Hi everyone, in current LuaTeX versions, the result of `linebreak` filter is ignored and instead it is always treated as if no node has been returned (so the internal linebreak algorithm is applied). A minimal example: \directlua{ callback.register("linebreak_filter", function(nodes) return node.hpack(nodes) end) } Some very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very very long paragraph. \bye You would expect the whole paragraph to be set in a single line, but normal paragraph breaking is applies instead. This is caused by a change in revision 6686 from Mar 29 2018 "experimental) possible fix for for lua stack issue + consistent error handling (HH)". There in the function lua_linebreak_callback, the change return ret; } + lua_settop(Luas, s_top); p = lua_touserdata(Luas, -1); if (p != NULL) { a = nodelist_from_lua(Luas); try_couple_nodes(*new_head,a); ret = 1; } - lua_settop(Luas, s_top); return ret; has been applied. I am not sure what kind of stack issue this was supposed to fix, but as a side effect this moves the check for the node return value after `lua_settop`, so the return value is already removed from the stack when the test is done. So it is ignored. Best regards, Marcel Krüger