Error 400: what to do when gpt-4o fails

A 400 means the request itself is invalid — a missing field, a wrong type, or a parameter this model does not support. Retrying will not help; the request body must change.

400 Bad Request means the server refuses to process the request because the body itself is invalid. Unlike 5xx, a 400 will not resolve by retrying — you get the same response every time. The parameters must be fixed first.

gpt-4o is served by OpenAI. Everything on this page — triggers, fixes and measured data — is compiled from the real runtime behaviour of this model at the gateway layer.

At this gateway, the most common trigger is: A required field is missing (model or messages). The recommended first action is: Check the request body against the parameter table on this page.

Common causes

• A required field is missing (model or messages)
• A parameter has the wrong type
• A parameter is not supported by this model
• messages is malformed or role has an invalid value

How to fix

• Check the request body against the parameter table on this page
• Remove unsupported parameters and retry
• Make sure messages is an array with valid role values
• Start from a minimal request, then add parameters one by one

Retry with exponential backoff

The snippet below retries when gpt-4o returns 400, up to 5 attempts, with an increasing wait plus random jitter so concurrent calls do not retry in lockstep. Read the base URL and API key from environment variables — never hardcode them.

import os, time, random
import requests

BASE  = os.getenv("OPENAI_BASE_URL")   # e.g. https://<your-gateway>/v1
KEY   = os.getenv("OPENAI_API_KEY")
MODEL = 'gpt-4o'


def chat(messages, retries=5):
    """Retry with exponential backoff + jitter."""
    for i in range(retries):
        try:
            r = requests.post(
                BASE + "/chat/completions",
                headers={"Authorization": "Bearer " + KEY},
                json={"model": MODEL, "messages": messages, "stream": True},
                timeout=60,
            )
            if r.status_code == 429 or r.status_code >= 500:
                time.sleep(min(2 ** i + random.uniform(0, 1), 30))
                continue
            r.raise_for_status()
            return r.json()
        except requests.exceptions.Timeout:
            time.sleep(min(2 ** i + random.uniform(0, 1), 30))
    raise RuntimeError("gave up after " + str(retries) + " retries")


print(chat([{"role": "user", "content": "hello"}]))

Key facts for this model

FAQ

Other errors on this model

• gpt-4o: error 429 — causes and fixes
• gpt-4o: error timeout — causes and fixes
• gpt-4o: error 500 — causes and fixes
• gpt-4o: error 502 — causes and fixes
• gpt-4o: error 503 — causes and fixes
• gpt-4o: error 504 — causes and fixes
• gpt-4o: error 401 — causes and fixes
• gpt-4o: error 403 — causes and fixes

Other models with the same error

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• deepseek-v4-pro
• grok-4.3
• llama-3.3-70b-instruct
• qvq-max
• qwq-32b
• glm-5
• MiniMax-M3
• kimi-k3
• hy3
• doubao-seed-evolving
• mimo-v2.5
• claude-opus-4-6
• gemini-2.5-flash

Back to the model page

Data updated: 2026-10-10 18:35