Discord error 50035: Invalid form body

One or more fields in your request failed validation. This is Discord's general schema-validation error and it is the most informative one in the API, because the response carries a nested errors object that spells out exactly which field was rejected and why. The path to the field is expressed as nested keys, with array indexes as keys (embeds > 0 > description), ending in an _errors array of {code, message} pairs.

Typical inner codes: BASE_TYPE_MAX_LENGTH (content over 2,000, embed description over 4,096, custom_id over 100), BASE_TYPE_REQUIRED (a required field is missing), BASE_TYPE_BAD_LENGTH, NUMBER_TYPE_COERCE (a string where a number was expected, often a snowflake sent as a number), EMBED_FIELDS_MAX (more than 25 fields), APPLICATION_COMMAND_INVALID_NAME (uppercase or spaces in a slash command name), BASE_TYPE_CHOICES (value not in an enum), and EMBED_TOTAL_SIZE (combined embed text over 6,000).

It also fires when the Content-Type header is wrong or missing for the body you sent. If the errors object is absent, suspect the header.

Read the errors object like a file path. errors.embeds.0.description._errors[0] means: the embeds array, index 0, its description field, first violation. Keys mirror your payload exactly, array indexes appear as string keys, and every path terminates in an _errors array of {code, message} pairs. A payload can carry several violations at once, so a short recursive walk beats eyeballing:

function walk(node, path = []) {
  if (Array.isArray(node._errors)) {
    console.log(path.join('.'), node._errors);
    return;
  }
  for (const key of Object.keys(node)) walk(node[key], path.concat(key));
}
walk(err.rawError.errors);   // discord.js DiscordAPIError

discord.py flattens the same structure into the exception text (In embeds.0.description: Must be 4096 or fewer in length.), so logging str(error) is usually enough there.

{
  "code": 50035,
  "errors": {
    "embeds": {
      "0": {
        "description": {
          "_errors": [
            {
              "code": "BASE_TYPE_MAX_LENGTH",
              "message": "Must be 4096 or fewer in length."
            }
          ]
        }
      }
    }
  },
  "message": "Invalid Form Body"
}

// missing field example from the docs:
{
  "code": 50035,
  "errors": {
    "access_token": {
      "_errors": [
        { "code": "BASE_TYPE_REQUIRED", "message": "This field is required" }
      ]
    }
  },
  "message": "Invalid Form Body"
}

Put a validation layer between your feature code and the Discord client that enforces the documented limits (see Discord limits) and truncates or rejects before the request is made. Make sure your error logging prints the errors object, not just message; without it 50035 is a guessing game.