Switching the runtime mid-conversation in Build with AI only updated local React state, so the picker could show runtime B while every subsequent message still executed on the runtime frozen at session create time.
- Add PATCH /api/agent-builder/sessions/{id}/runtime to rebind the hidden builder carrier (runtime_id/runtime_mode, model cleared since model ids are per-runtime). Creator-only, builder carriers only, target must be in-workspace, usable by the member, and online; a reply in flight returns 409.
- Serialise rebind against send: both take LockChatSessionForRuntimeBind on the chat_session row and SendDirectChatMessage re-reads the agent inside that transaction, so a send blocked behind a rebind cannot resume and stamp its task with the runtime the switch moved away from.
- Leave chat_session.runtime_id stale on purpose so the daemon starts a fresh provider session on the new runtime while Multica-side history and the draft survive.
- Frontend updates the draft only after the server reports the bound runtime, blocks sending during a rebind, disables the Mine/All filter alongside the trigger, and explains why the picker is locked during a pending reply.
Closes#5773
A quick Stop before the agent's first token no longer races a late reply. Started-but-empty cancellations defer the empty/non-empty judgment until the daemon acks its transcript flush (or a grace-period sweeper fires), then settle to a single outcome. Empty outcomes persist a durable, creator-authorized draft restore (fetched/consumed via a dedicated endpoint, reconnect-safe and at-most-once) instead of broadcasting the prompt over the workspace bus.
Closes#5219
