Amazon CloudWatch Alarm Name lenght restrictions

There are two parts in your question, first is cloudwatch alarm name limit, which is 255 characters. Second is SNS, which is 160 characters.

When cloudwatch alarm gets into alarm state and send trigger to sns, there SNS character limit comes in to the play. There is a limit of 160 characters per message and your message might be getting split into two parts(1/2, 2/2 80 characters each), but one single message can only have 160 characters.

Here is the SNS Documentation, which you can refer.

Attaching snapshot for your quick reference.

Hope you find this useful.

Abhishek