The Twilio Programmable Messaging platform has a limit of 1600 characters in a single incoming or outgoing message. This limit applies to all Programmable Messaging channels, including SMS, MMS, WhatsApp, Facebook Messenger, etc.
However, for SMS messaging, Twilio recommends sending messages that are no more than 320 characters to ensure the best deliverability and user experience. See below for details.
Outgoing message requests that exceed 1600 characters will fail with Error 21617.
Incoming messages to Twilio that exceed 1600 characters will be truncated to 1600 characters, and will be marked as failed with Error 21617.
Additional Considerations for Long SMS Messages
Although Twilio supports messages up to 1600 characters for all Messaging channels, including SMS, it is important to understand that sending very long SMS messages can negatively impact deliverability and user experience. Sending long SMS messages also affects message price, due to an SMS-specific concept called message segmentation.
Twilio recommends keeping SMS messages to a length of 320 characters or less.
What happens if I exceed the recommended 320 characters per message?
Occasionally sending messages longer than 320 characters will not cause problems. However, the longer your SMS messages are, the harder they are for your recipients to read and digest quickly. In addition, some wireless carriers and devices may behave differently than others when receiving long SMS messages. Regularly sending very long SMS messages may reduce your overall delivery rate.
Lastly, note that the length of an SMS (and whether any special characters, such as emojis, are present) will affect its price, because of a concept called segmentation.
For more details about SMS segmentation and considerations when sending long SMS messages, please see Does Twilio support concatenated SMS messages or messages over 160 characters?