cinderchat is a sequence-keyed mailbox built on top of cinder's own relay. Each side writes its messages to its own slot; the other side drains it whenever it's next online — no live connection required, no missed messages just because someone closed their laptop. Every message waits up to 7 days for pickup, the same ceiling cinder itself has always had.
Pairs the same way cindertransfer does: each side generates a one-time key and independently derives the same secret (ECDH) — nobody transmits it, nobody reuses it. From there, messages are written and read under keys derived from that secret, one slot per message, in order.
cinderchat is the first thing in this family that asks your browser to remember anything across visits — your contact list, kept locally, never sent anywhere. That's a real, deliberate trust boundary, not an oversight, and it's why this page doesn't call it finished the way the others are. Try it, but know what you're trying.