WarmAcpBackend (AgentBackend) holds one persistent goose acp / qwen --acp child + ClientSideConnection + ACP session per (chat,agent); initialize+session/new once, reused across turns. Abort = session/cancel the prompt only (never kills the child); child exit -> agent_sessions.status='crashed' -> re-spawn next turn. Dispatcher routes goose/qwen chat-tab tasks to the pooled warm backend via pure shouldUseWarmBackend (needs session_id+chat_id); one-shot runExternalAgent kept as fallback for arena/MCP/new_task. handleSessionUpdate extracted to a shared pure acp-event-map.ts (one-shot path byte-identical). SDK: installed @agentclientprotocol/sdk@^0.22.1 has stable resumeSession/loadSession; resume moot in the warm hot path, deferred to Phase 3. 15 new tests (warm-acp-routing, acp-event-map); 180 coder tests pass; tsc + build clean.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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