---
title: "400 Bad Request"
canonical: "https://lemwatch.com/http-status-codes/400"
category: "client-error"
last_verified: "2026-08-11"
source: "https://lemwatch.com"
---

# HTTP 400 Bad Request

**Summary** — The server could not understand the request due to invalid syntax or malformed data.

## What it means

HTTP 400 Bad Request means the server cannot process the request because the client sent something invalid — malformed JSON, missing required fields, invalid characters in the URL, or exceeded size limits.

## Common causes

- Malformed JSON in request body
- Missing required parameters
- Invalid URL encoding
- Request body exceeds size limit
- Invalid header values

## How to fix

- Validate request data before sending
- Check for special characters in URLs
- Ensure Content-Type header matches body format
- Review API documentation for required fields

## SEO impact

Pages returning 400 will not be indexed. If users see 400 errors, it indicates a broken form or link.

## Monitoring tip

LemWatch alerts on 400 errors from your monitored pages. Frequent 400s may indicate broken forms or outdated API integrations.

**Related:** [401](https://lemwatch.com/http-status-codes/401.md), [403](https://lemwatch.com/http-status-codes/403.md), [404](https://lemwatch.com/http-status-codes/404.md), [422](https://lemwatch.com/http-status-codes/422.md)

_Canonical HTML page: https://lemwatch.com/http-status-codes/400_