Twilio error 11206 means Twilio received a response from your webhook server but the response did not conform to valid HTTP protocol structure. Unlike 12100 or 12300, which are about the content of the response, 11206 is about the HTTP envelope itself being malformed: invalid status line, garbled headers, or a response body that arrives in a format Twilio's HTTP client cannot parse as a valid HTTP message. This error always originates in your server's response generation logic or web framework configuration.
What Causes This Error
A common cause is a web framework or middleware component that writes raw bytes to the response stream before the HTTP headers have been sent, which corrupts the HTTP response structure by inserting content into the header section of the message. PHP applications are particularly susceptible to this when a BOM (byte order mark) is present at the start of a PHP file, or when a whitespace character or echo statement appears before the opening PHP tag, causing output to be flushed before the Content-Type and status headers are written. Misconfigured chunked transfer encoding where the chunk size headers are malformed or the terminating zero-length chunk is missing causes Twilio's HTTP client to report an HTTP protocol violation as it cannot determine where the response body ends. Web application firewalls or reverse proxies positioned between your server and the public internet that rewrite or inject HTTP headers in a malformed way are also a source of 11206 errors that can be difficult to diagnose because the violation is introduced between your application and Twilio rather than in your application code itself.
How to Fix It Step by Step
Use curl with verbose mode (curl -v https://your-webhook-url) to retrieve the raw HTTP response from your webhook endpoint and inspect the response headers and status line for any malformation: look for unexpected bytes before the HTTP/1.1 status line, duplicate or malformed Content-Type headers, and incorrect Content-Length values that do not match the actual body length. Enable output buffering in your webhook handler and ensure all HTTP headers are set before any body content is written: in PHP, call ob_start() at the top of the script and use header() for all response headers before any echo or print statements; in Node.js, set all response headers using res.setHeader() before calling res.write() or res.end(). Check every middleware in your application's request pipeline for any component that might write directly to the response stream without going through the standard header-setting mechanism, including logging middleware, error handlers, and authentication layers. If you use a reverse proxy (Nginx, Apache, Caddy), test your webhook URL both through the proxy and directly to the origin server to determine whether the protocol violation is introduced by your application or by the proxy configuration.
How to Prevent It from Recurring
Adopt a response builder pattern in your webhook code where all response metadata (status code, headers, body) is assembled in a structured object and emitted as a single, well-formed HTTP response at the end of the handler, rather than writing to the response stream piecemeal across multiple code paths. Use a well-maintained web framework that handles HTTP response serialization correctly by default, such as Express in Node.js, Flask or Django in Python, or Rails in Ruby, rather than writing raw HTTP responses manually, since these frameworks handle encoding, header ordering, and chunked transfer encoding correctly out of the box. Add an integration test in your CI pipeline that makes an actual HTTP request to your webhook handler in a test environment and validates the HTTP response structure using an HTTP client library, not just the response content, catching protocol violations before they reach production. For PHP applications specifically, run php -l on all webhook files to check for syntax errors and use a code editor with BOM detection to confirm no files have a byte order mark prepended.
When to Call a Specialist
If curl and browser tests show a valid HTTP response but Twilio still logs 11206, the protocol violation may only appear under the specific HTTP/1.1 negotiation behavior Twilio uses when making POST requests with a form-encoded body, which can trigger edge cases in some frameworks' response handling that do not appear in standard GET requests. A specialist can set up a proxy server between Twilio and your webhook to capture the exact bytes Twilio receives and compare them against the RFC 7230 HTTP/1.1 message format spec, pinpointing the specific malformation. You should also escalate if the 11206 errors started appearing after a recent server or framework upgrade, as new versions of web servers and reverse proxies occasionally change default header behavior in ways that introduce HTTP protocol violations for specific clients. Protocol-level HTTP debugging is one of the more specialized areas of webhook troubleshooting that benefits from specialist tooling and expertise.
Conclusion
Error 11206 is an HTTP envelope malformation that is traced and fixed by inspecting the raw response your server sends and ensuring headers are written correctly before any body content. If this error is blocking your production system, contact our team and we will diagnose and fix it within the hour.
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