LINE error 400: The request body has X error(s)

LINE parsed your JSON successfully but found one or more validation failures in it, and the number of failures replaces the X. This is the workhorse 400 of the Messaging API and the one error where LINE tells you precisely where to look: the reference says "further details are shown in the details[].message and details[].property properties." Each element of details is one fault - property is the JSON path of the offending field (messages[0].text, areas[0].action.uri), and message is the rule it broke (May not be empty, invalid uri, or an enumeration of allowed values).

Read the details array before anything else; debugging this error from the top-level message alone is guesswork. The array indexes map one-to-one onto your messages array, so messages[1].type means the second message object in the request. Common rule families you'll see quoted in details[].message: required properties that are empty or missing, values outside an allowed enum, strings over their character caps (text messages cap at 5,000 characters counted in UTF-16 code units - see text limits), arrays over their size caps (more than 5 message objects per request, more than 13 quick reply items, more than 12 bubbles in a Flex carousel), and malformed or non-HTTPS URLs in properties that require "HTTPS (TLS 1.2 or later)".

Two quota-related distinctions: this error means the request never sent anything, so nothing was charged; and it is deterministic - the same body will fail the same way every time, so retrying without changing the payload is pointless. If your JSON is so broken it can't be parsed at all, you get The request body could not be parsed as JSON instead, and a wrong Content-Type header produces The content type, XXX, is not supported.

{
  "message": "The request body has 2 error(s)",
  "details": [
    {
      "message": "May not be empty",
      "property": "messages[0].text"
    },
    {
      "message": "Must be one of the following values: [text, image, video, audio, location, sticker, template, imagemap]",
      "property": "messages[1].type"
    }
  ]
}

Build message objects through one shared constructor that enforces the documented caps (text, Flex sizes, quick reply) rather than assembling JSON ad hoc, and wire the free validate endpoints into CI for every template you ship. Truncate user-generated text in UTF-16 code units with margin for LINE emoji alternative text, and reject non-HTTPS media URLs at input time.