THREADSAFE=0 was correct when sx had no threads; with std.thread (S6) and std.http's pooled dispatch (S7b), concurrent connections corrupted sqlite's unprotected globals (caught live: distd under ab -c20 died with free-of-unallocated inside yy_reduce). Serialized mode is sqlite's own default and safe for every consumer; per-connection use across threads is the supported pattern.