Mods: installation, updates and troubleshooting

“Game GML change … a new version of MOMI required”

If MOMI reports “Game GML change caused by game update. Unable to install. A new version of MOMI required when available.”, stop the installation attempt and check versions before changing folders. This is not the same symptom as an empty mods list.

1. Note your installed game version and MOMI version. Close the game and preserve a dated save backup.
2. For game 1.0.5, the author released MOMI v0.16.4 with updated MMAPI support on September 18. Obtain it from the author's release page, not an older tutorial's download.
3. Check each enabled mod separately. This release removes monster.draw; affected mods need author updates. Mods calling the old gpu_set_extra also need updating for the engine's gpu_set_draw_kind change. Updating MOMI alone does not fix these mods.
4. Keep incompatible packages disabled. Follow remove and restore to establish an unmodded baseline, then reinstall compatible packages one at a time.
5. If the version-matched setup still produces the message, save the exact error, game/MOMI versions and enabled-mod list for the author's issue tracker. Do not repeatedly reinstall the same unsupported versions or delete your save.

Completion check: installation finishes without the compatibility error and a disposable save reaches gameplay. If your game is newer than 1.0.5, this release note does not establish support: check the author's newer release notes first.

Official status and the documented toolchain

NPC Studio's new-engine notice gives modding a clear public place in the game's future. The studio invited mod creators to learn the new file structure, said every old-engine mod would need an update, and warned that modded saves could behave unexpectedly during the migration. It also told mod users to uninstall or update their mods before launching the new engine.

MOMI and the MMAPI hook framework it installs are community projects in Garethp's public GitHub repository, while Nexus Mods is a third-party host with a Fields of Mistria category. Treat all three as unofficial even when a developer discusses modding in general. Use NPC Studio's own pages only for the studio's engine and save-safety guidance.

If a modded session stutters, use the performance section below to create a reversible baseline before changing settings.

Pass the compatibility gate first

The July 2026 engine migration changed the file structure and made old packages unusable, and MOMI's tagged documentation says mods released before that migration need an update while new-engine packages use manifest.toml. Old tutorials built around data.win, YYToolkit, Aurie, registry injection, or pre-migration packages are excluded from this guide.

MOMI v0.16.4 updates MMAPI for game 1.0.5, but does not promise compatibility for every mod or later game patch. Check the repository and each mod page for the versions you are about to install.

Download checklist

1. The mod page was updated for the July 2026 new engine and names the game version it supports.
2. Its Requirements section names MOMI, MMAPI, another framework, or companion mod versions when needed.
3. Its files and changelog are from the same author page, not a mirror or a repost.
4. Its removal notes explain whether an existing save keeps custom items, data, or other dependencies.
5. Skip the package when its page does not explain compatibility or removal.

Back up saves before changing files

Close the game before making a backup. NPC Studio's Windows support instructions identify %LOCALAPPDATA%\FieldsOfMistria as the folder containing saves and settings, so copy that whole folder to a separate dated location and leave the copy untouched. Also record the game version, MOMI version, enabled mod list, and the exact version of each package.

This save copy has a different job from Steam file verification. A backup can return player data to an earlier state, while verification checks the installed game files and reacquires files that are missing or changed. Verification does not roll back a save, remove data written inside a save, or recreate content supplied by a removed mod.

For Steam Deck or Linux, use the current game support page and the Steam storage view to locate the save instead of copying a path from an old tutorial. Keep the backup outside the game installation on every platform. Confirm that the copied folder contains the save you intend to test before changing files.

Install safely

Use the author's Nexus page or canonical GitHub repository, and read the Files, Requirements, and Changelog sections together. A verified signature on a release commit does not authenticate a separately downloaded mirror. Stop when the file source does not match the author page.

