Bad config file (TOML / JSON parse error)
Mods store their settings in config/<modname>.toml or .json. If the file can't be parsed at boot — a stray quote, a duplicate key, a value out of the allowed range, or a format change between mod versions — the mod throws on load. Sometimes the server crashes hard; sometimes the mod fails silently and breaks features later. Either way, the fix is in config/.
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How to fix it
- 1
Read the error to find the bad file
The trace names the config file and (usually) the line number that failed. config/cold_sweat.toml line 47, config/sodium-options.json invalid JSON, etc. That's your target.
- 2
Restore the config from a backup
If you have a recent backup, the config from yesterday almost certainly parsed fine. Restore just that one file from the latest pre-crash backup — don't restore the whole world.
- 3
Delete the file to regenerate defaults
Mods recreate a default config on next boot when their config file is missing. Move the broken file out of config/ (don't delete — keep it for reference), restart, and the mod writes a fresh one. You'll lose your customizations, but the server boots.
- 4
Check for format changes after a mod update
Some mod updates change config schema between versions — a key gets renamed, a section gets restructured. If you just updated the pack and the config now fails, the format may have changed. Compare your config against the new default the mod ships in defaultconfigs/ or its source repo.
Alternative causes
These can produce the same error message — worth ruling out if the steps above don't resolve it.
ForgeConfigSpec silent-reset (value out of declared range)
Forge's config system silently resets values that fall outside the declared min/max range, but logs a warning. If a config value you set isn't being respected, this is usually why — check logs/latest.log for 'reset to default'.
BOM or encoding issue after Windows editing
Editing config files in Notepad sometimes adds a UTF-8 BOM that breaks the parser. Use VS Code or Notepad++ with UTF-8 (no BOM) encoding instead.
Frequently asked
Can I edit config files while the server runs?
Most mods read config at boot only. Live edits don't apply until restart. A few (Cold Sweat, OptiFine) reload on demand, but assume restart-required by default.
What if multiple config files are broken at once?
Usually means a shared cause — a mass-replace, a failed backup, or a disk-full event that truncated multiple writes. Fix the cause first (check our disk-full guide), then restore the configs from backup.
Where do default configs come from?
Most mods ship them inside the .jar at config/defaults/. When config/<modname>.toml is missing, the mod copies the default out. Some modpacks ship a defaultconfigs/ folder that overrides those.
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Last reviewed 2026-06-05. If a step is wrong or out of date, tell us — we'll fix the article and the auto-pattern at the same time.