11251: Fatal protocol violation

Twilio encountered an unrecoverable HTTP protocol violation while speaking to the target URL you configured. This usually means Twilio could not complete a valid HTTP exchange with your webhook or callback endpoint, or the response included invalid cookie data.

• Your target URL returned malformed or incomplete HTTP data.
• A network interruption broke the connection before Twilio received a valid response.
• The configured URL sent http:// traffic to an HTTPS-only port.
• Your response included an invalid cookie character such as \t, \r, \n, \v, or \f.
• Your response set a cookie with an empty name.

• Make sure the target URL returns a complete and valid HTTP response. If Twilio expects TwiML, return valid TwiML. If Twilio only needs an acknowledgment, return 200 OK or 204 No Content.
• Confirm that the URL scheme and port match your server configuration. Do not send http:// traffic to an HTTPS-only port.
• Review your application, proxy, load balancer, and network path for interruptions that can corrupt or truncate the HTTP exchange.
• Remove invalid control characters from any Set-Cookie header, and make sure every cookie has a non-empty name.
• Test the endpoint directly with a browser, curl, or Postman so you can inspect the raw response and catch malformed headers or unexpected redirects.
• Review the Twilio Debugger and request inspector to see the request Twilio made and the response your application returned.

• Webhooks Overview
• Twilio Debugger
• Webhooks security