Based on https://gist.github.com/TehNut/340f62d4167b225cb6d3f54119fff5a0

fabric-mod-json-schema.json

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "$id": "http://example.com/root.json",
  "type": "object",
  "title": "Fabric Mod JSON Metadata Schema",
  "description": "Schema used for validation of Fabric Mod's `mod.json` file.",
  "required": [
    "id",
    "version"
  ],
  "oneOf": [
    {
      "required": [ "initializer" ],
      "not": {
        "required": [ "initializers" ]
      }
    },
    {
      "required": [ "initializers" ],
      "not": {
        "required": [ "initializer" ]
      }
    },
    {
      "not": {
        "required": [ "initializer", "initializers" ]
      }
    }
  ],
  "properties": {
    "id": {
      "$id": "#/properties/id",
      "type": "string",
      "title": "Mod ID Schema",
      "description": "Validates Mod ID",
      "examples": [
        "my_mod_id"
      ],
      "pattern": "^([a-z][a-z0-9-_]{1,63})$"
    },
    "version": {
      "$id": "#/properties/version",
      "title": "Version Schema",
      "description": "Validates version string",
      "$ref": "#/definitions/version-string"
    },
    "requires": {
      "$id": "#/properties/requires",
      "title": "Requirement List Schema",
      "description": "Validates declared dependencies list",
      "$ref": "#/definitions/dependencies"
    },
    "conflicts": {
      "$id": "#/properties/conflicts",
      "title": "Conflicts Schema",
      "description": "Validates declared incompatiblities list",
      "$ref": "#/definitions/dependencies"
    },
    "languageAdapter": {
      "$id": "#/properties/languageAdapter",
      "title": "Language Adapter Schema",
      "description": "Validates declared canonical name of Language Adapter",
      "$ref": "#/definitions/class-name"
    },
    "mixins": {
      "$id": "#/properties/mixins",
      "type": "object",
      "title": "Mixins Schema",
      "description": "Validates declared mixin properties",
      "required": [],
      "properties": {
        "client": {
          "$id": "#/properties/mixins/properties/client",
          "type": "string",
          "title": "Client-Side-Mixin Schema",
          "description": "Validates the name of mixin config used on client",
          "examples": ["client.json"],
          "pattern": "^(.*)$"
        },
        "server": {
          "$id": "#/properties/mixins/properties/server",
          "type": "string",
          "title": "Server-Side-Mixin Schema",
          "description": "Validates the name of mixin config used on server",
          "examples": [
            "server.json"
          ],
          "pattern": "^(.*)$"
        },
        "common": {
          "$id": "#/properties/mixins/properties/common",
          "type": "string",
          "title": "Common-Mixin Schema",
          "description": "Validates the name of mixin config used on both side",
          "examples": [
            "common.json"
          ],
          "pattern": "^(.*)$"
        }
      }
    },
    "side": {
      "$id": "#/properties/side",
      "type": "string",
      "enum": [
        "client",
        "server",
        "universal"
      ],
      "title": "Side Schema",
      "description": "Vadliates sideness",
      "default": "universal",
      "examples": ["client", "server", "universal"],
      "pattern": "^(client|server|universal)$"
    },
    "lazilyLoaded": {
      "$id": "#/properties/lazilyLoaded",
      "type": "boolean",
      "title": "Lazily-Loaded Schema",
      "description": "Vadliates the lazy-loading property",
      "default": false,
      "examples": [true, false]
    },
    "initializer": {
      "$id": "#/properties/initializer",
      "title": "Initializer Schema",
      "description": "Validates entry point of initializers",
      "$ref": "#/definitions/class-name"
    },
    "initializers": {
      "$id": "#/properties/initializer",
      "type": "array",
      "title": "Initializer Schema",
      "description": "Validates entry point of initializers",
      "items": {
        "$ref": "#/definitions/class-name"
      }
    },
    "name": {
      "$id": "#/properties/name",
      "type": "string",
      "title": "Name Schema",
      "description": "Validates the human-readable name of Mod",
      "default": "",
      "examples": ["My Mod"],
      "pattern": "^(.+)$"
    },
    "description": {
      "$id": "#/properties/description",
      "type": "string",
      "title": "Description Schema",
      "description": "Validate description of said mod",
      "default": "",
      "examples": ["Lorem ipsum"],
      "pattern": "^(.*)$"
    },
    "links": {
      "$id": "#/properties/links",
      "title": "Link Schema",
      "description": "Validates all provided URIs that relates to the mod",
      "oneOf": [
        {
          "type": "string",
          "format": "uri",
          "examples": [ "http://example.com" ]
        },
        {
          "type": "object",
          "required": [],
          "properties": {
            "homepage": {
              "type": "string",
              "format": "uri",
              "examples": [ "http://example.com" ]
            },
            "issues": {
              "type": "string",
              "format": "uri",
              "examples": [ "http://example.com" ]
            },
            "sources": {
              "type": "string",
              "format": "uri",
              "examples": [ "http://example.com" ]
            }
          }
        }
      ]
    },
    "recommends": {
      "$id": "#/properties/recommends",
      "title": "Recommended Dependencies Schema",
      "description": "Validates recommened dependencies list",
      "$ref": "#/definitions/dependencies"
    },
    "authors": {
      "$id": "#/properties/authors",
      "type": "array",
      "title": "Author List Schema",
      "description": "Validates author list",
      "items": {
        "$ref": "#/definitions/person"
      }
    },
    "contributors": {
      "$id": "#/properties/contributors",
      "type": "array",
      "title": "Contributor List Schema",
      "description": "Validates contributor list",
      "items": {
        "$ref": "#/definitions/person"
      }
    },
    "license": {
      "$id": "#/properties/license",
      "type": "string",
      "title": "License Schema",
      "description": "Validates license name",
      "default": "",
      "examples": [
        "Thonkery Public License"
      ],
      "pattern": "^(.*)$"
    }
  },
  "definitions": {
    "version-string": {
      "type": "string",
      "pattern": "^(.+)$"
    },
    "class-name": {
      "type": "string",
      "pattern": "^([^0-9](?:\\.[^0-9])+)$"
    },
    "person": {
      "type": "string",
      "pattern": "^.+$"
    },
    "dependencies": {
      "type": "object",
      "patternProperties": {
        "^([^A-Z]+)$": {
          "oneOf": [{
              "$ref": "#/definitions/version-string"
            },
            {
              "type": "array",
              "items": {
                "$ref": "#/definitions/version-string"
              }
            },
            {
              "type": "object",
              "required": ["version", "side"],
              "properties": {
                "version": {
                  "$ref": "#/definitions/version-string"
                },
                "side": {
                  "type": "string",
                  "enum": ["client", "server", "universal"]
                }
              }
            }
          ]
        }
      }
    }
  }
}