I asked this same question on the JSON schema google group, and it was answered quickly. User fge asked that I post his response here:

Hello,

The current specification is draft v4, not draft v3. More specifically, the validation specification is here:

https://datatracker.ietf.org/doc/html/draft-fge-json-schema-validation-00

The web site is not up to date, I don't know why... I'll submit a pull request.

With draft v4 you can use this:

{
    "type": "array",
    "items": {
        "oneOf": [
            {"first": [ "schema", "here" ] }, 
            {"other": [ "schema": "here" ] }
        ]
    }  
}

For instance, this is a schema for an array where items can be either strings or integers (it can be written in a more simple way though):

{
    "type": "array",
    "items": {
        "oneOf": [
            {"type": "string"},
            {"type": "integer"}
        ]
    }
}

This is the correct answer. My corrected schema now includes:

"transactions" : {
    "type" : "array",
    "items" : {
        "oneOf" : [
            {
                "type" : "object",
                "properties" : {
                    "type" : {
                        "type" : "string",
                        "enum" : ["BUILD", "REASSIGN"]
                    }
                }
            },
            {
               "type" : "object",
               "properties" : {
                 "type" : {
                   "type" : "string",
                   "enum" : ["BREAK"]
                  }
               }
            }
        ]
    }
}

You have defined your schema correctly, except that it doesn't match the data you say you are validating. If you change the property names to match the schema, you still have one issue. If you want to allow "toll" and "message" to be null, you can do the following.

{
  "type": "array",
  "items": {
    "type": "object",
    "properties": {
      "loc": {
        "type": "string"
      },
      "toll": {
        "type": ["string", "null"]
      },
      "message": {
        "type": ["string", "null"]
      }
    },
    "required": [
      "loc"
    ]
  }
}

However, that isn't related to the error message you are getting. That message means that data you are validating is not an array. The example data you posted should not result in this error. Are you running the validator on some data other than what is posted in the question?

Does the second schema correctly identify that "arr is an array of objects that have an integer named type an integer, and either an object named Steps or an object named Heartrate"?

Yes, somewhat. You do not state which properties must exist, and which properties cannot exist together at the same time, which we can do with the "required" and "oneOf" keywords.

Here's an updated schema, and also fixing an error you made in the "Heartrate" section (you said "steps" instead of "heartrates"):

Copy{
  "$schema": "https://json-schema.org/draft/2019-09/schema",
  "type": "object",
  "properties": {
    "arr": {
      "type": "array",
      "items": {
        "type": "object",
        "required": [ "type" ],
        "oneOf": [
           { "required": ["Steps"] },
           { "required": ["HeartRate"] }
        },
        "properties": {
          "type": {
            "type": "integer"
          },
          "Steps": {
            "type": "object",
            "required": ["steps"],
            "properties": {
              "steps": {
                "type": "integer"
              }
            }
          },
          "HeartRate": {
            "type": "object",
            "required": ["heartrates"],
            "properties": {
              "heartrates": {
                "type": "integer"
              }
            }
          }
        }
      }
    }
  }
}

"type": "array",
"items": { // this schema applies to all items
  "type": "object",
  "required": [ "address", "first_name", "last_name" ],
  "properties": {
    "address": { "type": "string", ... },
    .. all your other property definitions here...
  },
  "oneOf": [
    { "required": [ "phone_ number" ] },
    { "required": [ "cell_ number" ] }
  ]
}

See https://json-schema.org/understanding-json-schema/reference/combining.html and https://json-schema.org/understanding-json-schema/reference/array.html

Without anyOf you're out of luck. The best you can do is use one schema that covers all of the options. It not a great solution, but it's better than nothing.

You have it slightly wrong. Currently your schema says... for the pipeline array, either, all of the items must be foo or all of the items must be baz.

You need to change your location of anyOf and items...

items applies to every item in the array. anyOf checks that any of the subschema values are valid against the instance location (in this case, each item in the array).

{
  "$id": "https://example.com/schema",
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "properties": {
    "pipeline": {
      "type": "array",
      "items": {
        "anyOf": [
          {
            "type": "object",
            "additionalProperties": false,
            "properties": {
              "foo": {
                "type": "string"
              },
              "someOption": {
                "type": "integer"
              }
            },
            "required": [
              "foo"
            ]
          }, {
            "type": "object",
            "additionalProperties": false,
            "properties": {
              "baz": {
                "type": "string"
              },
              "anotherOption": {
                "type": "integer"
              }
            },
            "required": [
              "baz"
            ]
          }
        ]
      }
    }
  }
}