---
"@context": https://schema.org
"@type": TechArticle
"@id": https://www.twilio.com/docs/api/errors/11251#article
headline: "11251: Fatal protocol violation"
url: https://www.twilio.com/docs/api/errors/11251
inLanguage: en
dateModified: 2026-06-09T11:00:42.000Z
author:
  "@type": Organization
  name: Twilio Developer Education Team
publisher:
  "@type": Organization
  name: Twilio
---

# 11251: Fatal protocol violation

Log Level: ERROR

## Description

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.

### Possible causes

* 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.

### Possible solutions

* 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.

#### Additional resources

* [Webhooks Overview](/docs/usage/webhooks/webhooks-overview)
* [Twilio Debugger](/docs/usage/troubleshooting/debugging-your-application)
* [Webhooks security](/docs/usage/webhooks/webhooks-security)