The fabric.mod.json file is a mod metadata file used by Fabric Loader to load mods. In order to be loaded, a mod must have this file with the exact name placed in the root directory of the mod JAR.

• schemaVersion Needed for internal mechanisms. Must always be 1.
• id Defines the mod's identifier - a string of Latin letters, digits, underscores with length from 1 to 63.
• version Defines the mod's version - a string value, optionally matching the Semantic Versioning 2.0.0 specification.

• environment: Defines where mod runs: only on the client side (client mod), only on the server side (plugin) or on both sides (regular mod). Contains the environment identifier:
  • * Runs everywhere. Default.
  • client Runs on the client side.
  • server Runs on the server side.
• entrypoints Defines main classes of your mod, that will be loaded.
  • There are 3 default entry points for your mod:
    • main Will be run first. For classes implementing ModInitializer.
    • client Will be run second and only on the client side. For classes implementing ClientModInitializer.
    • server Will be run second and only on the server side. For classes implementing DedicatedServerModInitializer.
  • Each entry point can contain any number of classes to load. Classes (or methods or static fields) could be defined in two ways:
    • If you're using Java, then just list the classes (or else) full names. For example:

      "main": [
          "net.fabricmc.example.ExampleMod",
          "net.fabricmc.example.ExampleMod::handle"
      ]

    • If you're using any other language, that is compatible with Java and has a Fabric adapter, then you should use following syntax:

      "main": [
         {
            "adapter": "kotlin",
            "value": "package.ClassName"
         }
      ]

• jars A list of nested JARs inside your mod's JAR to load. Before using the field, check out the guidelines on the usage of the nested JARs. Each entry is an object containing file key. That should be a path inside your mod's JAR to the nested JAR. For example:

  "jars": [
     {
        "file": "nested/vendor/dependency.jar"
     }
  ]

• languageAdapters A dictionary of adapters for used languages to their adapter classes full names. For example:

  "languageAdapters": {
     "kotlin": "net.fabricmc.language.kotlin.KotlinAdapter"
  }

• mixins A list of mixin configuration files. Each entry is the path to the mixin configuration file inside your mod's JAR or an object containing following fields:
  • config The path to the mixin configuration file inside your mod's JAR.
  • environment The same as upper level environment field. See above. For example:

    "mixins": [
       "modid.mixins.json",
       {
          "config": "modid.client-mixins.json",
          "environment": "client"
       }
    ]

The key of each entry of the objects below is a Mod ID of the dependency.

The value of each key is a string or array of strings declaring supported version ranges. In the case of an array, an “OR” relationship is assumed - that is, only one range has to match for the collective range to be satisfied.

In the case of all versions, * is a special string declaring that any version is matched by the range. In addition, exact string matches must be possible regardless of the version type.

• depends For dependencies required to run. Without them a game will crash.
• recommends For dependencies not required to run. Without them a game will log a warning.
• suggests For dependencies not required to run. Use this as a kind of metadata.
• breaks For mods whose together with yours might cause a game crash. With them a game will crash.
• conflicts For mods whose together with yours cause some kind of bugs, etc. With them a game will log a warning.

• name Defines the user-friendly mod's name. If not present, assume it matches id.
• description Defines the mod's description. If not present, assume empty string.
• contact Defines the contact information for the project. It is an object of the following fields:
  • email Contact e-mail pertaining to the mod. Must be a valid e-mail address.
  • irc IRC channel pertaining to the mod. Must be of a valid URL format - for example: irc://irc.esper.net:6667/charset for #charset at EsperNet - the port is optional, and assumed to be 6667 if not present.
  • homepage Project or user homepage. Must be a valid HTTP/HTTPS address.
  • issues Project issue tracker. Must be a valid HTTP/HTTPS address.
  • sources Project source code repository. Must be a valid URL - it can, however, be a specialized URL for a given VCS (such as Git or Mercurial).
  • The list is not exhaustive - mods may provide additional, non-standard keys (such as discord, slack, twitter, etc) - if possible, they should be valid URLs.
• authors A list of authors of the mod. Each entry is a single name or an object containing following fields:
  • name The real name, or username, of the person. Mandatory.
  • contact Person's contact information. The same as upper level contact. See above. Optional.
• contributors A list of contributors to the mod. Each entry is the same as in author field. See above.
• license Defines the licensing information. Can either be a single license string or a list of them.
  • This should provide the complete set of preferred licenses conveying the entire mod package. In other words, compliance with all listed licenses should be sufficient for usage, redistribution, etc. of the mod package as a whole.
  • For cases where a part of code is dual-licensed, choose the preferred license. The list is not exhaustive, serves primarily as a kind of hint, and does not prevent you from granting additional rights/licenses on a case-by-case basis.
  • To aid automated tools, it is recommended to use SPDX License Identifiers for open-source licenses.
• icon Defines the mod's icon. Icons are square PNG files. (Minecraft resource packs use 128×128, but that is not a hard requirement - a power of two is, however, recommended.) Can be provided in one of two forms:
  • A path to a single PNG file.
  • A dictionary of images widths to their files' paths.

You can add any field you want to add inside custom field. Loader would ignore them. However it's highly recommended to namespace your fields to avoid conflicts if your fields (names) would be added to the standard specification.