HookBus: A Vendor-Neutral Lifecycle Event Router for Heterogeneous AI Agents (Version 4, router reframing and AxonFlow differentiation)

HookBus is a vendor-neutral lifecycle event router for heterogeneous AI agents. The router is a transport and fan-out layer only; governance is performed by pluggable subscribers (enforcement gates, DLP, audit, cost attribution, knowledge-base injectors), not by the bus itself. Version 4 corrects the architectural framing from "governance-aware bus" (used in v1-v3) to "router with governance subscribers", and adds Section 2.7 differentiating HookBus from execution control planes with internal event buses, specifically AxonFlow's EventHub (first released in AxonFlow v4.2.0 on 10 February 2026, platform/shared/execution/event_hub.go, BUSL 1.1). The three-axis differentiation is: (i) HookBus is cross-runtime / cross-session, AxonFlow EventHub is per-execution-ID; (ii) HookBus accepts external publishers from independent AI-assistant runtimes (Claude Code, Cursor, Amp, Hermes, OpenClaw), AxonFlow EventHub accepts only internal publishers (MAP and WCP trackers); (iii) HookBus subscribers return synchronous permit/block decisions consolidated by a priority-weighted deny-wins algorithm back to the publisher through the bus, AxonFlow EventHub's Publish() has no return value and Subscribe() returns a read-only channel (AxonFlow's policy gate, StepGate, is a separate HTTP endpoint not composed with the bus). Prior version contributions retained: Standard Event Type Taxonomy (Section 4.1) covering six canonical lifecycle events and LLM Usage Metadata Contract (Section 4.2) for cross-publisher cost attribution and usage audit. Released under CC0 as defensive publication so that the event type names, envelope field names, router architecture, and metadata contract cannot be monopolised by any single vendor through later patent filings. The bus architecture itself is covered by UK Patent Application GB2608069.7 (filed 8 April 2026, Agentic Thinking Limited).