target MC version before including it in any verdict or recommendation. Run this for EVERY mod assessed and EVERY gap candidate — never skip.

Procedure

For each mod, call Modrinth:

GET https://api.modrinth.com/v2/project/{slug}/version?game_versions=["{mc-version}"]&loaders=["{loader}"]

If the slug is unknown, find it first:

GET https://api.modrinth.com/v2/search?query={mod-name}&facets=[["project_type:mod"]]

Take the top result's slug.

Read the response

Result	Status	Action
Versions returned, game version matches exactly	✓ exact	Use latest version + date
No results → retry with minor series (e.g. 1.21.1 for target 1.21.11) → found	~ minor	Flag as unconfirmed
No results on Modrinth → check CurseForge / GitHub	? external	Note source
Nothing found anywhere	✗ none	Block — see rules

Check dependencies

For every ADD candidate, also check its required dependencies:

GET https://api.modrinth.com/v2/project/{slug}/version?game_versions=["{mc-version}"]&loaders=["{loader}"]

Read dependencies array. For each required dep not already in the user's mod list, run this same procedure on it. If any required dep has ✗ none, the candidate cannot be recommended.

Rules

• ✗ none → NEVER recommend KEEP, ADD, or REPLACE with this mod. Flag as INCOMPATIBLE (user mod) or drop from gap list (candidate).
• ~ minor → May include, but flag in report as "unconfirmed for exact version".
• ? external → Include with source noted.
• Missing dep with ✗ none → Drop the candidate entirely; note why.

Report column

Add to every row in the Category Report:

| Compat | Latest for target | Date |
| ✓ exact | 0.21.3+1.21.11 | 2026-02 |
| ~ minor | 0.21.2+1.21.1 | 2025-11 |
| ✗ none | — | — |

Baseline failure (why this skill exists)

In a real audit run, 6 category agents recommended ADD for 14 mods without verifying exact 1.21.11 builds. The versions:1.21.11 Modrinth search facet filters results but does not confirm that a downloadable JAR for exactly 1.21.11 exists — a mod can appear in search results via minor-series matching. This skill closes that gap.