/branch (minimal): a conversing mnemon child

A branch is an ordinary parented registry child that converses. /branch [prompt] forks the trunk’s conversation into a new tab — mnemon-style context inheritance — but withholds reply, so the child parks Held for the human while its registry entry lives and quiesces the moment the entry is gone. No Station, no parent-less registration, no per-agent generations: four small mechanisms and one command, all resting on a single enabling concession.

This is the minimal cut. The full plan lets branches survive the trunk’s /clear and drop out of the trunk model’s agents view; that machinery existed almost entirely to buy those two properties. Give them up and the branch collapses to a plain child.

Conversing is derived from the tool view

returns() reads whether the agent holds reply (the Gate::Returns tool), not its parent/interactive position. The tool view is the single source of truth, so parking, the reply-nudge, and the advertised tools cannot disagree: an agent built without reply converses, full stop, wherever it sits in the tree (agents). This is what lets a conversing agent exist that is not the trunk — a branch is interactive, reply-withheld, and parented. park_mode reads interactive && !returns(); a conversing agent whose entry has been reaped reads !is_live and quiesces rather than parking Held on a dead entry (a zombie).

The enabling decision: /clear on the trunk clears everything, branches included

That single concession keeps a branch a plain registry child, and it dissolves — for free — the two problems that made a naive “mnemon minus reply” unsound:

• The /clear zombie. /clear runs clear_subtree(trunk), which removes the branch entry as an ordinary descendant; within one poll the branch’s park reads is_live == false, quiesces, and its tab retires. Only the trunk can /clear, so the global clear generation stays correct.
• The 1-hour ceiling. A branch registers with the ceiling off: a conversation must not lose a turn at the hour mark, and a user-seated turn is human-typed, not model recursion the ceiling exists to bound. register takes an explicit ceiling: bool — sub-agents pass true, a branch false.

Empty settle epilogue: no branch death touches the trunk

A branch pushes no AgentResult upward. Its result is never wanted, so the settle epilogue is gated empty: Kind::Died still fires, but the render-reply / commitment / parent_mailbox.push / settle block runs only for a returning child. This is why no branch death — /clear, /close, or process exit — ever pollutes the trunk’s context: not because a late result is caught by generation admission, but because the branch never posts one. The sub-versus-branch split is derived once, at the spawn site, from child.returns(): the same tool-view fact drives the ceiling knob, the tab kind, and the empty epilogue, so none can drift from the child’s actual tools.

The command surface

• /branch [prompt] forks the trunk’s conversation: mnemon context inheritance, the trunk’s capabilities verbatim (user-seated — no permissions argument, no narrowing), an own provider handle seeded from the trunk’s current model (cache hits), and fuel = creator − 1 so a branch may itself spawn. A bare /branch parks and waits on its tab; /branch <prompt> seeds the prompt as the fresh final turn. It is handled on the trunk at the turn boundary, host-side like the rest of the slash-command surface, and carries no fuel gate (human-typed).
• /close on a branch tab is the one command admitted off the trunk — a pure frontend registry op (remove_subtree: cancel_entry then remove {focused} ∪ descendants), never routed to a worker, so it needs no per-tab control. It errors on the trunk and on a plain sub-agent tab; the frontend tells a branch apart by a branch-tab set on Tabs, seeded when the branch’s Born arrives.

Steering is otherwise the ordinary registry-mailbox path — typed lines are the branch’s next turns. Any other recognised worker command typed on a branch tab is refused (“not available on this tab”) by the unified parser, never mailed to the model.

Deliberately accepted warts

Named here so nobody rediscovers them as bugs:

• /quit on the trunk kills branches with the process (their user.logs still flush). It has a named fix in the full plan if branches earn the upgrade.
• Ctrl-C (and Esc) no longer reach a branch’s turns from the trunk at all — cancel-per-tab made both cancel keys per-tab, so a branch turn is interruptible only from the branch’s own tab.
• The trunk’s model still sees a branch in agents and may message it (harmless, occasionally useful); agent_cancel can cancel its turn but cannot kill it — Held parking ignores the token.

See also

cancel-per-tab (per-tab cancellation, which ships first and is why Ctrl-C no longer reaches a branch), agents (the tool-view derivation and the uniform node), subagent-memory-modes (the mnemon inheritance a branch reuses). Maps: agent, tools, frontend.