Set_context is truncating my string?

Question

I have a procedure where I receive several parameters and buid a query based on this params:
IF keyword IS NOT NULL THEN
     dbms_session.set_context('PESQ_CTX', 'keyword', keyword);
     query := query || 'CONTAINS(ed_fotos.detalhe, sys_context(''PESQ_CTX'', ''keyword'')) > 0 AND ';
END IF;
As far as I can see when I register the keyword variable using set_context, the variable string is truncated. Keywors is a mixt of several comparisons: '({SANDY} within description OR {SANDY} within headline etc... )'
When the variable string is large, it is truncated and the variable looses the closing parentesis, causing an Oracle text error:
ORA-29902: error in executing ODCIIndexStart() routine
ORA-20000: Oracle Text error:
DRG-50901: text query parser syntax error on line 1
Is there anyway to avoid this problem? Does the variable registered has a limit of 255 characters? Using short variables I dont receive any error.
If I build the query without the context it works, but set_context should improve the performance
IF keyword IS NOT NULL THEN
     --dbms_session.set_context('PESQ_CTX', 'keyword', keyword);
     query := query || 'CONTAINS(ed_fotos.detalhe, ''' || keyword || ''') > 0 AND ';
END IF;

Answer 1

The default return size for sys_context is 256.
if you need more than that, you need to provide the optional third parameter, the length (up to 4000)