Is there a limit to the length of the ClientId or Channel name?

For clientId: there is no limit; however the clientId is counted as part of the size of the message and will contribute towards the total message size.

 

For channel name: see https://support.ably.io/en/support/solutions/articles/3000046824-what-restrictions-are-there-on-channel-names-

 

 

See https://support.ably.io/en/support/solutions/articles/3000035792 for more details on message size limits by account type.