Discord error 50109: Request body contains invalid JSON

Discord tried to parse your request body as JSON and failed. This happens before any field validation, so you will not get the helpful errors object that 50035 provides. The body is syntactically broken: trailing commas, single quotes, unescaped control characters or newlines inside strings, a JavaScript object literal that was string-concatenated instead of serialized, or a double-encoded string (a JSON string containing JSON).

It also appears when the body is fine but the Content-Type is application/json and the body is actually form-encoded, or when a multipart request's payload_json part contains malformed JSON. Bots using raw HTTP clients and templating systems to produce request bodies are the main source; library users almost never see this.

{
  "message": "The request body contains invalid JSON.",
  "code": 50109
}

Construct payloads as native objects and serialize at the last moment in one shared HTTP helper. Add a test that round-trips a sample payload through JSON.parse. Treat any string-typed "body" variable in request code as a smell.

When integrating third-party templating or webhook-forwarding systems, validate their output as JSON before relaying it to Discord so their bugs do not surface as yours.

Invalid JSON almost always means a human built a string; serializers do not make syntax errors. If the payload comes from a webhook-forwarding or templating system rather than your own code, validate its output before relaying, as covered in the webhook debugging guide.