Why a conformance suite at all

Your server works with your client. Ship it, and the first Safari user, Go bot, or reverse proxy that connects sends something legal but weird — a ping mid-fragmentation, a 16-bit length that could've been 7-bit, a close frame with a 1-byte payload — and your parser corrupts state or hangs.

RFC 6455 is full of MUSTs that never come up in happy-path testing. That's why Autobahn|Testsuite exists: ~500 scripted cases that probe every rule — valid frames you must accept, malformed frames you must reject with the right close code, and torture cases (fragmented pings, zero-length payloads, over-long encodings). "Passes Autobahn" is the community's bar for a real WebSocket implementation; every serious library (ws, websockets, gorilla, tungstenite) publishes its report.

The discipline it enforces is the mindset shift of this lesson: a protocol implementation isn't done when it works — it's done when every ILLEGAL input produces the SPECIFIED failure. Your exercise is a miniature Autobahn: given a frame's header bytes, output VALID or INVALID <reason> with the checks applied in a strict order, first failure wins — exactly how a conformant parser short-circuits.