Hi.
CIAO insists on a maximum of 512 characters, while Quintus (and I believe Sicstus) allow atom values up to 64K.
The latest versions of Ciao (from 1.5p35 on) allow atoms of unbound length. Maybe you are using an old version (possibly 1.4)? If so, we recommend you to update to the latest Ciao Prolog version, (Ciao Prolog 1.5p167 as of now), available at
http://www.cs.nmsu.edu/~clip/Software/Ciao/ http://www.clip.dia.fi.upm.es/Software/Ciao/
If you are using relatively recent versions, or if this last version does not solve your problems, please send us a short example of the code with which you are experiencing troubles.
Best,
Manuel Carro & the Ciao Prolog Team