Fixing 400 "Request body is not valid JSON"

These 400s mean the API could not read the request body, or the body was missing something it needs before it can do anything else.

The messages you can get

Request body is not valid JSON. The body did not parse, or was the literal null.
Field "model" is required. The JSON parsed but had no model string.
On /v1/messages: messages: Field required, and must be an array. when messages is missing or not a list.
On /v1/responses: input: Field required, and must be a string or an array of items.

HTTP 400
{
  "error": {
    "message": "Request body is not valid JSON.",
    "type": "invalid_request_error",
    "code": null,
    "param": null
  }
}

Common causes

Shell quoting. In Windows cmd, single quotes do not quote, so a curl -d '{...}' arrives broken. Use a file with -d @body.json.
Sending form data. Posting a dict as form fields instead of JSON (for example requests.post(url, data=payload) in Python) sends no JSON. Use json=payload.
String building. A message containing a quote or newline breaks hand-built JSON. Serialize with a JSON library.
Empty body. A request sent before the payload was attached.

How it shows up

SillyTavern builds valid JSON itself, so you will rarely see these there unless a proxy or extension rewrites the body. In SDKs the body is always serialized for you; seeing these errors usually means hand-written HTTP code. The OpenAI SDKs raise BadRequestError and do not retry.

Step by step fix

Print the exact body your code sends.
Run it through a JSON validator, or parse it back with your language's JSON library.
Check it has model as a string and messages as an array of objects with role and content.
Set Content-Type: application/json.
Test the minimal body:

{"model":"outlaw-1","messages":[{"role":"user","content":"hi"}]}

The docs have working examples for curl, Python and Node.

Telling it apart from similar errors

Key problems are checked first, so a request with both a bad key and bad JSON shows the 401. A model id that is present but wrong is model_not_found. Note that on /v1/chat/completions the API does not validate the rest of the body itself; a malformed field the model rejects (other than size or images) comes back as a 503 Model overloaded, so if a 503 never clears, check your body.

FAQ

Is a 400 charged?

No. These are refused before any model call.

Why did I get a 401 instead of the JSON error?

The key is checked before the body is read. Fix the key first, then the body.

Fixing "Unknown model" (model_not_found)This error means the model field held an id the API does not serve. The key and balance were fine; the name was not. Fixing a 401 "Invalid or disabled API key" errorA 401 means the API could not match the key you sent to a live key on an account. Nothing was generated and nothing was charged. Fixing 503 "Model overloaded. Please try again in a minute."A 503 means the model behind the API failed to answer, and the API had already retried before telling you. It is usually brief.