Decompose the Work

Write an indented capability tree. Add “— observable result” to each leaf so the tool can tell whether the pieces are testable.

Frame→Scope→Unknowns→Decompose→Choose→Test→Debug→Learn↺

"Build a chat app" is one sentence; this tree expands it into six capabilities. Realtime Delivery is the one you cannot yet build — which is the point of naming it. Split by Capability →

Indented outline

Build a chat app
  Authentication
    Sign up — A new email and password create an account that can log in.
    Log in — Correct credentials yield a session; wrong ones yield an error and no session.
    Session expiry — An expired session is refused on the next request.
  Conversations
    Create direct conversation — Two users get exactly one conversation between them, however many times it is requested.
    Create group — A group with three members is visible to all three and to nobody else.
    List conversations — A user's list shows every conversation they are in, most recent first.
  Messages
    Send message — A sent message is stored with sender, conversation and timestamp.
    Reject non-member — A user outside the conversation cannot post to it.
    Message limits — A 10,001-character message is refused with a message naming the limit.
  Realtime Delivery
    Push to connected members — A member with an open connection sees the message without reloading.
    Reconnect — A client that drops and reconnects receives the messages it missed.
    Ordering — Two messages sent in order arrive in that order on every client.
  History
    Load recent — Opening a conversation shows its last 50 messages.
    Paginate older — Scrolling up loads the previous 50 without duplicates or gaps.
  Read State
    Mark read — Opening a conversation clears its unread count for that user only.
    Unread counts — A message sent to a closed conversation increments its unread count by one.

Score

100

Leaves

Testable

Every leaf is testable and the tree is by capability — the structure has nothing obviously wrong with it.

No structural problem found. That does not prove the decomposition is meaningful; it only means the shape is usable.