Event-driven AgentLoop with mailbox drain pattern

[bonk-moltbot]

What

Replace the closed while-loop AgentLoop with an event-driven architecture. Everything that enters the context window goes through PendingMessage as a single pipeline:

Event → Subscriber → PendingMessage(kind: background|active) → Drain → Promote → Message

One path for everything: user messages, agent messages, tool calls, tool responses, memories, skills, goals, sub-agent responses. No exceptions, no bypass routes.

Problem

• AgentLoop is a closed while-loop invisible to EventBus — tool calls happen inside, subscribers can't react
• Pending messages hang when agent is idle — waiting for user input to trigger processing
• between_rounds is a hack to peek inside the closed loop
• Messages enter context through multiple paths (direct persistence, phantom injection, pending messages)

Key design decisions

• Two kinds of PendingMessage: active (triggers LLM request — user messages, sub-agent responses, tool responses) and background (appears in context silently — memories from Mneme, skills from Melete)
• AASM state machine on Session: :idle / :awaiting / :executing — replaces manual locks, determines interrupt strategy
• Mailbox drain pattern: SQLite serialized writes provide concurrency safety without mutex
• Two interrupt types: mailbox interrupt (stop drain loop) and tool execution interrupt (synthetic tool_response)

Prerequisites (done)

• Assemble tool pairs by tool_use_id, not message order #419 — Assemble tool pairs by tool_use_id ✅

References

Full exploration: thoughts/shared/notes/2026-04-03/qa-brainstorm-event-loop-mailbox.md