X_SET_COOKIE_HAND_BUILT
Means
a Set-Cookie attribute is spelled in a literal, or a Set-Cookie header is written by name, outside packages/core/src/cookie.ts and packages/http/src/set-cookie.ts
Typical cause
a header built by hand ('Path=/; HttpOnly', `Max-Age=${s}`) — the bytes core's serializer would have refused or encoded go out as written, and two spellings of one header drift
Fix
setCookie / deleteCookie from @ultimat3/http, or serializeSetCookie below http; then bun run set-cookie-literals --json