The server will not start
A mod throws while loading and the process exits. All you get is one stack trace, so you bisect by disabling mods one at a time until the server finally comes up.
Open source · GPL-3.0 · Windows and Linux
Know which mods break before you upgrade the server
Point modcheck at your mods folder. It checks every jar against Modrinth and CurseForge for the Minecraft version and loader you are moving to, then tells you what is ready, what needs an older build, and what has to be fixed first. When it cannot find an answer it reports Undetermined — it does not guess.
Zero third-party dependencies · Portable · Never writes to your mods folder
| Mod | Verdict | Current → usable on target | Released |
|---|
Open the full check page
Sample data · first 8 of 30 mods
Why this exists
Upgrading is a one-line change. Finding out what broke is the rest of your night.
Bump the version, restart, done — ten minutes. Then a mod turns out not to support the new version. Best case it quietly stops working. Worst case the server will not boot at all. Either way, you are reading logs long after you meant to be done.
It starts, but something quietly stops working
Plenty of mods do not crash. They just stop taking effect. You find out when a player asks where their claims went — and by then the world has been running for days.
Checking by hand does not scale
Open each project page, scroll the version list, match the game version, match the loader, then work out whether a build is stable or a beta. Thirty mods is an evening — and you redo it on every upgrade.
The verdicts
Six verdicts, each with a next step
Real life is rarely yes or no. Mods are often only a beta supports it yet or the newest release is the one that broke it. modcheck keeps those cases apart instead of flattening them into one vague answer.
Undetermined is not a failure
When modcheck cannot find a release record for a mod on your target version, it does not pick a plausible-looking older build to fill the gap, and it does not assume the mod is fine. It writes Undetermined, lists it separately in the summary, and leaves the call to you.
Guessing one version wrong costs you a night of log reading. It would rather admit it could not find out.
How it works
Three steps to the answer, a fourth to fetch it
No account, no cloud sync, no so-called smart recommendations. Everything it checks, you could check by hand — it would just cost you an evening.
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Read your mods folder
It scans the jars and works out what each mod is, which version you are running, and which loader it targets. Read-only.
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Look every mod up
Each mod is checked against Modrinth and CurseForge for the exact game version and loader you are targeting. Target version names come from Mojang's official version manifest.
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Report and advise
Every mod lands in one of six states with a one-line next step. Then you get the overall answer: can you upgrade today, or not.
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Fetch the recommended builds for you
Optional, and where most of the time goes: it grabs only the recommended build for Ready and Check rows, skips betas and snapshots, and pulls straight from Modrinth's own CDN into a folder you choose — never your mods folder.
Two editions
CLI or GUI
Same data, same six verdicts. The only difference is how you run it.
Command line (CLI)
One executable you can drop anywhere. No interface — output goes straight to your terminal.
- Size~18 MB
- DependenciesNone — no third-party deps
- Formsingle file
- Best forremote hosts, scripts, CI
Graphical (GUI)
Unzip and run. All six states in one table you can filter, with per-row detail.
- Size~70 MB
- Dependenciesbundled
- Formunzip and run
- Best forfirst run, reading every line
What it does not do
The limits, written down so you do not have to test them
The most useful thing a checker can tell you is what it will not do for you. All five below are true — design choices, not modesty.
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It does not touch your mods folder. Read-only: no moves, no renames, no deletions.
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It does not install or swap mods. It tells you which build to use; downloading and replacing stays with you.
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It does not redistribute mod files. It reads public release metadata — no bundling, no mirroring, no re-hosting.
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It does not check snapshot versions. Snapshot compatibility churns too fast for a verdict to stay true for a day, so it only answers for stable releases.
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It does not resolve dependencies recursively. It checks the mods actually present in your folder. It will not walk the dependency graph for you — because then you could not verify the result yourself.
Three-minute start
Five steps to your first run
No sign-up, no environment variables, no server config changes. Through all of it, modcheck does exactly one thing: read.
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Download and unzip
The CLI is a single file. The GUI is unzip-and-run. Neither needs an installer.
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Point it at your mods folder
Tell modcheck where your mods live. It only reads that directory, and it cannot change anything inside it.
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Pick the target version and loader
Say 26.3 and Fabric. Version names come from Mojang's official manifest, so you cannot pick one that does not exist.
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Let it finish
It looks up every mod on Modrinth and CurseForge. More mods means more time.
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Work the list
Fix the blocked ones, judge the betas yourself, re-run later for anything undetermined. Upgrade once the list is clear.
Check first, upgrade second
Both editions read the same data. Neither one will hand you a fake Ready.
GPL-3.0 · Portable · Config lives in a config folder next to the binary