Tags on the sites are, at most, 25 characters.

Permitted characters are a-z 0-9 + # - .

However, this is not part of the API contract insofar as if the site definition of a tag changes so will the API's. It is advisable, for correctness, to have a code path to handle tags > 25 characters in length. Simplest would be to truncate them.

All that being said, there are no plans to change tag lengths at this time.

Tags on the sites are, at most, 35 characters.

Permitted characters are a-z 0-9 + # - .

However, this is not part of the API contract insofar as if the site definition of a tag changes so will the API's. It is advisable, for correctness, to have a code path to handle tags > 35 characters in length. Simplest would be to truncate them.