Issue

Some Twilio Studio users may observe intermittent HTTP 400 responses and Error 11200 alerts from the Studio Flow webhook endpoint after calls complete using the Connect Call To widget. This article explains why these errors occur, their impact, and recommended mitigation steps.

Symptoms

• After a call is forwarded and completed via the Connect Call To widget, Twilio Monitor or logs show HTTP 400 responses and Error 11200 events.
• The error occurs during post-call callbacks to the Studio Flow webhook.
• The call flow and caller experience are not impacted; calls complete successfully.
• These errors may trigger false-positive alerts in monitoring systems.

Impact

• The error is benign: it does not affect the actual call flow or the caller’s experience.
• The main impact is operational, false-positive alerts that can make it harder to distinguish genuine webhook failures from expected timing races.

Product

Twilio Studio

Environment

Twilio Console

Cause

This behavior is due to a timing-related race condition:

• When a Studio Flow execution ends, any subsequent requests to the Studio Flow webhook URL (such as callbacks from the Voice platform) will return HTTP 400 if the call status is no longer “ringing.”
• If a callback arrives just after the execution has closed, Studio returns a 400 response by design. This results in Error 11200 being logged.
• This is most likely to occur when backend processing or network delays cause the callback to arrive milliseconds after the execution ends.

Resolution

To reduce the likelihood of this race condition and the resulting 400/11200 errors:

1. Add a Delay Function Widget:
   Place a Delay Function widget immediately after the Connect Call To widget in your Studio Flow.
2. Connect Both Transitions:
   Connect both the "Connected Call Ended" and "Caller Hung Up" transitions from the Connect Call To widget to the Delay Function.
3. Purpose:
   This keeps the Studio execution active for a short period after the call ends, increasing the chance that any post-call callbacks arrive while the execution is still open, thus avoiding the 400 error.

For implementation details and an example, see:
Twilio Functions: Add Delay Quickstart

Additional Information

• This workaround reduces, but may not completely eliminate, the race condition.
• The error does not affect billing, call duration, or Studio execution limits in normal use.
• Twilio engineering is aware of this behavior and is evaluating platform-side improvements to suppress these benign alerts in the future.

If you continue to see frequent Error 11200 events after implementing the workaround, or if you observe any impact to your call flows or caller experience, please reach out for further assistance.