11206: HTTP protocol violation

This error occurs when a valid HTTP exchange can't be completed with the webhook URL configured for your messaging workflow. For incoming messages, Twilio makes an HTTP request to your webhook and expects a valid response. Invalid cookie data in that response can also trigger this error.

• Your webhook endpoint 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 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 your webhook URL returns a valid HTTP response. When you respond to an incoming message with TwiML, return a supported content type such as text/xml, application/xml, or text/html.
• Confirm that the URL and port match the protocol you configured. Do not send HTTP traffic to an HTTPS-only port.
• Check your application, proxy, and network path for interruptions that can corrupt the HTTP exchange.
• Remove invalid control characters from cookies, and make sure every cookie has a non-empty name before your application returns the response.
• Review the failed request and response in Messaging Logs to inspect what Twilio received from your endpoint.

• Twilio's request to your incoming message Webhook URL
• TwiML for Programmable Messaging
• Webhooks security