HTTP 400 Bad Request

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

What 400 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 it

• 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 Frequent 400s may indicate broken forms or outdated API integrations.