Current MOMI installation sequence

1. Download MOMI from the Garethp GitHub releases page and select the stable release documented by the mod author.
2. On Windows, create mods inside the Fields of Mistria game folder beside FieldsOfMistria.exe.
3. On Steam Deck or another Linux system, use mods inside the game folder or mistria-mods in the home directory, as the tagged MOMI documentation allows.
4. Extract each supported package so its folder sits directly inside that mod directory and contains manifest.toml at its root.
5. Open MOMI, confirm the displayed game and mods locations, select only the packages you checked, and use its Install control.
6. Launch with one new mod at a time and test with a disposable save before opening the save you care about.

Do not run installers from URL shorteners, file lockers, chat attachments, or random mirrors. Do not disable antivirus, firewall, browser protection, or operating-system security to force an executable to run. Stop at a warning, recheck the canonical release and author page, and never share a Steam password, session cookie, two-factor code, or purchased game files for mod support.

Update after a patch

Game updates can invalidate a working setup even when MOMI itself still opens. NPC Studio recommends uninstalling or updating mods before a new-engine launch, and MOMI's tagged instructions say to run the installer again after a game update. Wait until each mod page names the new game or loader version before reinstalling it.

1. Close the game and make a new dated save backup.
2. Record the last working game, MOMI, and mod versions.
3. Uninstall the managed mod layer before launching a major engine update.
4. Update MOMI and each mod only from its canonical release page.
5. Start vanilla first, then add one version-matched mod at a time.

Remove and restore

Rollback sequence

1. Exit the game and make another dated copy of the save folder before troubleshooting.
2. Use the current MOMI Uninstall control so the installer can reverse the layer it manages.
3. In Steam, open Properties, then Installed Files, and run Verify integrity of game files.
4. Start the verified game without mods and confirm that a disposable or copied save reaches gameplay.
5. Reinstall only version-matched mods, one at a time, checking the game after each addition.

Removing one mod from a continuing save has no universal safe rule. Custom items, scripts, or per-save data may remain after its files are gone, so use that mod author's removal sequence and preserve the untouched backup until the save has been tested. If the vanilla game starts but the modded save does not, stop adding files and report the game, MOMI, and mod versions to the relevant author rather than deleting the only save.

Performance troubleshooting

Stutter without mods: test the same route

Record the installed title-screen version, hardware, display mode and one short route before changing anything. Use the current installed release as the baseline, not a forced downgrade to 1.0.2. Sudden pauses, sustained low FPS and horizontal tearing are different symptoms; note which one changes after each trial. Keep a save backup and use the clean-install check above if mods are involved.

NPC Studio's current troubleshooting page recommends checking minimum requirements and updating the system/graphics driver first. On Windows, select the game's executable under Graphics Settings → Options → High performance.

Restore previous values between failed trials. Then test default midpoint brightness/saturation, maximum In-Game Scale, or a lower resolution/smaller window with Smooth Camera Tracking Off. Disabling VSync, Snap and the cap together is the developer's last-resort option and may worsen stutter.

If it persists, use the linked bug-report form with your system details and reproducible route. Include the actual version and whether the clean, unmodded test still fails; do not delete saves to test performance.

MOMI cannot find the game or mods

These checks follow the linked MOMI v0.16.4 troubleshooting instructions. Use the mod author's supported versions when choosing a download.

Change one variable per test. Keep the working backup untouched, and never diagnose performance by deleting the only save.

Retest after v1.0.5 before changing more settings

The September 18 patch targets frame pacing, high-refresh displays, play below 60 FPS and 30 FPS settings on handhelds. It also fixes the slowdown when opening Treasure Boxes. First update in Steam, then compare the same location and action with your earlier result; record the version and chosen FPS setting. These fixes do not guarantee identical performance on every PC.

For crashes when kissing, check for the subsequent hotfix appended to the v1.0.5 announcement. The patch also addresses festival-day spouse schedules, Rock Stack combat and modded inventory sizes. A fix for one modded-inventory crash does not establish compatibility for all mods. Official patch and hotfix notes.